CODEWITHSUNDEEP
iosaudioavfoundationcore-audio
Gerard
Gerard

Reputation: 349

How to obtain audio chunks for analysis in core audio or AVFoundation

I need to analyse chunks of audio data of (approximately) 1 second with a sample rate of 8kHz. Although the audio will be recorded in real time, it will only be used for detecting specific events. Hence, there are no strict latency requirements. What would be the best framework to use in this case?

I already started learning Core Audio and I worked through the book Learning Core Audio. With the minimal amount of Swift documentation available on the internet I was able to set up an AUGraph for iOS to record audio with the remote I/O audio unit and to get acces to the raw samples with the output render callback. Unfortunately, I got stuck to create chunks of 1 seconds of audio samples to perform the audio analysis. Could a custom AudioBufferList be used for this? Or could a large ringbuffer be implemented on the remote I/O audio unit (like it is required in case of a HAL audio unit)?

I also tried to adopt AVFoundation with AVAssetReader to obtain the audio chunks. Although I was able to obtain samples of a recorded audio signal, I did not succes in creating a buffer of 1 second (and I even don’t know whether it would be possible to do this in realtime). Would AVFoundation be a good choice in this situation anyhow?

I would appreciate any advice on this.

A main problem for me is the fact that I try to use Swift but that there is not much example code available and that there is even less documentation. I feel that it would be better to switch to Objective-C for audio programming, and to stop trying to get everything in Swift. I am curious whether this would be a better time investment?

Upvotes: 2

Views: 1453

Answers (2)

hotpaw2
hotpaw2

Reputation: 70673

For analyzing 1 second windows of audio samples, the simplest solution would be to use the Audio Queue API with a lock-free ring buffer (say around 2 seconds long) to record samples. You can use a repeating nstimer task to poll how full the buffer is, and emit 1 second chunks to a processing task when they become available.

Core Audio and the RemoteIO Audio Unit is for if you need much shorter data windows with latency requirements on the order a few milliseconds.

Upvotes: 3

jaybers
jaybers

Reputation: 2211

Core Audio is a C API. Objective-C is an extension of C. I find that Objective-C is much nicer for working with core audio than swift.

I created a cross platform c lockless ring buffer. There is sample code that demonstrates setting up the ring, setting up the mic, playing audio, and reading and writing from the ring.

The ring records that last N number of seconds that you specify. Old data is overwritten by new data. So you specify that you want the latest 3 seconds recorded. The sample I show plays a sine wave while recording through the microphone. Every 7 seconds, it grabs the last 2 seconds of recorded audio.

Here is the complete sample code on github.

Upvotes: 2

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