What audio quality is available on newer iPhones?

Solution 1:

TL;DR: It appears the iPhone 6 supports 24-bit audio and 48 kHz playback.

So I found a way to empirically test playback.

This first screenshot shows a recording of playback over iPhone USB into Quicktime. You can see from the spectrogram that there is data in the 22-24k range, which means that playback is at least 48 kHz.

This next screenshot shows playback over USB straight into audacity. Basically, I played -95 dB and -97 dB tones at both 16- and 24-bit audio. -97dB is below the minimum volume of 16-bit audio. While all the other tones made a sound, the 16-bit, -97 dB chunk of the recording shows no sound, while 24-bit -97dB does play a sound.

Solution 2:

There appear to be vendors who sell USB audio ADCs and DACs that claim to be iOS compatible (via the Lightning Camera connection kit) and also claim sample rates of 96k and 192k.

Solution 3:

There appears to be no theoretical limit to the bit depth and sample rate an iPhone 7 can produce. The stock Apple apps and the built-in DAC used for the speakers and their headphones only support 16-bit at 48 kHz. That being said, devices plugged into USB/lightning port use their own DACs and iOS provides an interface in which an app (or your own code) can access the source file directly before conversion.

I can't link you directly to Apple developer resources for specific methods/classes, as I no longer have a developer account but this article seems to have a lot of the basic info, even though it's presented as more of a rant than anything else:

http://www.recode.net/2016/9/14/12904906/apple-iphone-7-audio-sound-quality-opportunity-airpods