User`s manual

AMBE-2000™ Vocoder Chip
User’s Manual Version 4.9
DVSI Confidential Proprietary, Subject to Change Page 11
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synchronization with the data stream before it can properly synthesize the speech waveform. The Unframed mode only
commits a single bit per frame to maintain data alignment. In higher error rate channels the performance will be improved by
adding more bits per frame to the alignment information (which is more easily performed when using Framed mode).
Additional flexibility is given to the channel interface to the encoder and decoder by allowing the AMBE-2000™ Vocoder
Chip to run in Passive or Active modes. In Passive mode, data strobes are provided by an external source, while in Active
mode, data strobes are provided by the AMBE-2000™ Vocoder Chip. The serial interfaces can be run in Passive or Active
modes. See Section 4 for full details and timing for both Framed and Unframed data.
2.2.4 Speech and FEC Rate Selection Overview
The total coded bit rate is the sum of two components, the Speech Data bit rate and the Forward Error Correction (FEC) Data
bit rate. The addition of FEC data to the speech data allows the decoder to be able to correct a limited amount of errors within
each frame should they arrive corrupted. If the channel is expected to have more errors then more bits should be dedicated to
FEC. At the same time, voice quality will increase if the number of speech bits is increased.