2.0.3 - 10.8.4 MS ADPCM Conversion causing major distortion

I can’t hear what I would call crackles or “major distortion”. I assume you are using headphones? If I use headphones I can hear more clearly what I hear in HiFi speakers when I select 8.5s to 10s in the track. There are several “soft whooshes” after the piano note (or whatever it is) and these are more noticeable listening to the Audacity file. In fact if you set the upper limit of Spectrograms in Preferences to 20000 Hz you can see several spikes between 8.5s and 9.5s that are not in the Twisted file.

Also we didn’t have the original PCM file for comparison so it’s harder to guess what we are listening for.

Thanks to Steve.

Gale

The issue can be reproduced:

  1. Generate 30 seconds of Brownian noise, amplitude 0.8.
  2. Apply “Fade Out” to the noise.
  3. Export as MS ADPCM
  4. Import the file and change the track to Spectrogram view.
  5. Zoom out vertically (if necessary) to 22 kHz range.

Vertical bars can be seen in the spectrum. When played, the bars sound like short quiet bursts of white noise. These “bars” are not present in the original track.

It’s certainly not conclusive, but I came across a reference so some applications that require ADPCM files to have a very small “frame size”. Apparently this avoids some technical difficulties when streaming packets, but has the down side that it can cause glitching. It may therefore be that these glitches are due to libsndfile maintaining compatibility at the expense of a little sound quality, but given that ADPCM is low quality anyway I doubt that they considered these minor glitches to be a significant problem. Alternatively it may be a minor bug in libsndfile, but I doubt that they will be spending much time worrying about it.


We have some example batch processing scripts on the forum - I’ll see if I can find them.

There’s a topic here that looks like it should be useful. In particular, see this post: https://forum.audacityteam.org/t/draft-howto-convert-audio-file-formats/17462/17 (and the responses that follow.

There’s also a more complex script that can work recursively through folders and create an output that mirrors the folder structure Batch Scripts with other programs - #6 by steve

I exported as MS ADPCM and noticed some distortion. However using ffmpeg it was fine. Also audacity IMA ADPCM seems to work fine