Capture Card Resolution -- was: Re: [mythtv-users] 2 Captur e card questions

Brian Foddy bfoddy at visi.com
Wed Mar 19 15:03:00 UTC 2003


Thank you people for several good responses, it makes sense to me now.

Brian


On Wed, 19 Mar 2003, Ralph Little wrote:

> Hi,
> I concur.
> There is a discussion on this here extracted from the virtualdub web site
> (VirtualDub.org):
> 
> 
> -----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8
> <-----8<-----
> Why do I get errors when I try to capture with a height above 240 (288 for
> PAL/SECAM)?
> 
>     This following only applies to devices based on the BT848(A) or
> BT878(A).
> 
>     NTSC video is composed of alternating fields of 240 scanlines, and
> PAL/SECAM of 288.  If you specify a height equal to or lower than that, the
> capture driver only snags one field.  If you specify a taller image, both
> fields are grabbed.  In either case when you specify a size smaller than the
> height of the field(s) captured, the result is then scaled down.
> 
>     The BT8x8(A) can only DMA to one destination at a time, and the output
> pixel format can only be set on a per-field basis.  If you specify Overlay
> mode, then the capture driver will instruct the chip to DMA one field to
> memory for capture, and the other field to the video card.  The catch is
> that if you are capturing both fields, both DMA lists have to point to main
> memory.  This means that overlay mode is impossible when both fields are
> captured, and depending on your exact driver, you will either have overlay
> disabled when the capture starts, or, as is more often the case, get a
> cryptic error about being out of memory.  Note that this is not a bandwidth
> issue -- you can capture 640x240 in overlay mode, but not 80x480.
> 
>     To "fix" the problem, specify Preview mode; this unfortunately is slower
> since the CPU is now doing a blit from the captured frame to the video card.
> It may be that your system is not fast enough to handle this, in which case
> you will need to disable the video preview entirely and capture blind.  A
> secondary consequence of all of this is that you see exactly what is being
> captured in Preview mode, while you are actually seeing the other field in
> Overlay.  If you have a strange source where one field is much cleaner than
> the other, it is possible with an Overlay display that you will see a clean
> display on screen and end up capturing crap.  (I have had this happen.)
> 
> This may not apply to you if you have such a chip built onto your video
> card, since in that case the chip can DMA into video memory, and then other
> video hardware can transfer from there into main memory for capture.
> 
> -----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<-----8
> <-----8<-----
> 
> Hope this helps...
> Ralph
> 
> -----Original Message-----
> From: Unit3 [mailto:unit3 at demoni.ca] 
> Sent: 19 March 2003 13:21
> To: Discussion about mythtv
> Subject: Re: Capture Card Resolution -- was: Re: [mythtv-users] 2 Capture
> card questions
> 
> 
> Brian Foddy wrote:
> 
> >Thanks for the reply, but I don't think your proof is valid.  I'm sure
> >the Myth will replay whatever resolution it thinks its capturing, but
> >that says nothing to what the card is delivering...
> >  
> >
> I believe that someone had a good idea about deinterlacing which would 
> be adequate.
> 
> >Can someone clarify why all the specs state 320x240 for many of
> >these cards and where the confusion comes from?
> >  
> >
> I think I can actually answer this! ;)
> 
> When trying to find better software for my TV Wonder under Windows, I 
> found some documentation that basically said that if you were using the 
> video overlay mode of your video card for display, the card could only 
> capture one field of the incoming video, but not both. Essentially, 
> limiting the maximum vertical resolution to 240 pixels. The 
> documentation suggested that it was easily possible to capture both 
> fields if you didn't use overlay mode directly from the card, but 
> instead captured both, did some processing, and then output the result 
> back to the screen (which is what MythTV, Descalar, and WinDVR all do). 
> However, most companies who make these cards would like them to run 
> smoother on lower end systems (for purely marketing reasons I'm sure), 
> and so their supplied software uses overlay mode for display and is 
> limited to 240 vertical pixels. Not suprisingly, most companies then 
> also limit their software to capturing 320 horizontal pixels, since 
> capturing at 640 or 720x240 would be just silly. :)
> 
> I hope this makes some amount of sense. Also, I don't know if this "can 
> only capture one field when using overlay" issue is simple a windows 
> driver issue, a WDM issue, or a chipset issue. All I know is that it 
> does exist under Windows, as even generic DirectShow capturing software 
> will refuse to capture full frames when using video overlay. (I know, 
> I've tried)
> 
> Graeme
> 
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