Digital Signal Processing, compression, linear and nonlinear: terminology,

measurement and issues.

Richard Baker

University of Manchester

Outline

• A few common misconceptions• What is signal processing?• Advantages of going digital• Analogue to digital conversion• Compression – why and how?• Measurement issues

Common Misconceptions• “Only digital hearing aids are signal processing

aids”• “Digital is better than Analogue”• “Wide dynamic range compression (WDRC) =

digital”• “Nonlinear = digital”• “Programmable hearing aids are the same as

DSP hearing aids”• “Digital hearing aids cut out background noise”

What is signal processing?• Signal processing is exactly what it says, it may

be:– Amplifying– Filtering– Peak-clipping– Compression: output limiting, WDRC, etc– Frequency shifting– … – etc.

What is a digital hearing aid?

• A digital hearing aid simply converts the signal to a numerical form before processing it

• It’s the signal processing algorithm that is important

DSP DAAD

MIC. LP(Anti-aliasing)

LP(reconstruction)

ANALOGUEDIGITALANALOGUE

SPEAKER

What is compression?

• Compression: – the range of input sound intensities is “squashed”

into a smaller range of output intensities

– e.g. a range of input intensities from 0 to 100 dB SPL may be compressed into an output range of 50 to 100 dB SPL

– The output “dynamic range” is reduced compared to that of the input

Why do we need compression?

• Sensorineural hearing loss most often results from damage to outer hair cells in the cochlear

• This results in:– Loss of sensitivity at low sound intensities

– Abnormally rapid growth of loudness (recruitment)

– Loss of frequency selectivity (Hearing aids can’t do much about this one at the moment)

Loudness Growth• Typically, sensorineural loss results in

recruitment:– Low intensity sounds are inaudible– Moderate intensity sounds are heard as very quiet– High intensity sounds are perceived as similar in

loudness to that normal hearing listener• Implications for hearing aids

– High gain for low intensity input– Low gain for high intensity input– i.e. reduced dynamic range at output compared to

input

CompressionImpairedNormal

Moderate

Weak

Intense

Non-linear

Dillon (2001)

Hearing aid goals• Audibility - be able to hear important sounds e.g.

speech• Comfort - sounds comfortably loud• Safety - sounds prevented from being too loud• Intelligibility - maximise the intelligibility of

speech sounds• Quality - maximise the perceived quality of the

sounds (e.g. little distortion)• Consistency - same performance regardless of

listing conditions• ...• The same aims apply to both linear and nonlinear aids

Linear versus nonlinear

• Linear - gain is constant irrespective of input level (if we ignore very high levels)

• Nonlinear - gain changes as input level changes (may be compression or expansion)

• Remember, when talking in dB terms:

Output level = Input level + gain

Linear hearing aids

• Amplify all sounds by the same amount• Problem – louder sounds become too loud to be

comfortable• Solution – use some type of limiting to prevent

this• e.g. clip the peaks off the waveform when it goes

too loud - peak clipping – causes distortion

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

time (ms)

duration of increased signal intensity

peaks clipped resulting in distorted waveform

Peak-clipping

Peak clipping

The need for compression

• The problem with linear aids – the same gain is applied to all levels of input signal

• we need high gain for low input levels, and low gain for high input levels - compression

• we need some way of automatically turning down the gain of the hearing aid as the input intensity increases

• an automatic gain control or AGC

Automatic gain control (AGC)• AGC parameters• Attack-time – The time taken for the AGC to respond

to an increase in input level• Release time – the time taken for the AGC to

increase the gain again when the input level decreases

• Knee-point – below a certain signal intensity the amplifier behaves linearly, above this intensity the compression operates

• Compression ratio – above knee-point, output with an increase in input is typically less than 1 dB per dB change in input

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

time (ms)

attack time - time taken for gain to be turned down by AGC

release time - time taken for AGC to come out of compression and restore original gain

duration of increased signal intensity

AGC

Automatic gain control

I/O functions, output spectra & transfer functions etc.

• I/O functions - output vs input– at one frequency

• Output spectra - output across frequency– at one input level

• input/gain function - gain vs input– at one frequency

• Transfer function - output/input (i.e. gain) across frequency– at one input level

• All ways of plotting different aspects of hearing aid function

• Input-output function

• Output spectra

Types of compressionThe main compression strategies fall into two

categories:

• Compression limiting – high knee-point, high compression ratio (e.g. 10:1) – limits MPO

• WDRC – wide dynamic range compression, low knee-point, low compression ratio (e.g. 2:1) – aims to restore loudness perception in moderate loss

• AVC - automatic volume control - slow acting compression designed to adjust overall gain when moving from quiet to noisy environment.

Output limiting

50

60

70

80

90

100

110

120

30 40 50 60 70 80 90 100

+40 dB gain

knee-point at 70 dB SPL input

output limiting

linear

+0 dB+10 dB+20 dB

+30 dB

+40 dB

+50 dB

output limiting

input (dB SPL)

output (dB SPL)

WDRC

50

60

70

80

90

100

110

120

30 40 50 60 70 80 90 100

+40 dB gain

knee-point at 40 dB SPL input

limiting at105 dB SPLoutput

compression ratioof 2:1

+0 dB+10 dB+20 dB

+30 dB

+40 dB

+50 dB

WDRC

input (dB SPL)

output (dB SPL)

50

60

70

80

90

100

110

30 40 50 60 70 80 90 100

+0 dB+10 dB+20 dB

+30 dB

+40 dB

+50 dB

WDRC

input (dB SPL)

output (dB SPL)50 dB 65 dB 80 dB linear + PC

Linear & WDRC aids setto have same gain at 65 dB SPLinput.

