On Wed, Apr 1, 2009 at 2:47 AM, Clemens Ladisch <cladisch@xxxxxxxxxxxx> 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 > _______________________________________________ > Linux-audio-user mailing list > Linux-audio-user@xxxxxxxxxxxxxxxxxxxx > http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user > 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. _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
