> As I recall, someone reported a similar symptom to what you're
> reporting: i.e. a "noisy" video signal. One possible cause that was
> advanced on the mailing list was that Mersenne's heavy usage of the
> FPU or cache might be generating enough interference to cause the
> problem.
And besides, most anything running on the machine will cause the CPU to use
it's cache. That's just unavoidable.
> I'm very curious about this...
>
> Here's some speculation: since you're not reporting screen corruption
> (just picture instability), I'll speculate that the corruption is
> occurring either between the frame buffer memory (VRAM) and the
> RAMDAC, or in the analog video signal after it leaves the RAMDAC. The
> second makes the most sense, since that's an analog signal and is more
> susceptible to interference than the digital signals from the VRAM.
> The question in my mind is how the CPU can generate a strong enough
> signal to interfere with the video signal...
Your basic PII probably consumes 18W or so. That's a guess by the way,
don't quote me on that. Some of that gets radiated as heat, some will
invariably be EMF. I would hazard a guess that Intel never reckoned on a
video card being so darn close to the CPU. Either that, or the ATI
graphics card is especially sensitive to EMF.
One other consideration might be a slightly higher voltage applied to the
CPU. PII's core voltage is somewhere in the neighborhood of 2.1V I think,
but again, I'm guessing (I actually *do* have the PII specs, just not right
now). If the applied voltage were a bit higher, that could result in an
unusually high amount of EMF.
Either way...EMF from the CPU definitely appears to be the culprit. One
possibility which I emailed the original person was to place some braided
copper between the CPU and video card, and tie that to a good ground.
That's not practical though. :-) We don't all have copper braiding lying
around.
Another option is moving the card to another slot which is the BEST idea if
it's a PCI card. If it's an AGP card, you can't move it. Oh drat.
Remember those old hobby computers from back in the 70's that generated so
much EMF you could make a TV flicker from the other side of your house? Or
how you could place a radio nearby and run a program that generated so much
noise in such a way that you could actually try to play a tune? Oh, for
fun... (Okay, I'm not quite old enough to have used a hobby computer in
the 70's...but close, so close).
