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

[Dominic Sacré]

On Friday, 10. November 2006 19:10, Carlo Capocasa wrote:
> I have the same problem going over the internal sound card, and it is
> more intense there. The odd part is, the lower the latency, the more
> intense the hum; and even weirder, I CAN EVEN HEAR THE HUM WHEN THE
> COMPUTER IS HOOKED UP TO SPEAKERS THAT ARE TURNED OFF. So you can
> actually hear the hum; it's just that whatever sound adapter is hooked
> up, firewire or interal, amplifies it.

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!

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...

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!

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...

    Dominic