Note. Linear aid under-amplifiesat 50 dB SPL input andover-amplifies at 80 dB SPL input

• Therefore need to test at different levels:

– 50 dB SPL input - quite speech level

– 65 dB SPL input - moderate speech level

– 80 dB SPL input - loud speech level

Multi-channel processing

Why multi-channel?• different hearing losses at different frequencies• different compression strategies required for

different frequency ranges• theoretical reasons for differing frequency

response• …• … e.t.c.

From Killion et al, 1990

Test signals• Pure-tone - single frequency component• Swept-tone - pure-tone swept up or down in frequency• Speech-weighted pure-tone sweep - swept-tone following the

spectral shape of an average speech signal• White-noise - noise signal containing equal energy at all

frequencies• Pink-noise - noise with energy decreasing with increasing

frequency• Speech-shaped noise - noise with spectral shape of an average

speech signal• Modulated Speech shaped noise - spectral AND temporal

shape similar to that of speech

Test signals

• Test signals can be either:– Continuous - long(ish) duration with approximately

constant amplitude– Fluctuating - varying up and down in amplitude

(usually designed to mimic temporal fluctuations in natural speech)

• Least natural: continuous pure-tone• Most natural: fluctuating speech shaped

noise

Which signal to use?

• With a linear aid pure-tone test signals should produce the same results as noise signals

• With non-linear aids, the aid can respond very differently to different signals

Which signal to use?• e.g. in some situations, pure-tones may produce

an artificially high measurement of low frequency gain - “blooming”– Suppose a compressor follows a high-pass filter– A tone is swept upwards in frequency through the

cut-off region of the filter into the pass-band– As the tone is in the cut-off region the input to the

AGC is low - thus the gain is high– In the pass-band the input to the AGC is high so

the gain is low– Result: Using a swept tone it appears that the low-

pass filter isn’t working – – use a broad-band signal!

blooming!

So, use a broad-band signal!

Which signal to use?• e.g. swept-tone versus noise

– Pure-tone - single frequency component therefore level well defined

– White-noise - many frequency components - measured level is sum of frequency components therefore level at one particular frequency is lower

– Overall level with noise signal also depends on analysis bandwidth

Implications of different signals

1. Output display for broadband signals is lower than tones - use gain display!

2. Output display depends on analysis bandwidth

3. For multichannel aids swept tone gives higher level signal through each band than broadband noise• At high levels tone may result in saturation

whereas noise doesn’t• Nonlinear aids may have different gain for tones

& noise even though they are nominally the same overall level

“extras”

• As well as different signal processing strategies modern hearing aids are available with many “extras” designed to improve their performance

• These also have implications for how the aids are tested and the signals used…

“extras”

• Noise suppression/cancellation– Algorithms attempt to “detect presence of speech”

and turn down the gain if no speech is present– Note

• Need to use realistic speech like signal to perform measurements – continuous noise will be suppressed, so need to have speech-shaped noise with fluctuating envelope (is such a signal available?)

• Turn the noise reduction feature off

“extras”

• Multi-program/memory aids

– Can allow 2 or more different processing algorithms to be used

– E.g. a second setting with extra gain for bouts of OME

– Note• Need to know what each of the memories are

supposed to do in order to test aid

“extras”

• Directional/Multi-Microphone technology

– Aims to improve signal-noise ratio by “picking out” sounds from the front, and reducing those from other direction

– Note• Need to be careful how aid is positioned in a

test box to get accurate measurements• Turn the directional microphone off!

“extras”

• Feedback management/cancellation– Notch-filters or complex feedback cancellation

algorithms have been developed that can reduce feedback and allow 10-20dB extra gain.

– This can allow additional gain, use of vents where they are normally not possible etc.

– Note: awareness of notch-filters is necessary & the feed-back suppression needs to be turned off for measurement purposes (is this possible for every situation?)

Feedback Management

Dillon (2001)

Feedback Cancelling

External leakage path

Internal feedback path

+

-

Dillon (2001)

Implications

• conceptual complexity - difficult to understand what the aid is doing

• complexity & adjustability - many different parameters to adjust to set up the aid

• lack of user adjustability - some nonlinear aids have no volume control - WDRC, in theory, should do away for the need for it

• test signal - need to chose the right test signal• lack of defined standards - no clearly defined

standards for measuring nonlinear aids

Ideal vs reality for testing aids

• Ideal situation:– full test-box & programming facility, ability to turn

off “extras”, modulated speech-shaped noise as test signal

• Likely situation for some (eg outreach or other services?):– “old” test-box, no programming facility, can’t turn

off “extras”, only continuous pure-tone or swept pure-tone available

Summary

• Signal processing• Compression

– Fits dynamic range of sounds into comfortable range of hearing

– AGC– Types of compression – output-limiting, WDRC

• Multi-channel processing• Implications

– conceptual, complexity, test-signals

• References

– Dillon, H. (2001) Hearing Aids, Thieme

– Sandlin, R.E. (2000) Hearing Aid Amplification, Singular

– Vonlanthen, A. (2000) Hearing Instrument Technonogy, Singular

– Venema, T. (1998) Compression for Clinicians, Singular

– Killion, M.C., Staab, W. & Preeves, D. (1990) Classifying automatic signal processors. Hearing Instruments, 41(8), 24-26

– Seewald, R. C (2001), A Sound Foundation Through Early Amplification 2000, Phonak AG, ISBN: 3-9522009-0-5

– Seewald, R. C. & Gravel, J.C. (2002), A Sound Foundation Through Early Amplification 2001, Phonak AG, ISBN: 3-9522009-1-3

• Standards

– BS EN 61669:2001 Electroacoustics – Equipment for the measurement of real-ear acoustical characteristics of hearing aids

– BS ISO 12124:2001 Acoustics – Procedures for the measurement of real-ear acoustical characteristics of hearing aids