November 19th, 2010, 06:21 PM
Windows DPC Troubles...
Hey all -
I'm going to start by describing my setup (and apologizing for a rather long post):
-HP Pavilion DV6810US running Windows 7
-M-Audio FastTrack Pro
I've run this same setup for a few (3ish) years now, and only recently (past 6mo. or so) started seeing this problem. It seems to have slowly gotten worse over time, which is why I didn't catch it right away.
So, I started having trouble getting accurate measurements with Smaart. At first it was brief, but one day (after a bit of down-time) I went to do some measurements and was almost completely unable to get anything but gibberish. Upon further investigation, I realized that the signals I was receiving were being corrupted. That is, the corruption was occuring after entering the FastTrack and before being stored to disk.
The waveforms I was getting looked like big chunks were missing, so I thought, "hm. Buffer under/over-run maybe? But I have plenty of CPU headroom, and no problems in the RAM department, so what's clogging up my data?"
Well, as it turns out, Windows doesn't provide its drivers with a direct way to stream data to peripheral devices. Instead, what most drivers do is use what's called a Deferred Procedure Call (DPC) to send chunks of data every so often. (It seems to be a similar concept to an interrupt routine, only a bit different in how/when it is triggered.) The problem is, these DPC's are shared with other software running on the PC, so when some other process's DPC gets called and it takes forever to do whatever it wants to do, guess what gets shafted? Your audio stream.
Long story short, I had a friend with a shiny new laptop (I believe the Toshiba Satellite L630; Win7) run a test program to measure his DPC latency and found that, although not nearly as bad as mine, his computer still exhibited problems once in a while.
Finally, the reason I bring this up here is mostly to figure out how much of a problem this really is. Is my situation few and far between, or have other people experienced similar intermittent audio problems? I have found enough instances of this problem on various recording forums to be certain that it is not limited to my particular computer, or even to windows 7, yet I've never seen any mention of it on system design forums. Any particular reason?