[mythtv] Semi-OT: Which HDTV card to buy for Myth?
Cory Papenfuss
papenfuss at juneau.me.vt.edu
Sun Jan 30 15:10:51 EST 2005
On Sun, 30 Jan 2005, Taylor Jacob wrote:
> Quoting Cory Papenfuss <papenfuss at juneau.me.vt.edu>:
>> From what I've read on it today, it has a subset MPEG decoder. It
>> won't play video, but it will interpret enough of the MPEG-TS stream to
>> filter out specific PIDs. In other words, if one is receiving a station
>> that has a simultaneous SD and HD within the same "channel," then the card
>> itself can choose only one or the other.
>
> The Flexcop handles the MpegTransport Stream. It is not a "subset MPEG
> decoder". The flexcop can filter up to 38 PIDs at the same time. This means
> you can record more than 1 station at a time. If you exceed 38 pids then you
> are in trouble. Most channels will be 2 PIDs (Audio + Video). So you do the
> math.
OK... poor choice of wording, perhaps. I would consider the PID
filtering a sort of MPEG processing... maybe not decoding, but an MPEG
stream processor. Am I correct in saying that the FlexCop in the
Air2PC-ATSC can selectively filter out PIDs (in a multiple station/channel
setup), whereas the pcHDTV card cannot? The latter simply dumps the raw
MPEG-TS stream without any filtering?
Feel free to correct any errors here, but I'm trying to get a
handle on the differences between the two cards. In particular, the
"open-ness" of the hardware/driver. From my understanding:
The pcHDTV card has the following:
- Standard i2c-controlled RF frontend for tuning and downconversion to
baseband.
- CX88-based PCI/DMA inteface between the Oren chip and the PCI bus
- CX88-based analog capture (as a side-benefit from using it as an
interface)
- Oren baseband demod chip that can do VSB and potentially QAM. BUT, this
chip is configured via a closed chunk of binary code. Thus the dependence
on Oren and why the pcHDTV-2000 doesn't and probably won't support
unencrypted QAM.
The Air2PC card has the following:
- Standard i2c-controlled RF frontend like pcHDTV card
- nxt2002 baseband digital demodulator (comparable to the Oren chip). It
can do VSB and QAM (unencrypted), but is also programmed via binary-only
firmware. The nxt2002 "driver" basically just loads the firmware.
- FlexCop ASIC to do both MPEG-TS PID filtering and PCI/DMA interfacing.
Is that about right? Honestly, I'm a bit concerned about the
longevity and support of drivers dependent on firmware snarfed from other
drivers. Once the broadcast flag is permanent, who's to say that the
firmware won't get "fixed" to enforce it. Call me paranoid, I guess... :)
-Cory
>
> All digital TV cards do not do anything with Mpeg. They are simply transporting
> a datastream. It could be anything they send, it just happens to be Mpeg we use
> it for.
>
> Taylor
>
*************************************************************************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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