如何使用kAudioUnitSubType_LowShelfFilter的kAULowShelfParam_CutoffFrequency参数来控制Core Audio中的低音？ [英] How to use kAULowShelfParam_CutoffFrequency parameter of kAudioUnitSubType_LowShelfFilter which controls bass in Core Audio?

问题描述

在提出这个问题之前，你必须经历过这个问题。如何使用kAudioUnitType_Effect的kAudioUnitSubType_LowShelfFilter控制核心音频中的低音？慢慢地&稳定地掌握音乐的低音控制。但是我的目标却没有成功。现在我知道我必须改变 kAULowShelfParam_CutoffFrequency 来改变低音。

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.

以下代码我在5到7天之前使用过。此代码正常播放音乐，但不能正确改变低音。看看这段代码： -

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);
}

现在在这段代码之后我知道我必须改变频率来改变低音，然后我使用下面的代码片段，但在下面的代码片段音乐没有播放，它也给予 excess_bad 。我只是写了我对代码所做的更改。看看当前的代码片段： -

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 ;
}
}

此代码现在我正在使用但是这不是改变频率。它甚至停止播放音乐并在这行代码中给出了overs_bad错误..

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

请有人帮我这个告诉我如何更改kAULowShelfParam_CutoffFrequency以便我可以调整低音音乐通过滑块。任何有关这方面的帮助都会非常明显。

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.

谢谢：）

推荐答案

在使用 AudioUnit 之前，您需要创建它。如果您使用 AUGraph ，您的代码将类似于：

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

您的代码失败的原因是该行

The reason your code is failing is that the line

AudioUnit lowShelfAU;

使用未确定的值初始化 lowShelfAU 。 AudioUnit 是指针类型，因此在没有初始化的情况下，它指向未知的内存区域。我认为在声明变量时始终初始化变量是一种编程最佳实践，以捕获这些类型的错误：

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;

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