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

Unit3 unit3 at
Wed Mar 19 13:21:22 UTC 2003

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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