Converting "unsupported" Video files for Apple TV 2

Let's explore the least painful way to convert incompatible files like mkv and avi to Apple TV compatible mp4 videos. Also, the solution I'm using is a Mac only application, so if you're on another platform, I'm afraid you'll need to keep searching.

Hello again and welcome back to my nearly abandoned blog!  A couple posts per year ain't bad, right?

For my last post of the year, I wanted to share some of what I learned about beating various video formats (.mkv, .avi, etc) into something that both imported to iTunes and would play on the Apple TV 2. The main thing we want to do is avoid re-encoding the video if it's already stored in the original file with the correct codec.

I want to try and break this post up into sections to make it a little easier to digest since I tend to be a bit verbose sometimes.  I'll break this up into sections for an overview of my experience, technical details on codecs, transcoding and conversion tools and finally audio formats (which is the main problem I had with conversion).  If you don't really want the back story or the details on codecs and tools, jump to the summary section at the end of the post that explains the process.

My Foray into Apple TV
This Christmas, I found that Santa left me a shiny new Apple TV under the tree.  Interestingly, I found it does stuff that my TiVos already also do, but the Apple TV is much more elegant, and streaming content located on my Mac super fast and much more user friendly.  That is to say once I was able to actually get content loaded into iTunes and playable on my new device.

Since I have been using PyTivo  for ages to get various video copied over to the TiVo boxes in the house, I've amassed a fair collection of content in a number of different formats.  Of course all of this content is perfectly legitimate, I would never condone any sort of piracy or circumvention of the DMCA, that is just wrong, mmmkay?

At first, I just started dropping my files into Handbrake using either the built-in presets (high profile was the recommended one on the forums) or by downloading and using specific presets for new Apple devices.  That worked great, the converted files played just fine on the Apple TV, save for two problems.  First was that since I was re-encoding stuff that was already compressed, there was a loss in video quality (though it was slight) and second was the process took an eternity.  With my dual core iMac it would take a few hours to convert a full-length 720P video.

Being an impatient person, I decided to school myself in the world of video conversion, so with some quality time with Google and some trial and error I was able to come up with a solution that will save countless hours of encoding, I hope.  Now is a good time to point out that none of this stuff is really original though, just things I found online. I will try my best to provide links wherever possible, but if I missed something, please let me know.  So let's move on to some of the technical details.

Tech Overview (or really basic things I should have known but didn't)
To understand the most efficient way to get video compatible with the Apple TV, I found it was important to learn about some basics like the ATV's capabilities, the difference between codecs and containers and some of the more common formats for (totally legit) files out there.

Supported Video Formats for the Apple TV 2:  There are basically a few highlights to take away from the spec details.  The only videos ATV can play must be MP4 files (.mp4, m4v, or mov), have to be encoded as h.264 and have an AAC soundtrack with a bit rate no higher than 160kbps.  Alternately, the files can have an AC3 soundtrack ONLY IF your Apple TV is connected to a receiver or TV that can decode that format.  That last part is where I ran into most of my trouble, more on that later.

Ok, let's dissect the supported format details a little bit more to try and get a feel for what we need to look out for.  The first part of the specs are what I tend to refer to as the container, which is probably not technically correct, but it's my blog dammit and I can call it whatever I want.  So by container I'm referring to the actual file type that stores the video and audio information that makes up a video file.  There are a number of containers running around out there in the wild, there are of course the ones directly supported by Apple like mp4, then there are the more popular and unsupported ones that we are mostly interested in like mkv and avi.

As mentioned, these container files contain video and audio streams, in many cases, the video stream included in a container unsupported by Apple TV is actually compatible with the Apple TV.  If a video file includes a stream encoded with h.264 (you may also see it called x264) then assuming it's not encoded at an outrageous bit or frame rate, it will be playable without transcoding into a different format.  If your file uses a different codec like XviD or DivX, you'll have no choice but to re-encode the entire file.

The same thing applies to the audio codec stored in the file, if it's compatible, it won't need to be converted.  The bad news is that I haven't seen a lot of mkv files with stereo AAC.  This particular point is where I ran into issues, especially finding a utility that could do video pass through while re-encoding the audio.  The good news is it takes WAY less time to just transcode audio when converting a video to an mp4.

ProTip: Most video players (like VLC or Quicktime) have an option to bring up information on a particular video which should tell you exactly what codecs are being used as well as bit rates and frame rates.

Get info in Quicktime X (using Perian). We would need to transcode the video in this file since it's DivX.  The audio is compatible, however.

Ok, that's great but...
So let's cover some of the applications and techniques required to actually make this thing work.  There are a bunch of different things you can use, depending on what you need to do and your individual preferences.   Tools that I personally use a lot are Handbrake (for full on encoding), Quicktime X (with Perian installed to allow me to load most types of videos) and finally, MKVtools.  Of the three applications listed here, MKVtools can actually do everything and will be the focus of the rest of this post.

