[opendtv] Re: VHF vs UHF coverage
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 25 May 2009 19:45:09 -0400
Richard Hollandsworth wrote:
> Since a real analog picture is somewhat more black than white (to
> prevent bloom), a "typical" on-air signal would have a long term PAR
> of about 4 dB, which is 3 dB LOWER than DTV. Hence a 5 MW (peak)
> analog transmitter would be closer to 2 MW (long term) average power.
I'm having difficulty reconciling this with A/54 Figure 8.2 (that you
pointed out).
I'll use your estimate that long term NTSC PAR is on the order of 4 dB.
Certainly, in extreme cases, the scrambled DTV signal will create
instantaneous high peaks. That's when probability conspires to to create
a lot of maxed out voltage signals after scrambling, huffman coding, and
after mapping bit combinations to trellis coded 8-VSB symbol voltages.
Since the ATSC pilot is highly attenuated, compared with an analog
carrier, those peaks will look large compared with the average, in the
ATSC case. But the 8 dB theoretical max PAR should occur virtually
never, according to the figure. So it shouldn't be a burden to
transmitter power sources.
The figure also says that a PAR of 4 dB or greater, which is what you
estimate for NTSC long term, will only occur about 5 percent of the time
in ATSC. So to me, that means that a 4 dB PAR would also be a bit of an
anomaly for ATSC, given that 95 percent of the time, the PAR would be
less than that?
Somehow, I think that the comparison of DTV vs analog should take into
account the average PAR values measured over a standard time window,
like seconds? Something that matters to the source of power to the
transmitter. And then you're still left with trying to match analog
image quality against a black screen for digital.
Bert
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