ADPCM

1
ADPCM

• The More Advance Codec Than PCM Is
• The Adaptive Differential PCM (ADPCM) codec.

• ADPCM Reduces The Per Call BandhWidth From
  64Kbps To 40, 32, 24, 16 Kbps.

• ADPCM Also Uses 8000 samples Per Second Like
  PCM

• But Unlike PCM It Uses 5, 4, 3, 2 Bits Per
  Sample

2
BITS PER SECOND IN ADPCM

• So The Bits Per Second In ADPCM By Using Less
  Than 8-
• Bits Per Sample.

8000 samples/sec x 5 bits/sample
40,000 40 Kbps 8000 samples/sec x 4
bits/sample 32,000 32 Kbps 8000
samples/sec x 3 bits/sample 24,000 24
Kbps 8000 samples/sec x 2 bits/sample
16,000 16 Kbps.
3
NEED TO DEVELOP NEW CODECS

• New Codecs, Have Been Developed That Produce
  High Quality Voice At Very Low Bit
  Rates.

• Other Codecs Are

• The Conjugate Structure Code-Excited Linear
  Predictive Codec. (CS-ACELP)
• Multipulse- Maximum Liklihood
  Quautization(MPMLQ)
• Algebric Code Excited Linear Prediction(ACELP)
• Low-Delay Code Book Excited Linear
  Prediction(LD-CELP)

4
(No Transcript)
5
Speech Compression Algorithms
Original (64000 bps)

• This is the original speech signal sampled at
  8000 samples/second and u-law quantized at 8
  bits/sample. Approximately 4 seconds of speech.
• This is speech compressed using the Adaptive
  Differential Pulse Coded Modulation (ADPCM)
  scheme. The bit rate is 4 bits/sample
• This is speech compressed using the Linear
  Predictive Coding (LPC10) scheme. The bit rate is
  0.3 bits/sample
• This is speech compressed using the Code Excited
  Linear Prediction (CELP) scheme. The bit rate is
  0.6 bits/sample

ADPCM (32000 bps)
LPC10 (2400 bps)
CELP (4800 bps)
6
CODECS V/S BANDWIDTH
7
Voice Compression Technologies
Unacceptable
Business Quality
Toll Quality
64
PCM (G.711)
(Cellular)
Bandwidth (Kbps)
32
ADPCM 32 (G.726)
24
ADPCM 24 (G.726)
16
ADPCM 16 (G.726)
LDCELP 16 (G.728)
8
CS-ACELP 8 (G.729)
LPC 4.8
0
Quality
8
Bandwidth Requirements
Voice Band Traffic
Encoding/ Compression
Result Bit Rate
G.711 PCM A-Law/u-Law
64 kbps (DS0)
G.726 ADPCM
16, 24, 32, 40 kbps
G.729 CS-ACELP
8 kbps
G.728 LD-CELP
16 kbps
G.723.1 CELP
6.3/5.3 kbps Variable
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Delay
Sender
Receiver
Network
First Bit Transmitted
Last Bit Received
A
A
Network Transit Delay
t
Processing Delay
Processing Delay
End-to-End Delay
10
Voice Transport and Delay
Cumulative Transmission Path Delay
0
100
200
300
400
500
600
700
800
Time (msec)
Delay Target
11
Delay VariationJitter
Sender
Receiver
A
B
C
Sender Transmits
t
A
B
C
Sink Receives
D1
D2 D1
t
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Speech Codec Comparison

• Codec Algorithmic TypeRate
  Delay(ms)
• G.711 A-Law / µ-Law 64 0
• G.722 SB-ADPCM 64/ 56/ 48 0
• G.723.1/ AMP-MLQ/ACELP 6.3/ 5.3 37.5
• G.726 ADPCM 16/ 24/ 32/ 40 0
• G.727 Embedded ADPCM 16/ 24/ 32/ 40 0
• G.728 LD-CELP 16 lt 2
• G.729 CS-ACELP 8 15
• G.729 ACS-ACELP 8 15
• G.729 BCS-ACELP 8 15
• G.729 ABCS-ACELP 8 15

13
Waveform Coding