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
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
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:
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