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I want to experimentally prove that the iPhone can play 24 bit audio through the USB port (and also >44.1 sample rate but that is more trivial). How can I test this?

I have a decent budget so a DAC with bit depth indication and MFi would work but I am also curious about doing it "in-house".

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Bit depth is specific to certain file types, most notably Wav. Because the standard procedure of a music application is to play a compressed version of a file such as Mp3 or Aiff it throws bit depth out the window and instead outputs signal at a kbps rate instead such as 192 or 320kbps. As far as DAC goes I don't think it will improve the quality of an already compressed file format, especially when the initial DAC is inside the Iphone. if you wanted to do this you would have to rip out the current DAC and replace it with a new one otherwise, then import a completely uncompressed 24bit sound such as a wav file. If not then the standard depth is 16bit and the standard rate is 320kbps

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Unlike some earlier models of iOS devices, the DACs and ADCs in the iPhone 6s and 7 support native sample rates of 48000. You can experimentally test for this by writing an iOS app that configures the RemoteIO Audio Unit for a 48k sample rate, then synthesize sine waves at frequencies a bit above 22.050kHz and observing the audio output on an oscilloscope.

Also, the default native format for iOS Audio Unit audio samples (as documented in Apple's AudioToolkit framework header files), after iOS 5 or so, switched from 16-bit integer to 32-bit floating point. So a custom app can feed high resolution audio samples to the audio output (and again, you can try observing the output on an oscilloscope).

  • Interesting. So we have documentation confirming potential of 32-bit depth and at least 48k sample rate. I never thought of testing the sample rate that way. I will have to try and see how high it can go – popctrl Mar 21 '17 at 14:07
  • I conducted some experiments based on your post and also other research I did. See my answer if you're interested. – popctrl Apr 27 '17 at 19:24
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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.

enter image description here

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.

enter image description here

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