Presentation on theme: "Richard Baker University of Manchester"— Presentation transcript:
1 Richard Baker University of Manchester Digital Signal Processing, compression, linear and nonlinear: terminology, measurement and issues.Richard BakerUniversity of Manchester
2 Outline A few common misconceptions What is signal processing? Advantages of going digitalAnalogue to digital conversionCompression – 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:AmplifyingFilteringPeak-clippingCompression: output limiting, WDRC, etcFrequency shifting…etc.
5 What is a digital hearing aid? A digital hearing aid simply converts the signal to a numerical form before processing itIt’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 intensitiese.g. a range of input intensities from 0 to 100 dB SPL may be compressed into an output range of 50 to 100 dB SPLThe 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 cochlearThis results in:Loss of sensitivity at low sound intensitiesAbnormally 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 inaudibleModerate intensity sounds are heard as very quietHigh intensity sounds are perceived as similar in loudness to that normal hearing listenerImplications for hearing aidsHigh gain for low intensity inputLow gain for high intensity inputi.e. reduced dynamic range at output compared to input
9 Compression Normal Impaired Non-linear Intense Moderate Weak Dillon (2001)
10 Hearing aid goalsAudibility - be able to hear important sounds e.g. speechComfort - sounds comfortably loudSafety - sounds prevented from being too loudIntelligibility - maximise the intelligibility of speech soundsQuality - 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 comfortableSolution – use some type of limiting to prevent thise.g. clip the peaks off the waveform when it goes too loud - peak clipping – causes distortion
14 The need for compression The problem with linear aids – the same gain is applied to all levels of input signalwe need high gain for low input levels, and low gain for high input levels - compressionwe need some way of automatically turning down the gain of the hearing aid as the input intensity increasesan automatic gain control or AGC
15 Automatic gain control (AGC) AGC parametersAttack-time – The time taken for the AGC to respond to an increase in input levelRelease time – the time taken for the AGC to increase the gain again when the input level decreasesKnee-point – below a certain signal intensity the amplifier behaves linearly, above this intensity the compression operatesCompression ratio – above knee-point, output with an increase in input is typically less than 1 dB per dB change in input
17 I/O functions, output spectra & transfer functions etc. I/O functions - output vs inputat one frequencyOutput spectra - output across frequencyat one input levelinput/gain function - gain vs inputTransfer function - output/input (i.e. gain) across frequencyAll ways of plotting different aspects of hearing aid function
21 Types of compressionThe main compression strategies fall into two categories:Compression limiting – high knee-point, high compression ratio (e.g. 10:1) – limits MPOWDRC – wide dynamic range compression, low knee-point, low compression ratio (e.g. 2:1) – aims to restore loudness perception in moderate lossAVC - automatic volume control - slow acting compression designed to adjust overall gain when moving from quiet to noisy environment.
25 Therefore need to test at different levels: 50 dB SPL input - quite speech level65 dB SPL input - moderate speech level80 dB SPL input - loud speech level
26 Multi-channel processing Why multi-channel?different hearing losses at different frequenciesdifferent compression strategies required for different frequency rangestheoretical reasons for differing frequency response…… e.t.c.
28 Test signals Pure-tone - single frequency component Swept-tone - pure-tone swept up or down in frequencySpeech-weighted pure-tone sweep - swept-tone following the spectral shape of an average speech signalWhite-noise - noise signal containing equal energy at all frequenciesPink-noise - noise with energy decreasing with increasing frequencySpeech-shaped noise - noise with spectral shape of an average speech signalModulated 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 amplitudeFluctuating - varying up and down in amplitude (usually designed to mimic temporal fluctuations in natural speech)Least natural: continuous pure-toneMost 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 signalsWith 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 filterA tone is swept upwards in frequency through the cut-off region of the filter into the pass-bandAs the tone is in the cut-off region the input to the AGC is low - thus the gain is highIn the pass-band the input to the AGC is high so the gain is lowResult: Using a swept tone it appears that the low-pass filter isn’t working –use a broad-band signal!
33 Which signal to use? e.g. swept-tone versus noise Pure-tone - single frequency component therefore level well definedWhite-noise - many frequency components - measured level is sum of frequency components therefore level at one particular frequency is lowerOverall 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 bandwidthFor multichannel aids swept tone gives higher level signal through each band than broadband noiseAt high levels tone may result in saturation whereas noise doesn’tNonlinear 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 performanceThese 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 presentNoteNeed 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 usedE.g. a second setting with extra gain for bouts of OMENoteNeed 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 directionNoteNeed to be careful how aid is positioned in a test box to get accurate measurementsTurn 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 ManagementA better solution is to turn down only a narrow band, where feedback would have occurred. Better still is to limit the maximum gain in this band, such that an automatic hearing aid is free to vary the gain up and down, but never exceed this preset maximum gain.Dillon (2001)
42 Feedback Cancelling External leakage path + - Internal feedback path An even better solution is to cancel the external feedback via an internal feedback path using a filter that has the same gain but opposite phase.Of course, when the jaw moves, the leakage path may change, so the hearing aid needs to continually monitor the input and output, and change the characteristics of the filter accordingly.Dillon (2001)
43 Implicationsconceptual complexity - difficult to understand what the aid is doingcomplexity & adjustability - many different parameters to adjust to set up the aidlack of user adjustability - some nonlinear aids have no volume control - WDRC, in theory, should do away for the need for ittest signal - need to chose the right test signallack 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 signalLikely 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 hearingAGCTypes of compression – output-limiting, WDRCMulti-channel processingImplicationsconceptual, complexity, test-signals
46 ReferencesDillon, H. (2001) Hearing Aids, ThiemeSandlin, R.E. (2000) Hearing Aid Amplification, SingularVonlanthen, A. (2000) Hearing Instrument Technonogy, SingularVenema, T. (1998) Compression for Clinicians, SingularKillion, M.C., Staab, W. & Preeves, D. (1990) Classifying automatic signal processors. Hearing Instruments, 41(8), 24-26Seewald, R. C (2001), A Sound Foundation Through Early Amplification 2000, Phonak AG, ISBN:Seewald, R. C. & Gravel, J.C. (2002), A Sound Foundation Through Early Amplification 2001, Phonak AG, ISBN:StandardsBS EN 61669:2001 Electroacoustics – Equipment for the measurement of real-ear acoustical characteristics of hearing aidsBS ISO 12124:2001 Acoustics – Procedures for the measurement of real-ear acoustical characteristics of hearing aids