Digital Signal Processing, compression, linear and nonlinear: terminology, measurement and issues.

1. Digital Signal Processing, compression, linear and nonlinear: terminology, measurement and issues. Richard Baker University of Manchester

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

3. 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”

4. 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.

5. 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

6. 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

7. 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)

8. 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

9. Compression Normal Impaired Intense Non-linear Moderate Weak Dillon (2001)

10. 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

11. 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

12. 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

13. Peak clipping

14. 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

15. 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

16. Automatic gain control

17. 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

19. Input-output function

20. Output spectra

21. Types of compression The 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.

22. Output limiting

23. WDRC

25. 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

26. 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.

27. From Killion et al, 1990

28. 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

29. 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

30. 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

31. 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!

32. blooming! • So, use a broad-band signal!

33. 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

35. Implications of different signals • Output display for broadband signals is lower than tones - use gain display! • Output display depends on analysis bandwidth • 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

36. “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…

37. “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

38. “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

39. “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!

40. “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?)

41. Feedback Management Dillon (2001)

42. Feedback Cancelling External leakage path +  - Internal feedback path Dillon (2001)

43. 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

44. 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

45. 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

46. 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