WO1994030029A1

WO1994030029A1 - Hybrid noise cancellation system for headsets

Info

Publication number: WO1994030029A1
Application number: PCT/US1994/005642
Authority: WO
Inventors: Jeffrey N. Denenberg
Original assignee: Noise Cancellation Technologies, Inc.
Priority date: 1993-06-03
Filing date: 1994-05-24
Publication date: 1994-12-22

Abstract

A hybrid noise cancellation system (60, 70) adapted for use in headsets and incorporating multiple noise cancellation algorithms operatively connected and adapted to completely cancel low frequency and high frequency, rapidly changing noise.

Description

HYBRID NOISE CANCELLATION SYSTEM FOR HEADSETS

This invention is directed to a hybrid noise cancellation system for headsets. It involves the simultaneous use of multiple noise cancellation algorithms to achieve better results than those obtainable with either one alone. The example application used is a headset for emergency vehicles with two major noise sources; the electronic siren (canceled by "Digital Virtual Earth" as described in U.S. Patent No. 5,105,377 and incorporated by reference herein) and low frequency noise from the vehicle engine (canceled by an analog feedback system). The two work well together when care is taken so that the Digital Virtual Earth (DVE) does not "hear" the low frequency noise and can concentrate on the rapidly changing tonal noise of the siren. This "hybrid" approach to noise cancellation has the merit that the two work better together than either alone.

Accordingly, it is an object of this invention to provide a system employing different multiple noise cancellation algorithms to achieve results superior to single algorithm use.

It is another object of this invention to combine a digital cancellation algorithm and an analog feedback system to cancel noise.

These and other objects will become apparent when reference is made to the accompanying drawings in which

Figure 1 is a block diagram of a basic analog feedback noise control system,

Figure 2 is a block diagram for a DVE noise cancellation system, and

Figure 3 is a block diagram of a hybrid noise canceling system.

Existing Algorithms for Headsets Analog

Equalized analog feedback (and feed forward) systems have been applied to headsets for several years. They are simple, non-adaptive systems which are effective at using active components to reduce low frequency noise (up to 500 Hz and somewhat higher) in both open and close back configurations.

Feedback Cancellation

Analog feedback noise cancellation headsets are in production by several companies in open and closed back form. Figure 1 shows a block diagram of a basic analog feedback noise control headset.

This is a classical feedback control system. It introduces a control signal (Anti- noise) at the headset and measures the result (at the residual microphone). Since the feedback loop has gain the system continuously attempts to reduce any disturbance (Noise) picked up by the residual microphone to zero. The system is limited, however, in its ability to handle disturbances with high frequency components due to delay in the headset configuration. The feedback loop must be equalized (EQ 1) to prevent the loop from being unstable (oscillating). In feedback control terms the system must have gain and phase margin for stable operation.

In a practical system the headset/microphone combination has about 0.1 msec of delay which results in a tradeoff between noise attenuation (usually 10 to 15 dB), operational bandwidth (about 500 Hz), and reliable operation.

Also shown in Figure 1 are two ways to avoid having the active headset cancel desired sounds which are played through the same headset (e.g. Communications or Music). Since the headset has a known cancellation effectiveness, a simple low frequency boost can be provided (EQ 3) to compensate for the attenuation of the active system. This simple approach will work but the resulting system will suffer reduced signal to noise and dynamic range. The second and somewhat better approach is to provide a second equalizer (EQ 2) which matches the headset path characteristics so that the desired signal's contribution to the residual can be subtracted out before the anti- noise is generated.

Feed forward

Some systems have chosen to use a feed forward microphone instead of the residual microphone in the classic feedback design. In this variation the microphone picks up the anti-noise in-phase with the noise. The relationship between this result and the result heard at the ear is measured and an appropriate equalizer is used to provide cancellation. This approach trades off a decrease in the achieved cancellation for a small increase in stability and signal to noise. The result is generally similar to that obtained in feedback systems.

Open Back Versus Closed Back Designs

A closed back system can provide significant noise reduction passively at frequencies above 500 Hz. Open back headsets allow desired communications through (along with any undesired noise). The application environment determines which is appropriate. There is another difference when analog active cancellation is used.

The equalizer (EQ 1 in Figure 1) design must accurately match the characteristics of the headset or the system will not function. An open back headset has a very stable transfer function. The transfer function of a closed back headset is strongly dependent on the effectiveness of the seal. Any slight opening in the seal (e.g. due to the user wearing glasses) significantly changes the low frequency response of the headset and will render the system inoperative. An open back system is therefore easier to produce as a

SUBSΠTUTE SHEET (RULE 26) reliable product but is not viable for applications with high frequency noise since there is little passive attenuation provided by the headset.

