CA2111829A1 - Multiband programmable compression system - Google Patents

Abstract

An improved audio compressor (100) of the type which is useful in a hearing aid and having a programmable compression ratio is disclosed. In the preferred embodiment, the compressor is used in a hearing aid device of the type which comprises a bandsplit filter (106) to divide an input audio signal into a plurality of frequency bands (108, 110). Associated with each frequency band is a limiter means (220) which provides variable gain amplification of the input signal as a function of the amplitude of the input signal and as a further function of a programmable control signal. A
feedforward means (206) is disposed in parallel to the limiter means (220), said feedforward (206) means providing a fixed gain signal. The output of the limiter means and the feedforward means are combined at a summing node (212) which forms the input to a variable gain amplifier. Associated control circuitry allows to limiter gain to be programmed to meet the needs of an individual user. The compressor has a variable compression ratio in the form of a sigmoid function.

Description

WO 93J00779 ~ 2 1 ~ 1 ~ 2 ~ Pcr/usg2/oss2l AN lMPROVED MULTIBAND PROGRAMMABLE COMPRESSION SYSTEM
FIELD OF THE INVENTION
This invention relates to audio signal processing and specifically to an integrated circuit multiband compressor system for use in devices for improving the S hearing of the healing impaired.
BACKGROUND OF THE INVENTION
Audio compressors are well known devices that may be advantageously employed in devices for assisting the heax~ng impaired. In a compressor, the gain of the device varies as a function of the amplitude of the input signal, the gain being 10 g~eatest for a low level input signal and smallest for a large amplitude signal.
In many cases the response of the ear of a hearing impaired person will be substantially dif~erent in terms of sensitivity and freguency response from that of a normal person. For a person with what is termed sensorineural hearing impairment, soft sounds are rendered inaudible, while loud sounds may be subjectively just as loud 15 as for a normal person. Conversational levels may be very soft or even inaudible for a person with scnsorineural hearing impainnent. Consequently, if linear amplification is used to assist such a person, loudness relationships are perceived as distorted, and loud sounds may be rendered uncomfortaWy and, in some cases, painfully loud. What is 1~ . .
necessary for comprehension of speech by the hearing impaired is to raise the 20 amplitude of soft speech cues to the level of audibility. Beyond this, further improvements may *~had by reestablishing loudness relationships. These concepts may be further understood by referring to Edgar Villchur, "Signal processing to improve speech intelligibility in~perceptive deafness~" Journal of the Acoustical Socie~y of America, Vol. --53, pp.~ 1646-1657.~
25 ~ Studies have shown that hearing aids with audio compression provide improved syllabic comprehension for;persons with sensorineural heanng losses. The use of audio compressors for the- hearing impaire~ is described extensively in a repo~
written by Walker ~and Dlllon, entitled "Compression in Hearing Aids: An Analysis, A
: Review, and Some Recommendations", NAL Report NQ. 90, published by the 30 Aus~alian Commonwealth Department of Health, National Acoustics Laboratories, i~:

WO 93/00779 ~ PCT/US92/055ZI~

21l1823 2 June 1982. Another clinical study, recently completed by B. C. J. Moore, J. S.
Johnson, T. C. Clark, and V. Pluvinage is reported in a paper, "Evaluation of a dual channel full dynamic range system for peo~le with sensorineural hearin~ loss," to be published in the Journal of the Acoustical Sociely of Ame~ica. This paper is S conclusive on ~e benefits of audio multiband compression.
Audio compressors may also be advantageously employed to tailor the characteristics of a hearing aid device to compensate for unique deficiencies ofindividual users as well as to simulate normal hearing under a variety of situations such as quiet or noisy environments. For example, in many cases a hearing impaired 10 person will only experience a hearing loss at high frequencies and at low levels. For such a person it is desirable to provide a device which amplifies sound only at low leve~s and high frequencies. At hi~h levels and hi~h frequencies, the gain of the device is reduced typically to a value close to unity. At low frequencies, the ~ain may be held near unity for all input sound levels. Thus, for this case, compression is lS introduced at high frequencies only, and the gain is near unity at high sound levels throughout the audio spectrum.
Since each hearing impaired person has a unique hearing response, a compression system with adjustable compression ratio and frequency response is hi~hly desirable. A multiband compressor system of wide dynamic range is descri~ed 20 in U.S. Pat. No. 4,882,762 dtled "Multi-Band Programmable Compression Systemn, issued Nov. 21, 1989, to the present applicant. This patent (the disclosure of which is hereby incorporated by re~erence), describes a system, currently in manufacture, for .
improving the hea~ing of pauents~ wi~ sensorineural hearing impairment. The system consists of an input trans;ducer to convert ambient acoustic sigrlals to elechicai signals, 25 ~ electronic ampli~le~ stages to establish- appropriate signal levels at various points in the system, a multiband compressor, and an output transducer to convert the ampli~led - ~electrical signals back into acoustic or mechanical form which can be heard by the hearing impaired person.
- The multiband compressor uses a plurality of compressor circuits of the 30 type described in U~S. Pat. No. 4,882,761, "A Low Voltage Programmable WO 93/00779 ~ 2 !3 PCr/US92/0S521 ~ -compressor issued November 28, 1989, also to present applicant. The compressor circuits shown in the '76l patent, (the disclosure of which is also hereby incorporated by reference), each perform effectively three functions in their respective frequency bands: first, each circui~ acts as a compressor circuit with programmable compression 5 ratio; second, each provides an amount of gain that is programmable to allow tailoring to the individual user; and ~ird, each provides the user volume control function. For purposes of this discussion, the term compression ratio is the ratio of a change in input signal level to the r~sulting change in output level.
The wide ran~e of gains that must be provided by the conlpressor circuits in perforrning these three functions has been found to adversely affect their performance, particularly as compressors, as described below.
In view of the extremely small size required of hearing device circuits which must fit in the human ear, the design concept of the device shown in the '761 patent was to incorporate these multiple functions in a single circuit. The improved compressor circuit described herein removes this limitation by separating these functions into æparate circuits, thereby greatly improving perfo~nance qua1ity, reducing distortion, increasing flexibility of gain, simplifying programmability and increasing manufacturing yidds, while taking no more silicon area on an integrated circuit "chip" than the earlier design. ~ ~
To appreciate how the circuit performs its function, we ~lrst describe the workings of a limiter h~aving pro~rammable characteristics as required in the present invention and how such a limit is used in a compressor having a progran~nable compression ratio. . ~ ~ ; ;,,; - i -. - ~ - In order to~provide signal limiting, a variable gain circuit controllable by an electrical signal is required. - Such circuits are well known in the art; o~e circuit ~at is par~icularly adaptable to the low supply volta~es used in hearing aids is-described in U.S. Pat. No. 4,~68A17, Va~iolosser, issued Sep. l9, 1989 to the present applicant and Baez.- As described in the '417 patent (the disclosure of which is hereby incorporated by reference), the ~current gain of the variolosser is equal to the ratio of two control currents, designated as IA and IB. with the overall current gain equal to WO 93/00779~ . t',`,^~ ~ ~ ~,. ,, ", ,, .,~ PCI~/US92/05521 . . .
211182~ 4 I~JIA. If we rectify and filter the incoming signal to this circuit to provide an envelope signal, and use this envelope signal to increase IA as the input signal increases, the current gain will decrease as the input signal increases. Hence, the slope of a curve of the output level vs input level begins to diminish at higher levels, until the output level 5 tends to remain constant above ~ given input level.
The gain falls as the input level increases because IA~ in the ratio ~ A, increases with input level. IA has two compGnents, a fixed dc current and a signal dependent current. At Yery low sound levels, the fixed dc component Of IA
predon~inates so that the gain is constant there. It is only when the signal dependent 10 part of IA becomes significant in relation to the dc part that the gain begins to fall.
We define P~b, the lower break point, as the sound level at which the signal dependent part Of IA becomes equal to the dc part. Note that so far we have described what may be referred to as a "soft limiter", i.e., a limiter in which the gain changes gTadually as the input signal increases. In this soft limiter I,~ is a fixed or DC current. From our 15 definition of the lower brealc point, it can be seen that the response at the break point is 6 dB below the intersection of the asymptotic values of the curve as the input level goes-to zero. This follows from the fact that at the break point IAO = IA3 and IA = IAO +
: ~ Ihsi where IAO is the dc or constant component of IA. and IAS is the signal rela~ed component Of IA ThUS, at the break point IA = 2IAO~ i.e., the overall gain is reduced ;~ ~ 20 by 50% or 6 dE~. ~
In the pro~rammable compressor of the above mentioned '761 patent, the limi~ng action causes the slope of the output level ~s input level at higher levels to begin to dhnunish as~the~value of IA rises above its dc value as described above.
Howeve ,~in;the~'761~device~at~some higher input level the value Of IB is caused to 25; - rise ~above its dc value `under the influence of the input signal. This in~oduces a second, upward break point into the curve that we term PUb, the upper break point. At higher levels beyond PUb ~the slope of the output level vs input level begins toasymptohcally approach a linear value. The mathematical function that de~lnes these characteristics, i.e., a sigmoid function, tends to be almos~ linear in the region between WO g3/00779 2 1 1 1 ~ PCr/US92/05521 ~J

