PS20251 Datasheet, PDF (14/25 Pages) List of Unclassifed Manufacturers – Dual COFDM demodulator with PID filters and MRC diversity

PS20251
3.16Viterbi Decoder
The Viterbi decoder accepts the soft decision data from the OFDM demodulator and outputs a decoded
bit-stream.
The decoder does the de-puncturing of the input data for all code rates other than 1/2. It then evaluates
the branch metrics and passes these to a 64-state path-metric updating unit, which in turn outputs a 64-bit
word to the survivor memory. The Viterbi decoded bits are obtained by tracing back the survivor paths in
this memory. A trace-back depth of 128 is used to minimize any loss in performance, especially at high
code rates. The decoder re-encodes the decoded bits and compares these with received data (delayed) to
compute bit errors at its input, on the assumption that the Viterbi output BER is significantly lower than its
input BER.
3.17MPEG Frame Aligner
The Viterbi decoded bit stream is aligned into 204-byte frames. A robust synchronization algorithm is
used to ensure correct lock and to prevent loss of lock due to noise impulses.
3.18 De-interleaver
Errors at the Viterbi output occur in bursts and the function of the de-interleaver is to spread these errors
over a number of 204-byte frames to give the Reed-Solomon decoder a better chance of correcting these.
The de-interleaver is a memory unit which implements the inverse of the convolutional interleaving function
introduced by the transmitter.
3.19Reed-Solomon Decoder
Every 188-byte transport packet is encoded by the transmitter into a 204-byte frame, using a truncated
version of a systematic (255,239) Reed-Solomon code. The corresponding (204,188) Reed-Solomon
decoder is capable of correcting up to eight byte errors in a 204-byte frame. It may also detect frames
with more than eight byte errors.
In addition to efficiently performing this decoding function, the Reed-Solomon decoder in PS20251 keeps
a count of the number of bit errors corrected over a programmable period and the number of
uncorrectable blocks. This information can be used to compute the post-Viterbi BER.
3.20 De-scrambler
The de-scrambler de-randomizes the Reed-Solomon decoded data by generating the exclusive-OR of this
with a pseudo-random bit sequence (PRBS). This outputs 188-byte MPEG transport packets. The TEI bit
of the packet header may be set if required to indicate uncorrectable packets.
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