Sunday, July 19, 2009

Fix for Dell Inspiron 6400 audio glitches

Since installing Vista on my Dell Inspiron 6400 laptop I've had problems with sound interruptions. They ranged from short clicks and pops to significant interruptions approaching a second in length. Disabling WiFi made them go away, so I assumed it was some conflict between the Dell 1500 (BCM4328) Draft 802.11n WiFi card and Sigmatel STAC 92XX C-Major HD Audio.

At one point I updated the WiFi driver and the sound interruptions went away once I connected to an 802.11g network. This was pretty good. If I didn't need the WiFi card I could disable it, and if I needed it, I would connect. However, when I bought a 802.11n router, the interruptions started happening a lot, even when I'm connected. The solution again was updating to the latest drivers, although that's not straightforward for reasons I'll explain later.

This time, I found a very useful tool: the DPC latency checker. I highly recommend it if you're experiencing audio glitches. Looking at the graph it produces is much faster and easier than listening for glitches. Here's what I saw:

When I disabled WiFi, the latency went back into the green. This showed me that it's entirely the WiFi driver's fault. Yes, updating audio drivers had helped, but I shouldn't expect audio drivers to be able to cope with this.

With some further online research, I found that people running Windows on Macbooks were having the same problems, and the driver fixed it. They had a slightly different BCM43xx chip, but I thought the same driver might support my chip. So, I searched Dell's support pages for the version number, downloaded the latest file and installed it. Here's what dpclat.exe showed me:

Yeah, that's not ideal, but it's a big improvement. More importantly, the audio interruptions are gone!

This would all have been much easier if Dell helped me find the latest drivers. It seems that soon after they stop selling a model, they stop saying that their drivers support that model. So, soon after I got my laptop, the drivers which support the Dell 1500 WiFi card stopped saying that they support my laptop. It gets worse. Soon, drivers stopped saying that they even support the Dell 1500 WiFi card. Dell claims that the driver I'm using is for various other wireless cards and for another laptop. It actually seems to be a universal Broadcom driver.

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