First post, please help...

[Faithwyn]

When converting from truehd to ac3 for example, does this reduce the picture quality of the 4K film ?

And, IS it even possible to download a 4K MKV file, which is in truehd, and convert just the audio to ac3 ?
thanks

Yes, it takes a few steps, but doesn't take long.
Use the above shot of MKVtools as a reference.
Add your '4Kmovie.mkv' to the top box. In the bottem box uncheck everything BUT the audio stream.
Click 'Start Multipexing', that will create a file: '4Kmovie.mka' which is JUST audio.
Then use a app the change the format/codec, for my test I used 'Format Factory' to convert a
DTS-MA audio file (that my old TV can't handle) to a 2 channel AAC file.
Now go back to MKVtools (which I left open) and add your new 2 chan. file to the top box.
In the bottom box check everything BUT the old audio stream, hit multiplex again.
Now, you have a file with 4K video and a stereo Audio.
If you decide to leave the TrueHD stream checked make sure the stereo track is set to 'Default'
as you can have as many as 30-32 Audio tracks in a .MKV file.
My older TV will NOT let me select audio or subtitle tracks and will only play the default tracks.
Make sense?

[venomx]

When converting from truehd to ac3 for example, does this reduce the picture quality of the 4K film ?

And, IS it even possible to download a 4K MKV file, which is in truehd, and convert just the audio to ac3 ?
thanks

Yes, it takes a few steps, but doesn't take long.
Use the above shot of MKVtools as a reference.
Add your '4Kmovie.mkv' to the top box. In the bottem box uncheck everything BUT the audio stream.
Click 'Start Multipexing', that will create a file: '4Kmovie.mka' which is JUST audio.
Then use a app the change the format/codec, for my test I used 'Format Factory' to convert a
DTS-MA audio file (that my old TV can't handle) to a 2 channel AAC file.
Now go back to MKVtools (which I left open) and add your new 2 chan. file to the top box.
In the bottom box check everything BUT the old audio stream, hit multiplex again.
Now, you have a file with 4K video and a stereo Audio.
If you decide to leave the TrueHD stream checked make sure the stereo track is set to 'Default'
as you can have as many as 30-32 Audio tracks in a .MKV file.
My older TV will NOT let me select audio or subtitle tracks and will only play the default tracks.
Make sense?

I muxed the audio stream with mkvtools.
Installed format factory, and chose to convert the MKA file to mp3

Will update how i get on

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In MKV Tool, once you add the mkv and the new audio file which one do you choose ?

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I did all of that, it plays audio now but its way out of sync

[Faithwyn]

I never have to mess with those settings you pasted above.
My 'Test.MKA' file which I converted to .AAC came out the same length and synced perfectly.
IF the audio timing is off equally throughout the film there is the 'Delay' option in the right column.
It's in milliseconds , so a value of 1000 would delay the audio by 1 sec.
-1000 would advance the audio by 1 sec. -750 = 3/4 sec, and so on.
I have no idea why yours is not synced since both tracks come from the same source.
I only use the delay feature when Muxing streams from 2 different sources and when
I do, it's kinda trial & error to get the timing JUST right!

I have found that trial & error IS a good way to learn these tools when we first start messing with video editing.

[venomx]

I tried again and it's working ! I had to convert the audio to a " high quality MP3 " Stream

Thanks so much man

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That tutorial should be stickied in my opinion, it has really helped me out

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Just one question, does doing this affect the picture quality at all ?

I'm asking because the data rate is identical from the original and finished copy, but the total bitrate is lower