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

Ralph Little ralph.little at
Wed Mar 19 13:54:50 UTC 2003

I concur.
There is a discussion on this here extracted from the virtualdub web site

Why do I get errors when I try to capture with a height above 240 (288 for

    This following only applies to devices based on the BT848(A) or

    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.


Hope this helps...

-----Original Message-----
From: Unit3 [mailto:unit3 at] 
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)


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