AMBE-2000 Digital Voice Systems, Inc., AMBE-2000 Datasheet - Page 42

• Manufacturer Part Number: AMBE-2000
• Description: 2.0 - 9.6 KBPS Single ic For Toll-quality, Low Data Rate, Real Time Speech Compression
• Manufacturer: Digital Voice Systems, Inc.

7.4
The Voice Activation Detection (VAD) algorithm along with the Comfort Noise Insertion (CNI) feature of the AMBE-2000™
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 Control Word 1 of the Framed Output format
described in section 5.3.3.
VAD can be enabled in one of two ways. A logic high on the hardware pin VAD_EN (pin 86), subject to the restrictions of
section 7.1, enables the VAD function. Once the AMBE-2000™ has begun operating, control word 2 is used to enable/disable
VAD as described in section 5.2.9.
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.
The synthesis of a Comfort Noise frame by the decoder is not dependant on VAD being enabled. The decoder will produce a
comfort noise frame if it receives an in-band silence frame (produced only by an encoder with VAD enabled).
7.5
The AMBE-2000™ Vocoder Chip is capable of detecting, transmitting, and synthesizing DTMF tones. DTMF features are
always enabled. Detection of a DTMF tone by the encoder sets the Encoder DTMF Detect flag described in section 5.3.3. The
DTMF tone detected along with amplitude information is placed in the DTMF control word as described in section 5.3.8.
Additionally, the encoder passes the DTMF data in-band (within the regular voice data bits) so that normal DTMF tones pass
seamlessly from the encoder to the decoder for synthesis.
The decoder synthesizes a DTMF tone in response to reception of an in-band DTMF tone frame or by setting the DTMF
control word as described in section 5.2.8.
7.6
Power savings can be achieved during times of longer inactivity of the AMBE-2000™ chip by placing it into one of three
available Low Power Modes. The chip can be placed into low-power and stand-by modes via hardware or software Control
Words. In low power modes the A/D-D/A port will be disabled, concurrently halting any processing of voice frames in either
direction. Depending on the low power state selected, either a Wake Up Control Word or a hardware reset on RESETN is
necessary to return the AMBE-2000™ to normal operation.
7.6.1
The standard sleep is the only low power mode that can be entered into either through hardware or software. The AMBE-
2000™ Chip can be placed into Standard Sleep mode either by setting SLEEP_EN (pin 83) high, subject to the restrictions of
section 7.1, or through software by using Control Word 2 with bit 3 set to 1 as described in section 5.2.9.
SLEEP_EN should be tied high if you plan to configure the A/D-D/A chip from Standard Sleep mode upon power-up or reset.
When using software SLEEP_EN with A-law or µ-law codecs, it is important to note that if packets are sent to the decoder
while it is in sleep mode, noise will be heard at the output. It is recommended that no packets be sent to the decoder until it is
commanded to wake up.
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DVSI Confidential Proprietary, Subject to Change
Voice Activation Detection (VAD), Comfort Noise Insertion (CNI)
Dual Tone Multiple Frequency, Detection and Generation
Normal Power and Power Saving Modes
www.dvsinc.com
Standard Sleep Mode
AMBE-2000™ Vocoder Chip
User’s Manual Version 4.0
Page 42