US2866848A - Method of improving intelligence under random noise interference - Google Patents

Description

Dec. 30,

L. J. FOGEL METHOD OF IMPROVING INTELLIGENCE UNDER RANDOM NOISE INTERFERENCE Filed April 2, 1954 F/G. l

v GATING 1%- AMPLIFIER CIRCUIT Jq PEAK POWER DETECTING CIRCUIT F/6.2- N2 I24 n4 Low LEVEL GATING D|2 AM I:(

no SQUELCH CIRCUIT H6 PEAK POWER DETECTING CIRCUIT Low LEvEL GATING D 'T' souELcH CIRCUIT q 2Io- 2IG 220 ZIB-I' Low PEAK POWER PASS v oETEcTING FILTER CIRCUIT INVENTOR,

LAWRENCE J. FOGEL.

A TTOR/VE).

United States Patent METHOD OF IMPROViN G INTELLIGENCE UNDER RANDOM NOISE INTERFERENCE Lawrence J. Fogel, Jackson Heights, N. Y. Application April 2, 1954, Serial No. 420,764

' 3 Claims. c1. 179-1 (Granted under Title 35, U. s. Code 1952 sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any, royalty thereon.

This invention relates to a method for improving the intelligibility of speech and more particularly to a method of improving the intelligibility of speech being amplified by electronic circuitry and having low signal to noise ratio.

' The problems of improving the relative quality of the signal in electronic transmission of sound has been ap proached in many different Ways. There are many systems for reducing noise and some for improving intelligibility by reducing noise, but most of them appear to lose sight of the basic premise that the improvement of intelligibility in sound transmission depends to a large extent on the transmission of consonant characteristics which provide a high degree of intelligibility as com pared with vowel sounds which, contrarily, have the larger amount of sound energy.

7 The basic noise or scratch reducing systems, standard in-audio amplifiers for many years, use a low-pass filter to reduce the high frequency noise energy but do this at the expense of the consonant energy which would improve intelligibility. Other more recent noise suppressing systems pass all the sound energy during a high level energy interval then cut off the higher frequencies or the entire signal when it falls below a certain level. These are also predominately actuated by vowel sounds and lose the value of consonant energy as far as the intelligibility of threshold signals is concerned.

These suppressing systems have also been applied to amplifiers where speech is divided into bands. Such systems include: the patent to Llewellyn 1,968,460 where each sub-band of speech energy is separately squelched, which favors the vowel sounds; the patent to Beers 1,961,329 where the high frequency is decreased when the signal is low, which againfavors the vowel'sounds and the patents to Hammond 2,008,825 and Doba 2,173,472 which divide the speech energy into bands with expansion and compression within each band to accentuate the stronger parts. All of these systems appear to be actuated by the vowel sounds and to accentuate them at the expense of the consonants which are more important as far as intelligibilityis concerned.

The method of improving intelligibility relied on here ,is based on the suppression of the peaks of energy ,which are dominant vowel sounds so that the remainder 'ofthe signal and even some of the noise, where important consonant sounds may be partially masked, may be accentuated. This method relies on the psycho-acoustical ability of the mind to fill in short gaps in sound that are omitted from a familiar pattern of speech.

It is therefore an object of this invention to provide a means for improving the intelligibility of speech.

It is a further object of this invention toprovide a means for the improvement of intelligibility of speech under high ambient noise conditions. I I

It is afurther object of this invention to j provide a means for improving the intelligibility of speech by reice . sounds wherein'maximum intelligibility lies.

It is a further object of this invention to provide means for improving theintelligibility of sounds by constructing an amplifier to reduce peaks of; energyWhiGh contain predominate vowel energy so that the remaining consonant energy may be accentuated. I Y

It is a further object of this invention to provide a system for-supplementing a standard noise suppressor by eliminating the peaks of energy as well as the nulls of energy. K

