Re: is there a buffer?

John Gruenenfelder (johng@bach.as.arizona.edu)
Sat, 9 Sep 2000 17:21:55 -0700 (MST) 

On Sun, 10 Sep 2000, Mikko Hänninen wrote:
>Lucas Vogel <lucas7@home.com> wrote on Sat, 09 Sep 2000:
>> I am using Afterstep 1.8.1 on SuSE 6.4. I seem to have problems with playing
>> mp3's while dragging windows around. I use FreeAmp to play music, and whenever I
>> drag a Netscape window or something it seems to screw up FreeAmp's buffer,
>> making it play like a skipping CD.
>
>I have a similar problem.  Whenever there is a large screen update (I
>switch pages with the pager, or a large xterm window is being completely
>redrawn, etc.) there is skipping with mp3 playing.  I think this is
>because of a DMA "conflict" -- the screen update uses DMA for the
>update, and while this is happening the mp3 player software can't use
>DMA for the sound playing.

The reasoning I've heard is bus saturation.  You move a large window so the
video card must transfer a lot of data.  For 1024x768x32 that's about 3.1MB
for a full screen update, which happens many times as you drag the window.

Most modern video cards have an option for bus mastering.  This is usually on
by default.  What this means is that when the video card is transferring all
of that data, it takes complete control of the bus until the transfer is
done.  This is hand because it results it better performance for your video
card.  Because of this, giving your music player a larger buffer won't help
because that data still has to travel over the bus where the bottleneck is.

To fix this you can:
1) work in a lower color depth resulting in less data.
2) turn off bus mastering.  Usually in your BIOS there is an option to "Assign
   IRQ to VGA".  Turning that off should disable bus mastering.  If not, there
   should be an X server option.  Of course, this will reduce the performance
   of your video card.


--John Gruenenfelder    Research Assistant, Steward Observatory, U of Arizona
elrond@azstarnet.com    johng@bach.as.arizona.edu
"This is the most fun I've had without being drenched in the blood
of my enemies!"
        --Sam of Sam & Max

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