I'm creating a old school music emulator for the old GWBasic PLAY command. To that end I have a tone generator and a music player. Between each of the notes played I'm getting a chirp sound that mucking things up. Below are both of my classes:
ToneGen.h
#import <Foundation/Foundation.h>
@interface ToneGen : NSObject
@property (nonatomic) id delegate;
@property (nonatomic) double frequency;
@property (nonatomic) double sampleRate;
@property (nonatomic) double theta;
- (void)play:(float)ms;
- (void)play;
- (void)stop;
@end
ToneGen.m
#import <AudioUnit/AudioUnit.h>
#import "ToneGen.h"
OSStatus RenderTone(
void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData);
void ToneInterruptionListener(void *inClientData, UInt32 inInterruptionState);
@interface ToneGen()
@property (nonatomic) AudioComponentInstance toneUnit;
@property (nonatomic) NSTimer *timer;
- (void)createToneUnit;
@end
@implementation ToneGen
@synthesize toneUnit = _toneUnit;
@synthesize timer = _timer;
@synthesize delegate = _delegate;
@synthesize frequency = _frequency;
@synthesize sampleRate = _sampleRate;
@synthesize theta = _theta;
- (id) init
{
self = [super init];
if (self)
{
self.sampleRate = 44100;
self.frequency = 1440.0f;
return self;
}
return nil;
}
- (void)play:(float)ms
{
[self play];
self.timer = [NSTimer scheduledTimerWithTimeInterval:(ms / 100)
target:self
selector:@selector(stop)
userInfo:nil
repeats:NO];
[[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
}
- (void)play
{
if (!self.toneUnit)
{
[self createToneUnit];
// Stop changing parameters on the unit
OSErr err = AudioUnitInitialize(self.toneUnit);
if (err)
DLog(@"Error initializing unit");
// Start playback
err = AudioOutputUnitStart(self.toneUnit);
if (err)
DLog(@"Error starting unit");
}
}
- (void)stop
{
[self.timer invalidate];
self.timer = nil;
if (self.toneUnit)
{
AudioOutputUnitStop(self.toneUnit);
AudioUnitUninitialize(self.toneUnit);
AudioComponentInstanceDispose(self.toneUnit);
self.toneUnit = nil;
}
if(self.delegate && [self.delegate respondsToSelector:@selector(toneStop)]) {
[self.delegate performSelector:@selector(toneStop)];
}
}
- (void)createToneUnit
{
AudioComponentDescription defaultOutputDescription;
defaultOutputDescription.componentType = kAudioUnitType_Output;
defaultOutputDescription.componentSubType = kAudioUnitSubType_DefaultOutput;
defaultOutputDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
defaultOutputDescription.componentFlags = 0;
defaultOutputDescription.componentFlagsMask = 0;
// Get the default playback output unit
AudioComponent defaultOutput = AudioComponentFindNext(NULL, &defaultOutputDescription);
if (!defaultOutput)
DLog(@"Can't find default output");
// Create a new unit based on this that we'll use for output
OSErr err = AudioComponentInstanceNew(defaultOutput, &_toneUnit);
if (err)
DLog(@"Error creating unit");
// Set our tone rendering function on the unit
AURenderCallbackStruct input;
input.inputProc = RenderTone;
input.inputProcRefCon = (__bridge void*)self;
err = AudioUnitSetProperty(self.toneUnit,
kAudioUnitProperty_SetRenderCallback,
kAudioUnitScope_Input,
0,
&input,
sizeof(input));
if (err)
DLog(@"Error setting callback");
// 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 = self.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 (self.toneUnit,
kAudioUnitProperty_StreamFormat,
kAudioUnitScope_Input,
0,
&streamFormat,
sizeof(AudioStreamBasicDescription));
if (err)
DLog(@"Error setting stream format");
}
@end
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
ToneGen *toneGen = (__bridge ToneGen *)inRefCon;
double theta = toneGen.theta;
double theta_increment = 2.0 * M_PI * toneGen.frequency / toneGen.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
toneGen.theta = theta;
return noErr;
}
void ToneInterruptionListener(void *inClientData, UInt32 inInterruptionState)
{
ToneGen *toneGen = (__bridge ToneGen *)inClientData;
[toneGen stop];
}
Music.h
#import <Foundation/Foundation.h>
@interface Music : NSObject
- (void) play:(NSString *)music;
- (void) stop;
@end
Music.m
#import "Music.h"
#import "ToneGen.h"
@interface Music()
@property (nonatomic, readonly) ToneGen *toneGen;
@property (nonatomic, assign) int octive;
@property (nonatomic, assign) int tempo;
@property (nonatomic, assign) int length;
@property (nonatomic, strong) NSData *music;
@property (nonatomic, assign) int dataPos;
@property (nonatomic, assign) BOOL isPlaying;
- (void)playNote;
@end
@implementation Music
@synthesize toneGen = _toneGen;
- (ToneGen*)toneGen
{
if (_toneGen == nil)
{
_toneGen = [[ToneGen alloc] init];
_toneGen.delegate = self;
}
return _toneGen;
}
@synthesize octive = _octive;
- (void)setOctive:(int)octive
{
// Sinity Check
if (octive < 0)
