CODEWITHSUNDEEP
iosmemorydiskaudiokitaudiounit
Matty H
Matty H

Reputation: 334

How does one allocate memory in an Audio Unit extension (realtime process) for audiofile playback?

I am working on creating an audio unit v3. I have made decent progress up to this point, I made a host app that loads all my 3rd party plugins. I created an auv3 plugin that processes audio and can be loaded by other hosts.

I now want to make an au that loads audio data from disk and scans the data at random positions with sample precision (timestretch, granular stuff, etc). I thought it would be a cool sample playback addittion for contributing to AudioKit.

So this would be making something at the level of AKSampler in the AudioKit framework. While looking through AK source, I feel like I am missing something.

While browsing github, I ended up at places like here: https://github.com/AudioKit/AudioKit/tree/118affee65f4c9b8d4780ef1a03a6d03004bbcee/AudioKit/Common/Nodes/Playback/Samplers

then I looked in here: https://github.com/AudioKit/AudioKit/blob/118affee65f4c9b8d4780ef1a03a6d03004bbcee/AudioKit/Common/Nodes/Playback/Samplers/Disk%20Streamer/AKDiskStreamerAudioUnit.mm

which brought me here: https://github.com/AudioKit/AudioKit/blob/d69dabf090a5e78d4495d938bf6c0aea9f672630/AudioKit/Common/Nodes/Playback/Samplers/Disk%20Streamer/AKDiskStreamerDSPKernel.hpp

and then eventually here: https://github.com/AudioKit/AudioKit/blob/d69dabf090a5e78d4495d938bf6c0aea9f672630/AudioKit/Core/Soundpipe/modules/wavin.c

I am not looking for info about AKSampler, specifically, just how in general audio files are being loaded and how it makes sense with the realtime nature of the au extension process..

I couldn't find any IPC/XPC code anywhere, so I am guessing that its not about circular buffers connecting to other processes or something.

Does AudioKit allocate memory in the realtime process for audiofile playback? This would seem to go against all the warnings from experienced audio programmers(articles like http://www.rossbencina.com/code/real-time-audio-programming-101-time-waits-for-nothing), but I can't figure out what is being done in AudioKit and generally in iOS..

what am I just not understanding or finding? :D

Upvotes: 1

Views: 212

Answers (1)

hotpaw2
hotpaw2

Reputation: 70733

Opening files and allocating memory for file reads should be done outside the real-time audio context, perhaps during UI file selection, never inside an Audio Unit callback.

One way to get random access to samples inside an AU callback is to memory map the file (mmap C API), and then touch every sample in the memory map before passing the memory pointer (unsafe raw etc.) and file length (mapped bounds) to the audio unit. Then you can do virtual random access file reads inside the callback with a fixed latency.

One way to touch every sample in an array by doing a checksum (and perhaps discarding the result later). This memory read is required to get the iOS virtual memory system to swap blocks from the file VM into RAM, so that storage system reads won't happen in the real-time context.

Upvotes: 1

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