the lower and uppa break points, with a point of inflection midway be~veen thesebreak points, as described be}ow. The input level corresponding to this inflection point will be termed Pv~ the pivot point. By varying the distance between the upper and lower break points, P,b and PUb, we can vary the slope of the output-input curve S between the break points over a relatively wide range of audio input amplitude. It can be shown that the reciprocal of the slope of the input-output curve is the compression ratio"u. Thus, by varying PUb relative to P~b (or vice versa), it is possible to obtain a continuous variation of the compression ratio. Programming the compression ratio can therefore be accomplished by variation of one or both of these break points, which, it 10 will be remembered, are set by dc currents applied to the aforementioned variolosser.
The prior art variable compressor applied ~he rectified envelope signal to change both ~A and IB whereas in accordance with the present invention, IA provides the limiting action, while IB provides for programming the circuit to provide a desired compression ratio. This improvement has several other advantages that will be 15 discussed below.
In addition to providing programmable compression ratios, the prior art circuit also allowed programming of gain, simply by varying the fixed or dc por~ions of I,~ and IB. i.e., lowering IAO or increasing 1, 0 causes the gain to increase.
Because of the wide range over which the gain of thçse circuits must 20 vary in the prior alt system in providin~ the three aforementioned functions, the values of control currents IA and IB must vary over a fairly wide range. This was found to ~ lead to intermoduladon ~between the control signals and spurious dc offset volta~es - leading to distortions, discussed below, that becarne quite severe for some gain or compression programmed values when the circuits were not exactly in balance. Thus, 25- while the circuit(-of~the '761 patent works very well to provide the desired functions, it , has been found that extremely careful adjustment of the circuit parameters is required :~ to main~un hi~h quality signal processin~. By requiring that IA and IB VarY over a range wide enough~to fulfill ~the demands of different amounts of hearing impairment, exacting speci~lcations are placed on the integrated circuit processin~ to obtain high ~:

W~ 93/0077~ , - Pcr/v5s~/~s52l i 211182~ 6 quality sound. These exacting specifications increase the manufacturing expense and reduce the manufacturing yield.
SUMhlARY OF THE INVENTION
Brie1y stated, the present invention provides an improved multiband S compress~r system ~hat is capable of improved distortion perfonTlance, greater gain flexibility, higher performance quality, and greater ease of manufacture. The present invention also provides an improved signal processing system when compared with the prior art. The invention contemplates a hearing device that includes a microphone for converting sounds to an electrical signal and a preamplifiler to amplify these signals.
10 T~e preamplifier feeds the input of a multiband compressor system having an output that feeds further signal processing circuits that may include an equalizer and an output ampli~ler that, in turn, feeds an electroacoustic ~ansducer, or loudspeaker.
Signals from the preamplifier are fed to a band split filter having a plurality of outputs, one for each band. The outputs of the band split ~ilter feed a plurality of signal limiters whose outputs are summed to forrn the output of the multiband compressor system.
In one aspect of the invention, the output of eaeh band of the band split filter is fed not only to a limiter, but also to ~ resistor. The output of each limiter is summed with the signal from the resist~r ~hat is a replica of the lirni~er input signal.
In t~is way, the output of each band can be separately adjusted.
In an ~ternate arrangement, a si~nal from the input of the band split filtcr is summed with ~he limiter outputs to create a multiband compression system wherein ~e compression ratio of each band is separately con~rollable by controlling - t~e gain of each linniter., ~ ~ ~;~ - i~. ,~'.'! !: '!' ' . "'".' ' '`,' 'i.~ '~-; In still another~aspect of the invention, one or more limiters may be left out for frequencies where limitin~ is not needed. In many cases of hearing impai~ment, for exan~ple, compression is not needed at the lowest frequeneies, so that a simple resistor may fonn the sole;path from the lowest band output of the band split filter to ~he output summation point.

WO 93/00779 ~ 2 9 PC~/US92/05521 In the preferred ernbodiment each of the compressors comprises two variolossers connected in series. The first is used as a limiter for signal compression with a prograrnmaUe compression ratio. The compression ratio of the ~Irst variolosser can be varied using a single prograrnmable variable. The second variolosser provides 5 a variable ~audiologically programmed) ~ain, a gain trim, and user volume control.
Thus, in addition to providing programmable compression ratios, this circuit also allows programming of gain. Moreover, the circuit provides independent programming of gains at high signal levels, such as at 80 dl~SPL, and at low levels such as 50 dBSPL. (llhe units generally used to measure acoustic amplitude are 10 decibels of sound pressure level or "dBSPL", and this term will be used herein.) The circuit further provides a dc control of gain from an independent source, allowing user control of the volurne.
Accordingly, it is an object of this invention to provide an improved multiband programmable audio compression system.
It is another object of tne present invention to provide an improved multiband programmable compression system -wherein gain and compression ratio ineach respective f~equency band may be adjusted independerltly.
It is also an object of the present invention ~o provide a si~nal compressor for hearing instruments having a pro~rammable compression ratio with 20 superior distortion characteristics.
It is still another object of this invention to provide high quality signal processing with less exactin~ circuit specifications, thereby to increase manufacturing yields.
It is -a fur~er object of this invention to provide a programmable signal 25 compression system whose compression ratio can be varied with a single program variable. -~
- ~ n - It is a further object of this invention to provide a multiband compression system whose equalization characteristic varies with signal level.
, , .

