Re: Versions and Reflector Sites

bgodette@idcomm.com
Mon, 13 Oct 1997 10:48:58 -0600


At 10:55 AM 10/13/97 -0400, you wrote:
>Re: USB cameras
>
>>I haven't seen these yet just heard about them. My guess is there will be
>>as big of a market for them eventually that there have been for the low
>>cost Quickcam (even though I've heard their frame rates might be even
>>worse than the Quickcam and demand more from the processor).
>
>>From what I've seen, excatly the opposite. See Xirlink USB camera
>(408-324-2100) soon to be released (not yet tested with CU-SeeMe tho).
>30Fps digitzized in camera, no card, just the USB port. No extra load on
>processor.
>
>Gary

Not from what I understand about USB. AFAIK USB is PIO, just like a
parallel port, the CPU is responsible for transfering the data from the bus
to memory, no DMA transfers or shared memory blocks like you'd have for a
real capture board. The main difference between USB and a parallel port, is
you're guarenteed interrupts for USB, you're not for a PPort, and for
*packets* of data instead of 4 or 8 bits (depending on if it's an SPP, EPP,
or ECP port) of data. Then, depending on what type of compression (if any)
is used by the video device, the video capture device driver has to
decompress the signal into something usable by the application if necessary.

For most cases it should have less CPU load than a PPort, but saying "no
extra load" isn't being realistic. Also remember that this is only an
12Mbit bus one bit wide (total bandwidth is 12Mbit), while a 16bit ISA card
has a bandwidth of 8Mbit * 16bits (128Mbit). Raw uncompressed 24bit video
at 160x120 @ 30fps requires 17.28Mbit of bandwidth for just the data
portion, ignoring packet headers and such. YVU9 brings that down to roughly
6.5Mbit, however that doesn't look nearly as good.