Loss in quality when merging mp3s?

[SebastianCmentowski]

Hi,
I used bragi-books to merge multiple mp3s to one audiobooks (two iterations, first transforms the mp3s to one single m4b, the second pulls chapterdata from Audible). However, I notice that the audioquality degrades significantly (much more "hissing" in s/z sounds).

Where could this come from?

[djdembeck]

Did you use the Docker image?

[SebastianCmentowski]

Yep. I also pinned the issue down to the mp3->m4b conversion.
Another audiobook in AAC/M4A is fine + the temporary individual m4b files already have the degraded quality.
My current fix is converting the mp3s manually through ffmpeg (which takes much longer but solves the issue).
Afterwards the merging should work with bragibooks

[djdembeck]

Which ffmpeg settings are you using? m4b-tool (what we pass commands to) is just a wrapper for ffmpeg, so I’d be surprised if it’s more than a misconfigured flag on our side.

[SebastianCmentowski]

I'm not using ffmeg very often, so I basically took the first setup I found online:
ffmpeg -i file -c:a aac -vn output