[LAU] jdelay - measured latency of a usb interface

Arnold Krille arnold at arnoldarts.de
Tue Mar 24 09:21:41 EDT 2009


On Tuesday 24 March 2009 13:18:37 Frank Barknecht wrote:
> Arnold Krille hat gesagt: // Arnold Krille wrote:
> > I _seriously_ doubt that all usb devices have the same latency. Indeed
> > they do alls have the same latency introduced from the usb-protocol added
> > to their specific latency, but that specific latency still depends on the
> > actual hardware and the driver. And yes, there are different usb-audio
> > drivers in the linux kernel, just search for *usx2y* for example. And the
> > hardware latency is determined by the converters and the processing chip,
> > just like with any other sound device, be it pci, pcmcia or firewire
> > based...
> 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. The same can be said about the firewire interfaces: Because of the way 
firewire works all have the same latency[*].
But still that is the latency of the bus/transport. You have to add the 
latency of the actual hardware and I am very certain that there are different 
converters/chips in use in the different usb audio devices. And even with using 
the same chips, the programming in the firmware can be different resulting in 
different latencies.

[*] Probably this statement is not as true for firewire as it is for usb as 
firewire seems to allow synchronuous and asynchronuous transfer, so its 
probably two different protocol-specific latencies in the fw-world. But for the 
details you have to ask ppalmers or wagi or Jonathan Woithe who all know more 
about firewire than me.

I just know from actually programming (or at least trying to program) an 
atmega with an ad-converter connected by a bus that inside the device you have 
an additional latency, no matter which device.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 197 bytes
Desc: This is a digitally signed message part.
Url : http://lists.linuxaudio.org/pipermail/linux-audio-user/attachments/20090324/9077131b/attachment.pgp 

More information about the Linux-audio-user mailing list