[LAU] USB, realtime, practical considerations

Matthew Smith matt at smiffytech.com
Wed Nov 19 17:16:12 EST 2008

Hi Folks

Whilst my 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 

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?
  - Can USB 2.0 take more than this (like another device?)
  - Anyone know how to set the sampling rate on this device through
    Linux?  Just specify in Jack setup?

* 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
  - 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?



Matthew Smith
Smiffytech - Technology Consulting & Web Application Development
Business: http://www.smiffytech.com/
Personal: http://www.smiffysplace.com/
LinkedIn: http://www.linkedin.com/in/smiffy

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