There will be many people who have never had the pleasure of viewing a slide tape show and when viewed on a ten foot screen with Hi Fi sound in a darkened room or hall it can have a tremendous impact.
In this age of DVD, Digital TV and Computer Technology slide tape may seem to be old fashioned but it has certain qualities of communication that are very rewarding.

Over on the right are two versions of the Phillips synchronizer for use with reel-to reel tape decks to put a pulse on the spare track.

One of the commercial dissolve units was the Duofade (seen on the left is the second one produced- still working) This was a manual device which consisted of two diaphragms which were individually controlled via cables. The remote control from each of the projectors are placed near the centre control so that the slide can be changed quickly after the diaphragm for that projector had been closed. With skill it was possible to achieve both beautifully smooth dissolves but also quick changes with individual control of each projector. There was one snag - you really had to know your sequence. You could also use the 1,000 cycle pulse as a guide but concentration through the sequence was paramount especially if the sequence of images was long.

One of the first amateur automatic dissolve devices was the Animatic Convar (pictured right) . It had limitations and there were incompatibilities between different Convars. The signal on tape from one Convar might not play back correctly using a different Convar. However it was a start and one could fix different plugs to suit particular projectors.

One of the early pro units was the Staysatom (shown left).You set the rate of dissolve by turning the dials (left on the unit) but could not change them during a sequence. This was the major disadvantage. It was pretty reliable and mine was still working after many years in the loft when last tried.

Projectors have changed over the years and there were basically three types - the Carousel with its round tray holding 80 slides , the Lietz, Agfa type with the pushpull magazine ( pictured right) and the combined unit such as the Rollei or Royale
Access to the control unit is usually via a socket on the side of the projector

A semi professional unit for two projectors was the electrosoinc ES69. It was primarily used for industry and education as was its larger brother unit ES3609 which also had a tape recorder and loudspeaker. They were very reliable and I still have a working model.

Some people call it AV- Audio Visual ,others slide -tape which is probably more accurate as AV also covers film TV etc. This type of communication was originally conceived during the Victorian era and I have an old projector which takes old glass slides - see separate page. It was revived by the French in the fifties and appeared in the UK in the mid sixties. I remember being hooked after seeing a show in Manchester the mid sixties which showed two and a quarter slides with music.
My first method of syncing slides with a sound track way back in the sixties was to use a device that put a 1000 cycle pulse on one track of the tape. On playback one could either set the projector to change on reaching the pulse or one could listen for the pulse and do the change manually.
I later made a home made 'dissolve device made out of cardboard and bits of elastic band ! It didn't last for too many performances. The first show I did was in March 1969 at my camera club in Chorlton, Manchester .It was a combined effort with Eddie Roper. We only had one Letiz projector each !
At an Audio Visual convention at Avon Tyrell in 1972 John Dawkins showed an unusual method of dissolving using crossed polarisers but one of the best devices pioneered by Sir George Pollock was the Duofade (more below)

A rear projection system was used which does require a reasonable working distance between the screen and projection system so most shows took place in theaters and halls that had a fair sized stage.

The brainchild of John Sandell (http://www.diafade.co.uk) The Diafade unit (above left) came in two versions, one for two projectors and one for four projectors. An extension to the 4 projector version was the Dolphin (above right) which was an automatic programming unit. I used them in many 8 projector shows for Nikon using a 2 Diafades An earlier setup can be seen below right A four track tape deck was used for the sound and while the Diafade controlled the centre 4, a pair of ES69's controlled the outer 4.

A few years ago a typical two projector amateur setup might look like the one shown on the left.

Nowadays the reel to reel tape recorder has been taken over by cassette machines.