On Thu, 15 Jun 2000, Peter Schneider-Kamp wrote:
> I am using INVIDEO's USB Capture device (a small USB device
> with plugs for S-VHS and VHS video signals).
>
> Can I get this to work with Linux?
All video cameras (or capture dongles) use vendor-specific interface.
If the chipset inside the device is one of those few that are
supported (like OV511) then it might work, with some pain. Otherwise a
new driver has to be written.
However I don't think that USB video capture devices make any sense at
all. USB (as it is now, 8/12 Mbps) is way too slow for any serious
capturing. Quality is horrible. PCI bus-mastering grabber is much
better. Next version of USB - 2.0 - will be fast enough, but it is not
even available to developers, let alone consumers. Your INVIDEO
device won't be able to use faster speed of that new USB protocol
anyway.
Another side of the problem is compression. Somehow most USB/video
OEMs think that their compression algorithms are valuable intellectual
property. Don't know where they get such ideas, but anyway they are
free to think this way. Most of compression code is closed-source and
the specification is never published.
This is bad for capture devices because you'd want at least 30 frames
per second. Without compression all you can reasonably hope will be
about 6-8 fps even with microscopic 320x240 image size. TV standard is
something around 768x576, and if you choose -that size- you will be
getting one frame every 2 seconds. Not very nice probably. I don't
even dare to mention HDTV :)
Video cameras can tolerate low fps and small sizes because they are
used mostly for videoconferencing or occasional image capturing. Their
sensors are also small (like 320x240) and they use lossy Bayer filter
to extract color information. You wouldn't want to shoot the next
"Star Wars" movie with one of such cameras - they are -cheap- in all
aspects. However the signal that comes from your VCR or DVD player is
of much higher quality. Passing it through USB would be similar to
listening to classical music over the telephone. You lose both frame
rate and image size.
This is why compression is so essential for capturing devices.
Compression allows you to trade quality (sharpness, colors) for fps.
But few Linux drivers have decompression code (Philips is one?), and
even when they do the code comes in binary form. So even if one can
reverse-engineer (or obtain) the basic USB protocol of the device it
won't be possible (it also may be illegal) to implement decompression
code.
The best way to support the device is to make the OEM to write the
driver for Linux (opensource as much as they can, other stuff may
remain in object form) or to publish the specifications. Maybe they
have already done so; then the driver can be written relatively
easily.
However at my opinion the best way for -you- is to return this USB
device because it will NEVER give you quality even close to what users
of PCI-based BTTV capture boards enjoy (on Linux or Windows - it does
not matter.) I have one such board and I like it. It also has tuner so
you could watch TV in a window (or fullscreen). These boards seem to
be -cheaper- than USB dongles. Even notebooks (that have no PCI clots)
offer ZoomVideo (?) ports (but I don't know much about them - no
notebooks here.)
Thanks,
Dmitri
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