Synchronous Digital (Chaplin)

Headsets based on the Chaplin method for cancellation of tonal noise have been demonstrated for years and are in current production for use by MRI patients. These are effective for applications with predominantly tonal noise from one source where a synchronization signal is available from that source. They have a major benefit in that the cancellation is selective, letting through desired sounds while canceling the undesired noise. They become complex for application with noise from several sources and are ineffective in the presence of broad band noise.

Digital Virtual Earth (DVE)

Headsets based on the digital virtual earth (DVE) algorithm can be very effective in applications with widely different noise characteristics.

Low Frequency Noise

A DVE system can behave very much like an analog feedback cancellation system and cancel low frequency broad band noise. The upper limit for this behavior is somewhat lower than that for an analog system due to the added delays in a digital controller, but its performance is very reliable.

Simple Tonal Noise

When only a few harmonics are present a DVE system can adapt rapidly to the noise and cancel it. This is the approach used in the Noise Cancellation Technologies, Inc. Siren Canceling Headset which is quite effective even when the noise is rapidly changing. This algorithm also exhibits a form of selectivity, it will always attempt to cancel the loudest noise in its operational bandwidth. When that component of the noise is reduced to the level of another component it will then attempt to reduce both together. This process continues until the system either runs out of degrees of freedom (filter taps, two are needed per tone) or no further reduction can be achieved. Figure 2 shows a block diagram for a DVE noise cancellation Headset.

The DVE controller 50 contains microphone preamplifier 51, analog to digital and filter unit, 52, summer 53, digital to analog and filter unit 54 and output amplifier 55.

Figure 3 shows the combination of two different noise cancellation systems to achieve a very high degree of noise reduction.

The system uses an analog cancellation system 60 with microphone preamplifier 61, equalizers 62, 63 and 64, output amplifier 65, and audio gain 66. System 60 is connected to DVE controller 70 having A/D and filter 71, "A", 72, and D/A and filter 73.

Example: An Emergency Vehicle Drive Headset (Analog and DVE)

As presented above an analog headset can deal effectively with low frequency noise and DVE can deal effectively with the rapidly changing siren noise. A headset with both algorithms working together can reduce both the low frequency noise from the emergency vehicle engine and the siren resulting in a very effective product. Figure 3 shows a block diagram of the two interconnected systems.

Handling Interactions

Here the operation of the two algorithms is maintained independent by insuring that DVE "hears" the residual noise with the low frequency components reduced by the analog cancellation subsystem. DVE then only generates anti-noise at the higher frequencies (mainly in response to the siren noise) which, in turn, is mostly above the frequency range of the analog cancellation subsystem. Even if the frequency ranges overlap slightly the subtraction of the anti-noise from DVE (via EQ 2, along with any externally provided audio signal) from the residual signal prevents the analog subsystem from responding to the DVE signal.

The additional subtraction of the externally provided audio signal from the input to DVE (via a second EQ 2) minimizes the tendency for DVE to respond to this audio (the so-called "echo" problem in these headsets). This second subtraction can also be done in the digital domain if sufficient processing resources are available.

An additional benefit of this approach is the improved effectiveness of the DVE system. Since the low frequency noise is removed, the DVE system will cancel the tonal components to the reduced noise floor. The use of a high pass filter to provide a similar benefit adds loop delay and thereby reduces the ability of DVE to track the rapidly changing siren noise.

Using the Digital System to Optimize the Analog Subsystem

A further improvement can be obtained using the DSP power of the DVE subsystem at calibrate time to adjust the parameters of the analog cancellation subsystem to match those of the headset. This is required if the headset is a field replaceable item which is packaged separately from the cancellation electronics. The acoustical performance of the headset speakers and residual microphones can vary significantly from unit to unit. Setting the parameters for optimal performance can be done manually in the factory with instrumentation but is too sophisticated for an average user. Parameters to be adjusted are:

Loop Gain - The loop gain is the primary parameter as it determines the amount of cancellation achieved by the headset. If it is set too low there is little cancellation. If it is set too high the headset can be unstable and create a loud noise at the ear.

The gain of the speaker varies 1 to 2 dB between units and that of the microphone varies 4 to 5 dB. The adjustment therefore needs to have a 10 dB range. If the desired cancellation effectiveness is 12 dB, a variation of 1 dB (20%) in the adjusted loop gain yields a variation of 3 dB in the cancellation effectiveness.

