Apple has updated its podcasting specs and is sending out emails to video podcasters with tips on how to make them look great on Apple TV.

The tips include:

• Encode at 640√ó480 or 640√ó360. This will look much better than 320√ó240 on HDTV, and will still port to the iPod. While 720p looks great, they say, it won‚Äôt work on an iPod.
• Don‚Äôt make two formats for different resolutions – it dilutes the popularity of the podcast and reduces exposure in charts.
• Don‚Äôt add letterboxing to make videos to a 4:3 aspect ratio. Leave them at 16:9.

If you’re creating video podcasts, recognize that Apple’s recommendations are a compromise that sacrifice Apple TV quality for compatibility with video iPods. While they recommend using just one format to get the highest ranking within iTunes, there may be enough interest in HDTV that there could be benefits to moving to 720p widescreen and treating lower resolutions as legacy formats.

Here’s Apple’s updated spec:

Formatting Video for the iPod or Apple TV

Although iTunes can play a variety of .mp4, .m4v, and .mov video formats, Apple TV and the iPod require more specific formats.

The iPod supports up to 640×480, while Apple TV supports up to 1280×720. To optimize for both platforms, we recommend that your source file is at least 640 pixels wide and that you use the built-in iPod converters in Compressor (“H.264 for iPod”), QuickTime Pro (“Movie to iPod”) or iTunes (“Convert Selection for iPod”). Each of these maintains the aspect ration of your source file and results in an M4V file containing H.264 video (Low Complexity version of the Baseline profile) and AAC-LC audio. If you want to maximize the screen area of a wide-screen TV, your source file should have an aspect ratio of 16:9 (e.g., 640×360). If you want to maximize the screen area on the iPod, your source file should have an aspect ratio of 4:3 (e.g., 640×480).

Because it uses H.264 Main Profile, QuickTime Pro’s “Movie to Apple TV” converter will result in a video that will not sync with the iPod.

Refer to the specifications below if you are not using the built-in converters in Compressor, QuickTime Pro, or iTunes.

iPod can play the following video formats:

• H.264 video, up to 1.5 Mbps, 640 x 480, 30 frames per sec., Low-Complexity version of the Baseline Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats
• H.264 video, up to 768 kbps, 320 x 240, 30 frames per sec., Baseline Profile up to Level 1.3 with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats
• MPEG-4 video, up to 2.5 Mbps, 640 x 480, 30 frames per sec., Simple Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats

Apple TV can display H.264 video, up to 5mbps, 1280×720, 24 fps, Progressive Main Profile. Apple TV supports AAC-LC audio up to 320 Kbps. The gating factors for video are bit rate and frame rate. The following are some sample conversions generated by the QuickTime Pro when choosing “Movie to Apple TV”:

* Represents an average bit rate.

Creating Video Podcasts

Although iTunes can play a variety of .mp4, .m4v, and .mov video formats, video iPods require more specific formats. By following the steps in the Creating Video for iPod tutorial, QuickTime 7 Pro will automatically create an .m4v file containing H.264 video and AAC audio that is optimized for iPod. iPod can play the following video formats:

• H.264 video, up to 1.5 Mbps, 640 x 480, 30 frames per sec., Baseline Low-Complexity Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats
• H.264 video, up to 768 kbps, 320 x 240, 30 frames per sec., Baseline Profile up to Level 1.3 with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats
• MPEG-4 video, up to 2.5 Mbps, 640 x 480, 30 frames per sec., Simple Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats