Bonus tip: make Subler write to the "Automatically Add to iTunes" folder inside the "iTunes Music" folder and (guess what!) the new MP4 file is automatically added to iTunes. Then, simply choose to add more tracks, select the original file again, deselect the video track, let the program do its thing, and Bob's your uncle. What this means is that Subler lets you open a file and specify whether to convert the audio to AAC or keep it as AC3. Subler was originally a tool to add subtitle tracks to MP4 videos, but it is also very good at "muxing" audio and video tracks. However, a better solution for this is Subler. It is then possible to paste back the original AC3 track in the new video if you really know what you're doing. It is possible to convert the file without re-encoding the video with QuickTime Pro, but this is neither particularly easy nor especially fast. (Although the right plugins hide many sins when using iTunes in 32-bit mode, in 64-bit mode, these plugins are no longer supported.) The good news is that iTunes, the Apple TV, and iOS devices can handle the H.264 video, but the bad news is that they don't handle MKV files or AC3 video. This video was encoded in H.264 with AC3 sound in an MKV file. For instance, I downloaded a video from the Roskilde music festival from ClearBits, a place where freely distributable content can be found. Things get harder when you find yourself in possession of videos that only have an AC3 audio track. Handbrake, the powerful video conversion tool, will do this without much trouble. But when ripping DVDs to an Apple TV friendly format, you may want to preserve the original 5.1 audio track as well as create a stereo AAC track, just like Apple does. Most of the time, it shouldn't matter, as most audio sources are stereo or even mono, so a stereo AAC track is all that's needed. So what does this mean when creating your own videos? And the iPhone, iPod touch, and iPad can't decode AC3 audio either, so they depend on the stereo track as well. Apple itself shrewdly avoids this problem by always having a stereo AAC audio track in all the videos that it sells, so the Apple TV can simply fall back to the stereo track if the display device it's hooked up to can't handle AC3 audio. In this case, if you attempt to play a video with just an AC3 audio track, the Apple TV will display an error message and refuse to play the video file. However, if you hook up your Apple TV to a TV or other device that expects to receive uncompressed PCM audio, the Apple TV's unwillingness to decode AC3 can be a problem. Most of the time, this isn't an issue, because AC3 is also used in HDTV broadcasts, so most high definition TVs can decode it, as can specialized "surround receivers" with many, many speakers that decode multi-channel digital audio for a living. But the Apple TV doesn't actually know how to decode the latter-if you want to play multichannel audio through your Apple TV, you need to hook it up to a TV or an amplifier that can accept audio in AC3 format and decode it. The audio that accompanies X.264 video, on the other hand, can only be AAC or AC3 (Dolby Digital, often 5.1 channels surround). The Apple TV will play audio encoded as MP3, AAC, Apple Lossless, and several other formats. What's going on inside the diminutive media player, and what should those of us doing our own audio/video encoding keep in mind? Audio inconsistencies With AirPlay streaming, on the other hand, audio is much more flexible than video. The new Apple TV eats near-Blu-ray quality X.264 video for breakfast-but when it comes to audio, the device is a picky one.
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