OT: Composite/Component [was Re: [mythtv-users] Re: Image quality,
what effects it?]
mythtv at edsons.demon.nl
Fri Aug 13 14:31:54 EDT 2004
Jay R. Ashworth wrote:
>> ? I was basically referring to the quantity of data that had to be
>>passed. If it were true RGB (which it's not... probably a factor of 1/2 for
>>4:2:2), 720x480 * 30fps * 3 colors * 8bits/color = 31MBps... that's a lot of
>>data for a PCI bus. Even with a 1/2 reduction for YUV and 1/2 D1 of 352x480,
>>that's 7.5 MBps... still plenty. Trying to do HDTV would require
>>proportionately more. That pretty much means it would have to encode to MPEG2
>>(or MPEG4) on the fly to stream over the PCI bus. Lots of embedded CPU power
>>necessary for that... 4x-8x hauppauge I suppose.
>Yeah. The PcHDTV card is probably doing that.
No, it is not. It is receiving a 2, 4, 8 or 16 VSB modulated MPEG-2
Transport stream according to the ATSC standard and selecting the wanted
PIDs out of it.
SW support is only for 8-VSB (the only modulation currently used on
ATSC), which limits the TS to 19.4 Mbit/s. This means the card is
specific for terrestrial ATSC. On a cable the signal is likely to be
re-modulated in QAM-64. QAM-256 might also be used. Most cable companies
use QAM for digital modulation and will not support 2 different
modulation schemes at the same time. Also the ATSC standard (at least
when last I looked at it) does not mandate any particular modulation
when used over cable. For terrestrial use VSB is mandated. Sattelite
uses QPSK (QAM-2) modulation.
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