WO 93/0~779 P~/US92/05521 ~ ~P~

2~ ~23 ~`

It is a yet another object of this invention to provide a compressor with a continuously variable compression ratio that is easily manufactured in integrated circuit form.
It is a fur~her object of this invention to provide a compressor with a 5 continuously variable compression ratio that is oper~ble from a single energy cell.
BRIEF DESCRIPrION OF THE DRAWINGS
These and other obiests of the present invention may be iFwlly understood through the description below and the accompanying drawing in which:
FIG. 1 is a block dia~ram of a multiband compressor of the type which 10 is known in ~he prior art.
FIG. 2a is a simplified block diagram and Fi~ 2b is a more detailed block diagram of the compressor in accordance with one aspect of the present invention.
FIG. 3a and 3b are ~raphs of the output response and gain of limiter 15 c~rcuits, and FIGS. 3c and 3d are graphs of the output response and ~ain of programmable compressor circui~s of the present invention.
~ IG. 4 is a graph of the output response of the circuit of the present invendon for various prog~ammed compression ratios showing the relatively constant ~ or:Linear characteristic of the compr~ssion ratio over the dynamic range of ordinary 20 speech, and showing the de~ree of slope change achi~vable by the circuits in accotdance with ~e invendon.
.; FIG. S is a circui~ schematic diagram of a vatiolosser and a con~¢ol circuit for a compressor of one embodiment of the invention. ~:
o~ifu~ oFIG.~6 is:a:c~rcwt:diag~am of an amplitude:detector and buffer amplifier25 ' . ~ fo~: conb~o~ g the circuit of FIG. ~ . ? ~
FIG. 7 is a circui~ diag~am of the programmable gain section of dle signal processing system for each-channel of the multiband compressor system in a~co~ance~with one embodiment of the invention. ~ -~ IG. 8 is a graph of the ou~tput response of the circuit of the present 30 invention for various programmed compression ratios as in FIG. 4, but usin~ the YVO 93/0077~ i 2 ~ 1 1 8 ~ 9 circuit of FIG. 8 to make the gain of the circuit independent of compression ratio at an input sound pressure level of 80 dBSPL, FIG. 9 is a multiband dynan~ic range processor using a plurality of limiters and a single side channel bypass path in another embodiment of the invention, S DETAILED DESCRI~FION C)F THE INVENTION
A signal compressor system comprises a variable gain circuit with a control circuit responsive to the amplitude of the signal at some point in the system, The ci~cuit varies ~he gain of the system based on a desired relationship between the input and output signal levels over a range of input signal amplitudes, In the case of the present invention, the variable gain circuit or variolosser is a cu~ent multiplier whose ~ansfer function is linear. In order to provide signal limiting, a variable gain circuit con~llable by an electrical signal is required, Such circuits are well known in the art; one circuit that is particularly adaptable to the low supply vol~ages used in hearing aids is described in the aforementioned '517 patent.
As described in the '517 patent, the current gain of the variolosser circuit is equal to the ratio of two con~ol currents designated IA and IB. with ~e current gain equal to I~/IA. The control circuits of ~he present invention are adapted to programmably contr~l a plurality of variolossers of the type deseribed in the '517 patent to provide a~desired gain, compression r~tio, and signal level^dependent 20 freqllency response which may b~ adaptively modi~led for the needs of individualusers.
One example of a prior ar~ si~nal compressor system is shown in FIG.
1. This sys~em is desclibed in the above mentioned '762 patent. Refe~ing now to ;IG. 1, ~e programmable:~muldbanLI cvmpressor system 100 receives an ~udio signal 25 : wi~ n~icrophone 102. ~:The output of microphone 102 is coupled to a preamplifier and automa~ic gain conaol cir~uit ~AGC) 104 and auxiliary preampli~ler 115 which con~aols ~he AGC acdon. ~The~output of the AGC circuit 104 is coupled to, psoglannnable bandsplit filter 106 which separates the audio signal into a plurality of frequency bands whieh, in the p~eferred embodimenl, consist of a hi~b and a low 30 frequency band. Th~ low frequency band output of programmable bandsplit filter 106 WO 93/00779 ~ P(~/US92/05521 ~ ~
21~I82~ -is coupled to programmable low band compressor tO8 and the high fiequency band output of programmable band split filter 1û6 is coupled to high band programmable compressor 110. The ou~put signals of the low band compressor 108 and the high band compressor 110 are coupled to summing circuit 112, which combines these S output signals to produce a complete audio signal. The ouq)ut of the summing circuit 112 is coupled to amplif~er 114, which amplifies the composite audio signal to a level suf~lciellt to drive an annunciator 124 which is coupled to amplifier 114. The system 100 further includes voltage regulator 116 for supplying a re~gulated voltage to various circuits of ~e system 100. Such a prior art programmable multiband compressor 10 system is particularly adapted for implernentation on an integrated circuit, and all of the circuits enclosed in the area 118 may be constructed on a single integrated circuit substrate. The programmable multiband compressor system 100 shown in FIG. 1 is adapted to be powered by a single cell, low voltage battery 122.
The programmable multiband compressor system 100 is adapted to 15 receive a plurality of control signals which may be generated by an external control circuit 120. The con~ol circuit 120 is coupled to bandsplit filter 106 and to the low and high band compressors 108 and 110, respectively. The control si~nals generated by control circuit 120 control the fiequency split between the low and the high frequency bands as well as the gain and compression ratios of the low and high band 20 compressors 108 and 110 to generate a desired response for the system 100 to compensate for a large variety of hearing impairments.
'! ' 'i' .; '.' In the prior art system described, each of the compressors 108, 110 may be circuits of the type described in the '761 patent. . ~ -7 ~ A~ pair of compressor systems in accordance with the present invention 25 and suiuble for uæ as the compressors 108 and,llO in the overall system of PIG. 1are shown in block form in FIG. 2a. Signals from the high band output of the above described band split filter 106 are applied to input terminal 202A of high band compressor system 110, and signals from the low band output of band split filter 106 are applied to input te;minal 202B of low band compreissor system 108. Both 30 compressors systems 108 and 110 operate in the same fashion; the following s~

WOg3r00779 `'''''' ~ i3~ PCr/USg2/05~21 ~.

des~.ription made in reference to high band compressor 110 applies equally to low band compressor 108.
The signal is applied to both a si~nal limiter 204A, to be desclibed below, and a feedforward resistor 206A. Currents flowing from the output of limiter S 204A and from feedforward resistor 206A are added together at the summing node 212A and applied to the programmable ~ain section 210A. A control current is applied to compressor programming terminal 208A from control circuitry 120 to programmably vary the compression ratio of the compressor system. Output from gain section 210A is delivered to output terminal 216A. The gain of the programmable 10 gain section 210A can be varied by applying a current from control circui~y 120 to programrning or control terminal 214A. Signals from output terminals 216A and 216B
are sumrned and applied to amplifier 114.
A limiter suitable for use in the present invention is depicted in further detail in ~;IG. 2b, in which the mput volta~e si~nal on input terminal 202 generates a 15 current in input resistor 222, which current is applied to variolosser 220. Variolosser 220 includes two control terminals, 282 and 286, which are used to control the gain of : ~ the variolosser. The output of the variolosser is connected to a summing node 212 at the input of progran~nable gain section 210. Input voltage from terminal 202 also generates a current in feedforward resistor 206, which current is also applied to sunun~ng node 212.
Input volta~e on terminal 202 is also applied to an amplitude detector . 240 which rectifies the incoming si~nal.: The output of amplitude detector 240is connected to a capacitor 250 which :~llters the rectified signal. The other side of the .capacitor is connected~to a point of lefelcnce potential, 218. The rectified,~fil~ered 2S - signal is applied to..~a buffer amplifier-260.:~ .Thus, amplitude :detector 240 detects the peak amplitude voltage of ~the-input signal and delivers it to capacitor 250, which .~ holds ::this voltage for a predetermined ;time. The holding or "release" time of the circuit is generally ~set~ between 10 and 100 ms. Voltage on capacitor 250 forms the - input to buffer amplifier 260,:whose output is applied to current converter 280 and ~ ~ 30 used to vary the current of the first control terminal 282 of variolosser 220. A

: .
.