octive = 0;
if (octive > 6)
octive = 6;
_octive = octive;
}
@synthesize tempo = _tempo;
- (void)setTempo:(int)tempo
{
// Sinity Check
if (tempo < 30)
tempo = 30;
if (tempo > 255)
tempo = 255;
_tempo = tempo;
}
@synthesize length = _length;
- (void)setLength:(int)length
{
// Sinity Check
if (length < 1)
length = 1;
if (length > 64)
length = 64;
_length = length;
}
@synthesize music = _music;
@synthesize dataPos = _dataPos;
@synthesize isPlaying = _isPlaying;
- (id)init
{
self = [super init];
if (self)
{
self.octive = 4;
self.tempo = 120;
self.length = 1;
return self;
}
return nil;
}
- (void) play:(NSString *)music
{
DLog(@"%@", music);
self.music = [[music stringByReplacingOccurrencesOfString:@"+" withString:@"#"]
dataUsingEncoding: NSASCIIStringEncoding];
self.dataPos = 0;
self.isPlaying = YES;
[self playNote];
}
- (void)stop
{
self.isPlaying = NO;
}
- (void)playNote
{
if (!self.isPlaying)
return;
if (self.dataPos > self.music.length || self.music.length == 0) {
self.isPlaying = NO;
return;
}
unsigned char *data = (unsigned char*)[self.music bytes];
unsigned int code = (unsigned int)data[self.dataPos];
self.dataPos++;
switch (code) {
case 65: // A
case 66: // B
case 67: // C
case 68: // D
case 69: // E
case 70: // F
case 71: // G
{
// Peak at the next char to look for sharp or flat
bool sharp = NO;
bool flat = NO;
if (self.dataPos < self.music.length) {
unsigned int peak = (unsigned int)data[self.dataPos];
if (peak == 35) // #
{
self.dataPos++;
sharp = YES;
}
else if (peak == 45) // -
{
self.dataPos++;
flat = YES;
}
}
// Peak ahead for a length changes
bool look = YES;
int count = 0;
int newLength = 0;
while (self.dataPos < self.music.length && look) {
unsigned int peak = (unsigned int)data[self.dataPos];
if (peak >= 48 && peak <= 57)
{
peak -= 48;
int n = (count * 10);
if (n == 0) { n = 1; }
newLength += peak * n;
self.dataPos++;
} else {
look = NO;
}
}
// Pick the note length
int length = self.length;
if (newLength != 0)
{
DLog(@"InlineLength: %d", newLength);
length = newLength;
}
// Create the note string
NSString *note = [NSString stringWithFormat:@"%c", code];
if (sharp)
note = [note stringByAppendingFormat:@"#"];
else if (flat)
note = [note stringByAppendingFormat:@"-"];
// Set the tone generator freq
[self setFreq:[self getNoteNumber:note]];
// Play the note
[self.toneGen play:(self.tempo / length)];
}
break;
case 76: // L (length)
{
bool look = YES;
int newLength = 0;
while (self.dataPos < self.music.length && look) {
unsigned int peak = (unsigned int)data[self.dataPos];
if (peak >= 48 && peak <= 57)
{
peak -= 48;
newLength = newLength * 10 + peak;
self.dataPos++;
} else {
look = NO;
}
}
self.length = newLength;
DLog(@"Length: %d", self.length);
[self playNote];
}
break;
case 79: // O (octive)
{
bool look = YES;
int newOctive = 0;
while (self.dataPos < self.music.length && look) {
unsigned int peak = (unsigned int)data[self.dataPos];
if (peak >= 48 && peak <= 57)
{
peak -= 48;
newOctive = newOctive * 10 + peak;
self.dataPos++;
} else {
look = NO;
}
}
self.octive = newOctive;
DLog(@"Octive: %d", self.self.octive);
[self playNote];
}
break;
case 84: // T (tempo)
{
bool look = YES;
int newTempo = 0;
while (self.dataPos < self.music.length && look) {
unsigned int peak = (unsigned int)data[self.dataPos];
if (peak >= 48 && peak <= 57)
{
peak -= 48;
newTempo = newTempo * 10 + peak;
self.dataPos++;
} else {
look = NO;
}
}
self.tempo = newTempo;
DLog(@"Tempo: %d", self.self.tempo);
[self playNote];
}
break;
default:
[self playNote];
break;
}
}
- (int)getNoteNumber:(NSString*)note
{
note = [note uppercaseString];
DLog(@"%@", note);
if ([note isEqualToString:@"A"])
return 0;
else if ([note isEqualToString:@"A#"] || [note isEqualToString:@"B-"])
return 1;
else if ([note isEqualToString:@"B"] || [note isEqualToString:@"C-"])
return 2;
else if ([note isEqualToString:@"C"] || [note isEqualToString:@"B#"])
return 3;
else if ([note isEqualToString:@"C#"] || [note isEqualToString:@"D-"])
return 4;
else if ([note isEqualToString:@"D"])
return 5;
else if ([note isEqualToString:@"D#"] || [note isEqualToString:@"E-"])
return 6;
else if ([note isEqualToString:@"E"] || [note isEqualToString:@"F-"])
return 7;
else if ([note isEqualToString:@"F"] || [note isEqualToString:@"E#"])
return 8;
else if ([note isEqualToString:@"F#"] || [note isEqualToString:@"G-"])
return 9;
else if ([note isEqualToString:@"G"])
return 10;
else if ([note isEqualToString:@"G#"])
return 11;
}
- (void)setFreq:(int)note
{
float a = powf(2, self.octive);
float b = powf(1.059463, note);
float freq = roundf((275.0 * a * b) / 10);
self.toneGen.frequency = freq;
}
- (void)toneStop
{
[self playNote];
}
@end
To play little tune create a Music object and play...
