Re: [LAU] jdelay - measured latency of a usb interface

Hi,

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.

Arnold

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