WOg3/00779~ 323 12 PCI/US92/05521~

compressor programming current Ipc~ applied to compressor programming terminal 208, is converted to a control current in current converter 284 and applied to a second variolosser control terminal 286.
The control currents applied to valiolosser control terminals 282 and 5 286 control the current gain of the variolosser; the gain of the variolosser is equal to the ratio IB/IA, where Ial is the current applied to the second control terminal 286 and IA is the cu~Tent applied to the first variolosser control terminal 282. The value of IAO
is set by a combination of a dc output level from buffer amplifier 260 and by the prog~amming current Ipc~ which is also used to cont~ol the dc value of I~.
In the signal processing arran~ement just described the input signal is rectified and filtered to provide an envelope signal, and this envelope signal îs used to increase IA as the signal increases. Thus, the current ~ain will decrease as the input signal increases, and the output level will tend to remain constant above a given input signal level. This action is illustrated in PIG. 3, to which reference is now made.
FIG. 3a shows graphs of output leve} vs input level. Naturally, input and e~tput transducers are required to convert acoustic signals to electrical signals and vice versa, as well as other electronic amplifiler stages that provide appropriate signal levels to the above mentioned limiter and to the output electroacoustic transducer, or loudspeaker, as described with reference to the prior art circuit of FIG. 1. In the graph 20 of FIG. 3a the output of limiter 204 is shown by curve 302 for one value of control current from control circuitry 120, and by curve 304 for a second value of control current. Each of these~ curv exhibits a slope of unity at extremely low levels, and a slope of zero at ex~emely high levels. T he graph for si~nals through feedforward ! ~ iresistor 206 is shown by the straight line 306 which is seen to exhibit a unit positive 25 slope since the- output is ~directly propor~ional to the input. ~he sum of the resistor - signal 306 and the limiter si~nal under the first men~ioned condition 302 is shown by ~dashed line 308, and that~for the second condition 304-by dashed line 310. Note, the - two limiter curves 302 and 304, are identical in shape but translated in both vertical ~and horizontal direedons in a manner described below. It is clear that the ~wo dashed 30 line cun~es 308 and 3 iO, depictin~ the sums of si~nals for the two si~nal paths, exhibit , W093/00779 i 2 1 :~ 1 8 2 ~ Pcr/us92/05521 ~
, differing slopes and only relatively minor departures from linearity. The slopes of these two curves are the reciprocals of the compression ratios in the two cases depicted, showing the control of compression ratio afforded by the control circuitry 120 in accordance with the invention.
S In FIG. 3b, the same info~nation is presented, this time as a plot of gain vs input level. Solid line culves 312 and 314 correspond to the conditions of curves 302 and 304 of FIG.3a, respectively. The limiter gain is relatively constant at extremely low levels, and approaches -1 asymptotically at hi~h levels. The feedforward resistor gain 316 is, of course, constant over the whole range. Dashed linle curves 318 and 320 follow the combined gain of the limiter and the feedforward resistor 206 and correspond to curves 308 and 310 of FIG. 3a. For the limiter, th ga;n falls as the input level increases because IA. in the ratio ~ A increases with input level. In the region of very low levels, the constant component Of IA. which is a fixed dc value, predominates so that the gain is relatively constant at low sound levels. As 15 noted above, it is only when the sij~nal dependent part Of IA becomes a sijgni~lcant portion of the dc part that the gain begins to fall. We have defined P~b~he lower break point, as ~e sound pressure level at which the si~nal dependent part Of IA! becomes equal to the~dc part. Note that for the limiter of the present invendon, IB is a dc or fixed current which is set by the pro~ramming current Ipc~
In the programmable compressor described in above mentioned '761 patent, the value Of IA rises with input si~nai as in the present invention. However, in the circuit shown in the '761 patent the value Of IB also rises above its dc value at :some higher~input leveL ~e variable nature of I8 is used to introduce a second break ~ point into the ~ain curve that we have termed Pub, the upper break point. When this 25 ~ . ~second break point is introduced, the slope of the output level vs input level at higher levels ~egins to rise. Thus, the overall gain eune is a sigmoid function. It is known that such functions do not follow the asymptotes very closely. Rather, they tend to be almost linear in the region between the lower and upper break points, with a point of inflection midway between these break points.

WO93/OOM9 - ~ PCr/USs2/05521 ~
'2 1i132~

By varying the distance between the upper and lower break points, Plb and Pu~t we can vary the slope of the compressor curves (i.e., dashed line curves 308, 310) of FIG. 3a over a relatively wide range in dBSPL. As noted above, the reciprocal of the slope of the input-output curve is tenned the compression ratio, y.
S Thus, by varying PUb relative to P,b we obtain a continuous variation of the compression ratio. Programming the compression ratio can therefore be accomplished by variation of one or both of these break points, which are controlled by dc currents applied to the aforementioned variolosser. The prior art variable compressor applied the rectified envelope signal to change IB in order to provide an upper break point and 10 thereby set the compression ratio.
In the compressor of the present invention, a simpler method is employed to provide the upper break point and thereby effect thë variation of compression ratio. In this circuit the drop in gain is initiated as in the earlier method, by increasing the value of IA~ which increases the negative feedback around the 15 variolosser ampli~ler. The means by which the drop in gain is terminated at the end of the compression span, however, is not by increasing the forward gain by increasing IB as in the prior art device, but rather by sendin~ si~nals through a feedforward signal path, which may be~a simple~resistor 2û6 as shown in FIG. 2. While a simple resistor is used in the preferred embodiment, it will be apparent to those skilled in the art that 20 o~er f;xed gain devices,~such as a~linear amplifier, can also be employed.
To apPreClate the mathematical characteristics of the programmability of ~e compressors of the~present invention, we~now develop the sigmoid relationship for ~gain of the~ compressor. The current gain of the variolosser is ~iven by the ratio of - output to feedback ~I currents. ~ By inspection of FIG. 2b, the compression section 25 transconductance,~gc rth cu~ént output for a given voltage input Vc), is given by:

;~

WO g3/00779 ~ 2 1 1 1 ~ 2 ~ Pcr/usg2/oSs~l , , gc~----+~
Rln lAC Rs The output of the compressor section therefore consists of tw'o' components; the current through feedforward resistor 206 (R5), and the input current through resistor 222 (Rj~, multiplied by the current gain for input voltage Vc of the S compressor variolosser, IBJI~C. These two currents ar~ combined at the output summing node 212, i.e., the input of the gain variolosser 210. The two terrns of Eq. 1 must have the same sign since otherwise si~nal cancell?tion would take place; thus, the variolosser must not include a phase reversal when a simple feedforward resistor is used. As noted above, other feedforward devices may be used instead of resistor 206.
10 For example, if phase reversal in the variolosser is desired, the feedforward circuit m~y be a simple linear amplifier which also introduces phase reversal.
Flxed resistor 206, R~, gives a component of gain that does not vary with input level. The compressor variolosser has a gain that falls as the input signal rises above Plb, the lower break point. The slope of variolosser gain vs input level 15 approaches the value -l at high levels, since IAc rises directly with signal level. The ^! ovcrall level of the varioloss gain, for a given input level, depends on IB~. The combined gain of the ~two paths of Eq. 11 however, becomes flat at suf~lciently high input }evels as I~JIAC ~approaches zero. This constitutes the means by which the upper break point is set and~ the compression ratio is varied.
20 .~ In the present invention, ~ is not~ signal dependent but is proglanunable. ~ When IB~ is`zero, the compressor portion of the circuit is inactive, the gaiin is~ ~1~, and the devlce ~is pro~med for a `compression raho of 1 (i.e., nocompressionj. As lBc increases, the ~ain at low levels is' increased,' but the negatiYe s ope of gain with input level prevents the; cornpressor from affecting high level gain 25 ~ as much- Thus, IBC becomes the compression adjustment parameter. `' ' - ' ~ ~ - Feedback tail current IAC is made up of a fixed part, IAO. and a signal dependent part, IAs, so lhat~

i :

