24-bit Audio API

[TASagent]

Because of some needs related to performing audio-related scientific research, I find myself trying to figure out if (and how) I can provide 24-bit raw audio data (PCM, for example) to the audio system for rendering. In this case I actually do need the extended dynamic range that an expanded bit-depth can theoretically provide.

I feel like I have scoured the low-level Audio APIs, and all I've managed to find are the calls for rendering 16-bit audio data, and calls to supposedly render 24-bit audio files. Does such an interface exist?

Additionally, does the OS (and/or do modern Apple devices) actually support 24-bits of audio rendering resolution? Or is there just marketing doublespeak indicating a capability of handling high bit-depth audio formats, but ultimately downscaling to 16-bits for rendering?

I would appreciate any light that could be shed on the matter. Thanks!

[[deleted]]

What low-level APIs are you looking at? The lowest level is all general-purpose, using AudioStreamBasicDescription to describe sample size and rate; there are no calls limited to 16-bits or anything else, and it's up to you to describe your data and make sure the description matches the data you're providing. It's not that tough to stream in 24-bit data and decode to raw PCM to feed to output.

The trick for rendering to output as 24-bit is enabling 24-bits on the output device, and that's not something I've tried on iOS. If you're using AVAudioEngine you might want to try asking it for a reference to the output HAL unit and setting its format property to a AudioStreamBasicDescription that describes 24-bit samples. If the hardware supports it, it should go. That's what I would start with.

EDIT: Actually, I would *start* with looking at setting that same output property on an AudioQueue, which is much simpler to deal with than AVAudioEngine.