Symptom

Some user reported to us that, when they share the video (mp4 format) from EventX video library to the audience, the audience reported to hearing "hissing sound". However, the sound quality is absolutely normal when the organiser play the video on their side.

Root cause

The sound track of the video is in stereo and contains 2 sound channels, but the sound channels are having opposite phases (ie the two sound channels are out of phase with each other and they are not working or happening together as they should). When the organiser play the video via screen sharing or play from video library, the sound has been offset by browser's sound downmixing.

Solutions

• Simply use video in mono channel for the video output file. On OBS Studio, the video in mono channel can be configured as below
  
• If you would like to use video in stereo channel, make sure the sound channel is in phase.

Check if the sound tracks is out of phase

1. Extract the sound track from the video
   ffmpeg -i path_of_source -vn -c:a mp3 output.mp3
2. Use Audacity to view the left and right sound track
3. Zoom in to the sound track and check if it is out of phase. This is a sample of a out of phase sound track.