> I know that USB has about 12MB/s bandwidth, and some sort of
> isochronous protocol, specifically to support such things, but
> what is the real bandwidth available for (eg) audio data.  Is it
> isochronous on a sample-by-sample basis, or is there a packet
> protocol in there (that must introduce a modicum of latency)?
90% of the bandwidth can be used for iso traffic, but that is not
90% of 12Mb/s since there are other overheads (headers, bit stuffing,
etc).  I would guess that 7-8Mb/s is the maximum you can use for iso
traffic.  Time is divided into 1ms frames on USB.  So you fit all
the samples that should go out in the next ms in one packet, i.e.,
a few hundred bytes, and then send them off to the device.  Different
frames may need different number of samples if you want to keep
synchronization.

> Is it reasonable to configure a USB sound system for more than
> two channels?  Would DVD-style five-or-six channels work?  Would
> eight be possible?
Eight should be barely possible.  The USB audio standard has names
and numbers for about 8 different channels already.  So we are prepared. :-)

    -- Lennart