[opendtv] Re: News: CEA FORECASTS CONSUMER ELECTRONICS REVENUE
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 13 Jan 2007 12:04:26 -0500
At 11:14 PM -0500 1/12/07, Albert Manfredi wrote:
First of all, I assume the 704 X 480 analog SD occupying 6 MHz more
or less represents NTSC. So in fact, it is 704 X 480 at 60i. And for
good, clear reception, in the 6 MHz channel, you need about 40 dB of
C/N. Less than that and the image quality suffers very rapidly,
compared with an SD 480i.
Try a more realistic raster that better reflects the frequency
response of NTSC. 480 x 480 interlaced is about right.
And the video bandwidth is 4.2 MHz.
The reason why 704 (13.5 MHz sampling) is overkill for NTSC.
Compare that to 1920 X 1080 at 30p or 60i. The video bandwidth now
is 31 MHz, and all you need to decode this essentially error free in
a 6 MHz channel is 15 dB of C/N, give or take a few fractions of a
dB. And this is very predictable, from what my STB's signal monitor
tells me.
Rubbish. Take a look at a spectrum analyzer of ANY MPEG-2 encoded HD
source with a bit rate of say 17Mbps (about the max video payload for
ATSC). The only stuff you will see above 20 MHz is the noise from
quantization errors. Black to white pixel transitions can throw some
nasty high frequency spikes - these "details" were not in the source,
they are artifacts of compression. The C/N of the channel is
completely irrelevant to this discussion - the frequency response is
no longer a function of an analog channel, it is a function of the
digital encoder.
Regards
Craig
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