I updated my Windows Vista Business machine (Dell Vostro 1500, 32-bit) to SP1 a few months back, along with updating several device drivers to the latest ones available from Dell. Since then, I've noticed the following:
After the computer has had an uptime of several hours, I start to get audio crackling and distortion during some network access. The crackling corresponds to spikes in DPC latency.
There are no such problems right after a boot-up or restart; the DPC Latency Checker shows latency in the green when the computer has been up for a short amount of time.
The spikes get progressively worse with increasing uptime. Currently, I'm seeing peak values of 10000 - 16000 microseconds.
By uptime, I mean the length of time that the computer is actually on and awake, from boot-up. Time spent in standby mode does not seem to contribute to the increasing latency. Rebooting resets the crawling latency.
The spikes don't seem to happen for all network access. They are worst when initiating connections (e.g. loading web pages, connecting to an SFTP server, browsing a network drive). For example, if I am downloading a large file, the latency only occurs at the beginning of the download.
The spikes occur whether I am using a wireless or wired connection (Dell Wireless 1490, Broadcom 440x 10/100). No gigabit networking or anything that is known to cause audio problems.
There don't seem to be any IRQ conflicts involved. The "High Definition Audio Controller" (SigmaTel High Definition Audio CODEC) is on IRQ 21; both of the network cards are on IRQ 17.
The audio distortion occurs for every software audio source I've tested: iTunes, WMP, VLC player, Flash videos.
Any idea what might be going on and how I might fix this?
I suspect that the problem is somewhere deep in the networking stack. I've tried disabling all network adapters and pinging 127.0.0.1. Without any real network access, I still get DPC spikes around 10000 μs. This is kind of ridiculous.