[linux-audio-user] Re: The famous "Jack Hum" (Can't record and desparate)

[Carlo Capocasa]

> A while ago I had a very similar problem, on an Athlon XP desktop machine. 
> Whenever I ran jack at low latency, there was a high pitched noise coming 
> from somewhere inside the case. I was unable to pinpoint where exactly it 
> came from, but I'm pretty sure it originated somewhere on the mainboard, 
> maybe the CPU or the PCI cards.
> But it didn't just come from the case, it was also audible on headphones, 
> on both my PCI sound cards!

This is EXACTLY the problem I faced, and EXACTLY the response I had to
it. Where the hell is that faint hum coming from?

What was especially confusing is that it was audible both without
headphones, very slightly, and with headphones, amplified.

> At 256 frames/second, the noise was barely audible... At 128 it was quite 
> easily noticable, and at 64 it became downright annoying. At 32 or 16 
> frames/second it was even worse...

This is exactly the behavior I experienced. Very slight at higher
latencies; however, I tried it at my friend's club and over the
high-powered amps it is still a clear annoyance, not to mention not
being able to sing and run effects over it at low latency.

> But the most intriguing part of the whole story: Not only did the noise 
> get louder at a lower latency, it also seemed to precisely double its 
> frequency when running at halve the number of frames. I even tried to 
> measure the frequency of the noise. As far as I remember, when running 
> jack for instance at 44100Hz, periodsize 32, the noise was at exactly 
> 44100/32 = 1378Hz!

That is absolutely fascinating. I WILL investigate this; if it
coincides, we have a VERY strong case that this is the same problem.

> Anyway, this whole issue went away after I disabled athcool (a utility to 
> reduce heat and power consumption on Athlon CPUs) while running jack. But 
> still all of this remains a mystery to me...

What would be the corresponding utility for a Pentium 4 mobile (used
before centrinos, slightly modified for notebook assemblage but still a
full Pentium 4) be? How could I completely de-activate power management
and open the case and put some ice cubes in? (IE manage power myself,
with a room fan, or liquid cooling, or whatever...)

Carlo