iOS Tone Generator with variable Oscillation Patterns

问题

I have a Tone Generator Application that generates a tone based on Slider Value for frequency. This part of the application works fine. I'm redering tone using

#import <AudioToolbox/AudioToolbox.h>

OSStatus RenderTone(
void *inRefCon, 
AudioUnitRenderActionFlags  *ioActionFlags, 
const AudioTimeStamp        *inTimeStamp, 
UInt32                      inBusNumber, 
UInt32                      inNumberFrames, 
AudioBufferList             *ioData)

{
// Fixed amplitude is good enough for our purposes
const double amplitude = 0.25;

// Get the tone parameters out of the view controller
ToneGeneratorViewController *viewController =
    (ToneGeneratorViewController *)inRefCon;
double theta = viewController->theta;
double theta_increment = 2.0 * M_PI * viewController->frequency / viewController-    >sampleRate;

// This is a mono tone generator so we only need the first buffer
const int channel = 0;
Float32 *buffer = (Float32 *)ioData->mBuffers[channel].mData;

// Generate the samples
for (UInt32 frame = 0; frame < inNumberFrames; frame++) 
{
    buffer[frame] = sin(theta) * amplitude;

    theta += theta_increment;
    if (theta > 2.0 * M_PI)
    {
        theta -= 2.0 * M_PI;
    }
}

// Store the theta back in the view controller
viewController->theta = theta;

return noErr;
}



- (void)createToneUnit
{
// Configure the search parameters to find the default playback output unit
// (called the kAudioUnitSubType_RemoteIO on iOS but
// kAudioUnitSubType_DefaultOutput on Mac OS X)
AudioComponentDescription defaultOutputDescription;
defaultOutputDescription.componentType = kAudioUnitType_Output;
defaultOutputDescription.componentSubType = kAudioUnitSubType_RemoteIO;
defaultOutputDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
defaultOutputDescription.componentFlags = 0;
defaultOutputDescription.componentFlagsMask = 0;

// Get the default playback output unit
AudioComponent defaultOutput = AudioComponentFindNext(NULL, &defaultOutputDescription);
NSAssert(defaultOutput, @"Can't find default output");

// Create a new unit based on this that we'll use for output
OSErr err = AudioComponentInstanceNew(defaultOutput, &toneUnit);
NSAssert1(toneUnit, @"Error creating unit: %ld", err);

// Set our tone rendering function on the unit
AURenderCallbackStruct input;
input.inputProc = RenderTone;
input.inputProcRefCon = self;
err = AudioUnitSetProperty(toneUnit, 
    kAudioUnitProperty_SetRenderCallback, 
    kAudioUnitScope_Input,
    0, 
    &input, 
    sizeof(input));
NSAssert1(err == noErr, @"Error setting callback: %ld", err);

// Set the format to 32 bit, single channel, floating point, linear PCM
const int four_bytes_per_float = 4;
const int eight_bits_per_byte = 8;
AudioStreamBasicDescription streamFormat;
streamFormat.mSampleRate = sampleRate;
streamFormat.mFormatID = kAudioFormatLinearPCM;
streamFormat.mFormatFlags =
    kAudioFormatFlagsNativeFloatPacked | kAudioFormatFlagIsNonInterleaved;
streamFormat.mBytesPerPacket = four_bytes_per_float;
streamFormat.mFramesPerPacket = 1;  
streamFormat.mBytesPerFrame = four_bytes_per_float;     
streamFormat.mChannelsPerFrame = 1; 
streamFormat.mBitsPerChannel = four_bytes_per_float * eight_bits_per_byte;
err = AudioUnitSetProperty (toneUnit,
    kAudioUnitProperty_StreamFormat,
    kAudioUnitScope_Input,
    0,
    &streamFormat,
    sizeof(AudioStreamBasicDescription));
NSAssert1(err == noErr, @"Error setting stream format: %ld", err);
}

Now I need to modify the patterns in the application like Dog Whistler Application. Can anyone tell me what things do I need do to modify the wave patterns following this source code?

Thanks in advance

回答1:

You would probably need different RenderTone implementations for each specific pattern. The implementation in your code produces a sampled pure sinusoidal wave with no modulation. There are various patterns you could generate, it depends on your needs what will you implement.

For example, generating shorter or longer beeps would require that you generate 'silence' (write 0-s to the buffer) in your 'for' loop for the sinusoidal for a certain number of frames within the loop and then generate the sinusiodal samples again and then silence again... (this is like chopping the signal)

You could also make an amplitude modulation (tremolo effect) by scaling the sample values with a factor computed with another sine signal (with much lower frequency).

Another example would be to produce a 'police siren' sound by modulating the frequency of the generated sample (vibrato effect), essentially the value of your variable theta_increment, also according to a low frequency signal. Or, simply using two different values for it alternating as with the 'beep' effect above.

Hope, this helps.

来源：https://stackoverflow.com/questions/12121544/ios-tone-generator-with-variable-oscillation-patterns