1~

WO g3/00779 ~ ' PCI~/US92/05~i21~'~f ., . i 2 1 ~ 2 3 16 IAC = IAO ~ IAS (2) Where IAO is the fixed part Of IA~ that sets the lower break point for the compressor; IAS
is t~le signal dependent part. Using Eq. 1, we can write:

8C RS~ RL1(1A~+IAO)) If we normalize the peak signal current, IAS~ to the fixed current, IAO~ by defining a normalized signal variable, vp = IAS~IAO, we obtain:

c Rs (vp ~

This expression, which is plotted as curves 318 and 320 in FIG. 3b, gives the gain as a ratio of a numerator with a fixed and a si~nal-dependent part, and a 10 denominator having a diffent f!xed but the same signal-dependent part. The lower break point, defined as the input level at which Vr = 1, can be constant with compression as it is in the device of the '761 patent. As described below, the lower break point can also be varied with compression ratio to secure a better variation of gain with level over the programmable compression ran~e.
Eq. A: is the sigmoid re!ationship but it is secured more simply in the present invention than~ the prior art. ;As can be s~en by inspection of the equation, when I,~ ~is zero, the~gain is not a function of Vc, the input signal variable.
FIG.~ 4~ to~?~Qhich ~reference is now made, is a graph showing the output level as a function of input level for various values of compression ratio obtained for 20 different values of programming current~ provided to control terminal 208 of FIG. 2.
lhis ~lgure shows the degree~of~relative linearity of the input-output culves, i.e., change in gain as ?~function of input level, over a relatively wide range of ~ compression ra~os obtained with the compression system of the present inven~ion.

:

W093/00779: ' ' 2 1 1 1 ~ 2 9 Pcl/usg2/o~s2l 3~

FfG. S is a circuit schematic diagram of a variolosser and a control circuit for a compressor in one embodiment of the present invention, Signals are fed to the circuit output via feedforward resistor 206 and to variolosser 220 by input resistor 222, Variolosser 220 was described in the '517 patent, Variolosser 220 5 includes an ampli~ler, sao, an output differential cell or output cell, 520, and a feedback differential cell or feedback cell, 540. The amplifier comprises a differential pair, 502, including transistors 514 and 516 disposed with their emitters connected in common, and a current mi~ror load, 504, which may include dc offset adjustment or trim resistors 506 and 508, The tail current for differential pair 502 flowing from the 10 emitters of transistors 514 and 516 is provided by a current generator, 512, provided by a transistor in a manner well known to practitioners of the art, The base of transistor 516 is returned tO point of reference potential 218 through resistor 510, whose function is to balance the dc volta~e drop of base current in resistor 222, by a nominal1y equal and opposite dc drop of base current in resistor 510, Output cell 520 includes a differential pair 522, comprising transistors 526 and 528 disposed with their emitters coupled together, 3nd a current n~irror load, 524. The base of transistor 528 is returned to a point of reference potential 218, while : ~ the base of transistor 526 is connected to the collector of transistor 514, the output of a~nplifier 500,- -The output of output cell 520 is coupled to output terminal 212 from 20 the collector of transistor 526. The tail current of output cell 520, IBC. is supplied by transistor 532, whose base is connected to diode connected transistor 534, The emitter of transistor 534 is connected through resistor 536 to the negative supply voltage, Tail ~:: current ~ is thus controlled by prograrnmin~ current IpC fed into control terminal 208.
~ Transistors 532 and 534 comprise current converter 284 that translates programrning 25 cuuTents applied to terminal 208 to output cell control current ~
;eedback cell 540 includes a differential pair 542, comprising transistors and 548, and a cu~ent~mirror load 544. The base of transistor 548 is retumed to a point of reference potential 218, while the base of ~ansistor 546 is colmected in common w~th the base of transistor 526 to the collector of transistor 514, ~he output of 30 ampli~ler 500, The output of feedback cell 540 is coupled to the base of ¢ansistor 514 W(:~ 93/00779 ~ f; ; ; PCI/US92tO5521 !i'~
211l82~ 18 from the collector of ~ansistor 548. Feedback cell 540 thereby provides feedbackaround amplifier 500 and renders the input at the base of transistor 514 a summing node. The tail current of output cell 540, IAC. is supplied by transistor 552, whose base is connected to diode connected transistor S54. Tail current IAC is thus controlled by 5 buffer arnplifier output current ID. fed into control terminal 278. Transistors S52 and 554 comprise current converter 280 that translates signal enuelope currents derived from buffer amplifier 26û to feedback cell control current IAC
Transistor 556 has its base connected to the base of diode connected transistor 534, and its emitter connected to ground through resistor 558.~ The collector of transistor 556 is connected to the base of transistor 552. The function of this arrangement is to subtract some current from the buffer amplifier output current, thereby loweAng the value of lower break point P,b as the compressor programmingcurrent is increased. It is noted ~lat the buffer amplifier output also produces a dc current in the absence of any input signal and that this sets the maximum value Of IAO
Resistor S58 allows the amount of current subtraction to be designed to tailor the lower breakpoint to a desired value as the compression is varied. Thus, the value of IAO. î.e., the dc component Of IAC varies and is also se~ by the programming current Ipc~
In the preferred embodiment, rosistor 558 is selected to set the lower break point at 50 dBSPL for maximum compression, i.e., when l,~ is at itS maximum allowable value.; 20 In operadon, the output and feedback cells, 520, 540 are identical except for ~eir tail ~currents, supplied respectively from current converters 28û and 284. With the gain of amplifier SOO reasonably high, the current gain of the variolosser~is equal to the ratio~of the output and feedback tail currents, ~ A ThUS.
~ when envelope signals from the buffer amplifier are presented to current converter 280, tail current IA inc~eases and~the gain of variolosser 220 is reduced. .
~ It is;well known that the relationship between the input voltage to a differential pair and *e output current therefrom is nonlinear. The relatively wide dynamic range of the varioiosser 220 is ascribed to the relatively precise cancellation of nonlinearities o f the output and feedback cells. Assume that a pure sinusoidal signal current appears at the output of the output cell 520. The voltage at the bases of WO 93/00~79 ; 2 1 1 :1 ~ 2 !~ Pcr/us92/o552l ;~r transistors 526 and 546 (arising from some input signal) must be predistorted toproduce this pure sinusoidal signal. The output current of the feedback eell must behave similarly to the output cell, and must also have a pure sinusoidal output. But the output of the feedback eell is virtually the entire input signal current to the 5 variolosser so that the input and output are both essentially sinusoidal. Thus; in normal operation, the variolosser has very low distortion characteristics. Otherdistortions do arise, to be diseussed below, that are involved with the operation of the variolosser as a compressor We first turn our attention to the amplitude detector and buffer amplifier.
Signals from the band split filter 106 at the input 202 of the compressor system are applied to the amplitude detector 240 and buffer ampli~ter 260 as shown in FIG. 6, to which we now turn. The amplitude detector is shown as a half-wave rectifier, but other types of amplitude detectors, known to those skilled in the art, could also be used without departing from the spirit of the present invention.
Amplitude detector 240 generates a voltage related to the positive envelope of the audio input si~nal. Specifically, the amplitude detector ~enerates a control that follows the positive peak envelope of the audio input si~nal. The circuit is adapted to receive an audio input signal at temlinal 202. This signal is further coupled to the variolosser 220 and the feedforward resistor 206 as described above.
Amplitude detector 240 compnses a differential pair 602, including transistors 604 and 606 disposed with their emitters connected together. The collectors of transistors 604 and 606 are` connected to a mirror load 608. The tail culTent of the differential pair is supplied by a transistor 614 whose collector is connected in conunon~ to~ the ;emitters of transistors 604 and 606. The base of transistor 614 is connected to a sou~e~of potential at terminal 680 that allows transistors connected to it to generate a predet~rmined current. Resistor 616 allows the adjustmen~ of the tail cutT,ent ~om this ~pre~etermined amount. The output of the differential pair 602, i.e., the collector of ~ansistor 604, is coupled to the base of output ~ansistor 610, whose emitter is connected to the posiuve supply and whose collector is connected to a filter capacitor 618 ;and a load resistor 62û.

