Bob Miller wrote:
>One other little problem with legacy receivers and the
>future/present needs of the US DTV spectum.
>
>Doug Lung in "China's new DTV Standard and DOCRs" brings
>up the troubling need for DOCRs. He thinks they are required
>for a successful OTA HDTV future.
For the record, I didn't buy your other arguments, but we've been over them
time and time again.
What you need for digital OCRs is a reasonable amount of pre-echo tolerance,
where the amount of pre-echo tolerance you have is proportional to how much
power the OCR can transmit, compared with the power density in that location
generated by the main transmitter. This is the same consideration you have
to give to SFNs that use unsynchronized transmitter.
The safe way to design this sort of network is to use what the French call
the umbrella configuration. The big stick is used for the bulk of coverage
(e.g. the Eiffel Tower), and the OCRs are low powered enough that they do
not interfere with each other and don't radiate so far that the main
transmitter will create what looks like strong pre-echo many usec prior to
the repeated signal.
The 5th gen receivers can stand strong pre-echo out to roughly 15 usec or
so, realistically, which means that beyond 3 miles from the OCR, you really
want that repeated signal to become weak compared with the main
transmitter's signal. That's so the main transmitter will again seem like
the dominant signal, and the OCR will create a benign post-echo. As the
relative power of the pre-echo drops, 5th gen receivers can tolerate quite a
bit of pre-echo. Like -50 usec.
The CRC demoed these in Ottawa, using the LG 5th gen demod. They got good
results with two OCRs creating coverage that overlapped with the main
transmitter, which is not half bad.
Yes, this is a strong suit of COFDM -- pre-echo tolerance -- although 1st
and 2nd generation DVB-T receivers (according to the French CSA document
http://www.csa.fr/pdf/Rapport-GT2-Aspects_Radiofrequence_de_la_TNT.pdf
Section 2.3.1.5) also had trouble with pre-echo tolerance.
It is of course not unreasonable to expect that as time goes by, as things
change, people will require STBs even if they own an integrated set. We have
discussed this many times too. If AVC is introduced, for example. I think
OCRs or gap fillers can be used with 8-VSB, and would be easiest to
implement when terrain masks the main transmitter's signal. Like in a
valley. Using directional antennas with the OCR helps too. No matter 8-VSB
or COFDM, designing a solid multi-transmitter single channel network in no
trivial task.
Bert
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