Download videos podcasts (xvid coded AVI) to iTunes & play it via Apple TV

if iTunes is supporting (have the codec) such file types.

Not iTunes. You can play AVI on a Mac with Quicktime if you install [Perian codecs for Quicktime|http://www.perian.org] - plugins to add functionality to Quicktime for playing additional audio and video formats

[Quicktime audio problems, Perian, VLC|http://discussions.apple.com/message.jspa?messageID=9013669] - read note about uninstalling prior DIVX support before installing Perian.

First you'll need to download and install the Xvid codec on your computer. That will let you play the files on your computer, but I don't think iTunes will pick them up. (someone please correct me if I'm wrong.)

I have used Visual Hub to do this in the past, but it is no longer available.

Once the Xvid codec is on your computer, you should be able to open the file into Quicktime and then save it off as a .m4a file. This is very time consuming, and sometimes yields unusable files, but it does work.

There may be new ways to pursue this, but I'm afraid I don't know what they are.

Doc

You can download the Perian version of the codec here: http://perian.org/.

The codec is just a little bit of plug-in code that tells quicktime how to read your files. Nothing to worry about.

You want to watch the files on your Apple TV, but your Apple TV doesn't know how to read .AVI files. Unfortunately, Apple didn't give you a way to teach the Apple TV about .AVI files. This means that you have to convert the files into a different format on your computer. Last time I mistakenly said you'd have to convert the files to .m4a. Once the codec is loaded on your computer, you'll want to open up the file and save it off as an .m4v, or a .m4, or a .mov file. The new version of the file (the .m4v) you can then drag into iTunes. Then iTunes syncs the file to your Apple TV, and you can watch it on the Apple TV.

Doc

Use MediaInfoMac to get information on the codecs used in the AVI. Many AVIs used mp3 for audio which Macs don't like. The video might already be in mp4 format (xvid is a mp4 variant). You might be able to use a program such as Mpegstreamclip to do conversions. If both audio and video are in mp4 format (e.g., xvid and aac) then you just need to use save in the mp4 wrapper (or mov) instead of recoding everything. If the audio is in mp3 then you may need to recode the audio but pass through the video, and save in mov or mp4 wrapper.

This may sound a bit complicated but that is the nature of multimedia these days. The easiest answer is just to buy the movie in a format compatible with iTunes.