[opendtv] Re: 20050627 Mark's Monday Memo

Bob Miller wrote:

> If we had an 8-VSB receiver as good as a 1999 COFDM
> receiver I would not be in doubt about a venture
> with the  5th gen receivers we have tested so far
> and would not be up late tonight talking to the Far
> East about the latest.

Context is everything.

First of all, in urban canyon reception, COFDM is doing
exactly what it was designed to do. That's where it's
*supposed* to shine. So if it does, it should not come
as a great surprise.

But even in spite of that, real tests by the CRC, an
impartial organization, provided this result in their
September 2003 article in the IEEE Transactions on
Broadcasting:

"As shown in Fig. 4, the DVB-T receiver could handle
pre and post ghosts over a wider range. It had a window
of 74 us working within a delay range of -74 to +74 us,
and capable of resolving zero dB ghost for a signal
having a high C/N of 31 dB, and less than 1 dB ghost
for a signal with a C/N of 22 dB."

By comparison, the CRC test on both the LG and the
previous Linx receivers showed that two 0 dB echoes,
i.e. three equal strength symbols arriving 1 usec
apart, as in Brazil E, could successfully be decoded
with a C/N of only 25 dB. And less stressful profiles
required C/N margins in ranges well below 20 dB.

So this shows a mixed bag. The 8-VSB receivers still
cannot match the echo tolerance range of COFDM with
GI set to 1/16 (50 uS for LG, 74 uS for COFDM), but
they can beat your 1999 COFDM performance for the
very difficult Brazil E profile handily, with 6 dB of
extra margin.

This is what I'm getting at. Depending on specific
surroundings, your sweeping assessment might apply or
not. In GENERAL, these tests show what most people
already know: COFDM is great in areas of high
multipath. It is very robust. But it is best used
where signal strength is not a big issue. That's why
it's good for applications like WiFi. If signal
strength is an issue, then even in *some* high
multipath environments, the new 8-VSB receivers might
have an edge. Unless you do the careful tests,
sweeping generalizations are not credible.

Bert

 
 
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