Equalizer - The maximum stable cancellation bandwidth at a particular loop gain is determined by the flatness and delay in the equalized loop. Since the headset and microphone (and other electronic components) vary from unit to unit (and with age), a classical equalizer can only be designed for the average conditions. The loop gain must therefore be set low to insure stability. A simple one stage equalizer has three key parameters:

Height - The peak amount of equalization needed. Width - The width of the equalization "bump". Location - The center frequency of the equalization "bump".

If a more complex equalizer is needed due to the headset characteristics, the number of parameters increases. A parameterized study of the variation of the headsets is required to determine which of these will actually need to be automatically adjusted in the final product.

The adjustment of parameters is done by replacing resistors with multiplying Digital to Analog Converters (DACs) in the equalizer circuit. The analog "gain" through each DAC is then controlled by a digital number stored in a register at each DAC by the digital control circuitry using heuristic rules and the result of transfer function measurements made at calibrate time.

Conclusion

A hybrid of two algorithms can be built which is simultaneously effective on two very different classes of noise. Low frequency, broad-band noise can be canceled by an analog cancellation system (or a DVE system optimized for minimum delay) while a DVE system can cancel simple tonal noise such as that produced by an electronic siren without being disturbed by the low frequency noise. The synergy of the combination also gives an opportunity to improve the operation of the analog cancellation system by using the processing resources of the DSP at calibrate time to compensate for component variations and aging of the system. The combined operation of these two subsystems therefore results in an improved noise cancellation headset configuration. Having described the invention attention is directed to the following claims.

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Claims

1. A hybrid noise cancellation system for canceling unwanted noise entering headsets, said system comprising a first noise canceling electronic means adapted to cancel low frequency noise, and a second noise canceling electronic means adapted to cancel rapidly changing periodic noise, said first and second noise canceling electronic means being operatively connected and adapted to completely cancel low frequency and high frequency rapidly changing noise through the provision of an output signal which can activate a transducer to produce noise to acoustically cancel said unwanted noise.

2. A system as in claim 1 wherein said first noise canceling means is an analog system means.

3. A system as in claim 2 wherein said analog system means is a feedback control system.

4. A system as in claim 2 wherein said analog system means is a feedforward system.

5. A system as in claim 3 wherein said analog feedback control system includes means for amplifying a residual noise signal from a residual microphone, an audio gain means, cancellation gain means, low frequency boost means and a match transfer function means to allow the system to continually adjust to cancel unwanted noise but to allow desired sound to be heard.

6. A system as in claim 3 wherein said second noise canceling electronic means comprises a digital virtual earth system.

7. A system as in claim 4 wherein said second noise canceling electronic means comprises a digital virtual earth system.

8. A system as in claim 1 wherein said second noise canceling electronic means comprises a digital virtual earth system.

9. A system as in claim 2 wherein said second noise canceling electronic means comprises a digital virtual earth system.

10. A system as in claim 1 wherein said second noise canceling electronic means is adapted and calibrated to optimally operate in the presence of said first canceling electronic means.

11. A system as in claim 2 wherein said second noise canceling electronic means is used to measure and optimize the performance of said analog canceling means.

12. A system as in claim 7 wherein the operative connection between said first and second noise canceling electronic means includes means to subtract an external audio signal from the input to the digital virtual earth system to alleviate the tendency of the latter system to respond thereto.

13. A hybrid noise cancellation headset system for use by a wearer to cancel unwanted noise while allowing desired noise to be heard, said system comprising a headset means adapted to be worn by a user and having at least one speaker means thereon adapted to be adjacent an ear of the user, a residual microphone means on said headset means adjacent said microphone means, and a first noise canceling control means adapted to cancel low frequency noise in the area of said microphone means, and a second noise canceling control means adapted to cancel rapidly changing noise, said first and second noise canceling control means being operatively connected and adapted to cancel both undesired low frequency and rapidly changing noise while allowing desired sound to be heard by the user.

14. A system as in claim 13 wherein said first noise canceling control means is a direct analog system.

15. A system as in claim 14 wherein said control means involves a feedback from said residual microphone means.

16. A system as in claim 13 wherein said second noise canceling control means is a digital virtual earth system.

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17. A system as in claim 16 wherein said system is adapted to cancel the loudest noise in its operational bandwidth.

18. A system as in claim 14 wherein said second system is a digital virtual earth system whereby the low frequency undesired noise is acoustically canceled by said analog system and rapidly changing, slightly higher frequency noise is canceled by said digital virtual earth system.