[wdmaudiodev] Re: usbaudio 4ms in Vista / Win7 myth?
- From: DJ Sisolak <dsisolak@xxxxxxxxxxxxxxxxxxxxxx>
- To: "wdmaudiodev@xxxxxxxxxxxxx" <wdmaudiodev@xxxxxxxxxxxxx>
- Date: Mon, 26 Oct 2009 18:31:42 -0700
This is not a Myth for Vista Gold. The driver did send 4ms requests in this OS
release. The problem was that the extra DPC time required to service the extra
requests was deemed as a performance hindrance and vendors requested that the
original 10ms request size from XP be restored. This was done in the ensuing
service pack (I believe it was SP1). The 10ms request was selected because it
matched the engine cadence and thus was the most efficient number for the
driver to use for the most common uses of audio capture, being usually webcams
and messenger type applications. Windows 7 however, has a means to set the
request size down to 3ms or any value between 3 and 10ms. There is a new
Property that can be sent to the driver pin: KSPROPERTY_AUDIO_BUFFER_DURATION
(see MSDN http://msdn.microsoft.com/en-us/library/ee220334.aspx). The caveat
being that you will sacrifice CPU time for latency. This value is also used in
the audio engine to match the exclusive mode period to the packet request count
as closely as possible in this same range.
Thx,
DJ
This posting is provided "AS IS" with no warranties, and confers no rights.
From: wdmaudiodev-bounce@xxxxxxxxxxxxx
[mailto:wdmaudiodev-bounce@xxxxxxxxxxxxx] On Behalf Of Matt Gonzalez
Sent: Sunday, October 25, 2009 7:45 PM
To: wdmaudiodev@xxxxxxxxxxxxx
Cc: wdmaudiodev@xxxxxxxxxxxxx
Subject: [wdmaudiodev] Re: usbaudio 4ms in Vista / Win7 myth?
I suspect that the 10 msec number is due to both the XP and Vista audio mixers
running at a 10 msec native buffer size.
Matt
On Oct 23, 2009, at 10:07 AM, First Last
<molley45@xxxxxxxxx<mailto:molley45@xxxxxxxxx>> wrote:
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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