Re: reverse engineering pwcx

Lee Revell
2004-08-28T18:04:07Z

On Sat, 2004-08-28 at 13:53, Joel Jaeggli wrote:
> On Sat, 28 Aug 2004, Lee Revell wrote:
> >
> > How do you account for the Slashdot poster's assertion that it's
> > physically impossible to cram 640 x 480 worth of data down a USB 1.1
> > pipe?
>
> 640x480 = 307200 pixels
> x 24 bits = 7372800 bits per frame (.9MB)
> x 30 fps = 221184000
>
> so that's 221mb/s for uncompressed 640x480. dv with 16bit pcm is 25Mb/s
> typically which is still a bit more than double what you can reasonably
> push through a usb1.1 port. raw you can push about 1.6 fp/s at 640x480
> through usb so your compression ratio needs to be order of 15 to 1 make
> it fit reasonably with room for overhead.

15 to 1 is impossible without lossy compression. I am not really a
video expert, but I know that it's impossible to losslessly compress
audio more than 50-75%. Therefore there is no way it's 'True 640 x 480'
as Philips' marketing claims. Unless you take 'true' to mean
'interpolated by the driver so your userspace program doesn't have to do
it'. This is a stretch.

Lee