An audio visual slide presentation can be most effective and yet it need not necessarily be expensive.

The term audio/visual or A/V has often been touched upon in these pages, but what does audio/visual actually mean, and how does it fit in with the purely audio techniques with which we are familiar? A wide range of communication aids is covered by the term in question, from a live human being equipped with script, mouth and flipchart to vast mixed-media presentations involving video, film and slides, all running in synchronisation and triggered by the push of a 'Go' button.

The majority of audio visual presentations are produced as educational or training aids and their purpose is to motivate an audience towards whatever end the designer has been commissioned to achieve. The '1984' advocates amongst us will also have spotted the interrogation and brainwashing possibilities of A/V, and this may not be as farfetched as might think. I have seen some pretty powerful presentations!

The purpose of this piece will be to deal principally with tape/slide presentations. A slide system of the type you may already have seen would consist of a number of slide projectors which operate in synchronisation with a soundtrack on tape. First of all though, it's worth popping back down the line a bit to trace the development of the tape/slide technique of A/V presentations.

In The Beginning

The simplest of presentations may not involve the use of any sound equipment as such. A good presenter armed only with a chalk board or flipchart, may be able to put across an utterly riveting talk, but not all of us are blessed with an overpowering charisma and a good speaking voice. This is where our familiar friend magnetic tape comes in.

Although some spontaneity must be lost through the use of a pre-recorded commentary, it has a number of distinct advantages. Most important of all is the voice artists themselves, a good one will manage to hold the attention of an audience even through a dull script. It should also be possible for a professional voice artist to read a script on 'The Decibel' or some other technical topic and make it sound as though he knows what he is talking about! Other advantages of a recorded commentary include freedom from accidental errors and omissions, and consistency where a presentation is to be repeated.

Once we have a clear and authoritative commentary on tape, our presenter with his flip chart or chalk board is likely to feel a little uneasy standing in front of an audience simply turning pages. The next stage, therefore, may be to substitute the flip chart or chalk board for an overhead projector or slides and satisfy the visual aspect in this way and both OHPs and slides allow the presenter to operate from a more comfortable position. Slides and OHPs can also be prepared in advance of the presentation, thus allowing for better visuals. This approach also cuts down the risk of errors.

Standard 35mm slides are capable of a very high quality image and when coupled with a good soundtrack very impressive results are possible. Our presenter, however, who started out using his own voice and artistic ability, is now reduced to simply pressing a button to change slides at the appropriate points during the soundtrack. Problems can occur here if the presenter momentarily nods off and wakes up to find he has missed a cue and that his slide changing finger is no longer in sync with his ears.

Autopresent

The next logical step forward is to control the slide changes from signals recorded onto the soundtrack. The presentation can now proceed completely automatically from the moment the Go button is pressed. Apart from the obvious advantage of no longer requiring a tame insomniac to present the show, the slides can be guaranteed to change, not only at the correct times, but also at the precise points at which they will create the most impact.

Now that the presentation is automatic it can be used in a variety of circumstances where the audience is not necessarily captive. Continuous running or viewer initiated shows are very popular at exhibitions and in business foyers. They are a slick and efficient method of making a point as quickly as possible and with maximum impact. However, in terms of tape/slide shows in general, these single projector presentations are small potatoes in comparison to the multiscreen, surround sound extravaganzas used by some companies to promote themselves or their products. The screens might be any size and shape and are often arranged to form great pyramids and panoramas. Each screen will normally have two or three projectors devoted to it and the sound will be courtesy of a sophisticated PA system running directly from a multitrack tape machine. The use of a multitrack enables effects to emanate from any position in which a speaker can be placed.

Pulsing

One aspect common to all tape/slide presentations, from single projector shows up is that the tape machine is being used for one extra job, besides providing the sound. The tape is also being used to store all the information needed to control the projectors, and the amount of control the tape can have over the projectors depends on the method of programming being used. The simplest method of programming is known as pulsing. These pulses are short bursts of a 1kHz tone which, when played via a decoder to the projector, trigger it to change or 'step' onto the next slide. A simple single projector presentation would be set up and pulsed as follows.

The slides are first arranged in order in which they are to be projected. (This step really does help.) The soundtrack consisting of the commentary, backing music and any sound effects should be ready on the tape machine and the pulsing unit is connected to both the projector and the tape machine as shown in Figure 1. At this point the system is ready for the control pulses to be recorded onto the control track on the tape. Of course, a spare track is needed for this information and this track must be capable of being recorded onto without interfering with the soundtrack. For this reason mono tape recorders are of no use for this type of A/V work, and stereo machines can only be used if each of their tracks is capable of being recorded onto independently and monitored in synchronisation. The advent of the 4-track simul-sync (Teac 3340S) proved most timely and it was rapidly adopted by the A/V industry (as well as starting the home recording revolution). Its successors have now become standards amongst the A/V fraternity. There are also specialised compact cassette based recorders available for this type of application. Often they use standard 4-track heads with tracks 1 and 2 being used for stereo sound. Track 3 is ignored and track 4 can be recorded onto independently to take the control signals.

