To start the digital stereo soundtrack in sync with my film, I made 1mm small perforations in the otherwise black optical soundtrack.
Thanks @Remork these bursts of optical sound are detected in Pd in the following way:
-one burst per second means it’s just a splice passing the optical head.
-two bursts per second means two perforations and will start the digital soundtrack.
-three bursts per second means three perforations and will end the digital soundtrack and stop the projector
After a lot of finetuning the threshold times and values, I got this working.
That is until light switches or power sockets were used anywhere else in the house.
Pushing switches or (un)plugging power sockets also generate bursts that give false detections and make start/stop the soundtrack etc.
If those bursts come from the circuitry that receives the optical sound in the projector, I might be able to bypass this by tapping straight in the optical sensor.
Or would those bursts come from the optical sound lamp itself? In this last case, I can forget about getting triggers from optical sound and choose between these options:
-Expensive option: the film normally has a black optical soundtrack, but I can ask the lab to print a 440Hz sine wave at the beginning of the film, and a 1000Hz sine wave at the end. This way the pitch can be detected in Pd to control the projector, room lights and soundtrack.
-Stupid option: use the button that visitors push to activate the soundtrack (after few seconds of silence) and shut the projector down at the end of the soundtrack.
The installation will run for 3 months, and as in this case there’s no feedback from the film to the computer, I have no idea if this stays in sync well enough. (There’s no talking, so it can be out of sync a few seconds.)
-I have also seen a system that has a microswitch with tiny wheel rolling along the film that detects the end of the film because of a small piece of film was cut away there, so the microswitch closes. I’m afraid this is hard to make (at least for me) and will put tension on the film loop.
Sorry if this is too far out. But it would be great to make my 50 year old projector interact with Bela. All advice welcome.