:
WO 93/0077~ ~ Pcr/us92/0~521 ~ ~' 2111S2~1 When a positive signal is received at terminal 202, transistor 604 conducts more heavily, pullin~ base current from transistor 610. This causes thecollector current of transistor 610 to increase, thereby charging capacitor 618. The time taken to charge the capacitor is telmed the attack time, and is quite rapid. In the 5 pref~lTed embodiment the attack time is less than one millisecond. When capacitor 618 is thus charged, and the input signal subsides, the base of transistor 606 becomes more positive than that of transistor 6û4, so that transistor 604 stops conducting. At the same time, diode connected transistor 609 of the mirror load conducts heavily and tries to get transistor 607 of the mir.or 608 also to conduct heavily Since there is no 10 place for the current to flow, transistor 607 saturates, pulling the collector voltage of ~ansistor to rise almost to the supply and cutting off transistor 610. Thus, capacitor 618 remains charged, its only discharge path bein~ resistor 620. The voltage on capacitor 618 decays slowly, and discharges in an amount of time called t~e release time. In the preferred embodiment the release time is set between 10 and 100 rns.
15 Thus, at any given time the voltage across capacitor 618 is related to the positive peak value of the input signal~ also termed the positive envelope of the signal.
The envelope si~nal on capacitor 61~ is coupled to the input of the buffer amplifier 260. The buffer ampli~ler 260 comprises an operational amplifi~er with unity feedback around it. Amplifier 260 complises an input differendal pair 632 20 includin~ ~ansistors 634 and 636 disposed with their emitters connected together. The colleetors of ~ansistors 634 and 636 ~re connected to a mirror load 638. ~he tail - c~nt of the differential pair is supplied by a transistor 644 whose collector is c~nnected in common to the emitters of transistors 634 and 636. The base of . transistor 644 is connected to bias line 6~û tliat allows transistbrs~ connected to it to 25 ~ generate a predeter~r~in~d current. Resistor 646 allows the adjus~nent of the t~il ;-~; cw~ent from this predetennined amount. The ou~ut of the differential pair 632 at thecollector of ~ansistor 634 is~ coupled to the base of output transistor 640, wh`ose emitter is conne~ted to-the posi~ive supply and whose sollector is connected in fçedback relation to ~he base of transis~or 636 throu~h resistor 648. The collector of 30 transistor 640 is also connected to a point of reference potential 218, through resistor WO 93/00779 ~ ,~; 2 1 i 1 ~ 2 9 PCr/USg2/05521~

650. A pull-down current is supplied by transistor 656. The base of transistor 656 is connected to a source of potential at terminal 680 that allows transistors connected to it to gener,ate a predetermined current. Resistor 658 allows the adjustment of the pull down culTent from this predetermined amount. A feedback stabilizing network 5 comprising the series combination of capacitor 652 and resistor 654 is connected from the collector to the base of transistor 640.
In operation amplifier 260 has unity feedback around it, so that the voltage at the collector of transistor 640 is essentially equal to that across capacitor 618, that is to say, equal to the positive envelope of the input si~nal. The current 10 through resistor 650 is therefore equal to the envelope volta~e divided by the resistance of resistor 650. The collector current of transistor 640 is therefore equal to the current through resistor 650 plus the pull-down current drawn by transistor 656.
Transistor 660 mirrors the collector current of transistor 640; the collector of transistor 660 is coupled to the collector of a second pull down ~ansistor 15 662 whose emitter is connected to ground through resistor 664. The output from the buffer amplifier is equal to the collector eurrent of transistor 660 (equal to that of ansistor 64(~) less the pull-down current of transistor 662, which can be adjusted by adjusdng the value of resistor,664. In operation, resistor 664 is made larger than resistor 658 in order to make the dc output of the buffer amplifier positive. i.e., of the 20 same sign as the envelope signal. This current helps set the lower break point for the compressor., It will be Iecalled that the lower brealc point is de~lned as the point at which the current induced by the envelope signal is equal to the dc cur,rent. It should be noted that the lower;~break'point is~also controlled by the compressor pro~amming cmrent Ipc~ as described above in connection with FIG. ~. by transistor 556 and resistor 25 558., Because of the unique design of the variolosser of the '517 patent in ,, which nonlinearities of the ~output and feedback cells cancel, nonlinear ~istortion of - si~nals amplified by the variolosser in the absence of control si~nals is minimal. The var,iolosser is a si~nal~ multiplier, however, and special attention must be paid to a type 30 of intermodulation distortion that arises as a result of the exercise of this signal : :

WO 93J0077g i ~ pcl/uss2/os521 i~
21ii~2~ 22 multiplication function. A particularly troublesome type of distortion arises from ~mperfections in the variolosser that can be represented as a dc input offset voltage.
This offset arises from imbalances in dc operating currents, geometrical imbalances in semiconductor device fabrication such as those of the two transistors making up the S input differential pair of the variolosser ampli~ler, a dc offset coupled into the variolosser input from the si~nal source (if it is dc coupled) and the like. This dc offset acts as an input signal added to the audio signal.
The output of the variolosser is the product of two signals, i.e., the audio signal itself (including the dc offset) and the control signal, which is derived 10 from the audio signal and is a representation of its envelope. Distortion arises in the form of the product of the audio signal with its own envelope. When the release time of the compressor is short, the envelope signal has a low frequency audio component that modulates the audio si~nal, resulting in low frequency components of distortion that are called difference tone intermodulation (DTIM) products. If two tones of15 roughly equal amplitude and closely spaced in frequency, 2700 Hz and 3000 Hz for example, are applied to the compressor input, the envelope of this composite signal . will vary at a rate equal tO dle frequency difference. or 300 Hz. When this envelop signal is Iectified and filtered to form a control signal applied tO the variolosser, sum and difference tones at 2400 and 3300~ Hz at the variolosser outputs. When speech or 20 music signal is being processed, these products will cause "muddiness" in the sound.
The amplitude of the 300 Hz component of the control signal will depend on the degree of ffltering by capacitor 618 and resistor 620. When this degree of filtering is small, as when the compressor release timeiis short, the 300 Hz component can besizable. but în ordinary practice, the ~lltering is sufficient to keep this distortion to 25 negli~ible proportions.
, A more serious si~nal de~radation may take ~place as a result of the envelope signal imultiplying the dc offset to produce~ a replica of the envelope signal itself at the OUtpllt. T his arises as the product of the envelope si~nal and a constant (the dc offset). Thus, in the example of the 2700 Hz and 300û Hz tolles, a 300 Hz 30 difference tone will appear at the output. With speech or music si~nals, such a low WO 93~00779 ~: ~ 2 1 1 1 ~ 2 '~ Pcr/US92/0552~

