Cassette recorders like ITT’s SL537assette AV or SL837 AV, used by tape-slide producers. have a four-track head. A pulse circuit is connected to one of these tracks. Pulses recorded on to the tape can be used to actuate a relay to control the slide projector.
The cassette that contains both the sound and the control pulses must be handled carefully and for that reason, it is wise to make a copy of it for normal use. Making such a copy is not so easy, however, because the pulses need to be copied precisely. When two AV recorders are available, making a copy becomes appreciably easier, provided that a small modification can be made to one of them.
The ITT recorders have a remote control input socket that is in parallel with a `to projector’ socket. Both sockets are 6-way DIN types. If only the relay contacts in the recorder are used to send control pulses to the projector, the ‘tact sw’ can be linked to one of the unused pins of the remote control socket (pins 1, 4, 5 and 6).
When the remote control is used, it needs to be ascertained whether there are free pins available. In the prototype, pins 4 and 5 are used—see Fig. 1 a. If you cannot be certain, it is safer to bring out the two relay contacts via a 3.5 mm jack socket. 
Once this work is done, pulses can be recorded on the tape from a second Once pulse output. But first, a short cable has to be made to connect the second recorder (pins 2 and 3 of the ‘to projector’ socket) to the modified socket on the first recorder In the two ITT case shown schematically in Fig. a l b . short Furthermore, for copying second sho record/playback) cable as shown in Fig. lc is needed. This cable els should of d the matched to the input le recorders. with the modified recorder set to record (audio as well as AV), the second recorder plays back the original cassette. The modified recorder will then make a perfect copy of the original tape. The cable connections in Fig. Have I been,? chosen so that two modified recorders can copy back and forth ad infinitum.
