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ON THE ARCHITECTURE OF THE CDMA2000
VARIABLE-RATE MULTIMODE WIDEBAND (VMR-WB) SPEECH
CODING STANDARD
Milan Jelinek, Redwan Salami, Sassan Ahmadi,
Bruno Bessette, Philippe Gournay and Claude
Laflamme
University of Sherbrooke, Canada - VoiceAge
Corp., Canada - Nokia inc., USA
Encoder Flow Chart

• VMR-WB
• Variable-Rate Multi-Mode Wideband Speech Codec
• New 3GPP2 WB Speech Coding Standard for 3G
  applications
• Main Features
• Near Face-to-Face Communication Speech Quality
• Source and Channel Controlled Operation (4
  Modes)
• 3GPP/ITU AMR-WB Directly Interoperable in Mode 3
• Average Bit Rates (ABR)
• Compliant with CDMA2000 Rate Set 2

VMR-WB Coding Techniques

• Source-Controlled Operation
• Hierarchical Signal Classification
• Operating on Frame-level

1. Voice Activity Detection (VAD)
2. Unvoiced Frame Decision
Based on the following parameters
Coding Type Bitrate kbit/s Description
Inactive Speech Coding CNG ER 1.0 -Noise excited LP filter -Smoothed over time
Inactive Speech Coding CNG QR 2.7 -As previous, but interoperable with AMR-WB CNG
Unvoiced Coding Unvoiced HR 6.2 -13 bit Gaussian codebook (4x/frame)
Unvoiced Coding Unvoiced QR 2.7 -As previous, but randomly chosen vectors
Voiced Coding Voiced HR 6.2 -Frame level signal modification -12 bit ACELP codebook (4x/frame)
Generic Coding Interoperable FR 13.3 -Similar to AMR-WB _at_ 12.65 kbit/s
Generic Coding Generic FR 13.3 -As previous FER protection
Generic Coding Interoperable HR 6.2 -As Interoperable FR, but with random algebraic codebook indices
Generic Coding Signaling HR 6.2 -As previous FER protection
Generic Coding Generic HR 6.2 -Pitch coded 2x/frame -12 bit ACELP codebook (4x/frame)

• Normalized Correlation

T open-loop pitch period estimate xi
perceptually weighted input signal

• Spectral Tilt

Eh average energy of last 2 critical bands.
El average energy of pitch-synchronous bins in
the first 10 critical bands
Active speech kbit/s 40 Speech Activity kbit/s
Mode 3 13.3 6.1
Mode 0 12.8 5.7
Mode 1 10.5 4.8
Mode 2 8.1 3.8

• Frame Energy Variation

• Noise Estimation Update Decision
• Based on parameters with low sensitivity to noise
  level
• Pitch period varying
• AND normalized correlation at pitch period low
• AND low estimated order of AR model
• AND signal energy stationary
• INDEPENDENT of VAD decision!
• - Robust to noise level variations
• - Conservative approach the noise estimation is
  updated only if quite sure the frame is inactive

E32(j) energy maximum in a bloc of 32-samples

• Relative Frame Energy - Erel

Decision
3. Voiced Frame Decision / Signal Modification
4. Low Energy Decision

• Channel-Controlled Operation
• 4 Operational Modes Controlled by Channel
  Conditions
• Transparent Memory-less Mode Switching
• Per-Frame Bit Rate Control Capability
•   Coding Types Relative Usage in Active Speech
• Mode Switching Performance

• Enhancements at Decoder
• Low Frequency Post-processing
• Enhancement of the periodicity in low frequency
  region

Performance (MOS scores from selection
test) CDMA Specific Modes (Modes 0, 1, 2), WB
Input
Performance (MOS scores from characterization
test)

• Voiced Decision is an Inherent Part of Original
  Signal Modification Algorithm
• Frame is coded as voiced if all constraints of
  the modification are satisfied
• Signal modification is done pitch-synchronously
• Pitch period evolution is piecewise linear
  (constant at frame end) to avoid pitch period
  oscillations
• Modified input is synchronous with original
  input at frame end
• Modification is transparent at least up to 30 of
  active speech frames (in the example bellow, no
  coding is used and 30 of active clean speech
  frames are modified)

• NB Input Test
• Modes 0, 1, 2, 3,
• Clean speech, nominal level
• Test on Interworking with AMR-WB _at_ 12.65 kbit/s
• -WB input, clean speech conditions

Purpose To avoid encoding unclassified frames
with low perceptual importance at Full Rate
Condition
Ref 0 AMR-WB _at_ 14.25 Ref 1 AMR-WB _at_ 12.65 Ref
2 AMR-WB _at_ 8.85
Test 0 VMR-WB Mode 0 Test 1 VMR-WB Mode
1 Test 2 VMR-WB Mode 2
Et sum of critical band energies for current
frame, in dB Ef long-term mean of Et for active
speech
Clean Speech Conditions
Example Typical example of a low-energy frame
encoded with Generic HR in mode 2

• Frame Errors Concealment
• Lost Frame Concealment
• Excitation energy and spectral envelope converge
  to estimated noise.
• Excitation periodicity converges to 0.
• Convergence rate depends on the signal class of
  last good frame.
• Recovery after erasure
• Careful energy control of synthesized speech.
• Artificial onset reconstruction in case of lost
  voiced onset.

Channel Error Conditions
 
Background Noise Conditions