Re: Real-time buffer playback in Swift?

Subject: Re: Real-time buffer playback in Swift?
From: Charles Constant <email@hidden>
Date: Tue, 16 Sep 2014 03:42:26 -0700

Thanks Jamie,

I probably should watched the WWDC videos again before asking the question.

They appear to discuss the information I was after in Session 502 (at 11m45s)

I think the way to do this is either

a) to continually schedule a small portion of audio with a completion callback, and then schedule the next portion when it completes (I'm guessing this won't work due to the time required to reschedule each callback?)

b) schedule as many portions as possible without waiting, and then handle the GUI updates only with their completion callbacks

I'd still be interested in hearing others' experiences with this. I'm wondering about the best way to do this without the playback becoming choppy, etc.

Cheers,

Charles

On Tue, Sep 16, 2014 at 12:30 AM, Jamie Bullock <email@hidden> wrote:

Hi Charles,,

Take a look at AVAudioFile and AVAudioPCMBuffer as described in WWDC 2014 Session 501 https://developer.apple.com/videos/wwdc/2014/

The only way I have found to access the raw floating point sample data is to use UnsafeArray, like so:

// Where buffer is an AVAudioPCMBuffer instance
let channels = UnsafeArray(start: buffer.floatChannelData, length: buffer.format.channelCount)
let samples = UnsafeArray(start: channels[0], length: buffer.frameLength) // Just the zero'th channel

for i in 0..buffer.frameLength {
let sample = samples[i]
// etc...

If anyone knows a better way I'd also be interested to know it.

Cheers,

Jamie
On 16 Sep 2014, at 07:11, Charles Constant <email@hidden> wrote:
Hello all,

I'm working on a digital audio editor in Swift. So far I do import/export of files using AudioToolbox. I only have makeshift playback currently, where I send the whole buffer contents at once (so there's a ridiculous amount of lag before playback starts). Now I'm at the point where I want to get proper playback sorted. As part of my research, I tried to port the examples in "Learning Core Audio" to Swift. Mostly this went okay, however the most interesting examples for playback are currently broken on Apple's end*.

Can anyone describe to me the most appropriate way to do real-time playback of a buffer (floats normalized to ±1.0) using Swift?

Something using callbacks would suit me well, as I'll need to update the playhead position in my GUI and I was planning on doing so by calling a Notification each time the system executes my callback. Also, I want to mix the channels together on my end, to ensure that what the user hears is precisely the same as what they get upon export to file. Is there some particular method in AVAudioEngine appropriate for this?

Thanks,

Charles

* These projects all rely either upon incorrectly bridged structs, or C Function Pointers:
6) CARecorder
7) CAPlayer
8) CAConverter
12) SimpleSineWavePlayer
13) AUGraph Play Through
14) AUGraph Play Through II
17) iOS Backgrounding Tone
18) iOS AU Pass-Through

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References:

>Real-time buffer playback in Swift? (From: Charles Constant <email@hidden>)

>Re: Real-time buffer playback in Swift? (From: Jamie Bullock <email@hidden>)

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