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

[Pieter Palmers]

Fernando Lopez-Lezcano wrote:

>On Sat, 2006-11-11 at 13:51 +0100, Daniel Wagner wrote:
>  
>
>>>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!
>>>      
>>>
>>Is an interrupt fired at each period? If so the cpu will use more power 
>>(or woken if it was asleep); a small power 'surge' which can of course 
>>have an influence on any analog hardware near by.
>>    
>>
>
>Really sounds like the power supply, the high frequency transformers can
>"sing" with changes in load if the windings are a bit loose. I think
>capacitors can also do that. 
>
>Somebody I know had a fanless computer in his room (a very old machine
>that really did not need fans if the case was left wide open) and the
>only remaining sound was some component singing with the network
>traffic...
>  
>
A simple test to see if the load is causing the hum:
Open a PDF in acroread (if you have it installed). I noticed that I can 
make some computer sing by dragging the view (hand tool) in acrobat 
reader. acroread seems to contignously redraw when you use the hand tool.

The solution: get a high quality power supply & possibly a high quality 
mainboard too. But as you are experiencing this with a laptop there's 
not much you can do.

Pieter

PS: Maybe this is the one good reason to choose Apple over Dell... ?