mh presentation template

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Front-end Audio Processing Reflections on
Issues, Requirements, and Solutions
Tomas Gaensler mh acoustics www.mhacoustics.com
Summit NJ/Burlington VT USA
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Front-end Audio Processing

• Processing to enhance perceived and/or measured
  sound quality in communication and recording
  devices

Then
Now
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Not So Famous Quotes (Acoustic Jewelry/Bluetooth
Headset)

• Gary Elko (mh/Bell labs colleague)
• At IWAENC 1995 Acoustic Echo cancellation will
  not be needed in the future when people wear
  acoustic jewelry
• Arno Penzias (1978 Nobel prize laureate)
• No one would want acoustic jewelry because
  people would think the users talking to
  themselves are crazy
• Im glad the success of Bluetooth headsets show
  that both were completely wrong!

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Classical Front-end Architectures - POTS
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Classical Front-end Architectures Cellphone 1995
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Classical Front-end Architectures Cellphone
2005 - 2010
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Cellphones and Handsfree

• Common problems
• Far-end listener does not hear near-end talker
• Near-end listener does not understand far-end
  talker
• Why?
• Form factor Size
• Limited understanding of physics and acoustics(?)

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RX/TX Levels, Coupling and Doubletalk

• Echo louder than near-end
• Linear AEC
• ERLE ? 20-30 dB
• After cancellation Residual Echo to Near-end
  Ratio (RENR)
• RENR ? 90-20-70 0 dB

Far-end ? 95100 dBSPL at loudspeaker

• 8590
• dBSPL at mic

• gt20 dB of residual echo suppression required
• Duplexness suffers

Near-end talker ? 5570 dBSPL at mic
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TX Dynamic Range and Noise

• Echo 90 dBSPL ? Peak echo ?105-110 dB
• No saturation of echo in TX path

Echo Level 90 dBSPL
Near-end speech Level 70 dBSPL
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TX Fixed-point Processing and Quantization Noise
Q-noise increases by 6log2(N) dB!

• N64 ? Q-noise increases by 36 dB
• Double-precision required

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RX Dynamic Range and Distortion
Digital gain
Analog gain
To AEC

• Small loudspeakers have rather high cut-off
  frequency (high-pass)
• EQ often required to get acceptable sound
  (frequency response). However EQ means
• Loss of signal loudness and dynamic range
• Increased (analog) distortion
• Many manufacturers compensate the loss of signal
  level by excessive digital gain and therefore get
  (digital) saturation

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What Can or Should be Done?

• Minimize acoustical coupling by good physical
  design
• TX
• Use noise suppression but not excessively
• Double-precision, block scaling, or
  floating-point
• RX
• Compression instead of fixed gain
• 10 or less loudspeaker/driver THD is desired

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What about Non-linear AEC Algorithms?

• Interesting problem proposed and worked on for
  many years
• Not practical in most AEC applications since
• Complicated model
• Gain and therefore saturation possibly in both TX
  and RX paths
• Added complexity and system cost
• Often slow convergence
• Difficult to fine-tune in field
• Even when non-linear cancellation works
  perfectly, the user still perceives a distorted
  loudspeaker signal!

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Classical Front-end Architectures Cellphone
2005 - 2010
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Single Channel Noise Suppression

• Basic single channel noise suppressor
• An extremely successful signal processing
  invention by Manfred Schroeder in the 1960s
• Musical tones is it a (solved) problem?
• How do we evaluate and improve quality?
• How about convergence rate?

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Background to Single Channel Noise Suppressors
enhanced speech
NS
speech
noise

• Block processing
• Frequency domain model
• Linear Time-varying filter
• Wiener filter

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Background to Single Channel Noise Suppressors

• Estimation of spectra is often done recursively
• Frequency smoothing

, when speech is not present
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Musical Tones Is it a (Solved) Problem?

• Examples
• Original (Sally Sievers reel, June-Sept. 1964
  by Manfred Schroeder and Mohan Sondhi at Bell
  Labs)
• Original noise (iSNR 6 dB)
• Schroeder 1960s
• Generic spectral subtraction Boll 1979
• IS-127 1995
• A problem of last century, only a constraint in
  design
• Controlling variance of suppression gains
• Any NS algorithm should be constrained not to
  have musical tones
• Must only have a small impact on voice quality

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Quality Metrics

• Most importantly Listen!
• SNR
• Total
• Segmental
• During speech
• Distortion metrics
• ISD (Itakura-Saito distance)
• ITU-T P.862 PESQ/MOS-LQO

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Quality Metric P.862 (PESQ/MOS-LQO)

• MOS-LQO (MOS Listening Quality Objective)
• Alg-1/2 Wiener methods with 12 dB noise
  suppression

• What can the best noise suppressor achieve?

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Quality Metric My Rule of Thumb

• Ideal MOS (PESQ) performance bound is given by
  shifting the unprocessed PESQ-curve to the left
• Example for 12 dB suppression
• 12 dB shift to the left

12 dB
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Convergence Rate

• Important performance criterion
• Non-stationary noise conditions
• Frame loss
• Main objective
• Maximize convergence rate while maintaining
  speech quality

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Convergence Rate A Useful Test

1. Input sequence
2. IS-127
3. Wiener Based
4. A spectral subtraction m-script retrieved from
   the internet

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Convergence Rate and MOS-LQO

1. Normal
2. Fast
3. MOS-LQO

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Current Applications and Drivers of NS Technology

• Where is NS going in industry now?
• Beyond 12 dB of suppression
• Multi-microphone solutions
• Two- or more channel suppressors
• Linear beamforming
• Applications
• Mobile phones (a few two-microphone models have
  reached the market)
• Bluetooth headsets great "new" application for
  signal processing (Ericsson BT headset 2000)

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Background to Linear Beamforming

• N Number of microphones
• Broadside linear beamforming (e.g. delay-sum)
• Directional gain 10log(N)
• White Noise Gain (WNG)gt0
• Practical size large (30cm)
• Endfire differential beamforming
• Directional gain 20log(N)
• WNGlt0
• Practical size small (1.5-5cm)

? Differential beamformers more suitable for
small form-factors
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Background to Linear Beamforming

• What do we gain?
• Less reverberation (increased intelligibility)
• Less (environmental) noise
• No (or low) distortion on axis
• Possible interference rejection by spatial
  zero(s)
• Some Issues
• Performance is given by critical distance!
• Increase in sensor noise (WNG, differential
  beamforming)

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Beamforming Critical Distance

• Critical distance (Reverberation radius)
  reverberant-to-direct path energy ratio is 0 dB
• DI Directivity Index gain of direct to
  reverberant energy over an omni-directional
  microphone
• Order of finite differences used. 1st ?2 mics,
  2nd ?3 mics etc)

Order DI dB
0 0
1 6 2.0
2 9.5 3.0
3 12 4.0
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First-Order Differential Beamforming
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Classical First-Order Beamformer Responses
Cardioid
Hypercardioid
Dipole
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Beamforming Demo DEWIND? processing