[LAU] USB audio interface and a buggy USB controller

Alex Tinsley cvtrig at gmail.com
Fri Sep 17 16:57:45 UTC 2010


A little info on USB Audio that will be helpful to those here and others 
reading this thread.

First off any noise that you hear through your USB Audio interface is 
because the grounding in the audio device is using the grounding on the 
laptop which is most commonly data ground wired to earth ground. So you 
hear all computer noise through the audio output. One way to alleviate 
this is to lift the ground on the laptop (unplug the power adapter from 
the laptop or get a ground lift adapter). Noise goes away. A lot of 
devices suffer from this, 1394 has same issue.

Second, regarding USB Hubs, USB Audio does not handle going through USB 
Hubs unless the hubs are designed properly. Regardless if it's 1.1, 2.0 
or 3.0.

Some Hubs are just splitters, they mult one port into two or more using 
the same power and bandwidth as the source port. These are typically 
passive devices (i.e. no power supply) and are not recommended for 
anything that requires full power of the USB bus like a USB Audio device.

However using USB Hub that is powered will improve your chances, even 
there you might have an issue unless you know exactly whether or not the 
Hub vendor made the device to properly replicate data and power to each 
output port.

Griffin used to make a hub called the UH-124, it was a 4 port USB 1.1 
powered hub that properly replicated the source port power and data 
stream to each of the 4 outputs. Dr Bott also made a 7 Port hub that did 
the same thing. Typically powered hubs offer a better chance of allowing 
USB Audio and USB MIDI to function properly regardless of operating system.

Another item to consider is that laptops especially PC laptops are 
designed so that there are 2 physical ports per internal USB controller. 
These USB controllers are bound to other devices on the laptop that are 
sharing power. It is not uncommon for PC Laptop USB ports to actually 
have reduced bandwidth (and in some cases with (Ultra Low Voltage 
notebooks (Sony makes one)) there isn't enough power on the USB bus to 
power a single USB Audio or USB MIDI device.

So no passive hubs. Don't waste your money unless you're absolutely 
mobile (in a forest, desert, etc) and you don't have a choice, your 
mileage will vary.

When using a powered hub, make sure you don't have anything plugged into 
the second port (above, below or next to) the one you're already plugged 
into when running a single USB Audio device or when running with a Hub. 
Powered hubs cost more (typically $25 - $50).

If you're not sure if you're getting the right one, each USB port 
carries 500ma at 5VDC. The PSU should be able to say what the total 
output current is. Divide output current by number of USB ports, if the 
number is 500 or greater per port then it's a good indication the vendor 
made the hub correctly. Their might be a little more than 500 to drive 
the leds on the unit. If not, get a different one. The only downside to 
powered hubs is that the lesser expensive ones have huge wall-wart power 
adapters. The better ones will have a smaller power adapter.

Another item to remember with USB Audio is that the devices typically 
pull 50 - 80% of the USB bus data stream for 16 bit 44.1 to 24 bit 48k 
recording. and up to 95% when running 24bit 88.2 or 96 or higher (on USB 
2.0) sharing that port with other devices will almost always interfere. 
Most notorious are USB Mice and Cameras. If you must use a USB Mouse or 
other USB media device (Camera, MP3 player, etc) plug it into another port.

Why is this this way you might ask? (geeking out here for a moment). The 
USB Audio spec per the USB.org website is not how Microsoft designed the 
intended use of USB in their OS. We're talking about Linux though, yes. 
However the computer you're installing Linux on was designed and 
intended for use with a Windows OS to make it ACPI compliant. Microsoft 
required starting back during Win 98SE that the OS must be able to 
shared 2 USB devices with not more than 40% max bandwidth per device and 
allowing 10% for overhead per device. That equals out to 100%. On 
Win98SE in the device manager you could actually view the device 
bandwidth for each USB device connected. That all went away with Win2K 
and the rest is history. That's why PC's have 2 USB ports per 
controller. Since Apple utilizes the controllers the way USB.org 
presented it's use, anyone using a x86 or x86_64 laptop will have an 
extra port to confuse matters when only one should be used per controller.

This applies to x86 and x86_64 machines, Mac Books do not have as many 
issues because they are designed with all of this in mind. Macs limit 
one USB port to one internal controller, that's why those machines have 
so few USB ports.

Hope this helps.

On 9/17/2010 1:32 AM, david wrote:
> Philipp Überbacher wrote:
>> Excerpts from david's message of 2010-09-15 19:32:44 +0200:
>>> Monty Montgomery wrote:
>>>> Oh, I should say I've also just recently seen similar problems (for no
>>>> apparent reason) when an XHCI (3.0) controller hands off a full-speed
>>>> (1.1) isochronous device.  Is the port this is being plugged into a
>>>> USB 3.0 port ('SuperSpeed')?
>>> Someone said something earlier about hubs. I have a USB2 hub plugged 
>>> into a USB2 port on my laptop. When I tried using a Behringer 
>>> UCA-202 (USB1.1) device through the hub, it could barely play, and 
>>> not record at all. Connected directly to a USB2 port, it plays and 
>>> records fine.
>>> So original poster just might be dealing with an internal hub.
>> That usb hub stuff seems to be really strange. I do have an external
>> hub, but I only use it for HDs.
>> The USB interface is connected directly to one of the laptops ports.
>> The laptop seems to have an internal hub, or two?
>> Anyway, it works just fine, as far as I can tell. Some jack2 test I ran
>> showed quite immense jitter, but I guess that's normal with USB devices.
>> $ lsusb
>> Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
>> Bus 006 Device 002: ID 0582:0074 Roland Corp. EDIROL UA-25
>> Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
>> Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
>> Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
>> Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
>> Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
>> Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
> jack2 on my non-RT kernel laptop has no jitter at all working with 
> UCA-202 connected to a USB2 port on the laptop.
> Lower signal levels and noise when connected via my self-powered 
> 9-port USB2 hub. Cheap hub, probably very bad ground or poor shielding.

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