[mythtv-users] cheap ($250) HD component video capture..

Scott Alfter mythtv at salfter.dyndns.org
Sat Oct 27 18:08:06 UTC 2007


Chris Ribe wrote:
> On 10/26/07, Jerry Rubinow <jerrymr at gmail.com> wrote:
>> On 10/26/07, Chris Ribe <chrisribe at gmail.com> wrote:
>>>
>>>> Is your core 2 duo six times faster than your PIII?  There are
>>>> anywhere from six to nine times the pixels to deal with, depending on
>>>> what resolution you were recording SDTV at..
>>> Actually, my math says there are 5.3 - 6 times as many pixels in an HD
>> frame
>>> than a 720x480 SD one.
>> But aren't the HD pixels bigger than the SD pixels (24-bits vs
>> 8-bits?), or do both get digitized to the same size?
> 
> As far as analog capture is concerned, you can sample at whatever color
> depth you like, but 8bits per channel is pretty standard for HD and SD
> alike.

In addition, Cb and Cr are subsampled by most codecs.  4:2:0 is the most common
type of subsampling: both the vertical and horizontal resolution get cut in
half, so each pixel of Cb and Cr covers four pixels (in a 2x2 block) from the
original image.  This works out to an effective 12 bits per pixel (8 bits of Y,
2 bits each of Cb and Cr) for the subsampled uncompressed data, regardless of
resolution, as each 96 bits of 24-bit RGB input gets reduced to 48 bits of
subsampled output.

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