Audio on ProRes encoded QuickTime video stutters on playback
We have discovered that our ProRes/PCM encoded QuickTime files played with VLC exhibit loud audio stuttering distortion, obscuring the underlying signal. The best way to describe it is that it sounds like the clicking sound made by a rotating impact lawn sprinkler, like this one:
I have measured the clicking as occurring at approxmately 5.6 times a second. Our files are encoded at ProRes 422 SQ for video; and PCM S24 BE (in24) for audio. These files playback perfectly with no distortion using the MacOS Apple QuickTime Player.
All our similarly encoded files exhibit this audio distortion on VLC on MacOS 10.12, Windows 7 and 10 desktops, and Linux Debian. So it seems that VLC is the issue and not the specific platform it runs on. One theory is that there is a bug in the audio demuxer.
I can't attach a file because of the size restriction on uploading (a 1 second clip would be 18 MB), but a representative 14 second clip can be ordered for ftp delivery at https://www.nodc.noaa.gov/oer/video/?oerkey=fileIdentifier:EX1402L3_DIVE01_20140412T133700Z_ROVHD_AUDIO.
On MacOS, I have tested with VLC 2.2.3 and 2.2.4 with the same audio stuttering. As a test, I downgraded the Mac's version of VLC to v2.0.0, supposedly the first version to support ProRes 422. Interestingly the there was a different audio error in v2.0.0- total silence. (Also, the video playback was poor, with a sort of rolling graininess throughout.)
This clicking distortion is not a big issue for our Mac users, since the Apple QuickTime Player handles ProRes files with no problem. But our Windows users can only use VLC to play ProRes-encoded video, and as fas as I know the only free software that can do that, now that QuickTime Player has been deprecated on Windows, is VLC. I believe that an earlier version of VLC used to handle these files - probably between v.2.0.0 and 2.2.3 based on my testing.
We are a scientific archive among whose customers include scientists and filmmakers who depend on high resolution video for species identification or broadcast, so we hope a fix can be found soon.
Thanks for continuing to support this wonderful and essential software.