Vista Audio Latency Issues

If you’re planning to run audio applications on your PC, you’ll want to keep your latency as low as possible. This is something that may require quite a bit of system tweaking, as neither Windows XP nor Vista is designed as a real-time operating system. These are primarily consumer or business platforms (depending on the flavour you buy) and are not engineered with musicians in mind. However, there are some things you can do to significantly improve Windows performance for audio…
(from DAW)

What is Latency Anyway?

I previously covered the basics of buffers and latency, but this is an issue that crops up on a regular basis. Simply put, latency is the delay between an audio event being requested and when it is performed; such as the gap between pressing the keyboard and your synth actually making the sound.

In computer terms, requests made to the processor are in the form of interrupts – an interrupt tells the processor to stop what it’s doing and focus on a new task instead. Interrupting does take up a certain amount of CPU overhead, so the system uses buffers to reduce the number of interrupts.

For audio at 44.1kHz, processing one sample at a time would require the processor to fetch audio 44,100 times per second, which is not practical for current technology. Therefore, the samples are buffered into larger groups which require less frequent processing, but which introduce a certain amount of delay into the chain. Larger buffers place less demand on the processor, but increase latency. If the processor isn’t fast enough to empty the buffer before the next one is due, the nasty clicks, pops and dropouts begin. This is why increasing buffer sizes often solves audio dropout problems. Mark Knutson describes XP’s audio handing on this page.

Vista Is a Latent First

There are a number of tools available that allow you to check system latency. Thesycon’s DPC Latency Checker is very useful in this regard; it’s a stand-alone executable that scans for unwanted latency spikes. You can even run it from a USB stick to check computers in the shop before you buy. Ideally, you want your laptop to stay under 500 microseconds – a desktop should be able to deliver far lower readings.

I ran DPC on Vista Home Premium “out of the box” on a Dell M1330, and the readings were fairly steady around the 1000 mark. While not disastrous, this is a bit higher than I would like. A fresh (and tweaked) install of Vista may have seen an improvement, but seeing as I couldn’t get Pro Tools to run reliably on Vista anyway, I decided to format the disk and install XP instead. This led to a radical reduction in latency; the M1330 is now running from 3-80 microseconds with occasional peaks to 200 microseconds.

There’s an interesting thread over at the SOS Forum where users are posting latencies for various laptop and desktop systems. Disabling wi-fi often reduces system latency; if you have spikes, you should disable system devices one by one until you find the offender(s). Installing new drivers for these devices may help; for Vista, Service Pack 1 is due soon, which may improve the situation somewhat. In the meantime, most of the tweaks in the XP Optimisation Guide are applicable to Vista also.


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