[mythtv-users] Comcast HD & firewire

[Daniel Kristjansson]

On Fri, 2006-11-03 at 13:03 -0500, Preston Crow wrote:
> On Fri, 2006-11-03 at 12:39 -0500, Jerry Rubinow wrote:
> > On 11/3/06, Dylan Semler <dylan.semler at gmail.com> wrote:
> > > I've been wondering about why there are no tuner cards with DVI or HDMI
> > > inputs for this reason.
> 
> > Because taking the raw, uncoded HD video streams and converting them to mpeg
> > in real time is too CPU intensive, and dealing with raw unencoded HD video
> > without compressing it first is too bandwidth intensive.
> 
> Just curious; what is the data rate for DVI at 1080i or 720p (whichever
> is greater)?  Does it really exceed disk bandwidth?  And if so, by what
> factor?

1920*1080*24*30*1.1 = 1,642,291,200 bps

Or about 196 Megabytes per second (about 706 Gigabytes per hour).

I don't think that is too far above a modern hard drive write speed,
if you striped a couple drives you could probably keep up. The only
real problem is that the market for such a thing is pretty small,
so you would probably need to build it yourself or pay a lot for
commercial solution intended for movie studios. You could probably
find a card that did MPEG compression for less money (i.e. under $10K).

Technically it is not hard to build an HD capture card, it's all
about the relatively small market size. Your best bet might be to
look at the GNU Radio project. They are working on analog signal
sampling for different reasons, but you want the same thing just
at a higher bandwidth (and at baseband, which is a little simpler).

FYI You could save about 1/3 of your bandwidth if you do YUV
    rather than RGB capture, this may avoid the RAID striping.

-- Daniel