Re: [LAU] USB, realtime, practical considerations

From: Clemens Ladisch <cladisch@email-addr-hidden>
Date: Thu Nov 20 2008 - 11:24:21 EET

Matthew Smith wrote:
> * It would be best to balance the devices across the 3 ports.

It depends. Usually, all ports are connected to the same controller and
have to share their bandwidth. The same is true for all ports of a
single hub.

> * My soundcard is a Behringer UCA202, capable of sampling at a maximum
> of 48kHz @ 16bits x 2 channels. I'm guessing that this will have the
> greatest data rate of all attached devices.

USB 1.1 can transport six channels. (In USB 1.1 mode, the SB Audigy 2 NX
just barely manages eight channels, but leaves practrically no bandwidth
for any other devices.)

> - Can USB 2.0 take more than this (like another device?)

No, all communication with USB 1.1 devices is at full speed, not high
speed.

It's different with full speed devices connected to a USB 2.0 hub; in
this case, the computer and the hub communicate at high speed.

To use multiple USB 1.1 devices that together would exceed the bandwidth
of a single USB 1.1 bus, you need to connect each one to a USB 2.0 hub.
(Or use two separate USB controllers, but only the latest mainboard
chipsets that support 12 USB ports have them.)

> - external USB QWERTY keyboard (don't have desk space for this
> when doing audio work, so hope it won't be transmitting any
> data.)
> - trackball (only transmitting when wiggled (I hope) - which should
> not occur when recording is going on.)

Input devices must be polled by the computer; not using them doesn't
save much bandwidth.

Furthermore, all communication with low-speed devices runs at low speed,
so they use up as much bandwidth as a full-speed with with eight times
the same data, or a high-speed device with 320 times the same data.

Best regards,
Clemens
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Received on Thu Nov 20 12:15:01 2008

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