frequency difference tone--the signal envelope -- may be clearly audible and can be very annoying.
Another serious distortion arises as a result of the multiplication of the envelope with the dc offset because the attack time for compressors is usually made 5 quite short compared with the release time. A sudden increase in the amplitude of the incoming signal will cause the envelope signal from the amplitude detector to exhibit a sudden xise, or a step, which has spectral components through much of the audio range. When this is multiplied by the dc offset, it appears directly at the variolosser output as a distortion signal with a broad frequency range coverîng much of the audio 10 spectrum. From experience, when this effect is made ne~ligible all other distortions arising from the multiplication of the signal plus offset by the signal envelope are also negligible. Because this distortion is associated with the attack time of the amplitude detector, we have termed it "attack distortion." With speech signals that change in amplitude in a sudden fashion, what is heard in addition to the speech is an annoying 15 "static" or scrapin~ sound.
_. This effect may be quantified by measuring the chunge in the dc output offset of the variolosser when a high audio level is suddenly applied to the variolosser.
The dc voltage at the output of the compressor is first measured under conditions of - zero input signaL Then a sinusoidal signal of a level equivalent to an 80 dBSPL
20 si~nal is applied to the compressor input and tbe dc voltage at the output of the compressor is again measured. The dc shift in the variolosser output is indicatîve of the amount of attack distortion l~ ~ The amount of attack distortion varies wîth the amount of compressîon and gain that is pr~gtammed~ into the compressor. This arises from two separate 25 causes.- ~First, the~amount of ~gain~in dle variolosser will obviously affect thé dc output : . shift. Second, a~more subtle effect is that the dc input offset valies with the program (the gain and compression,rado) selected as well as with the signal level applîed to the variolosser. It îs ~his second~ effect that makes the problem a most difficult one. The reduction of this effect by the compressor of the present invention comes abou~ by 30 elinunabng the aforemendoned change of ly with si~nal level. This ch:mge of Iy in WO 93/00779 j , ; PCr/US92/05521 ~

the pAor art is replaced in the present invention by the fixed resistive path of the feedforward resistor, RJj.
In the preferred embodiment of the invention, in order to program the gain of the device a second variolosser is used, one whose gain does not vary with 5 signal level. While the preferred embodiment employs a variolosser, it will beapparent to those skilled in the art that any programmable variable gain amplifier may bei used as the gain section. A circuit schematic diagram of the programmable gain section of the signal processing system of the preferred embodiment is shown in FIG.
7. It includes a variolosser 220 similar to that described above for the compression 10 section in FIG. 5. The control currents for variolosser 220 are generated entirely from a dc programming current applied to gain programming terminal 214 that sets the gain of the variolosser in response to programming for the individual patient, user volume control inforrnation in the form of a current from the control circuitry, and gain trimming information used in adjusting the gain in manufacture of the hearing device.
15 Because of the multiple purposes for controllin~ the gain, and the fact that the manufacturing gain trim must be uniform over the entire pro~amming range, that is to say a given programming current should represent the same gain change in dB oYerdle entire programming range, the control circuit is designed to have a gain change in dB that is linear with prog~ramming current over substantially the entire programming 20 and volume control range. This is accomplished through the manner in which the control currents are derived. Scaling of the dB change per microampere of con~olcurrent is accomplisked by scaling the resistance of a pair of control resistors, 716 and 736~
i~r~ t; ~ Control currents are generatsd from the programming currents in the - 25 following way. Tail currents IBg ~and IA8 are ~enerated by current converter ~ansistors - - 718 and 738, which have their collectors connected to control terminals 286 and 282 respec~ively,iand their emitters connected to the negative supply, or ground. Bases of transistors 718 and 738 are connected to diode-connected transistors 720 and 740, ~ respectively, which are connected to pnp current mirrors 710 and 73û, respectively.
30 The pnp cuITent milTors include diode connected transistors 712 and 732, whose `;

WO 93/00779 PCI /VSgZ/05521 ~f~
~ 21~1~2!) collector currents are delivered respectively to transistors 706 and 708 of differential pair 709. It is differential pair 709 that controls the gain of the programmable gain section. The tail current of differential pair 709 is provided by current sink transistor 724, whose base is connected to the bias source, and whose value is adjusted by S emitter resistor 726. This tail current is apportioned between the transistors of differential pair 709 by application of a voltage between the bases of transistors 706 and 708. The value of this voltage is established by the gain programming current in the following way. Programming currents are applied to diode-connected transistor 702 which forms part of mirror 704. Collector current of transistor 703 mirrors the 10 pr~gramming current, and causes a voltage drop in resistor 716, which lowers the collector current of transistor 706 relative to tr~sistor 708. These two currents, translated through their respective sets of currents mirrors, control the tail currents of the feedback and output cells of variolosser 220. It should be obvious to one skilled in the art that the emi~ter area ratios of mirrors 741, 730, 721 and 710, as well as 15 differential pair 709, can be modified to adjust the relative sizes of the feedback and output cell tail currents to secure a desired gain range for variolosser 220. In a prefer~ed embodiment, the rela :ive areas of the transistors are proportioned as shown in FIG. 7, in which, for exarnple, transistors 706 and 708 are denoted as X10 and X2 transistors, respectively. Thus, transistor 706 has ten times the emitter area of a 20 standard transistor, and transistor 708 has 2 times the area of the standard. Similarly, transistors 718 and 738 are X5 and X3 transistors, and transistor 714 is a X2 transistor. All other transistors in the chain of current mirrors are standard size - transistors. I n this way, with zero gain programrning current (the maximum gain `
setting) the ratio of feedback cell tail cu~ent to output cell tail current îs (3 x-2)/(10 x 25 -: ? 2 x 5) = 0.060. Resistor 736~pem~its base bias culrents of transistor 708 to flow to a sour~e of reference potential 218. - - - ~
The input output curves of FIG. 4 have a range of values of about 7 dB
at- 80 dBSPL. In order to be able to specify the hi~h level gain at 80 dBSPL
separately from the speci~lcahon of compression ratio (or, equivalently, the gain at 50 30 - dBSPL), it is advantageous to make a gain correction for the excess gain of the wo 93/00779~ PCr/US92/05521 2 ~ I 1 S 2 ~

