User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–25
Original 11/97
Vocoder
CDMA takes advantage of quiet times during speech to raise capacity. A variable
rate VOCODER is used; the vocoder’s output is at 9600 BPS when the user is
speaking. When the user pauses, or is listening, the data rate drops to 1200 BPS.
The data rates of 2400 and 4800 BPS are also used but not as often as the other
two. The data rate is based on speech activity and complexity. A decision is made
on the data rate every 20 msec. Normal speech has about a 40% activity factor. A
40% voice activity factor means that only 40% of transmission time is needed to
transmit the intelligible parts of speech.
Convolutional Encoder
Data in
9600
pbs
Data Out
9600 bps
D D D D
Data Out
9600 bps
D D D D
Forward Error Protection
CDMA07.DRW
Figure 23. Convolutional encoder
The forward CDMA link uses a half–rate convolutional encoder to provide error
correction capabilities. A half–rate encoder produces two output bits for every bit
input. This type of encoder accepts incoming serial data and outputs encoded data.
A convolutional encoder uses a shift register that contains a history of the bit
stream. It starts with all zeros and the data stream is shifted through. The two
9600 BPS output data streams are combined at a higher rate to provide a single
19,200 BPS data stream.