Using the information I had up to this point, I was using QuickTime to simply convert the container to MOV for existing h.264 video by using save as and selecting the movie format.  This resulted in a file that imported into iTunes and played just fine, hooray!  Then I tried to open it up on my Apple TV, it attempted to stream it, but quickly produced the dreaded "An Error Occurred Loading this Content. Try Again Later" message.  Researching the error message, I found this blog post that indicated the ATV might have trouble detecting the AC3 audio stream.  I switched the AC3 setting to on, but alas, my super cheap-o house brand TV couldn't handle an AC3 signal apparently as I continued to get the error message when attempting to play the video.

Create Apple TV magic with MKVtools
Ok, so I need to find a tool to allow me to pass through the video stream while transcoding the audio stream to a compatible AAC format.  I keep a number of tools around, but I found none of them really offered the ability to pass through video, most could pass through audio, but not video.  Some searching revealed that the old version of QuickTime Pro would let you do that when saving as, but that wasn't available to me.  I also have a copy of VisualHub laying around, which lets you use the pass through flags for both video and audio.  For some reason the resulting video files were unplayable in anything.  Since VisualHub has been abandoned for years, I decided to keep looking.

I stumbled across this thread on the Mac Rumors forum that was discussing methods of using a bunch of different command line open source tools to manually extract/convert the video and audio streams and then re-combine (mux) them back into a playable mp4 file.  Too much work for me :)  However, the thread was 18 pages long, and in the latter pages there was mention of a graphical front end for these tools called MKVtools.  

MKVtools is available with an unlimited trial that limits a few minor features (like queuing) but otherwise is fully functional.  The author wants to make sure the application works for you before you pony up the princely sum of... 5 bucks!

MKVtools can take a variety of formats as input and produce avi and most importantly mp4 files as the results.  It can do full encoding of video and audio or selectively let you select either or none for conversion to an mp4 file.  As an additional bonus, you can also keep (or convert to) an AC3 sound track and just add your AAC stereo stream as a secondary stream.  This solution worked for me and allows me 5.1 compatibility with my TiVo that is connected to a proper AV receiver.  As an added bonus you can also work with subtitles, though if you want to hard code a subtitle stream from an mkv file to your mp4, you will have to re-encode your video file.

The main MKVtools window with our sample file loaded

Once your video file is loaded, the main window will show you the specifics of the streams included in your video file.  Note the track info for the video stream.  It is listed as mpeg4.  This is NOT compatible with the ATV, if it were it would be listed as h.264.  Therefore, in this particular example we would need to do a full re-encode, this tool would work, or you could always use your favorite tool to re-encode.

The MP4 section

Here's the mp4 section where we'll be creating our compatible mp4.  We'll ignore the incompatible format for the moment and pretend we have a real h.264 stream.  To convert, check the boxes next to the audio and video streams in the top window.  Then simply click on the pass thru button in the video section, this video already has an aac stream, so we'll also select pass thru for audio and allow the application to just repackage the file as an mp4 file.  If the packaged audio were AC3, you could also leave pass-thru selected and simply check the add 2 Ch. track button, it will leave the existing audio alone and add an AAC secondary track which could be read by the Apple TV.  Finally if you have some other type of 5.1 soundtrack, like DTS, you could select either AAC 2ch or AC3 (5.1) and then check the add 2 Ch. track button.

Click convert and the resulting file will by default be saved to the same location as the original (along with the log file and intermediate audio and video files).  You can change the save location and the retention of the extra files in the preferences.

Thanks for sticking with me this far.  The summary of all this is below if you need a quick reference (or didn't want to read this massive article).

Summary
Ok, so here's the basic steps for conversion happiness in numbered list form.  The moral of the story is if you have a video already encoded in h.264, you want to avoid re-encoding the video if at all possible.

1. Download MKVTools
2. Open your video file in MKVTools and click the MP4 tab
3. Select the video and audio streams check boxes, note the codecs of each
4. If video is listed as h.264, select pass thru in the video section.  Otherwise, select one of the options to re-encode the video (or use your favorite other video encoding tool, you can't pass through the video for this file).
5. If the audio is listed as aac 2ch. no futher steps are required, select pass thru for audio then click convert.
6. If the audio is AC3, click the add 2 Ch. track button then click convert
7. If the audio is something else, select AAC 2ch and click convert, you may be able to convert 5.1 sound to ac3 5.1 but make sure to click the add 2 ch. track option, click convert.
8. import the resulting mp4 file into itunes when it's done.
9. View the file on the Apple TV.