AudioUnit is a good library, it provides a lot of predefined processes for us to deal with the audio data. But I have to say, Apple, you really leave a huge jungle in AudioUnit. You can almost find no useful documents for you to understand the APIs. If you want to use an AudioUnit, you do need to invoke a lot of function in a certain sequence. Otherwise, the AudioUnit will refuse to work.
After you properly set up the AudioUnits and AUGraph, you still might be confused when you want to just some properties of a AudioUnit on the fly, you will find out that you are falling into another jungle. You will hard to know which parameter to use and how to use it. Because All you can access the parameters of an AudioUnit is AudioUnitSetParameter. If you look into this API's document, you will see what the real jungle is:
Sets the value of an audio unit parameter.
OSStatus AudioUnitSetParameter (
The audio unit that you want to set a parameter value for.
The audio unit parameter identifier.
The audio unit scope for the parameter.
The audio unit element for the parameter.
The value that you want to apply to the parameter.
Set this to 0. To schedule the setting of a parameter value, use the AudioUnitScheduleParameters function.
A result code.
Available in iPhone OS 2.0 and later.
See, the first parameter is easy to understand, it means which AudioUnit you want to set/change a parameter with.
the second one will be a little confused, because you need to find out each AudioUnit has which kind of parameters at all. Well, to tell the truth, if you don't know a parameter name, there is no way for you to jump to the parameters defination! So, here is what might be help to you. [Click Here(Local help URL, if you don't have iPhone SDK documentations installed in your system, don't click this URL)]. And don't forget to bookmark it! ;-p