[self.music play:@"T180 DF#A L2 A L4 O4 AA P4 F#F# P4 O3 D DF#A L2 A L4 O4 AA P4 GG P4 O3 C#C#EB L2 B L4 O4 BB P4 GG P4 O3 C#C#EB L2 B L4 O4 BB P4 F+F+ P4 O3 DDF#A L2 O4 D L4 O5 DD P4O4 AA P4 O3 DDF#A L2 O4 D L4 O5 DD P4O4 BB P4 EEG L8 B P8 ML B1 L4 MN G#A ML L3 O5 F#1L4 MN D O4 F# ML L2 F# MN L4 E ML L2 B MN L4 AD P8 D8 D4"];
Any idea on how to remove the chirp between notes?
I think that the bit where you stop audio output between notes is the culprit:
if (self.toneUnit)
{
AudioOutputUnitStop(self.toneUnit);
AudioUnitUninitialize(self.toneUnit);
AudioComponentInstanceDispose(self.toneUnit);
self.toneUnit = nil;
}
Just leave the tone unit active and you'll have less chirping. You'll need some other way to generate silence, probably by having RenderTone continue to run but generate amplitude zero.
I was able to eliminate the slight chirps that remained by having it, on a frequency change, fade the amplitude down to nothing, update the frequenmcy, and fade back in again. This is of course what the old PC speaker couldn't do (except for a few people who rapidly switched it on again), but with a very rapid fade you can probably get the old-school effect without the chirps.
Here's my fading RenderTone function (currently using evil global variables):
double currentFrequency=0;
double currentSampleRate=0;
double currentAmplitude=0;
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 = 0.5;
// Get the tone parameters out of the view controller
ToneGen *toneGen = (__bridge ToneGen *)inRefCon;
double theta = toneGen.theta;
BOOL fadingOut = NO;
if ((currentFrequency != toneGen.frequency) || (currentSampleRate != toneGen.sampleRate))
{
if (currentAmplitude > DBL_EPSILON)
{
fadingOut = YES;
}
else
{
currentFrequency = toneGen.frequency;
currentSampleRate = toneGen.sampleRate;
}
}
double theta_increment = 2.0 * M_PI * currentFrequency /currentSampleRate;
// 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) * currentAmplitude;
//NSLog(@"amplitude = %f", currentAmplitude);
theta += theta_increment;
if (theta > 2.0 * M_PI)
{
theta -= 2.0 * M_PI;
}
if (fadingOut)
{
if (currentAmplitude > 0)
{
currentAmplitude -= 0.001;
if (currentAmplitude < 0)
currentAmplitude = 0;
}
}
else
{
if (currentAmplitude < amplitude)
{
currentAmplitude += 0.001;
if (currentAmplitude > amplitude)
currentAmplitude = amplitude;
}
}
}
// Store the theta back in the view controller
toneGen.theta = theta;
return noErr;
}
That little chirp is generally an artifact of mathematics. The ear essentially analyzes input signals in the frequency domain. A steady sine wave at frequency 220 Hz, for example, will sound like an A. However, when your sine wave is *un*steady, there are other frequencies that show up due to the boundary. In particular, you get a bit of a pop due to the very high frequency component of starting or stopping a sound abruptly.
The way I solved this in my synthesizer (in Javascript, not Obj-C, but the concept here is the same) is to fade the sound in over 300 samples or so on note on, and fade the sound out over 300 samples or so on note off. There's no way to truly eliminate boundary effects other than not having a boundary at all, but even a small and imperceptible amount of fade will render the boundary effect imperceptible as well.
来源：https://stackoverflow.com/questions/10051215/audiounit-tone-generator-is-giving-me-a-chirp-at-the-end-of-each-tone-generated