User`s manual

VC-55™ PR Voice Codec Board
User’s Manual Version 1.3
Page 15
Section 4 – Interface Protocol
Channel Interface
Digital Voice Systems, Inc.
The Speech Compression Specialists
Vocoder Board Channel Basic Operation
All channel packets contain data (coded speech) that is sent to the decoder to synthesize speech.
When a Channel packet is received the Vocoder Board decodes the packet and plays the synthesized
speech out the selected analog interface. At the same time the VC-55™ -PR Vocoder board can
encode the speech from the analog input and send it across the channel to be decoded by another VC-
55™ -PR .
Data Structure
Data packets sent as byte-aligned frames over the asynchronous RS-232 Uart Serial interface. This
helps maintains compatibility with asynchronous 8N1 framing and synchronization if channel errors
exist.
The first byte is the sync element (byte 0). It holds the 8-bit identification of the start of the packet. The
packet sync value of the first byte is always the same. The second 8 bit byte will either be 0x0F or 0xF0
indicating the encoded rate. The rest of the packet is the compressed voice data. An “x” is used in all
the tables in this description to indicate that a bit can take a value of 0 or 1.
Packet Structure
Byte Parameter Name
Bits
7 6 5 4 3 2 1 0
Hex Value
0 sync 0 1 0 1 0 1 1 0 0x56
1 rate x x x x x x x x 0x0F or 0xF0
2
Compressed voice bits
0-7
x x x x x x x x 0xXX
…
Compressed voice bits
8-15
… …
… … … …
10
Compressed voice bits
64-71
Last Byte when running Half-Rate Mode
… … … …
…
Compressed voice bits
72-79
x x x x x x x x 0xXX
… … …
19
Compressed voice bits
136-143
Last Byte when running Full-Rate Mode
Table 8 Packet Structure
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