Swift

June 8, 2014 Gene De Lisa 11 Comments

Swift and Core Audio

Like many of you, I’ve been knee deep into Swift this week. Once you get beyond the hello world things and try something a bit more complicated, you start to learn the language. So, why not go off the deep end and try to work with what is essentially a C library: Core Audio? Turns out that’s a good way to get the types grokked.

First, don’t try to use core audio in a Swift playground. I wasted a day trying to do this. It doesn’t work yet. So, create a project.

Update: In Swift 2 you can create an AUGraph in a playground. You just need this at the top of your playground.

import XCPlayground
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

1 2	import XCPlayground XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

I couldn’t do something as simple as this in the playground:

var processingGraph:AUGraph = AUGraph()

1	var processingGraph:AUGraph = AUGraph()

So, ok I put that in a Swift class as an instance variable and created it in the init function.

Most core audio functions return a status code which is defined as OSStatus. You need to the type on the var.

var status : OSStatus = 0
status = NewAUGraph(&processingGraph)

1 2	var status : OSStatus = 0 status = NewAUGraph(&processingGraph)

Or, if you want, you can cast noErr like this.

var status = OSStatus(noErr)

1	var status = OSStatus(noErr)

Here’s an adventure with Boolean.

The function AUGraphIsInitialized is defined like this:

func AUGraphIsInitialized(inGraph: AUGraph, outIsInitialized: CMutablePointer) > OSStatus

1	func AUGraphIsInitialized(inGraph: AUGraph, outIsInitialized: CMutablePointer) > OSStatus

So, you call it like this:

var status : OSStatus = OSStatus(noErr)
var outIsInitialized:Boolean = 0
status = AUGraphIsInitialized(self.processingGraph, &outIsInitialized)

var status : OSStatus = OSStatus(noErr)

var outIsInitialized:Boolean = 0

status = AUGraphIsInitialized(self.processingGraph, &outIsInitialized)

That works. But how do you check it?

Boolean is defined as an CUnsignedChar (in MacTypes.h)

So, you cannot do this:

if outIsInitialized {
    // whatever
}

if outIsInitialized {

// whatever

}

And you cannot cast it (could not find an overload…)

var b:Bool = Bool(outIsInitialized)

1	var b:Bool = Bool(outIsInitialized)

or with Swift’s “as”

var b:Bool = outIsInitialized as Bool

1	var b:Bool = outIsInitialized as Bool

I’m clearly overthinking this. Because this is all that is needed. D’oh!

var outIsInitialized:Boolean = 0
status = AUGraphIsInitialized(self.processingGraph, &outIsInitialized)
if outIsInitialized == 0 {
     status = AUGraphInitialize(self.processingGraph)
     CheckError(status)
}

var outIsInitialized:Boolean = 0

status = AUGraphIsInitialized(self.processingGraph, &outIsInitialized)

if outIsInitialized == 0 {

status = AUGraphInitialize(self.processingGraph)

CheckError(status)

}

Update: In Swift 2, you use DarwinBoolean instead.

var outIsInitialized = DarwinBoolean(false)
status = AUGraphIsInitialized(self.processingGraph, &outIsInitialized)
if !outIsInitialized  {
   status = AUGraphInitialize(self.processingGraph)
    CheckError(status)
}

var outIsInitialized = DarwinBoolean(false)

status = AUGraphIsInitialized(self.processingGraph, &outIsInitialized)

if !outIsInitialized {

status = AUGraphInitialize(self.processingGraph)

CheckError(status)

}

Another problem I had was using constants such as kAudioUnitSubType_Sampler while trying to create an AudioComponentDescription. The trick was to simply cast to OSType.

var cd = AudioComponentDescription(componentType: OSType(kAudioUnitType_MusicDevice),
componentSubType: OSType(kAudioUnitSubType_Sampler),
componentManufacturer: OSType(kAudioUnitManufacturer_Apple),
componentFlags: 0, componentFlagsMask: 0)
status = AUGraphAddNode(self.processingGraph, &cd, &samplerNode)
CheckError(status)

var cd = AudioComponentDescription(componentType: OSType(kAudioUnitType_MusicDevice),

componentSubType: OSType(kAudioUnitSubType_Sampler),

componentManufacturer: OSType(kAudioUnitManufacturer_Apple),

componentFlags: 0, componentFlagsMask: 0)

status = AUGraphAddNode(self.processingGraph, &cd, &samplerNode)

CheckError(status)

Resources

Here is my Github repository for this project. It’s a simple 3 button iPhone app that plays sine tones (based on the button’s tag value).

