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AMBE-3000F™ Vocoder Chip Users Manual

Version 3.4, April, 2014

itial Design Considerations

Issue

a PKT_CODECSTOP:

0x61 0x00 0x01 0x2B

Issue PKT_INIT:

0x61 0x00 0x02 0x0B 0x03

Issue a PKT_CODECSTART

0x61 0x00 0x02 0x00 0x2A XxXX*

*See Table 57 PKT_CODECSTART Field Data

4.5 Special Functions Description

The special functions of the AMBE-3000F™ Vocoder Chip, such as voice activity detection, echo cancellation, DTMF,

data/FEC rate selection, power mode control, etc. can be controlled either through hardware control pins and/or through the

packet interface. The hardware inputs are only accessed for input during the first 7 milliseconds after a hardware reset on

RESETn. For predictable operation these signals must remain stable over this time period. After this 7 milliseconds

initialization period changes on these pins are ignored, unless another reset is performed.

4.5.1 Voice Activity Detection & Comfort Noise Insertion

(DTX_ENABLE TQFP pin5, BGA pin C1)

The Voice Activity Detection (VAD) algorithm along with the Comfort Noise Insertion (CNI) feature of the AMBE-3000F™

Vocoder Chip performs useful functions in systems trying to convert periods of silence, that exist in normal conversation, to

savings in system bandwidth or power. VAD and CNI can be enabled by either hardware configuration pin (DTX_ENABLE

TQFP pin5 BGA pin C1) or as part of a control packet.

With the VAD functions enabled, when periods of silence occur, the encoder will output a silence frame (in-band). This

silence frame contains information regarding the level of background noise, which allows the corresponding decoder to

synthesize a “Comfort Noise” signal at the other end. The comfort noise is intended to give the listener the feeling that the call

is still connected, as opposed to producing absolute silence, which can give the impression that, the call has been “dropped”.

The decoder will produce a comfort noise frame if it receives an in-band silence frame (produced only by an encoder with

VAD enabled). The synthesis of a Comfort Noise frame by the decoder is not dependent on VAD being enabled.

If the VAD features are being used to reduce transmit power during times of conversational silence, DVSI recommends that a

silence frame be transmitted at the start of the period and approximately each 500-1000 milliseconds thereafter. This is to

ensure that the parameters regarding the levels of background noise are transmitted to the decoder for the smoothest audible

transitions between synthesized speech and synthesized silence.

The silence threshold value is -25 dBm0 in the VAD algorithm. Each frame that exceeds this level will be classified as voice.

If the frame level is less than -25 dBm0 the voice/silence decision will be determined based upon various adaptive thresholds.

4.5.2 Echo Canceller (EC_ENABLE TQFP pin120 BGA pin D5)

The AMBE-3000F™ Vocoder Chip’s voice coder contains an echo canceller that can be selectively enabled or disabled via

either hardware pin or setting of control command packet. The echo canceller 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 can

be cancelled, so circuits should be designed to minimize nonlinearities. The Echo Return Loss (ERL) of the analog circuit

must be 6dB or more for proper echo canceller operation. Linear Codecs will generally provide better performance than µ-law

or a-law codecs due to lower quantization noise.

The AMBE-3000F™ Vocoder Chip employs an adaptive echo cancellation algorithm to cancel echoes of the decoder output

present at the encoder input. The echo canceller is an Adaptive LMS echo canceller with a 16 ms (128 samples) filter. It

exceeds all the performance requirements specified by ITU-T recommendation G.165.

(Subject to Change) Page 32

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