AMBE-1000 Digital Voice Systems, Inc., AMBE-1000 Datasheet - Page 54

• Manufacturer Part Number: AMBE-1000
• Manufacturer: Digital Voice Systems, Inc.

6.4
The AMBE-1000 is not recommended for new designs.
Please refer to the AMBE-2000/2020 product line.
The AMBE-1000™ 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-1000™ Vocoder Chip for optimum performance.
Figure 6-A Typical Echo Path
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
chips due to lower quantization noise.
The echo canceller can be activated either through the hardware pin 92, ECHOCAN_EN,
or through the Command Frame interface described in section 4.1.14.4. See section
6.1 for important note about the ECHOCAN_EN pin.
Voice Activation Detection (VAD), Comfort Noise Insertion (CNI)
The Voice Activation Detection (VAD) algorithm along with the Comfort Noise Insertion
(CNI) feature of the AMBE-1000™ chip performs useful functions in systems trying to
convert periods of silence, that exist in normal conversation, to savings in system
bandwidth or power.
With the VAD functions enabled, periods of silence will be denoted by the encoder in
two ways. First, 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”. Second, the Encoder Silence Detected flag is set in the Status_0
Field of the Framed Output format described in section 4.1.4.
VAD can be enabled in one of two ways. A high signal on the hardware pin VAD_EN
(pin 91), subject to the restrictions of section 6.1, enables VAD. The Control Frame
described in section 4.1.14.4 describes how to enable/disable the VAD algorithm once
the AMBE-1000™ has begun operating.
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.
AMBE-1000
Decoder
Encoder
The AMBE-1000 is not recommended for new designs.
Please refer to the AMBE-2000/2020 product line.
Visit us at www.dvsinc.com
8kHz Speech Data
8kHz Speech Data
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A/D-D/A
Echo Path A
Echo Path B
4 to 2 wire
Converter
law or Alaw