Core Audio Swift

11 thoughts on “Swift and Core Audio”

Thanks for this!

Please post any thoughts on how to write to an AVAudioPCMBuffer. I’m stuck.

Bob, send me the code what you’re trying to do. Are you generating samples in code?
You’d do use the floatChannelData variable

buffer.floatChannelData.memory

1	buffer.floatChannelData.memory

Hi Gene,

Yeah, Im trying to generate samples into code. I can produce sound, but when I try and change the frequency variable I get a modulation effect. The other strange thing is when I change the frame count, the pitch changes. I’m guessing that has something to do with the buffer being looped.

Thanks for any help.

Bob

import UIKit
import AVFoundation

class ViewController: UIViewController {
    
    var currentFrequency:CDouble = 440.00
    let engine = AVAudioEngine()
    var player:AVAudioPlayerNode = AVAudioPlayerNode()
    var frequency:Float = 1000.00
    var isOn:Bool = false
                            
    @IBAction func stepperAction(sender : AnyObject) {
        let stepVal = Float(stepperView.value)
        println(stepVal)
        let nextBuffer = newBuffer(stepVal)
        player.scheduleBuffer(nextBuffer, atTime: nil, options: .InterruptsAtLoop, completionHandler: nil)
        player.scheduleBuffer(nextBuffer, atTime: nil, options: .Loops, completionHandler: nil)
    }
    @IBOutlet var stepperView : UIStepper = nil
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        setupEngine()
        let buffer = newBuffer(320.00)
        player.scheduleBuffer(buffer, atTime: nil, options: .Loops, completionHandler: nil)
        player.play()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    func setupEngine () {
        engine.attachNode(player)
        var mixer = engine.mainMixerNode
        var error:NSErrorPointer = nil
        engine.connect(player, to: mixer, format: mixer.outputFormatForBus(0))
        engine.startAndReturnError(error)
    }
    
    
    
    func newBuffer(thisFrequ:Float) ->AVAudioPCMBuffer {
        var mixer = engine.mainMixerNode
        var mySampleRate:CDouble = 44100.00
        var cap = UInt32(thisFrequ)
        var audioCap:AVAudioFrameCount = cap
        //var intCap:UInt32 = cap
        var buffer = AVAudioPCMBuffer(PCMFormat: player.outputFormatForBus(0), frameCapacity:100)
        buffer.frameLength = 100
        
        
        var sr:Float = Float(mixer.outputFormatForBus(0).sampleRate)
        var n_channels = mixer.outputFormatForBus(0).channelCount
        var sampleCount = 0
        var maxSampleCount = Int(buffer.frameLength)
        var waveLengthInSamples:Float = 44100.00 / thisFrequ
        while sampleCount < maxSampleCount {
            for (var i:Float = 0; i < waveLengthInSamples; i++) {
                if i < (waveLengthInSamples / 2) {
                    let max:Float = 1.0
                    buffer.floatChannelData.memory[Int(i)] = max
                } else {
                    buffer.floatChannelData.memory[Int(i)] = -1.0
                }
                
                sampleCount++
            }
        }
        
        return buffer
    }
}

import UIKit

import AVFoundation

class ViewController: UIViewController {

var currentFrequency:CDouble = 440.00

let engine = AVAudioEngine()

var player:AVAudioPlayerNode = AVAudioPlayerNode()

var frequency:Float = 1000.00

var isOn:Bool = false

@IBAction func stepperAction(sender : AnyObject) {

let stepVal = Float(stepperView.value)

println(stepVal)

let nextBuffer = newBuffer(stepVal)

player.scheduleBuffer(nextBuffer, atTime: nil, options: .InterruptsAtLoop, completionHandler: nil)

player.scheduleBuffer(nextBuffer, atTime: nil, options: .Loops, completionHandler: nil)

}

@IBOutlet var stepperView : UIStepper = nil

override func viewDidLoad() {

super.viewDidLoad()

// Do any additional setup after loading the view, typically from a nib.

setupEngine()

let buffer = newBuffer(320.00)

player.scheduleBuffer(buffer, atTime: nil, options: .Loops, completionHandler: nil)

player.play()

}

override func didReceiveMemoryWarning() {

super.didReceiveMemoryWarning()

// Dispose of any resources that can be recreated.

