[PlanetCCRMA] Re: [Alsa-user] delay in general

[Clemens Ladisch]

Alexander Carôt wrote:
> 3.) Has anyone information about the communication layer USB-audio puts on
> top of the PCI-bus and how much delay it takes ?

USB data is sent in packets; time is divided into one-millisecond frames.
USB audio sends the data for one millisecond in one packet (per frame).
The host controller does not (and can not) guarantee when in a frame the packet
is sent, so a packet that is to be sent in frame N must be prepared by the
driver during frame N-1 (or earlier) and will be played by the device in frame
N+1.  For capturing, the situation is the same.

> In other words : Can a USB device ever reach latencies like PCI-devices ?

No.


Clemens