How can I change non-realtime pitch/samplerate of a sound?

Question

I have an mp3 sound I'd like to play with pan, pitch/samplerate, and volume controls set once (not changing in realtime). I am using AVAudioPlayer at the moment, which works, but the rate setting performs time stretching instead of performing the samplerate change where the lower values cause a sound to get slower and lower-pitched, and the higher ones cause a sound to get faster and higher-pitched (kind of like tape speed). E.g., setting samplerate to 88200 HZ when your sound is actually 44100 will result in it playing at 200% speed/pitch. Is something like this possible with AVAudioPlayer, or is there another way to accomplish this? Here's what I have so far:

player=[[AVAudioPlayer alloc] initWithContentsOfURL:
[NSURL fileURLWithPath: @"sound.mp3"] error: nil];
player.volume=0.4f;
player.pan=-1f;
player.enableRate=YES;
player.rate=2.0f;
[player play];

Note: I'm not referring to the pitch method where time stretching is used in combination to keep the length of the sound aproximetly the same, or any "advanced" algorithms.

It doesn’t look like AvAudioPlayer provides that kind of functionality developer.apple.com/documentation/avfoundation/avaudioplayer — fdcpp
Changing playback speed is not necessarily as simple as changing sample rate. There are on a set number of sample rates supported so you will likely have to take a different approach. The most obvious way I can think of is with an Audio Unit that interpolates between sample values, either dropping or adding in value accordingly — fdcpp
@fdcpp Is there another way I can do it then? Perhaps another class? Another framework? — NS studios
If it is for non-real-time you could ingest the audio file, get the samples, apply the interpolation save a new audio file and play it back out. — fdcpp
For Frameworks: JUCE is perhaps heavy handed. OpenAL could do it, but is old fashioned. AudioUnits do seem to be the most likely contender — fdcpp

Rhythmic Fistman Rhythmic Fistman · Accepted Answer · 2021-01-01T11:26:46

AVAudioEngine can give you pitch, rate, pan and volume control:

self.engine = [[AVAudioEngine alloc] init];

NSError *error;

AVAudioPlayerNode *playerNode = [[AVAudioPlayerNode alloc] init];
AVAudioMixerNode *mixer = [[AVAudioMixerNode alloc] init];
AVAudioUnitVarispeed *varispeed = [[AVAudioUnitVarispeed alloc] init];

[self.engine attachNode:playerNode];
[self.engine attachNode:varispeed];
[self.engine attachNode:mixer];

[self.engine connect:playerNode to:varispeed format:nil];
[self.engine connect:varispeed to:mixer format:nil];
[self.engine connect:mixer to:self.engine.mainMixerNode format:nil];

BOOL result = [self.engine startAndReturnError: &error];
assert(result);

AVAudioFile *audioFile = [[AVAudioFile alloc] initForReading:url error:&error];
assert(audioFile);

// rate & pitch (fused), pan and volume controls
varispeed.rate = 0.5; // half pitch & rate
mixer.pan = -1;       // left speaker
mixer.volume = 0.5;   // half volume

[playerNode scheduleFile:audioFile atTime:nil completionHandler:nil];
[playerNode play];

If you want separate rate & pitch control, replace the AVAudioUnitVarispeed node with an AVAudioUnitTimePitch node.

How can I change non-realtime pitch/samplerate of a sound?

1 Answers