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Digital Audio Signal Processing: Introduction Version 2014-2015 Lecture-1: Introduction p. 4
Aims/Scope
Aim is 2-fold : • Speech & audio per se S & A industry in Belgium/Europe/… • Basic signal processing theory/principles : Optimal filters Adaptive filter algorithms (APA, Filtered-X LMS,..) Kalman filters (linear/nonlinear) etc...
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Hearing • Outer ear/middle ear/inner ear • Tonotopy of inner ear: spatial arrangement of where sounds of
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HA technology requirements • Small form factor (cfr. user acceptance) • Low power: 1…5mW (cfr. battery lifetime ≈ 1 week) • Low processing delay: 10msec (cfr. synchronization with lip reading)
DSP challenges in hearing instruments • Dynamic range compression (cfr supra) • Dereverberation: undo filtering (`echo-ing’) by room acoustics • Feedback cancellation • Noise reduction
Case Study: Hearing Instruments 9/14
Digital Audio Signal Processing: Introduction Version 2014-2015 Lecture-1: Introduction p. 14
DSP Challenges: Feedback Cancellation • Problem statement: Loudspeaker signal is fed back into microphone,
then amplified and played back again • Closed loop system may become unstable (howling) • Similar to feedback problem in public address systems (for the
musicians amongst you)
Case Study: Hearing Instruments 10/14
Model
F
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Similar to echo cancellation in GSM handsets, Skype,… but more difficult due to signal correlation
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DSP Challenges: Noise reduction Multimicrophone ‘beamforming’, typically with 2
microphones, e.g. ‘directional’ front microphone and ‘omnidirectional’ back microphone
Case Study: Hearing Instruments 11/14
“filter-and-sum” the
microphone signals
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• Constraints: power/bandwith/delay of wireless link • ..10kBit/s: coordinate program settings, parameters,… • ..300kBits/s: exchange 1 or more (compressed) audio signals
• Challenges: • Improved localization through cue preservation • Improved noise reduction + benefit from binaural unmasking • Signal selection/filtering, audio coding, synchronisation, …
Case Study: Hearing Instruments 13/14
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Adaptive filtering algorithms AEC Control AEC Post-processing Stereo AEC
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Overview : Lecture-6
Acoustic Feedback Cancellation • Ex: Hearing aids • Ex: PA systems • correlation between filter input (`x ’) and near-end signal (‘ n ’) • fixes : noise injection, pitch shifting, notch filtering, ...
amplifier
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Overview : Lecture-7
Active Noise Control • Solution based on `filtered-X LMS’ • Application : active headsets/ear defenders
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Overview : Lecture-7bis & 8
3D Audio & Loudspeaker Arrays • Binaural synthesis …with headphones head related transfer functions (HRTF) …with 2+ loudspeakers (`sweet spot’) crosstalk cancellation
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Aims/Scope (revisited)
Aim is 2-fold : • Speech & audio per se • Basic signal processing theory/principles : Optimal filtering / Kalman filters (linear/nonlinear) here : speech enhancement other : automatic control, spectral estimation, ... Advanced adaptive filter algorithms here : acoustic echo cancellation other : digital communications, ... Filtered-X LMS here : 3D audio other : active noise/vibration control
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Literature
Literature (General) (available in DSP-CIS library) • Simon Haykin `Adaptive Filter Theory’ (Prentice Hall 1996) • P.P. Vaidyanathan `Multirate Systems and Filter Banks’ (Prentice Hall 1993) Literature (specialized) (some available in DSP-CIS library) • S.L. Gay & J. Benesty `Acoustic Signal Processing for Telecommunication’ (Kluwer 2000) • M. Kahrs & K. Brandenburg (Eds) `Applications of Digital Signal Processing to Audio and Acoustics’ (Kluwer1998) • B. Gold & N. Morgan `Speech and Audio Signal Processing’ (Wiley 2000)
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