[wdmaudiodev] usbaudio 4ms in Vista / Win7 myth?
- From: First Last <molley45@xxxxxxxxx>
- To: wdmaudiodev@xxxxxxxxxxxxx
- Date: Fri, 23 Oct 2009 10:07:51 -0700 (PDT)
Hi,
I have an USB Audio Class device, which I've been using on XP x64 with
usbaudio.sys and the asio4all driver. I've been using usbtrace to look at the
actual packets to try and determine the actual latencies.
It seems usbaudio.sys always asks for 10 packets at a time (i.e 10ms of data)
from the IN endpoint, however, when using the asio4all driver I see that
usbaudio can send as little as 1ms at a time, which is great. I understand
that usbaudio is still usb 1.0, so anything less than 1ms is not possible given
that it cannot support any interval other than 1 (meaning only frames, no
microframes for packets).
So I have a couple questions regarding this...
1) Why 10ms only for the input stream? If you take into consideration the
round trip latency 10ms input + ~2ms audio processing + 1ms output + (whatever
time it actually takes to go up and down the stack), that's like at least 15ms
in a best case scenario with usbaudio, no? more? Anyone who plays an
instrument, even someone who doesn't know how long 15ms is or how many feet
from the speakers that equates to will tell you "that doesn't feel right". Why
so much latency on the input stream?
2) I've heard rumors that in Vista / Win7, that it uses packets in groups of 4
instead of 10, or 4ms at a time. How do you get this elusive number? I have a
Win7 test machine here, and darned if it doesn't still ask for 10 packets / ms
at a time. Is 4ms a myth? How is the NumberOfPackets determined? Is it
determined at the kernel level? in usbaudio? or perhaps even in the descriptors
of the device? Or is it enforced by DirectSound, or I suppose DirectKS or
WaveRT when using ASIO? How do I get that magic number?
Any suggestions welcome, thanks.
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