Digital Voice Systems Inc.
234 Littleton Road Westford, MA 01886
Tel: (978) 392-0002 Fax: (978) 392-8866 e-mail: [email protected]
Soft Decision Error Decoding with the
AMBE-2000/2020 Vocoder Chips
In modern communication systems the transmitter transmits information in the form of
symbols. The demodulator takes the received signal and tries to decide which symbol
was transmitted. In other words, the demodulator is making a decision based on the
signal received. For instance, in a binary system, the symbols could be represented by a
1 and a 0. Because of interference and many other factors, these signals can be
misinterpreted because of channel degradation. Significant improvement in FEC
performance can be added by setting up the receiver so that the demodulator is making a
finer estimation of the received energy prior to the decoder, this is called soft-decision
decoding. There is a link below that describe this in more detail.
http://www.mathworks.com/access/helpdesk/help/toolbox/commblks/usersguide/tutor135.shtml
The AMBE-2000/2020 utilizes a 4 bit soft decision decoder. The bits are defined as
follows:
Decision Value
(Binary)
Interpretation
0000 Most
confident
0
0111 Least
confident
0
1000 Least
confident
1
1111 Most
confident
1
Placing a logic high on pin 79 of either the AMBE-2000™ or AMBE-2020™ vocoder
chips enables the soft decision error correction on the decoder. Enabling the soft-
decision does nothing to the encoder packet. The packet will look like a normal encoded
packet. The user must implement circuitry at the receive end of the channel for making a
finer (4 bit) estimation of the received energy (see above link for an explanation of how
this can be done).
The AMBE-2000/2020 decoder packet structure is altered. The decoder expects each
voice data bit of the encoded packet to be represented by 4 soft decision (SD) bits. The
decoder will make the decision of whether or not a 1 or a 0 is represented by the SD bits.
Table 1 is an example of a soft-decision decoder packet.
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