programmable compressor section when it is programmed for high compression ratios.
This provides orthogonal control of the compression ratio and gain. This correction is made by transistor 750, whose emitter is connected to ground through resistor 752.
The base of transistor 750 is connected to the compressor programming input terminal 5 208. When the compressor programming current is applied, the collector current of transistor 750 increases; this current produces a voltage drop in resistor 716 which operates in an equivalent manner to the collector current of transistor 703 to reduce the gain of tne variolosser by 7 dB for maximum compression. Interrnediate values of compression require intermediate values of correction current, which is supplied by 10 transistor ?50. The ex~ctitude of the correction is demonstrated by the input output curves of FIG. 8, which show the output at 80 dBSPL for all values of compression to be within one dB of each other.
Another embodiment of the present invention is shown in FIG. 9, which shows a band split fil~er 906 having an input terminal 902, a first plurality of outputs, 15 each ~onnected to a limiter 908"b n of the type described above, and a secondplursl~y of outputs, each connected through ~lxed gain means 9lO, which may be asimple resistor as shown in FIG. 9, to the summing node 918. (In FIG. 9 only onesuch fixed ~ain means 910 is shown.) In this embodiment, it is contemplated thatcompression is unneeded in certain frequency bands, and therefore no limiters are 20 provided for such bands. This follows from the fact that in a large percentage of cases of sensonneural hearing~impairrnent low frequency hearin~ is unaffected.
Each of the limiters 908 has an output connected to a summing node 918 and a programmin~ terminal 914~b n for a control cu~rent to set the compression ratio;as described~above. ~A;fixed~conductive path is connected directly between dle 25 input te~nnal 902 and~the summing node, 918. Summing node 918 fonns the inputto circuit 920 that provides amplification, volume control, pro~rammable gain, a~manufacturing gain trim, and an equalizer. This embodiment has the advantage, particularly in a system with many bands, of not requiring a separate gain variolosser for each band. It also provides for separation of the high level equalizer function from 30 the band splitting of the compression function. The compensation for sensorineural WO 93~00779 ~ 2 1 1 1 8 2 ~ PCT/USg2/05521 , 27 impainnent may not match the équalization requirements ~or that person; ~he embodiment of FIG. 9 allows the two requirements to be separated.
Other uses and modi~lcations of the foregoing will be apparent to those skilled in the art without departing from the spirit of the present invention. Therefore, S it is intended that the invention be limited only by the following claims.

.

~ ' '` ~ " `' ' ; ~

.

Claims (24)

1. An audio frequency signal compressor comprising:
input means for receiving an input electrical signal having a waveform representative of acoustical information;
limiter means connected to said input means, said limiter means having an input signal breakpoint such that output signals from said limiter are substantially proportional to said electrical input signal for input signals substantially below said breakpoint, and output signals from said limiter are substantially constant for input signals substantially above said breakpoint;
control means for programmably varying the gain of said limiter;
signal feedforward means having input and output terminals, the input terminal of said signal feedforward means being connected to a source of an electrical signal having the same waveform as is applied to said input means, said signal feedforward means providing a signal at its output terminal which is directly proportional to said input electrical signal; and summing means for forming a sum of the signals from the output of said limiter means and the output terminal of said signal feedforward means, thereby forming a compressor having a programmable compression ratio.

2. The signal compressor of claim 1 wherein said signal feedforward means comprises a resistor.

3. The signal compressor of claim 1 further comprising gain means, connected to said summing means, for providing an amplified output signal representative of the combined signal from said limiter means and said feedforward means.

4. The signal compressor of claim 1 wherein said limiter means composes a variolosser having a gain set by the ratio of two control currents.

5. The signal compressor of claim 4 wherein one of said control currents is a programmable constant.

6. The signal compressor of claim 5 wherein said limiter comprises signal amplitude detection means for varying me value of the other of said control currents.

7. The signal compressor of claim 3 wherein said gain means comprises a variolosser having a gain set by the ratio of two control currents.

8. The signal compressor of claim 3 wherein said gain means comprises circuit means for providing orthogonal control of compression ratio and gain.

9. A signal compressor having a programmable compression ratio, comprising:
a variolosser having an input terminal for receiving an audio frequency electrical signal having a waveform, an output terminal, and first and second control means, the gain of said variolosser between said input and said output being controlled by the ratio of the currents through said first and second control means;
detector means for providing a control signal current responsive to the amplitude of said input signal;
means for applying said control signal current to said first control means;
programming means for providing a programming current to said second control means;
signal feedforward means having an input end and an output end, the input end of said feedforward means being connected to a source of an electrical signal having the same waveform received at said input terminal and the output end of said feedforward means being connected to said output terminal of said variolosser, for providing a signal at said output terminal proportional to said input electrical signal.

10. The signal compressor of claim 9 wherein said feedforward means comprises a resistor.

11. The signal compressor of claim 9 wherein said feedforward means comprises a linear amplifier.

12. The signal compressor of claim 9 wherein the control current to said first control means is a fixed dc value in the absence of an input electrical signal.

13. The signal compressor of claim 12 wherein the fixed dc value of the control current through the first control means is a function of the programmingcurrent.

14. The signal compressor of claim 9 further comprising variable gain amplifier means connected to said output terminal.

15. The signal compressor of claim 14 wherein said variable gain amplifier means comprises a second variolosser.

16. The signal compressor of claim 14 wherein said variable gain amplifier comprises circuit means to orthogonalize compression ratio and gain.

17. A multiband audio frequency signal compressor, comprising:
a band split alter for dividing an electrical signal, representative of an acoustical signal, into a plurality of band-limited electrical signals having separate frequency ranges;
a plurality of limiter means each having an input terminal connected to an output of said band split filter, and an output terminal;
each of said limiter means having a separately programmable maximum output level; and, a plurality of feedforward means, each feedforward means having an input end and an output end, the input end of each feedforward means being connected to anoutput of said band split filter, such that the signal applied to each feedforward means is in the same frequency range as the signal applied to a limiter means, and the output end of each feedforward means being connected to a limiter output terminal, for providing a signal at said output terminal which is proportional to said band limited electrical signal.

18. The multiband compressor of claim 17 further comprising a plurality of variable gain amplifier means connected to said output terminals.

19. The multiband compressor of claim 18 wherein said limiters and said variable gain amplifiers comprise variolossers.

20. The multiband compressor of claim 17 further comprising amplitude detector means for producing a control signal proportional to the peak amplitude of the input signal, and wherein the gain of said limiter is varied in response to said control signal.

21. The multiband compressor of claim 17 wherein the band split filter is.
programmable to divide the frequency spectrum into desired frequency ranges.

22. The multiband compressor of claim 17 wherein the number of limiters is less that the number of band-limited signals produced by said band split filter? and further comprising means associated with each frequency band which is not connected to a limiter for providing an output signal which is proportional to the portion of the input signal within the frequency band.

23. A hearing aid, comprising:
an input transducer for converting ambient sounds into an electrical signal representative of the sounds;
band split filter means for dividing said electrical signal into a plurality of band-limited electrical signals of predetermined frequency range;
a plurality of individually programmable compressors for providing compressed output signals based on said band-limited electrical signals;
control means for controlling the frequency ranges of the outputs of said band split filter means, the compression ratio of each of said compressors, and the gain of each of said compressors;
means for combining the outputs of said compressors;
an output transducer for converting the combined output of said compressors into an acoustical signal;
wherein each said compressor comprises a variable gain limiter means having as its input one of said band-limited electrical signals, said limiter means producing an output signal with a gain set by said control means, a circuit parallel to said limiter means for providing a signal at the output of said limiter means which is proportional to said band-limited electrical signal, and a variable gain amplifier whose input is the combined output from said limiter means and said parallel circuit.

24. The hearing aid of Claim 23 wherein each said limiter means comprises a variolosser having a gain proportional to the ratio of two control currents, and an amplitude detector for providing a first control signal proportional to the amplitude of the band-limited electrical signal, said first control signal being used to control one of the variolosser control currents, the other of said variolosser control currents being controlled by a second control signal from said control means and being independent of said first control signal, whereby the compression ratio of said variolosser is set by said second control signal.