Airplay Mirror question

[ALR26]

I understand that my late 2009 iMac does not support Airplay mirroring according to Apple's requirements. I have Mavericks and the latest versions of iTunes installed. However, iTunes does allow me to mirror both audio AND video to my AppleTV 3. So why can't I mirror my desktop as well? I would think movies are more graphical intensive than just a desktop display.

I already know about parrot, so this is not about getting 3rd party software. I'm just curious.

 

[priitv8]

iTunes doesn't mirror anything. It just sends your stream (which is already precompressed and prepackaged inside the mp4 file) over the net to appleTV for rendering.
Despite similar name, AirPlay Mirroring is (almost) completely different story.
You have your screen as unpacked RGB information (a bitmap, 32bits per pixel) in the framebuffer. Now you need to compress this into H.264 in real time, at least 24 times per second, before you can beam it over to aTV. You compress in order to reduce bandwidth required. At least 24 frames per second in order to eliminate flicker.
Here's where the hardware video compressor becomes handy. Unfortunately, such is present only in later generations of i-series CPU-s.
Without compression, sending eg 15" MBP's framebuffer would require
1440x900x32x24=995328000bps=949Mbps for video only. Add audio and overhead.

The long story short - for iTunes movies the compression occurs offline and time spent is irrelevant (mastering).
For Mirroring, this must happen in real time.