BWE of NB LBR codec

1. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 1
IEEE Speech Coding Workshop
Sept 17–20, 2000
Lake Lawn Resort
Delavan, WI
Jean-Marc Valin, Roch Lefebvre
University of Sherbrooke
Bandwidth Extension of Narrowband
Speech for Low Bit-Rate Wideband Coding

2. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 2
Outline
• Problem statement
• Proposed solution
• System performance
• Discussion

3. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 3
Problem Statement
• Telephone Band: 300 - 3400 Hz
• AM Band: 50 - 7000 Hz
• How to make sound like with 500 bits/sec?
• We need to recover information from both low and
high-frequency bands
(G.729)

4. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 4
Proposed Solution
0 1000 2000 3000 4000 5000 6000 7000 8000
10
20
30
40
50
60
70
80
90
100
110
Frequency (Hz)
Amplitude
(dB)
• 1) Do our best to recover the wideband information
from narrowband speech
• 2) Use coding for the information that cannot be
recovered
– Recovered information :
• Low-frequency band
• High-frequency excitation
– Coded information :
• High-frequency spectral
envelope

5. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 5
System Overview
• Inverse IRM filter is optional
– produces a flat response from 200-3500 Hz
Inverse
IRM
Filter
Low-frequency
regeneration
2
High-frequency
regeneration
8 kHz
narrowband
16 kHz
wideband
50-300 Hz
band
300-3400 Hz
band
3400-8000 Hz
band
Side information

6. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 6
Low-Frequency Regeneration (1/2)
• Assumptions :
– Only pitch harmonics need to be recovered
• In general, no more than two pitch harmonics below 200 Hz
– Absolute phase is not perceptually relevant
• Frequency of harmonics determined from pitch
analysis
• Amplitudes determined from feed-forward multi-
layer perceptron (output in log domain)

7. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 7
Low-Frequency Regeneration (2/2)
Low-frequency
harmonic synthesis
Scale
and sum
2
1st harmonic
2nd harmonic
Pitch
analysis
MFCC
calculation
Multi-layer
Perceptron
Pitch delay
Cepstral coefficients
Scale
factors
Narrowband
speech
Low
frequencies
Pitch gain
LP
filter
(1)
(1)
(16)

8. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 8
High-Frequency Extension
• Excitation-filter model (16 ms frames)
• Problem is separated in two parts
– Excitation extension (no side information)
– Spectral envelope coding (side information)
A(z)
Narrowband
speech
LPC
analysis
Excitation
extension B(z)
1
Spectral envelope
Extension
Side information
(High-frequency spectral envelope)
High-
frequency
band
High
pass

9. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 9
Excitation Extension
Narrowband
excitation
Absolute
value
Whitening
filter
wideband
excitation
2
High
pass
0 5 10 15 20
-0.5
0
0.5
1
0 2 4 6 8
0
2
4
6
8
10
0 5 10 15 20
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8
0
5
10
15
0 5 10 15 20
-0.2
0
0.2
0.4
0.6
0.8
0 2 4 6 8
0
1
2
3
4
5

10. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 10
Spectral Envelope Coding
• Spectral envelope calculated from the wideband
LPC coefficients
• Quantization of the 3000-8000 Hz range (40 points)
– Log domain
– 8-bit Vector Quantization (500 bits/s side information,
using 16 ms frames)
• Concatenation with envelope obtained from LPC
analysis on narrowband speech

11. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 11
Objective results
• Low-frequency band
– 3 dB RMS error on harmonic amplitude
• High-frequency band
– 3.6 dB RMS error on envelope
– No objective measure for excitation extension
(perceptually very close to original)

12. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 12
Subjective Results
female male
Original wideband
Recovered from
original IRM-filtered speech
Recovered from
G.729 coded speech

13. Speech Coding Workshop 2000 Jean-Marc Valin, Roch Lefebvre 13
Discussion
• Highlights
– Expand IRM-filtered telephone-band speech to AM band
– Very low side information rate (500 bits/s)
• Areas of improvement
– Use high-band spectral estimation before coding
– Use residual low-frequency information (below 300 Hz)
– Noise robustness
– Post-filtering