[pwc] Multiple webcams

[Mathieu Bouchard]

On Sun, 11 May 2008, Gilles GIGAN wrote:

> I just wanted to confirm whether or not you can use more than one webcam 
> at the same time on the same computer, all using the same pwc driver 
> instance ? So far, my tests with 2 webcams have failed.

You have to check how USB connectors are grouped on your computer. On the 
one that I had to use with 6 PWC cams at once, there were 8 USB plugs, but 
4 controllers. There's a bandwidth limit per controller, not just per 
plug.

You *have* to set the framerate of each camera in advance: just changing 
your rate of frame-query won't change the bandwidth. This is done with a 
PWC-specific call. Changing resolution will help even more. I don't know 
how much changing the compression-level (PWC-specific) helps, I didn't see 
any doc for that, and I don't know anything that measures the amount or 
percentage of bandwidth taken by a certain combination of options.

I have also encountered problems of power and signal strength for which 
diagnosis was pretty close to impossible, as error messages were evasive 
or errors were not checked for and some reactions were just weird: e.g. 
with some computers, all cameras halt at once, and on some others, a very 
bad multitasking occurs between them.

Counterintuitively, my laptop had a so much more better ability to handle 
cameras than the desktop computer, and was wildly above standards for 
allowed hubless wire length, and perhaps even total wattage; and this 
misled me at first. Don't assume that those things have anything to do 
with the strength of the power-supply.

If you have any problems with hubs, or with "active cables" (which are 
just single-port hubs), then make sure you disable USB2 in your BIOS (I 
hope that you have that option! not all computers have it).

This may be the second time I write all of this, but I don't recall. Maybe 
you can find some other mail of mine last year that had some other tips.

  _ _ __ ___ _____ ________ _____________ _____________________ ...
| Mathieu Bouchard - tél:+55.11.3578.6390, São Paulo, SP, Brasil