How to use kAudioUnitSubType_LowShelfFilter of kAudioUnitType_Effect which controls bass in core Audio?

Question

i'm back with one more question related to BASS. I already had posted this question How Can we control bass of music in iPhone, but not get as much attention of your people as it should get. But now I have done some more search and had read the Core AUDIO. I got one sample code which i want to share with you people here is the link to download it iPhoneMixerEqGraphTest. Have a look on it in this code what i had seen is the developer had use preset Equalizer given by iPod in Apple. Lets see some code snippet too:----

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

What kAudioUnitSubType_AUiPodEQ does is it get preset values from iPod's equalizer and return us in Xcode in an array which we can use in PickerView/TableView and can set any category like bass, rock, Dance etc. It is helpless for me as it only returns names of equalizer types like bass, rock, Dance etc. as i want to implement bass only and want to implement it on UISLider.

To implement Bass on slider i need values so that i can set minimum and maximum value so that on moving slider bass can be changed.

After getting all this i start reading Core Audio's Audio Unit framework's classes and got this

after that i start searching for bass control and got this

So now i need to implement this kAudioUnitSubType_LowShelfFilter. But now i don't know how to implement this enum in my code so that i can control the bass as written documentation. Even Apple had not write that how can we use it. kAudioUnitSubType_AUiPodEQ this category was returning us an array but kAudioUnitSubType_LowShelfFilter category is not returning any array. While using kAudioUnitSubType_AUiPodEQ this category we can use types of equalizer from an array but how can we use this category kAudioUnitSubType_LowShelfFilter. Can anybody help me regarding this in any manner? It would be highly appreciable.

Thanks.

Answer 1

Update

Although it's declared in the iOS headers, the Low Shelf AU is not actually available on iOS.

---

The parameters of the Low Shelf are different from the iPod EQ.

Parameters are declared and documented in `AudioUnit/AudioUnitParameters.h':

// Parameters for the AULowShelfFilter unit
enum {
  // Global, Hz, 10->200, 80
  kAULowShelfParam_CutoffFrequency = 0,

  // Global, dB, -40->40, 0
  kAULowShelfParam_Gain = 1
};

So after your low shelf AU is created, configure its parameters using AudioUnitSetParameter.

Some initial parameter values you can try would be 120 Hz (kAULowShelfParam_CutoffFrequency) and +6 dB (kAULowShelfParam_Gain) -- assuming your system reproduces bass well, your low frequency content should be twice as loud.

---

Can u tell me how can i use this kAULowShelfParam_CutoffFrequency to change the frequency.

If everything is configured right, this should be all that is needed:

assert(lowShelfAU);
const float frequencyInHz = 120.0f;
OSStatus result = AudioUnitSetParameter(lowShelfAU,
                                        kAULowShelfParam_CutoffFrequency,
                                        kAudioUnitScope_Global,
                                        0,
                                        frequencyInHz,
                                        0);
if (noErr != result) {
  assert(0 && "error!");
  return ...;
}