[wdmaudiodev] Re: usbaudio 4ms in Vista / Win7 myth?
- From: First Last <molley45@xxxxxxxxx>
- To: wdmaudiodev@xxxxxxxxxxxxx
- Date: Sun, 25 Oct 2009 11:19:20 -0700 (PDT)
Thanks Jerry,
I don't suppose all this is documented somewhere? Most of the documentation
I've read so far is worded to the effect of "Microsoft strongly suggests not
writing your own driver and using usbaudio instead".
Keeping a major detail like latency and synchronization problems "hidden" could
be very dangerous to vendors who are led to believe that making their device
UAA / WHQL compliant is a viable solution on Windows. It may be viable for OS
X, but on Windows, especially where kernel mode development is involved,
there's a huge difference in time and resources needed between simply making
the device compliant, and "rolling" your own driver.
I don't mean to be rude, but frankly, I don't know why there is even a question
of "Why would you want USB Audio 2.0 in Windows?". The question should be "Why
wouldn't you?".
________________________________
From: Jerry Evans <jerry@xxxxxxxxxxx>
To: wdmaudiodev@xxxxxxxxxxxxx
Sent: Fri, October 23, 2009 3:03:55 PM
Subject: [wdmaudiodev] Re: usbaudio 4ms in Vista / Win7 myth?
FL:
Your observations and inferences are pretty much spot on. The XP stack
as an inbuilt 10ms long buffer (at 44K1). This accords with the default
buffering used (AFAICR) in PCI audio driver samples. Why? Who knows. I
asked out the same thing years ago in this NG and ended up assuming it
was 'about right' for the processors of the period.
The only way to get less buffering is to roll your own USB driver. On 7
it might be possible that WASAPI exclusive mode means reduced latency.
That is the theory for PCI devices but I have not tested this
proposition. Exclusive mode is still, AFAIK, broken for Vista.
I do hope those nice people in the driver division take account of
latency in any forthcoming USB 2 class driver ..
Jerry.
First Last 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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