[opendtv] Re: ATSC-Mobile
- From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 13 Feb 2007 13:10:02 -0500
Tom Barry queiried:
>Except for memory for buffering I thought LDPC took very little effort,
linear time and just a bunch of xor's. What is the bottleneck?
The decoder end is the concern.
In 1958, Robert Gallager, then a Ph.D. student at the Massachusetts
Institute of Technology (MIT), created a class of codes that, like Hamming's
codes, add redundant bits that permit decoders to do even-odd checks to
guess which bits of the original message have been corrupted by noise. As
with turbo codes, Gallager's LDPC codes use cross talk between decoders to
gradually establish confidence about the likely bits of the original
message. However, in contrast to turbo codes, which use just two decoders,
LDPC codes use a separate decoder for each bit of the message, creating a
giant rumor mill. That requires thousands, or even tens of thousands, of
decoders.
Al Limberg
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