DVD/HD speed on Dell C840 tanks when fan comes on

Archived from groups: comp.sys.laptops (More info?)

qu0sbx102@sneakemail.com (Tom Whistler) wrote in message news:<5fc43a45.0405051524.2373aaf6@posting.google.com>...
> DVD playback stutters horribly (both audio and video) on my Dell C840
> when the fan is on.

Well, no suggestions from the world at large, so I just kept digging.
I discovered that one of the two fans on my C840 doesn't work, which
is most certainly contributing to the laptop getting too hot and is
part if not all of the problem.

I downloaded the i8kfangui program which allows direct control over
the fans on my laptop, and provides a running
temperature/fan-speed/CPU usage display as well. I also downloaded
the intel cpu frequency tool.

Using these together, I determined that when the C840 reached a
certain CPU temperature, the fan turned on high and the CPU speed
dropped. The CPU speed drop was what was causing the DVD stuttering.

I found I could get DVDs to play reliably by doing this:

+ manually setting the fan on high to get the temperature below where
it turns on automatically (sometimes 60*C, sometimes 75*C), in the
40's to 50's C.

+ hitting Fn-Z on the keyboard. This turns off the fan, sets the CPU
speed back to max, and does some sort of "normalizing" of the
temperature sensors.

+ Setting the CPU speed to "battery optimized". Setting it slower
("max battery" ) is too slow to play DVDs without stutter, and setting
it faster ("max performance" ) gets the CPU too hot even with the (one
working) fan on high, so the system then reduces CPU speed
automatically which causes stutter.

+ Turning the fan on to "high" manually via the i8kfangui program.
This will keep the CPU cool enough to prevent automatic clock speed
reduction, so the DVD plays smoothly.