[Openal] Sound crackling

Wed Apr 8 06:07:47 PDT 2009

Hi,

I'm not sure if the source code to the Apple implementation is available -
maybe someone else on the list knows.

re : looping.  Every implementation should play all of the samples when
looping is enabled otherwise this will cause clicks & pops.  Are you
enabling looping before playing a Source or during playback?

Dan
Creative Labs (UK) Ltd.

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             Søren                                                         
             Christiansen                                                  
             <sc at spanning.dk>                                           To 
                                       Daniel PEACOCK                      
             04/08/2009 01:33          <dpeacock at creativelabs.com>         
             PM                                                         cc 
                                       openal at opensource.creative.com      
                                                                   Subject 
                                       Re: [Openal] Sound crackling        

Thank you so much! It makes sense and also supports my testing results ...
decreasing the volume steps pr. frame/loop decreases the crackling. Is it
possible to get the implementation details for the different AL devices
somewhere (preferably apples openal impl)? I have a different problem,
maybe related to the same as above: Sometimes when I loop audio, some of
the the samples are simply cut away!. I can imagine that it has something
to do with buffer boundaries (which differ from hw to hw)? or?

thx again,

On Apr 8, 2009, at 1:36 PM, Daniel PEACOCK wrote:

      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.

      Notice
      The information in this message is confidential and may be legally
      privileged. It is intended solely for the addressee. Access to this
      message by anyone else is unauthorized. If you are not the intended
      recipient, any disclosure, copying or distribution of the message, or
      any action taken by you in reliance on it, is prohibited and may be
      unlawful. If you have received this message in error, please delete
      it and contact the sender immediately. Thank you.

      Creative Labs UK Ltd company number 2658256 registered in England and
      Wales at Belmont Road, Belmont Place, Maidenhead, Berkshire, SL6 6TB

      <graycol.gif>Søren Christiansen <sc at spanning.dk>

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                                                                [Openal]   
                                                                Sound      
                                                                crackling  

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      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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