Create mkv with no or blank video

[Halloway]

I'm experimenting with multichannel audio formats for playback on media players. One thing I'd like to try is creating a mkv file with no video and a m/c audio track. Is there a way I can create a mkv without a video track? If not, how might I create a blank or black mp4 video of the appropriate length that I could then mux with the audio?

[Ai Haibara]

There is an audio-only Matroska container spec (.mka), though I haven't really looked into how to create them, and I'm not sure how well they're supported by hardware players.

From the matroska.org FAQ:

Q: What file extensions does Matroska use?
A: We currently have 4 different extensions specified :

• .mkv : Used for Video files, as well those containing audio (movies) or video only
• .mka : Used for audio only files, can contain any supported audio compresion format, such as MP2, MP3, Vorbis, AAC, AC3, DTS, or PCM
• .mk3d : For files with stereoscopic (3D) video
• .mks :Used for Subtitles that an 'elementary' matroska stream

Q: What is the advantage of using the .mka file instead of the original audio formats, like mp2, mp3 etc?
A: You can embed lyrics or transcriptions (e.g. from srt subtitles) in the audio file. You can use chapters to separate parts of a track or a live album. In some case (MP3, AC3, DTS) the Matroska file may also be smaller than the original with much better/cleaner seeking support.

Here are some reasons that placing audio in MKA is useful:
1. The tags will be the same no matter what audio format you use. That means that if you write a program to read back tags, it only has to read them from one type of tagging system, no matter what type of audio is being used.
2. All tracks to a CD can be in a single file. You have the option of dividing the tracks into seperate Tracks, or seperate Chapters. You could make your own compilation in a single file, even using different audio formats, such as MP3 and Vorbis.
3. If you write a program to read audio of of MKA, then you don't need to understand how the framing works in the different formats because it is already done for you in Matroska.
4. It is easy to delete portions of the audio without reencoding because you just throw away those blocks. You don't even have to be able to play that format back, you could edit by just knowing timecodes.
5. Detecting differences between two audio streams would be easy because you could store both in a single file, start playback, and then just switch between tracks.
6. If you intend to combine the audio with video, then having is in MKA means you can merge it with an MKV, even if the application doesn't support the audio type.
7. In the case of MP3, MP2 AC3 and even some AAC, using "compressed headers" the MKA file may even be smaller than the original "raw" file, without losing any bit of information.

[Halloway]

Thanks for the tip. It seems that mkvmerge GUI supports .mka 'out of the box'. Simply add the audio files and it automatically chooses the .mka format. Surprisingly, my WD TV Live player also supports the format!