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Even 720p content is almost 1 million pixels per frame; 24 bits per pixel at 60fps is 180Mbps, which is clearly impossible. So how is the data transferred?

Some guesses:

  • Heavy compression.
  • Apple video playing APIs, which send the compressed stream is sent to the Apple TV, for the Apple TV to decode itself
  • Apple video playing APIs, which send a URL of the data location to the Apple TV.
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2 Answers

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AirPlay sends MP3, AAC and H.264 compressed data directly to receivers, thus a very high compression in some cases and very good for the rest.

Apple'd developer documentation covers the basics as well as some tips for preparing content so that it streams/forwards well.

http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/AirPlayGuide/Introduction/Introduction.html

The decoding of the streams is done by a chip (and encoded as well - the same on iOS as on OS X) so you can see which devices support AirPlay and deduce which GPU and CPU families have the hardware to do this both rapidly and in the case of iOS with low power usage.

This iOS support matrix is very handy to see each iOS version and the CPU/RAM for various models even though it doesn't call out AirPlay support specifically.

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It seems that AirPlay supports all applications though, not just those that are playing video via Apple-supported formats. Do you know how this works? (I'm not completely sure, but this form of stream (which also doesn't support "background" AirPlay) seems to happen at lesser quality.) – mjs Jan 23 at 22:39
It's pretty simple. The GPU/CPU has to render everything on the stream. iOS directs a taps on that stream and sends it over AirPlay as well as drawing to the local screen. Just as two monitors on a computer can mirror content or show two screens. So can AirPlay based on whether the system is in mirror mode or an app allows AirPlay and can draw two screens of content like Keynote for example. – bmike♦ Jan 23 at 22:44
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The video is not sent pixel by pixel, but is encoded with a suitable video compression algorithm specifically designed to use much less bandwith while still being able to reproduce the original video faithfully to the human eye.

Without having investigated a suitable guess would be that the h264 encoding is used.

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An iPhone can simultaneously decode and encode an HD video stream? I'd like to see a source for that! – mjs Jan 23 at 22:27
For the hard encoding step please consider IPhones can record video in full HD... – Thorbjørn Ravn Andersen Jan 24 at 4:11

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