[LAU] USB, realtime, practical considerations

[Lee Revell]

On Wed, Nov 19, 2008 at 5:16 PM, Matthew Smith <matt at smiffytech.com> wrote:
> Hi Folks
>
> Whilst my 2.6.24.7-rt17 kernel and disabling all hardware and services
> unneeded for audio seems to have gotten rid of those nasty xruns, I live
> in fear that they will return.  My main worry is how I organise my USB
> devices.
>
> My laptop (ThinkPad Z61m) has 3 USB ports.  I have several devices that
> need to be connected to them, meaning that I need to use USB hubs.
>
> Here are some assumptions (or guesses) that I have made, on which I
> would appreciate comment:
>
> * It would be best to balance the devices across the 3 ports.
>
> * 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.
>  - Should it have its own port?

Yes.  This will probably use more bandwidth than all the other devices combined.

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

No USB 2.0 audio devices have Linux support AFAIK.

>  - Anyone know how to set the sampling rate on this device through
>    Linux?  Just specify in Jack setup?
>

Should work.

> * Now that I know that I can run USB to MIDI devices through a powered
> hub (thanks David), I was thinking of of hooking the following into a
> 7-port hub:
>  - iPod cable (no iPod on the end, so no burden.)
>  - cellphone cable (as per iPod cable.)
>  - 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.)
>  - USB floppy drive (not in use when recording.)
>  - USB to MIDI interface
>  - Another USB to MIDI interface - is it too much to have 2 of
>    these on the same hub?  If not, would it take a third?
>

MIDI uses very little bandwidth, should work fine.

Lee