[LAU] jdelay - measured latency of a usb interface

Ken Restivo ken at restivo.org
Wed Mar 25 02:39:03 EDT 2009

On Tue, Mar 24, 2009 at 08:49:42PM +0100, Frank Barknecht wrote:
> Hallo,
> 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
o> > >   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. :)
> I know, that the manufacturers of USB cards like to brag with their hardware
> being faster than that of the competition, and they would never openly admit,
> that a 3 Euro BurrBrown chipset is just as fast as a 600 Euro device - but what
> if that's actually the way it is?

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?

In other words, the USB latency is the first-order issue in
determining latency, and any other factors are so small as to be
basically insignificant?

FWIW, I have used 3 different USB interfaces, and even though
I did not test them with JDelay, all worked identically with the
same jack settings (-n3 -p128) and resulting latency.

The three interfaces I've used are, by the way, an M-Audio
FastTrack Pro, a cheap Taianese US$6 USB dongle, and a Zoom H2.


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