[Openal] Sound crackling

[Søren Christiansen]

before playing ... I've recorded the output with Audio Hijack and its  
obvious that some samples are missing around the loop point. Its part  
of a game engine and CPU can be stressed (low framerate). Can samples/ 
mixing blocks be skipped, if the mixing exceed some  time limit?

On Apr 8, 2009, at 3:07 PM, Daniel PEACOCK wrote:

> 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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> <graycol.gif>Søren Christiansen <sc at spanning.dk>
>
>
> Søren Christiansen <sc at spanning.dk>
> 04/08/2009 01:33 PM
>
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> Daniel PEACOCK <dpeacock at creativelabs.com>
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> Re: [Openal] Sound crackling
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>
> 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>
>
> Søren Christiansen <sc at spanning.dk>
> Sent by: openal-bounces at opensource.creative.com
> 04/08/2009 11:19 AM
>
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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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