the previous 2 posters, pretty much just answered about using an aTV3, and not what you asked
If you're planning on running XBMC on your imac and
using airplay mirroring to get it to the TV, the answers to your questions are...
1.it will be 1080, but see #2
2. the image will be compressed, so it's likely to contain
artifacts. it will probably also be a bit jerky. a lot of this depends on your network connection. But... it will never look as good as directly plugging an HDMI cable in.
3. the audio using mirroring is stereo only.
not sure why airserver got in here, it's designed to be an airplay receiver, so you would use it to send video from your iOS device to your mac's screen. instead of sending to an aTV. It does send 5.1 if you're playing back a video with 5.1, but airplaying a video and airplay mirroring are 2 different things.
----
if you want good quality video, and 5.1 sound
you have a few options besides the aTV, and your choice depends on your technical expertise.
1.
roku, little technical knowledge.
you will have to use
Plex, it's based on XBMC and has a lot of the same features, but it runs in a client/server model. so you would run the server on your iMac. The iMac will then convert (if needed) the file to something that can play on your client.
it will even convert DTS and AAC 5.1 to Dolby 5.1 since that's what the roku supports.
there are also plex clients for windows, mac,s linux, iOS, and android, so you can watch on different players. you can start a video on the TV, stop it, and pick up in the same place on a different player.
(there's also a client for a jailbroken aTV2)
2.
raspberry pi, you'll need to be comfortable with a linux command line.
people have built versions of both
xbmc and the
plex client that will run on it.
3. Plex and DLNA
Plex will also serve to DLNA clients. so you may already have the hardware you need. Check your TV, Blu-ray player, or game console to see if it's got DLNA built in.
I was even able to get DTS-MA 7.1 using DLNA and my samsung blu-ray player. (my panasonic plasma only gives me stereo sound though)
this version is a bit funky, but it does work. You'll also need to look at the video quality preferences as the default is pretty low. look at the bottom of the menus where you select your different library sections for preferences. (this setting also will re-set itself, you may have to re-select the option you want every time you power down what you're using as the player)
