CODEWITHSUNDEEP
iosaudiosignal-processing
user3837954
user3837954

Reputation: 11

rapid volume changes cause artefacts in AVAudioPlayer

I have some pure tones (sine waves) that I need to fade in and out and also adjust their volume arbitrarily through a thumbwheel. I generate the PCM data along with a WAV header and then encapsulate it in an AVAudioPlayer which plays back fine at constant volume.

Now, as trivial as this sounds, I'm finding that changing volume rapidly on iOS 7 causes some pretty nasty artefacts. Imagine a tone playing with a slider controlling it's volume. Moving the slider around rapidly will cause the noise I'm describing. It's particularly obvious because the source signal is just a tone. If it were music, things would likely just get lost in the relative noise. Oddly, if I directly instrument the device volume via MPMusicPlayerController instead of trying to control things either through AVAudioPlayer's volume or even at the byte level, I get much smoother results and no artefacts. I suspect the device is doing something when it's volume is adjusted that I am not.I know this kind of quantization noise is an issue in audio processing and I'm wondering if anyone may have some advice.

I've also reproduced the issue using sample level playback via Novocaine. No matter what I do, I can't seem to get smooth, noise-free fade in/out characteristics. Any help would be much appreciated.

Upvotes: 0

Views: 187

Answers (2)

hotpaw2
hotpaw2

Reputation: 70703

If modifying audio volume at the sample level, make sure to smoothly change the volume, never suddenly from one sample to the next, otherwise the result will have discontinuities, which can be very noisy. e.g. gradually fade in any volume changes over many (perhaps a few dozen) milliseconds worth of samples, using either a linear ramp, or an ease-in-ease-out half-cosine curve, per-sample between level settings. Perhaps one controller is doing this automatically, but the other is not.

Upvotes: 0

Alexander
Alexander

Reputation: 583

Have you checked out AVAudioMix? I've used this for ramping volume with an AVPlayer, and it works incredibly well. The function to ramp volume over a specific time has been particularly helpful for fade in/out effects.

Upvotes: 1

Related Questions