}

func setupEngine () {

engine.attachNode(player)

var mixer = engine.mainMixerNode

var error:NSErrorPointer = nil

engine.connect(player, to: mixer, format: mixer.outputFormatForBus(0))

engine.startAndReturnError(error)

}

func newBuffer(thisFrequ:Float) ->AVAudioPCMBuffer {

var mixer = engine.mainMixerNode

var mySampleRate:CDouble = 44100.00

var cap = UInt32(thisFrequ)

var audioCap:AVAudioFrameCount = cap

//var intCap:UInt32 = cap

var buffer = AVAudioPCMBuffer(PCMFormat: player.outputFormatForBus(0), frameCapacity:100)

buffer.frameLength = 100

var sr:Float = Float(mixer.outputFormatForBus(0).sampleRate)

var n_channels = mixer.outputFormatForBus(0).channelCount

var sampleCount = 0

var maxSampleCount = Int(buffer.frameLength)

var waveLengthInSamples:Float = 44100.00 / thisFrequ

while sampleCount < maxSampleCount {

for (var i:Float = 0; i < waveLengthInSamples; i++) {

if i < (waveLengthInSamples / 2) {

let max:Float = 1.0

buffer.floatChannelData.memory[Int(i)] = max

} else {

buffer.floatChannelData.memory[Int(i)] = -1.0

}

sampleCount++

}

return buffer

}

Philip Hart says:

June 26, 2014 at 6:33 am

Hi Gene, Many thanks for posting this.

I am translating an Objective-C application which – like you – I am using to kick Swift’s tyres.

I had reached the complicated bit of calling the Audio library, and was a bit daunted. But your article has pointed the way!

Cheers,
Philip

Reply
1. Gene De Lisa says:
  
  June 26, 2014 at 7:27 am
  
  Most people would write another Weather or To Do app to learn a new language.
  Not us.
  We do the hardest thing on the platform. 🙂
  
  Glad to be of help
  
  Reply
Robert says:

July 3, 2014 at 7:05 pm

Gene, thanks for the article. I think you’re representing a lot of people, like myself and others above, who take CoreAudio seriously. I esp. take it seriously because I’m using iOS to improve my hearing and also “try” and mask my tinnitus (I’ve tried so many hearing aids; the industry is knee deep in earning huge gross margins – mfg’s and audiologists; not in quality).

Swift and the Swift Playground, REPL, are much more conducive to researching new ideas much like OOPS like Smalltalk and Actor once did. Please add me to your mail list if you should continue to get deeper into the subject. I’m looking into trying to bridge STK (Synthesis Toolkit).

If you find any other resources or books to be published, I’d love to know. Thx.

Reply
Dylan says:

July 9, 2014 at 3:39 pm

Thanks for this. A lot. I even thought I’d use that gittip button but it didn’t work… 🙂

Reply

How about for core audio’s AudioUnitSetProperty function. I cannot seem to convert this to swift.

Here is an example:

 var status = AudioUnitSetProperty(
   self.samplerUnit,
   AudioUnitPropertyID(kAUSamplerProperty_LoadInstrument),
   AudioUnitScope(kAudioUnitScope_Global),
   0,
   &instdata,
   UInt32(sizeof(AUSamplerInstrumentData)))

var status = AudioUnitSetProperty(

self.samplerUnit,

AudioUnitPropertyID(kAUSamplerProperty_LoadInstrument),

AudioUnitScope(kAudioUnitScope_Global),

&instdata,

UInt32(sizeof(AUSamplerInstrumentData)))

Pingback: Swift 2: AVFoundation to play audio or MIDI | Gene De Lisa
Nitridium says:

May 16, 2016 at 5:50 am

Hello Gene De Lisa!

Do you know, is it possible to play AVAudioPlayer’s MPMediaItem (Apple Music content) with equalizer?

Reply

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Swift and Core Audio

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