Which Userland Interface for USB Video Class Driver?

Tom Evans tevans.uk at googlemail.com
Thu Sep 25 09:14:08 UTC 2008

On Thu, 2008-09-25 at 11:57 +0800, Intron is my alias on the Internet
> As a driver writer, the AV encoding standard concerns me. For,
> the driver must be able to decode compressed frame into plain bitmap
> if the chip cannot decode frame on its own. Of course, the Chinese
> standard is so cheap that many hardware vendors are willing to integrate
> decoder into chips, I believe.

I'm not sure this is necessarily true anymore. From a European view
point, the vast majority of DVB-{S,T,C} cards on the market are 'budget'
cards; they simply demodulate an MPEG-2 Program Stream from the signal.
Certainly, under linux, neither of my DVB-T tuners (pci saa based, usb
dibcom), nor my DVB-S card (also pci saa based) can decode a single
frame of actual video data.

Even when the actual video is not MPEG-2, like HD streams encoded in
MPEG-4 AVC, it is contained in an MPEG-2 PS. All the drivers do is to
setup and tune the device to the appropriate channel, and provide a
device to read the PS from. It is up to whatever userland program which
is using the device to decode the data, if that is even required. I
certainly wouldn't like a driver to decode high bitrate MPEG-4 AVC if
all I am trying to do is record a show to disk.

As far as I can tell, ATSC tuners do precisely the same thing.



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