AMBE2000 Datasheet, PDF (39/47 Pages) List of Unclassifed Manufacturers – VOCODER CHIP

AMBE-2000™ Vocoder Chip
User’s Manual Version 3.0
Table 7-B Rate Selection Using Rate Info 0-4, AMBE-2000™ only
RATE_SEL4
Pin
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
RATE_SEL3
Pin
1
0
0
0
0
1
0
1
0
1
1
0
1
0
1
1
RATE_SEL2
Pin
1
0
0
1
0
0
1
0
0
0
0
1
1
1
1
1
RATE_SEL1
Pin
1
0
0
1
1
0
1
0
1
1
1
0
0
0
0
1
RATE_SEL0
Pin
1
0
1
0
0
0
1
1
1
0
1
0
0
1
1
0
Speech Rate
(bps)
2000
3600
4000
2400
4800
4000
3600
2400
6400
4000
4400
8000
4000
9600
3600
2400
FEC Rate
(bps)
0
0
0
1600
0
800
1200
2400
0
2400
2800
0
4000
0
6000
7200
Total Rate
(bps)
2000
3600
4000
4800
6400
7200
8000
9600
7.3 Echo Cancellation
The AMBE-2000™ Vocoder Chip provides a 16 millisecond echo canceller that is suitable for canceling the local echo caused
by a 2-to-4 wire hybrid and can achieve echo cancellation of approximately 30dB or more. Only the linear portion of the echo
is cancelled, so circuits should be designed to minimize non-linearities.
The AMBE-2000™ Vocoder Chip echo canceller operates by sending a 240 millisecond audible training sequence to the A/D-
D/A immediately following a reset. Best results will be achieved if the analog circuit causing any echo is stable at this time. If
the analog circuit changes substantially following this training , the echo canceller must be re-initialized, by resetting the
AMBE-2000™ Vocoder Chip for optimum performance.
Figure 7-A Typical Echo Path
AMBE-2000
Decoder
Encoder
8kHz Speech Data
A/D-D/A
8kHz Speech Data
Echo Path A
Echo Path B
4 to 2 wire
Converter
The Echo Return Loss (ERL) of the analog circuit must be 6dB or more (in diagram ERL = Echo Path A – Echo Path B) for
proper echo canceller operation. Linear A/D-D/A chips will generally provide better echo cancellation performance than µlaw
or Alaw chips due to lower quantization noise.
The echo canceller can be activated either through the hardware pin 78, ECHOCAN_EN, or through the Control Word
interface described in section 5.2.9. See section 7.1 for important note about the ECHOCAN_EN pin.
page 39
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