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Re: Using the EQAudioUnit with OpenAL on iPhone
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Re: Using the EQAudioUnit with OpenAL on iPhone


  • Subject: Re: Using the EQAudioUnit with OpenAL on iPhone
  • From: William Stewart <email@hidden>
  • Date: Mon, 24 Aug 2009 16:22:15 -0700

As a user of the OpenAL API you provide buffers that are then mixed and provided to the hardware to be played. The implementation uses AU3Mixer and AURIO audio units to implement this rendering. What you would *like* to do, is to put an EQ between the mixer output and the AURIO - the last step before the h/w.

Unfortunately, this is all implementation detail, and none of this is directly exposed to you. We'd have to provide an extension to let you do this. So, this can't be done at this point. I'd really like to understand why you want to do this - what affect are you trying to achieve?

Bill

On Aug 24, 2009, at 1:40 PM, Darren Baptiste wrote:

@Steve - thanks for the clarification. I guess I had the wrong mental picture of the process.
I can wrap my head around the idea of processing the contents of the buffer BEFORE calling the play method.


However, your suggestion that doing EQ work inside OpenAL instead throws me, because I don't see ANY EQ-type methods in the OpenAL framework provided in the iPhone SDK.

Is there another way of exposing these, or is there another library I must link to?

Cheers,
Darren


"Be the change you wish to see in the world" ~ Mahatma Ghandi

d{ ~_~ }b

On 2009-08-24, at 2:34 PM, Steven Winston <email@hidden> wrote:

Hi Darren,

My understanding is that you might be thinking about this wrong. When
the docs say that OpenAL uses RemoteIO, it shouldn't translate to
audio is passed into the "audio-in" line and then outputed in the
"audio-out." It's not a pass-through type of thinking. RemoteIO has
a render callback, the audio buffer that RemoteIO uses is created and
filled in elsewhere. Think of it as if you wanted to produce the
audio world's hello world sin wave. You're creating sound within the
render call back, not piped into the render pipeline.
Now something that you can also think about is OpenAL on any platform,
is a high level language for audio processing. Positioning is
specifically done by adjusting balance and fade in this specific case
(there's other ways to do it). Therefore, if you think about it like
that, what you want to gain access to is the post openal processed
buffers, (i.e. don't use alSourcePlay() but do all the processing you
want prior to that call). Now write your own play function that will
fill in a buffer with the data you would have played out, and use that
buffer in the AUiPodEQ (although to be honest, I'm fairly certain
overall that's probably not what you want as it'd be easier by far to
do whatever EQ work you want in OpenAL and forgo using AUiPodEQ).
Anyway, that'd be my first idea for how to solve this problem; Good luck!


Steve

On Mon, Aug 24, 2009 at 6:17 AM, Darren Baptiste<email@hidden > wrote:
I am using OpenAL to play sounds, due to the positioning features of the
framework. Work well. Is there a way to direct the output from OpenAL
through an AudioUnit (e.g. the AUiPodEQ) before output to hardware?




Or perhaps it has to go the other way round, first through the AudioUnit(s)
then through OpenAL?




I read somewhere that OpenAL outputs via the RemoteIO AudioUnit. Is there a
way to redirect through a custom AUGraph instead?




Darren


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