[App_rpt] USB hubs & USB audio

Sat Jul 26 01:19:12 UTC 2008

Hi Everyone,

Well, I just tried one of the usbgear hubs listed below. It ..almost..  
works. The 2 URI devices I'm experimenting with are identified and
functional EXCEPT that it sounds like the audio sampling rate is
wrong--way too low with aliasing distortion (or something). ugh. Probably
a driver confusion issue??

Anyway, the USB hub and hardware otherwise seems to be fine.
Here is an output from the usbtree utility:

[root at kb4fxc-rpt ~]# ./usbtree
/: Bus 02.Port 1: Dev 1, Class=root_hub, Drv=ohci_hcd/8p, 12M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Drv=ehci_hcd/8p, 480M
    |_ Port 4: Dev 2, If 0, Prod=USB2.0 Hub, Class=hub, Drv=hub/4p, 480M
        |_ Port 1: Dev 3, If 0, Prod=C-Media USB Headphone Set, Class=audio, Drv=snd-usb-audio, 12M
        |_ Port 1: Dev 3, If 1, Prod=, Class=audio, Drv=snd-usb-audio, 12M
        |_ Port 1: Dev 3, If 2, Prod=, Class=audio, Drv=snd-usb-audio, 12M
        |_ Port 1: Dev 3, If 3, Prod=, Class=HID, Drv=usbfs, 12M
        |_ Port 2: Dev 4, If 0, Prod=C-Media USB Headphone Set, Class=audio, Drv=snd-usb-audio, 12M
        |_ Port 2: Dev 4, If 1, Prod=, Class=audio, Drv=snd-usb-audio, 12M
        |_ Port 2: Dev 4, If 2, Prod=, Class=audio, Drv=snd-usb-audio, 12M
        |_ Port 2: Dev 4, If 3, Prod=, Class=HID, Drv=usbhid, 12M
        |_ Port 4: Dev 5, If 0, Prod=USB2.0 Hub, Class=hub, Drv=hub/4p, 480M

I'll investigate further since this is ultimately a config I may want to 
use as well.

David KB4FXC

On Tue, 22 Jul 2008, David McGough wrote:

> 
> So, I guess the next question is: Does anyone have a favorite multi-tt
> hub? ....I have used this one in the past for other linux applications and
> it seemed okay...I'll try one with the URI's....I guess the biggest issue 
> here is that the hub you buy today won't necessarily be the one on the 
> shelves tomorrow....A bit frustrating!
> 
> http://www.usbgear.com/USBG-217C.html
> 
> -David
> 
> 
> 
> On Tue, 22 Jul 2008, David Kramer wrote:
> 
> > I have successfully tested URIs with several high speed USB hubs.  But these
> > tests were performed without any other USB devices connected at the same
> > time so this may not be very usefull.
> > 
> > Skip points out the difference between full speed (12Mbps) and high speed
> > (480Mbs) devices.  This terminology is confusing.  Not all USB 2.0 compliant
> > hubs support high speed, so be sure the package says HIGH SPEED.
> > 
> > Very few hubs support multiple transaction translators, probably because the
> > chip sets are more expensive and almost nobody understands the need.
> > Theoretically, one TT is all that is needed provided you don't mix USB 1.x
> > and 2.0 devices on the same hub.  Keep in mind that USB supports several
> > data modes each with varying priorities and latency allocations.  Plus USB
> > physical layer is half-duplex so one-way communication never comes close to
> > 480Mbps.  The TT tries to handle all the traffic so don't expect good
> > results if you plug a bunch of web cams or other bandwidth hungry devices
> > into your hub.
> > 
> > Hope this help.
> > 
> > David Kramer
> > DMK Engineering Inc.
> > 310-544-1222
> > 
> > 
> > -----Original Message-----
> > From: app_rpt-bounces at lists.illiana.net
> > [mailto:app_rpt-bounces at lists.illiana.net] On Behalf Of Skip WB6YMH
> > Sent: Tuesday, July 22, 2008 1:22 PM
> > To: app_rpt at lists.illiana.net
> > Subject: [App_rpt] USB hubs & USB audio
> > 
> > > 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=1650536
> > 2
> > 
> > http://www.linux-usb.org/usb2.html
> > 
> > 73's Skip WB6YMH
> > 
> > _______________________________________________
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