[Openal]Popping noise problem

Tue Aug 4 03:44:44 PDT 2009

Hi All,

I'm not familiar with the iPhone OpenAL capabilities in terms of voice
counts and OpenAL Sources to know what you should expect.

In terms of the audible pop when stopping a playing Source ... I think you
are doing the right thing by fading it to silence.   Have you tried
experimenting with the duration of your fade (500ms sounds higher than
necessary) - but it does depend a lot on how often the OpenAL
implementation actually commits a new gain level.

An alternative idea, which will involve you doing a lot more work on the
application side is ...

If you stream all your audio to OpenAL via Source Buffer Queues you could
handle the fade to silence yourself.  e.g. Stream the audio in small
chunks, e.g 4 Buffers of 25ms.  (The smaller the buffer duration the better
- but will increase the chance of break-up)   When you need to stop a
source, set a flag, that indicates that you should feed it one more chunk
of audio - but before passing the data to the Buffer, ramp the sample
values to 0.   This would have the advantage that the fade could be done
much quicker as you would be able to introduce a per-sample fade out.  Then
you can stop feeding data to that Source, and wait for it to stop
naturally.

NOTE : You could try to keep the Source playing by starting to queue data
from the new sound effect (e.g straight after the old sample's fade-out
buffer has been queued).  However you will have to be careful about when to
commit any new source parameters (e.g gain, position, etc ...) as you
wouldn't want to commit these values while the old sound is still fading
out, or do it too late because otherwise the new sound will be audible with
the wrong parameters.

Dan
Creative Labs (UK) Ltd.

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             Pingwah Leronz                                                
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                                                                   Subject 
                                       RE: [Openal]Popping noise problem   
             08/04/2009 08:25                                              
             AM                                                            

I haven't experienced this , so this is just a wild suggestion,
but what happens if instead of just stopping the source, you assign an
empty buffer
to the source, then stop that?

eg. have a silence buffer?

it sounds kind of backwards, and I'm guessing stopping the silence buffer
would create the pop noise too?

Like I said, haven't experienced this, but its worth a try.

Andrew

> Date: Mon, 3 Aug 2009 22:12:08 -0700
> From: frankmail007 at yahoo.com
> Subject: [Openal]Popping noise problem
> To: Openal at opensource.creative.com
> CC:
>
>
> I'm using OpenAL in a few my projects for iPhone becuase it's portable
and easy to use. But the popping noise problem puzzled my for quite long
time although I managed to by pass the problem by some tricks in some of my
project. I'm here agina to seek for a decent solution.
>
> My problem is I always get popping noise when I'm trying to stop a source
which is still playing. I tried to adjust the gain to 0 or move the source
very far or even move the offset to the end of the buffer, the problem
can't be solved. ( There is about 10ms silence at the end of the sound
file).
>
> Then I tried to create a timer when I'm going t stop a source. The gain
of this source will be reduced gradually to near 0, for example 0.0001 and
then I move the offset to the end of the buffer. The popping sound will not
exist any more this time. However this is not the perfect solution because
it takes time to stop a source. In my case it'll take about 500ms to stop a
source. That means this source can't be reused within this duration. That
limits the maximum number of sources at the same time. I'm not sure what is
the maximum number of sources can be used in iPhone. Smebody told me it's
32. That means there are at most 32 sources can be used within 500ms which
leads to problem in my new project. My new project only need at most 12 to
16 sources at the same time. But due to the popping noise problem, it will
easily use more than 32 sources.
>
> Can anybody advise what should I do to overcome this problem? Or I should
use some other audio library? ( I tried AudioUnit, it's perfect to meet my
requirement but it's not portable.)
>
> Thanks,
>
> Frank
>
>
>
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