[Openal] Sound crackling

[Daniel PEACOCK]

AL devices are all implemented differently ... but most of them will not
commit 100 volume levels per second on a Source especially the software
based ones.   Most software mixers operate by mixing a block of audio at
regular intervals and use the currently set parameters at that time to
control the mixing.   These blocks can be different durations but could be
of the order of 25 milliseconds in duration (meaning 40 updates per
second).  Some AL devices will do parameter ramping to reduce artifacts due
to sudden changes in level from one mixing interval to the next, but some
may not (it is a performance vs quality trade-off).

The popping & clicking is possibly occurring due to large changes in volume
level.  This is in part being caused by the use of a linear volume fade.
You may get better results if you use a different type of fade, eg...

// called 100 pr sec
float flInc = (PI / (2.f * 100.f)); // 100 steps from 0 to PI/2
float flFade = 0.0f;
// fade in from silence to full volume in 100 steps
void Update()
{
      flFade += flInc;
      alSourcef(id, AL_GAIN, 1.0f - cos(flFade));
}

Dan
Creative Labs (UK) Ltd.

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             Søren                                                         
             Christiansen                                                  
             <sc at spanning.dk>                                           To 
             Sent by:                  openal at opensource.creative.com      
             openal-bounces at op                                          cc 
             ensource.creative                                             
             .com                                                  Subject 
                                       [Openal] Sound crackling            
                                                                           
             04/08/2009 11:19                                              
             AM                                                            
                                                                           
                                                                           
                                                                           




Hi all,
I'm encountering crackles and pops if I change the source.volume
frequently like this (pseudo code):

// called 100 pr sec
void Update()
{
             read out [volume]
             alSourcef(id, AL_GAIN, [volume] + 0.01);
}

Is this expected and/or can I do something to prevent it?
Im on a intel macbook pro.

cheers,

Søren C.


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