Audio-visual synchroniser for changing slides with taped commentary - uses pause in sound-track to cause slide to change, using switch open in absence of sound

摘要

The synchronisation of a slide projector with a sound track in an audio-visual message is achieved by changing slides whenever there is a sufficiently long pause in the sound track. An amplifier is connected to the signal from the sound track, and operates an electronic switch. In the absence of a sound signal the switch is open, allowing a capacitor to discharge. Unless the switch is closed by a sound level, hence re-charging the capacitor, the potential of the capacitor will fall low enough to trigger a mono-stable. This in turn operates a relay which commands the slide-projector. After a slide-change the switch is kept closed until the sound trck starts again, so that a long pause will not cause several slide changes.