[opendtv] Re: Let the games begin

  • From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Mon, 13 Nov 2006 21:11:38 -0500

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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