Comtech EF Data SDM-9000 - Manual (Page 212)

Options SDM-9000 Satellite Modem
A–26 Rev. 4
In punctured operation (rates 3/4 and 7/8), a rate exchange is required between the rate
1/2 convolutional encoded data and the encoded punctured channel data.
Note: The DVB specification also describes rate 2/3 and rate 5/6 puncturing, but the
SDM-9000 does not support these code rates at this time.
Table A-4 shows the ratios between the convolutional encoder’s clock and the symbol
clock for each exchange rate.
Table A-4. Rate Exchange Ratios for DVB Puncturing
Rate Ratio Comment
7/8 7:4
3/4 3:2
1/2 1:1 No symbols are punctured, therefore no rate exchange is required.
The reception process works by shuffling the encoded punctured channel data to the
appropriate positions for the rate 1/2 Viterbi decoder. Null symbol indicators are inserted
into the punctured symbol positions in the rate 1/2 data stream.
Again, for rates 3/4 and 7/8, a rate exchange is required for the symbol clock to Viterbi
clock hand-off. For rate 1/2, no symbols are punctured, and therefore, no rate exchange is
required.