[wdmaudiodev] Re: firewire audio at low latency
- From: Uwe Kirst <u.kirst@xxxxxx>
- To: wdmaudiodev@xxxxxxxxxxxxx
- Date: Fri, 12 Sep 2003 12:51:37 +0200
Evert van der Poll schrieb:
> Good to hear you found a solution. To what kind of latency this amounts to,
> speaking in terms of audio (DirectX, ASIO) applications? Just curious.
>
Hi,
WDM, DirectX: I'm not sure, I'll have to measure. This stuff is very complex. >=
ASIO latency.
ASIO:
I think I will have one buffer more latency than you would normally expect if
you have a PCI card.
Look at the following illustrations:
ASIO in general: you have only 2 buffers, one buffer latency
|----bufffer 0----|----buffer 1----|
Firewire:
|----buffer 0----|----buffer 1----|----buffer 2----|
buffer 0: this buffer is completed and can be reused
buffer 1: this buffer is processed by the ohci controller
buffer 2: this buffer is attached to the 1394 bus driver
Because you have the additional stage (attaching to the driver), you will always
have one buffer more latency.
I think you will get droppouts if you would attach a buffer first and fill it
afterwards. This will not work.
You must add the latency of the hardware (A/D and D/A conversation each add
about 50 sample as far as I know, can be omitted if digital I/O is used).
You may also add the time it takes to transfer a firewireframe (125us, about 4-6
sample).
Uwe
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