variometer tuner (2)

Automatic slide changers are extremely useful when a recorded commentary is used to accompany a programme of slides. Most auto slide changers make use of a control signal. This may be a high frequency low level signal, recorded on the same track as the commentary, which is unobtrusive but which may be extracted for control purposes by a filter. Alternatively, an audio tone may be recorded on a parallel track. The slide changer described here uses neither of these methods but senses the drop in signal level of the commentary when the commentator stops speaking for more than one second.