[mythtv-users] HDTV over S-video?

Shawn Flynn sflynn1 at rogers.com
Tue May 30 22:53:45 UTC 2006

On May 29, 2006 9:51 PM, Michael T. Dean wrote:
>On 05/29/2006 07:23 PM, Nick Rout wrote:
>> On Mon, 29 May 2006 16:32:16 -0400
>> Matt Pilet wrote:

<< snip >>

>>> I purchased a firewire card thinking 
>>> that I could use it only to find that Charter uses 5c encryption on all 
>>> non-analog channels - can't someone crack this for illegitimate use ;) 
>>> It's not like we're trying to "steal" free cable, we are PAYING a 
>>> premium for it. The cable box should decrypt it and pass it on the 
>>> whatever device we're smart enough to throw together. I think a 
>>> "loophole" would be to build a card that has RGB component inputs. I 
>>> haven't been able to find any such card yet.
>> something like this card looks like the right hardware for the job (if
>> it had a driver of course)
>> http://www.fi-llc.com/boards/Products/AccuStream170.php
>> Until you click the "Press Here for information on how to order the
>> AccuStream 170" and discover the price is $2995.00 !!
> And you'll be one of the first on your block to buy a couple of cases of 
> the new 750GB HDD's since you won't be able to encode the streams to 
> MPEG-2 in real-time, you'll have to dump them to disk uncompressed and 
> work your way through encoding later.
> And, even at $3K, it only does 1600x1200 at 30fps.  720p60 (one of the two 
> dominant HDTV formats in the US) is 1280x720 at 60fps.  1080i60 (the other 
> dominant format) is 1920x1080 at 30fps (with 60 fields/second).
> So, I guess even $3K won't buy you the right hardware for the job.
> Mike


I've had a chance to look at the limited specs that are available for the
AccuStream 170, and while I agree that it wouldn't be able to capture a
1080i signal at full resolution, it should have no issue capturing a 1080i
signal at a reduced resolution or a 720p signal at full resolution (minus
analog sampling loss).

The limiting factors of the card are a maximum resolution of 1600x1200
pixels, 120MHz sampling rate in double buffer mode (60f/s), 170MHz sampling
rate in single buffer mode (30f/s), and a sustained data transfer rate of

What is commonly called 1080i HDTV is officially defined by the SMPTE274M
standard. That standard defines both interlaced and progressive modes, but
since only the interlaced mode is typically broadcast OTA or via CableCo's,
I'll stick to that portion of the document. SMPTE274M states that the signal
is a fixed 1125 lines of resolution (1080 visible) with a frame rate of from
23.976f/s to 30f/s. The pixel clock and active horizontal resolution will
vary depending on the frame rate chosen in order to maintain the fixed 1125
lines of resolution. Typical values for active horizontal resolution are
1280px, 1440px and 1920px. Typical values for Pixel Clock will vary from
49.451MHz to 74.25MHz. Comparing these values to the specifications of the
AccuStream 170, we can immediately see that while the card could handle some
of the video modes defined by SMPTE274M, the only mode that you will
typically encounter (1920x1080x30 at 74.25MHz) falls outside of its
capabilities. You would have to capture at a reduced horizontal resolution
and possibly need to drop every other field in order to capture the signal.
It would probably still look ok (based on consumer sensibilities), but
certainly wouldn't be "full resolution".

720p video on the other hand is defined by the SMPTE296M standard. SMPTE296M
states that the signal is a progressive scanned signal with a fixed 750
lines of vertical resolution (720 visible) at a Pixel Clock rate of 74.25MHz
(with a 0.074MHz variance allowed). The total horizontal resolution and
Frame Rate are adjusted to maintain the fixed vertical resolution and Pixel
Clock. Although the total horizontal resolution is variable, the active
horizontal resolution (the visible part) is fixed at 1280 pixels. The frame
rate is adjustable from 23.976Hz to 60Hz. The most commonly seen broadcast
720p signals are 1280x720x60 at 74.25MHz or 1280x720x59.94 at 74.176MHz. Comparing
these signals to the specs of the AccuStream 170, they appear to be well
within the parameters needed for a full resolution analog capture.

So, it certainly appears that the hardware, although cost-prohibitive,
should be able to do the job of capturing HDTV from the YPbPr analog output
of a CableTV settop box. Now I'm just waiting for them to add a realtime
Mpeg2/4 encoder and drop the price by an order of magnitude. Of course, by
the time that happens, the MPAA et al will have greased enough palms to have
made the "analog hole" disappear. :(

Shawn Flynn

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