Mono tracks, transcoding and AI

[MarmiteSandwich]

I have transcoding and replaygain working with these parameters:-

Code:

/opt/bubbleupnpserver/ffmpeg
convOut=-af volume=replaygain=track
mp4:wav24/-, mp3:wav24/-, alac:wav24/-, aac:wav24/-

This works as intended except for some tracks which were encoded by iTunes into aac at 160kbps mono (see jpg). These are played by my WiiM Ultra in wav24 in stereo, but at double fast time, making them all sound like Micky Mouse on helium gas. I know this is probably related to ffmpeg rather than MinimServer, but maybe someone can suggest a fix, other than re-encoding them all!

Some of these tracks were actually encoded at 128k mono, but I noticed that MinimServer groups them all (160k and 128k) in Audio Data as 128k for some reason, although this is not an issue for me.

Any suggestions welcome.
Marmite.
Edit: jpg attached.

[MarmiteSandwich]

(31-10-2026 10:59)MarmiteSandwich Wrote:  Any suggestions welcome.

Well, I consulted Claude. I hope there are no rules about this with the forum.
This was the answer and it worked immediately:-

Quote:Replace "convOut=-af volume=replaygain=track" with "convOut=-af volume=replaygain=track -ac 2"

Apparently,

Quote:What’s likely happening

Double speed with a pitch about an octave up is the classic signature of mono samples being read as stereo. ffmpeg outputs a 1-channel WAV, but something downstream (BubbleUPnP Server or the WiiM) treats it as 2-channel. It then consumes each pair of consecutive mono samples as one L/R frame, so the audio plays at twice the speed and finishes in half the time. Stereo files never hit this, which fits your symptoms.

Fix to try first: force stereo output

Add -ac 2 to the convOut so ffmpeg upmixes mono to dual-mono:

Hope someone else finds this useful. It does tend to raise interesting questions about how we all use forums and how they will develop in the age of AI.

Anyway, problem solved.
Cheers,
Marmite.
PS this was a free Claude account, not a subscription.