[LAU] jdelay - measured latency of a usb interface

[Justin Smith]

On Wed, Apr 1, 2009 at 2:47 AM, Clemens Ladisch <cladisch at fastmail.net> wrote:
> Ken Restivo wrote:
>> On Tue, Mar 24, 2009 at 08:49:42PM +0100, Frank Barknecht wrote:
>> > Arnold Krille hat gesagt: // Arnold Krille wrote:
>> > > > I'm no expert on this, but IMO ALSA-dev Clemens Ladisch is one. He wrote:
>> > > >   "Due to how the USB protocol works, all USB audio devices have the same
>> > > >   latency."
>> > >
>> > > I know that clemens knows his stuff. But he is talking about the usb-protocol
>> > > here.
>> >
>> > He is saying that because the USB protocol works the way it works, all USB
>> > audio devices have the same latencies. I don't know how that statement is open
>> > to a different interpretation than that all USB audio devices have the same
>> > latency. :)
>
> Well, I was simplifying somewhat.  In the context of that mail, the
> 40 ms latency was not the responsibility of the USB device, so
> switching to another one would not help.
>
> When using the same sample rate and the same buffer/period settings,
> that data that the USB audio driver sends always uses the same timing,
> regardless of which device is used.
>
>> > I know, that the manufacturers of USB cards like to brag with their hardware
>> > being faster than that of the competition,
>
> These differences do exist, but mostly because of differences in
> the (Windows) drivers.
>
>> Is it possible that the USB protocol sets a *minimum* on latency,
>> and thus any speed improvements in the USB hardware are essentially
>> moot, limited by this lower bound imposed by the USB protocol?
>
> Yes.  The theoretical minimum latency is at least two or three
> milliseconds (depending on which assumptions you make); the additional
> latency introduced by the DAC itself is much lower than that.
> (With kernel 2.6.31 or so, the protocol-related latency of high-speed
> (USB 2.0) audio devices can be reduced to one eighth in some cases.)
>
> It is, of course, possible to have additional latency due to DSP
> processing or something like that, but nowadays, these things are done
> in software.
>
>
> Best regards,
> Clemens
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>

Before those USB-2.0 high speed numbers mean anything to those of us
on this mailing list, we will need USB-2.0 high speed sound cards that
work reliably with Linux. As it is there are very few, if any, sadly.
For whatever reason the manufacturers did not standardize on USB-2.0
sound cards the way they did on USB-1.1.