I got a question from a reader asking how Apple’s AirPlay streams audio. The question specifically asked about how audio files are converted, and whether AirPlay reduces their quality.
Apple doesn’t provide much information about AirPlay, and I found a number of articles and forum posts where people described complex testing routines to determine the bit depth and sample rate of music streamed to AirPlay devices, such as an Apple TV or AirPort Express. But you don’t need to go to such great lengths to figure this out. Simply open Audio-MIDI Setup on a Mac, and select AirPlay.
As you can see above, AirPlay streams at 16-bit, 44,100 kHz. However, what you don’t see is that AirPlay streams music in Apple Lossless format. What this means is that no matter what format your music is in, it gets converted by OS X – not by iTunes – to Apple Lossless, to ensure the highest quality. So lossless files will be streamed as lossless, as will AAC or MP3 files.
However, high-resolution files will be downsampled to 16/44.1. Interestingly, the Apple TV outputs audio in 48 kHZ, most likely because this is 48 kHz is the standard for movie and TV audio. Movies sold by the iTunes Store contain audio at 48 kHz, but only at 160 kbps.
It’s interesting to read why this is the case: http://www.tvtechnology.com/opinions/0087/digital-audio-sample-rates-the–khz-question-/184354. ↩