OT: Composite/Component [was Re: [mythtv-users] Re: Image quality, what effects it?]

[Cory Papenfuss]

On Fri, 13 Aug 2004, Jay R. Ashworth wrote:

> On Fri, Aug 13, 2004 at 08:31:54PM +0200, Rudy Zijlstra wrote:
>>> 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.
>
> Oh, of course: that card has to *decode* MPEG, which is (by design)
> much easier.
 	Actually, it doesn't have to decode it.  It may have to read some of 
the stream and pick out portions of it, but computational part is rendering the 
MPEG video into YUV.  All it has to do is downconvert the RF, demod the VSB 
into a digital bitstream, and send the selected PIDs of it down the PCI bus.

>
>> 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.
>
> I believe SCTE-40 specifies QAM-64, yes.  I do wish someone would do a
> card.

 	I emailed the pcHDTV guys and they said that the chip on it supports 
QAM.  I don't even know the IP licensing for the card and its use, but here's 
the response I got:

Hello Cory,

         I can get you a block diagram  but basically the card uses a philips
ATSC/NTSC tuner which then branches to send a NTSC signal to the bt878
chip and the ATSC signal through a filter then to the OREN OR1211 chip
which demods the ATSC and then sends the digital ATSC stream to the
BT878 chip to be DMA'ed to the host computer.   The OREN chip supports
QAM so it may be possible to receive QAM but we have not tried this yet
but may look into it and post the results if we get it to work.   The
BT878 documentation has been on the net,  We can't post the OREN
documents but can answer questions.

         Thanks,

         Jack Kelliher
         pcHDTV, Inc.


I suspect that terrestrial cable will use encrypted QAM with cablecard anyway, 
so getting uncrypted QAM support is probably moot.

-Cory