AMBE-2000™ Vocoder Chip

User’s Manual Version 3.0

page 8

Confidential Proprietary, Subject to Change

2 AMBE™-2000 Application Design Overview

2.1 Basic Operation

In its simplest model, the AMBE-2000™ can be viewed as two separate components, the Encoder and the Decoder. The

Encoder receives an 8kHz. sampled stream of speech data (16-bit linear, 8-bit Alaw, or 8-bit ulaw) and outputs a stream of

channel data at the desired rate. Conversely the Decoder receives a stream of channel data and synthesizes a stream of speech

data. The timing for the interfaces for the AMBE-2000™ Encoder and Decoder are fully asynchronous.

Figure 2-A Basic Operation

AMBE-2000

Encoder

8kHz Speech Data

Decoder

8kHz Speech Data

AMBE-2000

Decoder

8kHz Speech Data

Encoder

8kHz Speech Data

Compressed Data @ 2400-9600bps

Compressed Data @ 2400-9600bps

Typically the speech interface is an external A/D-D/A chip. The format of the incoming and outgoing speech data streams are

coupled, that is to say they must be the same format (16-bit linear, 8-bit Alaw, or 8-bit

commonly (but not limited to) an 8 or 16 bit microprocessor or other suitable ‘glue logic’ hardware capable of performing the

rudimentary formatting functions between the AMBE-2000™ channel format and the format of the system channel under

design.

Optional functions of the chip, such as echo cancellation, voice activation/detection, power mode control, data/FEC rate

selection, etc. are controlled either through hardware control pins (see Section 0) and/or through the decoder command

interface (see Section 4) Data sent into the decoder for function control purposes is distinguished from the data to be decoded

into speech through a channel format which is described in Section 4.

2.2

Initial Design Considerations

Some of the initial design considerations the application engineer will face are the following:

•

Choice of A/D-D/A chip.

•

Choice of Channel Interface.

•

Speech and FEC Rates.

2.2.1

A/D – D/A Overview

The choice of the A/D-D/A chip is critical to designing a system with superior voice quality. Given that Alaw and

companding chips are already incorporating some compression to reduce the number of bits per sample, it is recommended

that, when possible, a 16-bit linear device be used for maximum voice quality. When choosing a device, pay particular

attention to Signal to Noise ratios and Frequency Responses of any filters that may be present on the analog front end of these

chips. The Alaw and

is under other cost type restraints.