[App_rpt] USB hubs & USB audio

[Skip WB6YMH]

> Jim Dixon and I have had disapointing results with USB hubs during the
> testing of USB fobs with them. We don't know why they don't work
> properly and we don't have the time to figure it out right now. There
> may be hubs which work just fine, but we just don't know as the root
> cause of the problem has not been uncovered.  If Steve Henke W9SH
> knows something I don't them maybe he should speak up on the list. 
> Steve  WA6ZFT 

The problem is probably complex.  USB 2.0 hubs come in two flavors single-
transaction translators (single-TT) and multi-transaction translators 
(multi-TT).  Most USB audio devices are "full speed" devices (12 
mbits/sec), not "high speed" devices (480 mbits/sec) so the USB hub must 
translate the data rate between the host and the device.  The single-TT 
devices only have a single translator which is shared between all ports.  
A multi-TT hub has a translator per port.  So in theory a milti-TT hub is 
what you should want for optimal bus usage.

An additional complication is that the Linux kernel must be multi-TT hub 
aware to properly schedule things.  I'm not current with Linux 
development, but I've read discussions in the past about improving the 
kernel's USB scheduler in this area.

Here are a couple of links to info which may be useful:

http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16505362

http://www.linux-usb.org/usb2.html

73's Skip WB6YMH