Appending stereo AC3 to surround AC3 in mkvmerge

[onesloth]

I'm trying to append one video, with a 2ch AC3 audio track, to another, with a 5.1ch AC3 audio track, so that the files play sequentially without having to select a different audio track during playback when the second video starts.

Mkvmerge won't append the tracks because the track parameters do not match. Is there a way to append the tracks as they are, or do I have to upmix the stereo track?

If upmix is required can anyone recommend the best method to do so? I use BeSweet and Megui typically. I'm using AC3 audio to preserve the quality and mix levels of the original audio so a method that results in the upmixed stereo channels being played back identically to the original stereo channel is what I am looking for. Maybe a method that just inserts dummy channels to make it 5.1?

[onesloth]

Anybody got any ideas...

[TheFluff]

Appending two tracks with different settings (like different amount of channels, different samplerates, different resolutions etc) is not possible, and for good reasons.

You have to upmix the stereo track, or downmix the surround track. IIRC AC3 isn't too picky about what you can and cannot append so it should work as long as the number of channels, samplerate and bitrate are all the same in both tracks.

[multicone]

If the lazy matroska people would finally implement their long time planned 'codec state' element in the middle of the streams, this could work. This would also allow to change pixel resolution in the middle of a video stream, etc.