Page 266 - Anatomy of a Robot

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COMMUNICATIONS 251
The decompressor has a DSP compute engine that
MPEG decompressor
decompresses the MPEG video data. The output of the decompressor is a broad-
cast video signal suitable for viewing.
Figure 9-12 shows the distribution of errors in an MPEG satellite transmission and
helps explain why concatenated codes work so well. The figure shows the errors pres-
ent in the DVB communications link. Errors are shown as tic marks on the time graphs
to approximate the distribution over time. This shows the relative action of the various
concatenated coding blocks. The following describes the action of each block in the fig-
ure with concentration on the handling of errors:

MPEG compressor
RS encoder
Interleaver
Convolutional codes
Modulator
With luck and proper design, none of these five preceding blocks adds errors to the
data. The modulator is the one block capable of introducing partial errors in the sense
that it provides D/A functions. No analog signal is ever perfect. A good modulator will
not add any significant errors.

Channel

The data communications channel is taken to be a standard communications link with
errors added as the result of interference and noise. Data errors might occur at random
intervals, or in concentrated bursts. Such errors are as follows:

Random errors Random errors are the easiest to fix. The existing concatenated
codes are well suited to fixing random errors.
Bursts of errors The existing concatenated codes are reasonably well suited to
fixing bursts of errors. The convolutional codes tend to concentrate errors into short
bursts anyway. Naturally, if too many errors occur, they cannot all be corrected.
Regularly space errors The existing concatenated codes have the most trouble
with errors that occur at regular intervals. The RS block codes, in particular, are
weakest at correcting such errors. This is not to say that these codes will not take
care of errors distributed in such a manner. Just be careful designing a communi-
cation link if the noise is organized in some way.
Demodulator By and large, a demodulator will not add much noise to the sig-
nals in the channel. It will add a small amount, but by the time a demodulator is