[linux-audio-user] Re: [Ardour-users] Re: [Alsa-devel] usb audio sound quality

[Patrick Shirkey]

Jesse Chappell wrote:

> What is the nature of the quality degradation?  I assume that
> no xruns are occurring in all cases... you are sensing
> degradation that occurs with no obvious suspect?
> 

Yes no xruns. It seems to be directly related to the amount of available 
physical memory. I have been talking with AthlonRob about how the Linux 
memory management works and it seems that the kernel will cache almost 
all the available memory up to between 10 and 4 MB before it starts 
swapping.

It also appears that the kernel stores a certain amount of recently used 
data and I guess that by playing an audio stream it stores that to 
memory until something which needs it, like a gui app request the space. 
It will then transparently give up the memory it has cached giving the 
impression that no memory has been used to open an application.

So It seems I am in a bit of a catch22. In order to use JACK with no 
sound degredation I have to run with no gui. However the kernel will 
eventually use up the memory space which will probably cause 
degredation. However I haven't tested this as I haven't had the time. 
 From here it seems the kernel will cache memory at around 1MB per 
minute. Sometimes more, sometime less.



-- 
Patrick Shirkey - Boost Hardware Ltd.
For the discerning hardware connoisseur
Http://www.boosthardware.com
Http://www.djcj.org - The Linux Audio Users guide
========================================

Being on stage with the band in front of crowds shouting, "Get off! No! 
We want normal music!", I think that was more like acting than anything 
I've ever done.

Goldie, 8 Nov, 2002
The Scotsman