https://rogueamoeba.com/loopback/ - best, but expensive
https://github.com/ExistentialAudio/BlackHole - works, cheap
https://support.shinywhitebox.com/Installing-iShowU-Audio-Capture - another solution
https://vb-audio.com/ - yet another solution
Video input to OBS
Reasoning: on the NINJAM-server the audio is delayed intentionally. The influencing factors are the BPM chosen (by the musicians) and the BPI set (according musical and technical requirements*). So the audio-signal will always be delayed by a calculated, stable interval against the video-signal from a video-conferencing-tool like Zoom, Skype or Facetime, which has smaller but undefined, unmusical and unstable delays.
What we now need to determine is the amount of delay between video and audio to adjust/delay the video until it is synchroneous with the audio out of NINJAM.
Delay is usually expressed in milliseconds. So audio and video could share a common measure like milliseconds.
Delay in milliseconds can usually be easily estimated/determined for the video (by measuring or just reading what the software says).
To get the delay-value of the audio for different delays relating to BPM- and BPI-settings of the server the following calculation is required:
there are 60.000 milliseconds in one minute. If we take 120 beats per minute the calculation is
60.000ms / 120beats = 500ms
now, if a BPI of 4 relates to one musical measure in a 4/4-beat so that there are four beats in one BPI, we need to take the above millisecond value and calculate
4beats x 500ms = 2000ms
That means, if we assume the video has zero delay that we have to shift the video by 2000ms for 120 BPM in a 4 BPI environment to get audio and video syncronised.
*musical and technical requirements: the majority of music is relying on fixed time signatures/measures - 4/4 is an example, 3/4 (waltz) another; technical requirements are delays introduced by encoding and transporting the audio to the server and back from the server to the other musicians.
OBS does allow us to do this easily by inserting a video-delay-filter (example: 72BPM about 3333ms):
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