Other and further objects of this invention will become apparent from the following specification and the drawing in which Fig. 1 represents a block diagram of a typical circuit, Fig. 2 represents an additional circuit and Fig.- 3 represents a circuit'for use with a standard 'squelching system I Referring more particularly to Fig.1, a system is shown including a microphone IOen erg iZin g'an amplifier 12 whose signal goes through a gating circuit 14 to a loud- I speaker or other electromechanical transducer 16: The signal in the amplifier 12 actuate's a peak detecting circuit 18 which supplies a gating signal to the gating amplifier 14. In operation, a signal is applied by the microphone to the amplifier 12 which may include vacuum 'tubes or other devices well known in the art. The gating circuit 14 may also includewell known electronic circuitry. The

peak detecting circuit 18"monitors the signal infthe am plifier 12 and turns on and oil the gating'circuit to pass the lower component of the signal rather thanthe higher component to the loud speaker 16. I

It should be stressed here that the gating time duration must be limited to a very small fraction of time. I This time interval must be very much smaller in practice'than the time constantusually'allotted to squelch circuits, since the length of time that the mind'of'the hearer can psychoacoustically fill in an omitted sound without becoming conscious of an interruptionis very small. I I

Fig. 2 shows the circuit of Fig. 1 with similar parts similarly numbered and with'the addition of a low level squelching circhit ll t o f' a standard type forreducing or eliminating'the' fraction of the signal completely under the noise level. This will heIp reduce the more dominant noise factor as mother systems. More noise may be tolerated in this arrangement than-with a standard circuit because the overall noise is ultimately decreased during the vowel interval and the fragment of noise? that is passed also contains a high factor of" intelligibility.

In Fig. 3 the circuit has been varied by the addition'o'f a filter circuit 220 before the gate actuating squelch cirintroduced by theelectronic'circuitry in any=of several ways well known in the art. The peaks of energy ofthe signal are detected'by'the peak detecting circuit which then closes the gating circuit to "reduceor eliminate the peaks of energy normally applied to the loudspeaker or earphone 16 and, ultimately, the ear. I 1 :3;

Fig. 2 is a similar but is supplemented by the low level squelching circuit 124 which maybe of any offthe types well knownjin noise reducing'systems. 4

L Fig. S-isagain similar but has a low-pass filter 220 preceding'the peak detecting circuit to assure that low frequencyor vowel peaks;.and .-not the consonant peaks actuate the gating circuit. 1 I

. ep h r s t t t co ona t n sxis a ually of prime importance in speech, intelligibility whereas the vowel energy is relatively less important. The voltage peaks of bothvowels: and consonantsmaysbeisubr stantially the, same or flle peaks of consonant; voltage maybe even higher than the pcaksoi vowel yoltage, but the consonants appear as sharp ;t ransients ot extremely short time duration, while ,the yowelsounds are of considerably, longer duration. Therefore, themean energy is much higherduring a vowel intervalthan during a consonant interval. Theetiectcn the ear where the gain is turned up under difficult conditions rwould be somewhat of a saturation of the; ear from vowel sound to yowel sound to make the recognition of the consonant transient peaks even lesslikely. This ,rejsults in an overall decrease in intelligibility. In the subject invention the vowel peaks are detected and madeto actuate a gating circuit so theycan be reduced or eliminated in thetransdu'certo gvoiddistracting the ear from its ,morefimportant task of catching the consonants. 1 Theimportance of consonant energyin intelligibility is emphasized by rnany authorities anditis discussed and proven, for example, in the work otLicklider and Miller, beginning on page;10481oftheffHandbook of Experimental Psychology, edited by. ,S. S. Stevens and published by John Wiley 8: Sons in 1951'. The ,same article also points. out that the frequencies of speech under 1000 cycles contain 80% of the power while contributing only 10% to the articulation of speech.. 1

Relating this to applicants device, if the peak detecting circuit is used without filtering, as inFig. 1, it can be assumed, that 80% ofthe actuating. signal energy will be contributed by the lower frequencies ;and, in any case, the removal of the portion of the energy sections of the speech may only decrease the intelligibility, by;1 On the other hand, the elimination of the noise for a substantial portion ofthe, reception.time ,wil l ;inerease the intelligibility considerably more than thedecrease from altering e p ra m. v The well known oorrelationpf frequencies and vowel sounds is discussed in much detail by Fletcher in chapter 5. p ges 282-286 of his volume on1 .'.Spee ch and Hearing, published by Van Nostrandt C ,o Inc. in 1929. Fletcher also shows typical electronic high andlow-pass filters and shows the results of filtering on the'various components of speech. n I i I According to the teachings of thesetexts; and others, most of the energy in speech iscarried in a portion of the frequency spectrum that contributes the least to the intelligibility. This portion iofthe frequency spectrum is the lower portion, for the mosttpart below 1000 cycles and, generally speaking, comprising vowel sounds.

