How to use kAULowShelfParam_CutoffFrequency parameter of kAudioUnitSubType_LowShelfFilter which controls bass in Core Audio?

问题

You must had gone through this before coming to my this question.How to use kAudioUnitSubType_LowShelfFilter of kAudioUnitType_Effect which controls bass in core Audio? Slowly & Steadily getting the things right for bass control of music. But yet not got succeeded in my objective. Now i got to know that i have to change the kAULowShelfParam_CutoffFrequency to change the bass.

The following code i was using before 5 to 7 days. this code plays music properly but doesn't change bass properly. have a look on this code snippet:-

 - (void)awakeFromNib
{
printf("AUGraphController awakeFromNib\n");

mIsPlaying = false;

// clear the mSoundBuffer struct
memset(&mUserData.soundBuffer, 0, sizeof(mUserData.soundBuffer));

// create the URLs we'll use for source A and B
NSString *sourceA = [[NSBundle mainBundle] pathForResource:@"04 - Second Hand Jawaani - [rKmania.com]" ofType:@"mp3"];
NSString *sourceB = [[NSBundle mainBundle] pathForResource:@"Track2" ofType:@"mp4"];
sourceURL[0] = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, (CFStringRef)sourceA, kCFURLPOSIXPathStyle, false);
sourceURL[1] = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, (CFStringRef)sourceB, kCFURLPOSIXPathStyle, false);
[bassSlider addTarget:self action:@selector(selectEQPreset) forControlEvents:UIControlEventValueChanged];
}

// output unit
CAComponentDescription output_desc(kAudioUnitType_Output, kAudioUnitSubType_RemoteIO, kAudioUnitManufacturer_Apple);

// iPodEQ unit
CAComponentDescription eq_desc(kAudioUnitType_Effect, kAudioUnitSubType_AUiPodEQ, kAudioUnitManufacturer_Apple);

// multichannel mixer unit
CAComponentDescription mixer_desc(kAudioUnitType_Mixer, kAudioUnitSubType_MultiChannelMixer, kAudioUnitManufacturer_Apple);

printf("add nodes\n");

- (void)selectEQPreset;
{
AUPreset *aPreset = (AUPreset*)CFArrayGetValueAtIndex(mEQPresetsArray, value);
OSStatus result = AudioUnitSetProperty(mEQ, kAudioUnitProperty_PresentPreset, kAudioUnitScope_Global, 0, aPreset, sizeof(AUPreset));
if (result) { printf("AudioUnitSetProperty result %ld %08X %4.4s\n", result, (unsigned int)result, (char*)&result); return; };

printf("SET EQ PRESET %d ", value);
CFShow(aPreset->presetName);
}

Now after this code i got to know that i have to change frequency to change the bass, then i use the following code snippet but in the following code snippet music is not playing and its giving excess_bad too. i'm just writing the changes i made to the code.have a look on current code snippet:-

// iPodEQ unit
CAComponentDescription eq_desc(kAudioUnitType_Effect, kAudioUnitSubType_LowShelfFilter, kAudioUnitManufacturer_Apple);

- (void)selectEQPreset;
{
AudioUnit lowShelfAU;
assert(lowShelfAU);
float frequencyInHz = 120.0f;
frequencyInHz = bassSlider.value;
OSStatus result = AudioUnitSetParameter(lowShelfAU,kAULowShelfParam_CutoffFrequency,kAudioUnitScope_Global,0,frequencyInHz,0);
if (noErr != result) 
{
    assert(0 && "error!");
    return ;
}
}

This code now i'm using but this is not changing the frequency. Its even stop playing the music and giving the excess_bad error on this line of code..

 AudioUnitSetParameter(lowShelfAU,kAULowShelfParam_CutoffFrequency,kAudioUnitScope_Global,0,frequencyInHz,0);

Please anybody help me regarding this tell me how can i change the kAULowShelfParam_CutoffFrequency so that i can adjust the bass of music Via slider. Any help regarding this would be highly appreciable.

Thanks :)

回答1:

Before you can use an AudioUnit you need to create it. If you're using an AUGraph your code will look something like:

AudioComponentDescription filterDescription = { kAudioUnitType_Effect, kAudioUnitSubType_LowShelfFilter, kAudioUnitSubType_LowShelfFilter, 0, 0 };

AUNode filterNode = -1;
OSStatus result = AUGraphAddNode(mAUGraph, &filterDescription, &filterNode);
if(noErr != result) {
    // Handle error
}

AudioUnit filterUnit = nullptr;
result = AUGraphNodeInfo(mAUGraph, filterNode, nullptr, &filterUnit);
if(noErr != result) {
    // Handle error
}

// Set parameters on filterUnit

The reason your code is failing is that the line

AudioUnit lowShelfAU;

initializes lowShelfAU with an undetermined value. An AudioUnit is a pointer type so without initialization it points to an unknown area of memory. I think it is a programming best practice to always initialize your variables when they are declared, to catch these kinds of bugs:

AudioUnit lowShelfAU = nullptr;

回答2:

`AudioUnit lowShelfAU;` <-- that is an uninitialized garbage value

you need to actually create an AU instance (the Low Shelf) and add it to an AUGraph.

Note: The compiler/analyzer will identify this problem for you. I recommend people turn the warning levels waaaay up, then build, analyze, and remove all issues. Repeat until clean.

回答3:

`as you know ,apple create 3 audio units according their purpose. remoteIO --> for input and output;mixer --> for audio mixer ;for some audio effect is eq and Filter so,if u wanna cutoff the audio frequency,u need to create a filter unit and connect them together.

来源：https://stackoverflow.com/questions/12033705/how-to-use-kaulowshelfparam-cutofffrequency-parameter-of-kaudiounitsubtype-lowsh