Re: [alsa-devel] Alsa simultaneous use of usb-sound card - BW problem

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Ralf Mardorf wrote:
> On Mon, 2014-08-04 at 23:11 +0400, Vladimir Mosgalin wrote:
>>> IIRC the plug'n'play audio USB driver is limited to USB1, for USB2
>>> proprietary drivers are needed.
>>
>> Can't be true.. Here is example of USB 2.0 audio device (bcdUSB: 2.00),
>> completely plug and play:
>
> Perhaps the devices aren't recognised as 2.0 devices. I don't know how
> much channels duplex for unsynced USB devices do work sharing one USB,
> USB 1 or 2. An expert should chime in. Does "2.0 full speed-compatible"
> mean "USB-2 class compliant" or is a special driver needed?

USB 2.0 is not the same as as USB Audio 2.  (USB 2.0 adds the high-speed
mode, i.e., 480 Mbit/s.  USB Audio 2 is a specification that allows
conformant drivers and USB 2 devices to work together.)

Nowadays, Linux does support USB Audio 2.
(As do OS X, and FreeBSD; the only holdout is Windows.)

>>> perhaps the bandwidth really is to small.

Indeed, that is why the error message says "bandwidth".

Please note that USB 2.0 *adds* high speed mode, so it is still possible
to have devices running at full speed (12 Mbit/s).

Bandwidth is measured in time, not bits, so a bit from a full-speed
device takes as much bandwidth as 40 bits from a high-speed device.

A single stereo full-duplex device can easily max out a full-speed bus.
If you want to use two of them, connect them to *different* buses.
(Check the output of "lsusb -t" to see which bus they are connected to.)


Regards,
Clemens

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