It is obvious from theteachings of these texts that a substantial portion of .thGYOWOl sounds may be subtracted from the consonant sounds by' either detecting peak; energy or by frequencv discrimination. or both. On the other hand, it is fairly apparent that since this invention deals with psychological acoustical phenomena itisnot limited :to vowel sounds alone butincludestthe efiects of peak energy orlow frequency t ofthis described system allowsincrease of the average signalintensity sound level and thereby increases the average power that'm'aybe delivered under conditions reception'close to.the' threshold .of pain. In some ways thisinvention is the aural lequivalentof. .automatic .speedwritingireading. r y

As an additional feature during the cut-off period of the vowel sounds an artiticial substitute tone of' 'approxiinately cycles per second; "be added to the output to provide. a mental fill-in inerase the length of time i that the ear will tolerateinterprttttionsi "This'ispds ime in practice, since the eneaeonsu n enmit a: the ear and mind to identify wen known words make the mind substitute the probably vowel sounds for the artificial substitute tone.

A further modification of this signal reception technique might be particularly applicable for extremely 'slow speech where the vowel sounds could be reduced only to the average level rather than completely eliminated. This would require the gating circuit to be lay-passed by attenuated vowels signals.

It is also conceivable that a second channel of information, not necessarily speech, may be transmitted during the speech vowel gap time and thereby accomplish multiplexing.

Having thus described my invention, what is claimed is: I

1. In a system for increasing the intelligibility of speech in the presence of noise, an amplifier having an input and two outputs, a gating circuit having an input, an output and a control connection, a peak detecting circuit for transforming alternating current energy peaks of audio frequencies into direct current switching pulses, said detecting circuit having an input for low frequency audio signals and an output forthe direct current switching pulses a low pass filter connecting the input of said detecting circuit to one of the outputs of said amplifier, the output of said detecting circuit connected to the control connection of said gating circuit, the input of said gating circuit connected to the other of the outputs of said amplifier, and an electromechanical transducer connected to the output of said gating circuit, said gating circuit being in an on position when no signal is being detected and in an off position when a peak energy signal is being detected by said detecting means.

2. In a system for increasing the intelligibility of speech in the presence of noise, an amplifier having an input and two outputs, a source of signals connected to said input, a gating circuit having an input, a control connection and an output, a low-pass filter for low frequency audio signals having an output and an input connected to one of the outputs of said amplifier, a detecting circuit for transforming alternating current energy peaks of audio frequencies into direct current pulses, said detecting circuit responsive to energy peaks above the average level, said detecting circuit having an input for audio frequency signals and an output for direct current switching pulses, the input of said detecting circuit connected to the output of said low-pass filter, the output of said detecting circuit connected to the control connections of said gating circuit, a low level squelch circuit having an input connected to the other of the outputs of said amplifier and an output connected to the input of said gating circuit for cutting off signals below a given level substantially below said average level, said gatingcircuit being in an on position when said signals are between the said average level and said given level.

3. A squelch circuit for increasing the intelligibility of, speech in the presence of noise comprising a signal source, an audio frequency low pass' filter, coupled to said source, a transmission path coupled to saidlsiourcccontrol means in said path controlling the transmission of signals through said path, a gating means to actuate said control means to block said path, a circuit adapted to be actuated by signal energyfrom saidlow pass filter exceeding a given levelto produce-a rectangular pulse, and an audio frequency low pass filter means for applying a, portion of said pulse to said gating circuit to actuate itupon the signal energy exceeding said level, said gating circuit actr' ing to block said transmission path when actuated bysaid pulse and to unblock said transmission path at other times when no high energy signal is actuatingsaid circuit.

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