I'm not an expert on this, so correct me if this is wrong.
mp4 is a lossy codec.
m4a is lossless. An m4a file is a container format and can hold a variety of data.
A conversion from mkv (also lossless) to m4a should be a lossless process.
MMH converts mkv to m4a that play back nicely in full Atmos through kodi for me.
Am I missing some intermediate lossy conversion in MMH that takes place?
I hope it doesn't turn out to be too boring or confusing. Let's go...
MP4 file format is a container (MPEG-4). MP4 is not a codec.
As a container it can contain audio, video and other things such as subtitles and still images.
The Audio (or video) can be ‘encoded’ using different codecs. Each different codec can be lossless or lossy.
M4A file extension is normally used for a container MP4 that has a single ‘audio’ thing inside.
A single audio ‘track’. It would be as (MPEG-4 Audio)
You can just rename M4A to MP4 file extension (without any conversion tool) and that can be interesting for some players, such as Oppo that cannot play M4A single audio file, but plays the same file just renamed to MP4.
Normally, MP4 or M4A file containers are used to ‘contain’ tracks with lossy audio codecs, such as DD+ (Dolby Digital Plus) or DD+/JOC for Dolby Atmos lossy.
MKV is another type of container (for audio, video, subtitles, etc.)
MKV container is normally used to contain tracks with lossless codecs such as Dolby TrueHD (with or without Atmos), DTS HD-MA, but could also contain lossy codecs.
It is very instructive to install the
MediaInfo tool in your PC,
to ‘see’ what different video and audio
‘tracks’ a container file MP4,M4A, MKV has inside. You then can see what
‘codec’ is in each ‘audio or video track,
and see if it is lossy or lossless by its name.
Using MMH or other tools you have to differentiate
two types of conversion, if available in the tool:
- Just a ‘repack’ moving the same video/audio tracks from one container to other without changing them. Lossy remains lossy, and lossless remains lossless. Also known as 'remux'.
- Video/audio track conversion. By decoding it and reencoding using a different ‘codec’ you can convert from lossless to lossy, losing quality and reducing file size. Or from lossy to lossless, increasing file size but without increasing the quality.