Pulsing the tape is a very straightforward task. It simply involves setting the tape recorder to record on the control track whilst monitoring the soundtrack. Each time the next slide is required a button on the pulsing unit is pressed and a 1kHz tone is recorded. On playback, the pulsing unit takes on the role of decoder and listens for the 1kHz pulses, converting them into a signal which it sends to the projector telling it to move onto the next slide.

Dissolving

One step up from here is the use of two projectors. This has the great advantage visually of always allowing one projector to light the screen while the other is stepping. It also makes a range of visual effects possible, such as dissolves and superimposition. This step up also necessitates the introduction of another piece of equipment: the dissolve unit as shown in Figure 2. This controls the rate at which the lamp in one projector fades up as the other fades down thus producing the visual equivalent of a cross-fade. These can be made to occur at different rates (generally between about 2.5 and 12 seconds) but for each different rate of dissolve, a different pulse is required, either in terms of frequency or duration or sometimes as combinations of both. Additionally for any other effects such as superimposition or flashing, more sophisticated pulse systems are required. This system of using pulses works well until we want to introduce more projectors. Naturally, of course, one tends to move on from two to three projectors and this opens up the visual scope even more. Control-wise however, a new problem occurs. With two projectors dissolving from one to the other the commands can simply apply to each projector alternately. With three projectors, however, we may require one to be singled out to do something fancy whilst the other two carry on alternating. In order to achieve this a different system of control is required.

Multiplexing

Multiplexed encoding allows for many commands to be given in a short space of time. It also allows a great deal more information to be stored on the control track of the tape. With multiplexing, the control signal from the tape goes via a demodulator to all the projectors. Each of these carries its own individual control unit (see Figure 3) which can be addressed independently of the others. This method of encoding is powerful enough to run scores of projectors and the commands can still be stored on a single track of audio tape.

Yet another method of control is continuous tone programming. This can only be used to dissolve between two projectors and consists of a sliding tone which at each end of its range has one projector fully lit and the other dark. With the sliding tone at the midway point each projector will be at half brightness. Both pulsed and multiplexed programming have an advantage over the continuous tone method in that they are easier to edit on tape. Commands are only present on the tape when something has to happen and between the commands are periods of silence. This makes it quite easy to drop in commands when recording the control track. In this way it's possible to correct misplaced or incorrect commands without necessarily re-programming the whole presentation. With continuous tone programming it is necessary to edit at two points where the frequencies match and this is not so easy.

The reliability of controlling presentations from signals stored on tape is very good. In the past, drop-outs on tape would cause enormous problems. One missed command and a whole show could suddenly become nonsense. Trying to catch up by manually advancing the projectors is not as easy as it may sound. It's not always easy to tell how many commands may have been missed and trying to advance the projectors whilst commands are still coming through is about as much fun as eating worms. Using present day recording tape and equipment it would be extremely unfortunate to encounter a drop-out of sufficient length to disturb a pulsed programme as most of the tones are of almost half a second duration. With multiplexed programming, however, as was mentioned earlier, a lot of information is stored over a short length of tape. The possibility of a drop-out causing a mouthful of worms here is countered by the fact that because multiplexing is so fast there is time for each command to be repeated three or four times. The decoder compares adjacent commands and in true democratic style will go with the majority decision, rendering the system highly reliable.

It is not always the controlling data itself which goes onto tape to operate the projectors. Often a time code will be recorded which is fed to a computer on playback. The computer is programmed in A/V language and the data stored in the memory is retrieved in synchronisation as the time code dictates. In this way the information that actually controls the projectors is stored in computer memory and the information on tape is being used to trigger it. This has many advantages and the programme can be modified without having to re-record any of the sync information on tape. The complete control sequence can then be stored on cassette or floppy disk for further use.

Quality

Tape/slide is a very popular method of A/V presentation. One obvious reason for this is the very high quality of both picture and sound that are possible. 35mm slides offer much higher definition than either 16mm film or video and the sound for tape/slide can come from as high a quality recorder and sound system as is available. Couple with this the fact that the production costs for tape/slide presentations are far below those for film and video and its attractions become apparent.

This piece has dealt briefly with the development of the art of tape/slide presentation. It's afield in which the creative possibilities are enormous and in a future issue we hope to cover the production of a soundtrack for tape/slide use, so borrow a projector, dig out your old holiday slides and we will see if we can't knock up a bit of low budget Rodgers and Hammerstein.