Use of ALC_FREQUENCY property

[Jim B.-4]

[ We have OpenAL in beta test for the open source game, Alien Arena
(homepage: http://icculus.org/alienarena/rpa/
svn browse: http://svn.icculus.org/alienarena/ ).
It uses the crx engine, derived from the Quake 2 engine and runs
in Windows and Unix/Linux. So far, the testing is going well.
I have several questions, posted today under 3 separate
subjects. ]

The existing program allows menu selection of sound quality by setting the
maximum sampling rate. As near as I can tell, this is not useful in OpenAL. Is
it true that the ALC_FREQUENCY property is not settable in current OpenAL
implementations? Also, in the current versions, is it possible for
ALC_FREQUENCY to be anything other than 44100 KHz? It looks like OpenAL will
resample any file that is not at ALC_FREQUENCY when the Buffer is loaded. So,
there is no point in resampling before loading the Buffer -- correct? Also. I
am trying to decide if it would be worthwhile to check for ALC_FREQUENCY at
22050 KHz (or less) and low pass filter our 44100 KHz sound files.

Jim Bower



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[Jason Daly]

Jim B. wrote:
> The existing program allows menu selection of sound quality by setting the
> maximum sampling rate. As near as I can tell, this is not useful in OpenAL. Is
> it true that the ALC_FREQUENCY property is not settable in current OpenAL
> implementations?

I thought that it was, but it's set at context creation time, so you
have to destroy and re-create the context whenever you adjust it.

If that doesn't work, you might try closing the device and re-opening it
as well.

I admit I haven't tested this in a long time, so I'm just telling you
what "should" happen :-)


> Also, in the current versions, is it possible for
> ALC_FREQUENCY to be anything other than 44100 KHz? It looks like OpenAL will
> resample any file that is not at ALC_FREQUENCY when the Buffer is loaded. So,
> there is no point in resampling before loading the Buffer -- correct?

Not unless you have your own sample rate conversion code that sounds
better than what the OpenAL implementation gives you.


>  Also. I
> am trying to decide if it would be worthwhile to check for ALC_FREQUENCY at
> 22050 KHz (or less) and low pass filter our 44100 KHz sound files.
>  

I don't know whether you'll gain anything from that or not.

--"J"

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[Chris Robinson-5]

On Tuesday 19 May 2009 1:20:05 pm Jim B. wrote:
> The existing program allows menu selection of sound quality by setting the
> maximum sampling rate. As near as I can tell, this is not useful in OpenAL.
> Is it true that the ALC_FREQUENCY property is not settable in current
> OpenAL implementations?

It's not settable in OpenAL Soft, currently. I'm not sure if it has an effect
on the Windows implementation or not, though.. it may work there. I do have
plans for implementing it in OpenAL Soft, when I find a safe way to do it.

> Also, in the current versions, is it possible for
> ALC_FREQUENCY to be anything other than 44100 KHz?

Yes. OpenAL Soft allows the user to set a default frequency for playback via a
couple config files (eg. I use 48khz output), and it may also be modified
according to the hardware's needs. Calling alcGetIntegerv with ALC_FREQUENCY
should show these changes.

> It looks like OpenAL
> will resample any file that is not at ALC_FREQUENCY when the Buffer is
> loaded. So, there is no point in resampling before loading the Buffer --
> correct? Also. I am trying to decide if it would be worthwhile to check for
> ALC_FREQUENCY at 22050 KHz (or less) and low pass filter our 44100 KHz
> sound files.

There's no need to filter sounds before loading. Just load the sound with its
original sample rate, and OpenAL will take care of resampling and everything
for you.
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[Daniel PEACOCK]

For completeness ... the Windows versions ("Generic Software", "Generic
Hardware", and Creative's range of native devices) all ignore the
ALC_FREQUENCY context hint.   The "Generic Software" device is hardcoded to
output at 44.1KHz, the other devices all mix to a rate determined by the
hardware (which in some cases can be set by user options).

>> It looks like OpenAL will resample any file that is not at ALC_FREQUENCY
when the Buffer is loaded.

That's not true for any of the Windows implementations.  Re-sampling is
only down at mix-time, because you don't know until that time what
parameters will be set on the Source + Listener that will affect the
playback rate of each Source (e.g Pitch + Doppler).

>> I am trying to decide if it would be worthwhile to check for
ALC_FREQUENCY at 22050 KHz (or less)

I think all the AL devices you are likely to encounter will be mixing to at
least 44.1KHz.

Dan
Creative Labs (UK) Ltd.

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On Tuesday 19 May 2009 1:20:05 pm Jim B. wrote:
> The existing program allows menu selection of sound quality by setting
the
> maximum sampling rate. As near as I can tell, this is not useful in
OpenAL.
> Is it true that the ALC_FREQUENCY property is not settable in current
> OpenAL implementations?

It's not settable in OpenAL Soft, currently. I'm not sure if it has an
effect
on the Windows implementation or not, though.. it may work there. I do have

plans for implementing it in OpenAL Soft, when I find a safe way to do it.

> Also, in the current versions, is it possible for
> ALC_FREQUENCY to be anything other than 44100 KHz?

Yes. OpenAL Soft allows the user to set a default frequency for playback
via a
couple config files (eg. I use 48khz output), and it may also be modified
according to the hardware's needs. Calling alcGetIntegerv with
ALC_FREQUENCY
should show these changes.

> It looks like OpenAL
> will resample any file that is not at ALC_FREQUENCY when the Buffer is
> loaded. So, there is no point in resampling before loading the Buffer --
> correct? Also. I am trying to decide if it would be worthwhile to check
for
> ALC_FREQUENCY at 22050 KHz (or less) and low pass filter our 44100 KHz
> sound files.

There's no need to filter sounds before loading. Just load the sound with
its
original sample rate, and OpenAL will take care of resampling and
everything
for you.
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