AMBE-2000™ Vocoder Chip

User’s Manual Version 3.0

page 39

Confidential Proprietary, Subject to Change

Table 7-B Rate Selection Using Rate Info 0-4, AMBE-2000™ only

RATE_SEL4

Pin

RATE_SEL3

Pin

RATE_SEL2

Pin

RATE_SEL1

Pin

RATE_SEL0

Pin

Speech Rate

(bps)

FEC Rate

(bps)

Total Rate

(bps)

1

1

1

1

1

2000

0

2000

1

0

0

0

0

3600

0

3600

1

0

0

0

1

4000

0

1

0

1

1

0

2400

1600

4000

1

0

0

1

0

4800

0

1

1

0

0

0

4000

800

1

0

1

1

1

3600

1200

1

1

0

0

1

2400

2400

4800

1

0

0

1

1

6400

0

1

1

0

1

0

4000

2400

6400

1

1

0

1

1

4400

2800

7200

1

0

1

0

0

8000

0

1

1

1

0

0

4000

4000

8000

1

0

1

0

1

9600

0

1

1

1

0

1

3600

6000

1

1

1

1

0

2400

7200

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

8kHz Speech Data

Encoder

8kHz Speech Data

A/D-D/A

4 to 2 wire

Converter

Echo Path A

Echo Path B

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

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.