Wednesday, February 17, 2010

Check Your DPC Latency!

First off, I am not sure how much of this article will relate to MAC, but it is definitely applicable to Windows. DPC Latency is the delay in receiving Deferred Procedure Calls (DPC) that can cause delays (pops, cracks, etc.) in streaming audio and video. While managing a task, processors are interrupted regularly to perform higher priority processes. Even when the system is idle, interrupts are being sent. They're absolutely necessary for the PC's processor to work.

DPC is a device driver's callback routine on how to continue the process before interruption. If there are two devices (two audio cards for example) in conflict, there may be some delay in returning to the original process. Imagine that you are having a bilingual conversation with a stranger, but someone interrupts you to do something more important. Once you finish talking to that person, you try to remember what language you were originally talking in. You may have a conflict in your mind of whether to use English or Spanish, so you wait and think about it for a minute. This is DPC latency because you are choosing which device (language in this case) to continue the process. In a computer's case, it's deciding between software drivers that are running hardware.

In other words, this can devestate your DJ performance. I have three audio cards: an external, an internal, and an internal Audio/Video TV capture card. The A/V Capture card's drivers was conflicting withe the other two sound cards. I disabled the capture card's drivers in Windows Device Manager, and my DPC latency dropped from 2 ms and 7 ms latency spikes down to a steady 0.2 ms DPC latency. What's that mean? My audio and video stopped randomly popping and crackling and my computer began to run a little smoother.

To check your DPC Latency, go to http://www.thesycon.de/deu/latency_check.shtml and run the small program for a minute or two. If you are constantly in the green, you're ok. If you're spiking up in the yellow, you might want to look into resolving any driver conflicts. If you're in the red, you definitely want to try to fix this. It is probably causing some popping and crackling in your playback. Try disabling any unused audio/video/networking drivers. This is the simplist solution, but there may be more complex solutions. Native Instruments Traktor Pro Technical Issues forum has a sticky about DPC latency with Dell computers. Don't disable random drivers, as you might disable something important. To access device manager, right click "My Computer" and select "Properties". Select "Hardware" tab and "Device Manager" button.

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