Here, Supercar's Mike Mercury uses an unsuspecting friend to show off the lip-sync mechanism. The fat grey cylinder positioned vertically between the two eyeballs is the solenoid. As current is applied to it through one of the metal strings, it pulls on a small metal thong, thereby opening the puppet's lower lip which is attached to the chin with a small piece of pliable leather and is closed again by means of a spring. [If anyone can tell me who the hapless guy in the chair is, I would very much like to know so I can add that little bit of trivia].
In his New Scientist article of December 1965, John Read wrote: "Our 'actors' cannot talk so we must record the dialogue – usually in a civilised Bostonian accent – before we shoot the film. The sound-track is played back as the film is being shot and in order to synchronise the movement of the mouth with the words, we had to devise a method of electrically connecting the puppets' heads with the tape-deck amplifier and also, of course, a means of ensuring that the right puppet 'speaks'. This was accomplished by assigning each puppet one of four separate channels which were fed back into the lip-sync machine through a neutral line. As the pre-recorded dialogue was sent through one of the four channels, the right actor would speak."
In 2003, the Dagostini publishing company issued a short series of magazines devoted to Thunderbirds. In the 4th issue they published this drawing of the lip-sync mechanism. I found it particularly enlightening so when I could lay hands on a scan I took the opportunity to make a web version of it.
One of my international correspondents :), Israel based Mickey Raphaelovich (an electronics engineer himself), eMailed me the following technical explanation and diagram of the lip-sync box.
"To make their puppets 'talk', John Read and Reg Hill developed an electronic circuit which cleans and amplifies the soundwaves and translates them into DC pulses that drive the solenoid.
The signal, having a value of about 5-25 millivolts, passes the first RC (resistance-capacity) net [A] to filter out any noise so as to leave a clean signal (any noise could translate as a DC pulse and activate the mouth out of turn). After the signal has thus been cleaned, it is fed into an amplifier which boosts it to several volts. It is then directed into another RC net [B] to filter out noise which may have polluted it on its way through the amplifier's different components.
Following this second clean-up, the signal is fed into a diode bridge [C] which turns the signal's negative parts into positive parts. The processed signal is then sent into a resistance-diode net [D] which cuts off the sine wave head and leaves DC pulses. Finally, this pulse is fed into the puppet head through the strings on the side of the head.
Each pulse thus represents a word or a syllable. For example: in the first Thunderbirds episode Trapped in the sky Jeff Tracy says: Why, that's Tin Tin's airplane. When he says the word 'Tin Tin' his mouth opens twice, once for each syllable.
When more than one puppet needs to be hooked up to the lip-sync machine, it is switched in parallel with the others and can be activated using a simple on/off switch."
This modern-day version of the lip-sync machine was used during the puppet demo at the Andersons Are Go! event. One wire connects the tape recorder to the lip-sync box on the right, the two wires coming out of the box ran up to the puppeteers' bridge and were connected to the back headstrings of Penny and Virgil.
During the demo, Sylvia, keeping a close eye on the scripted dialogue, used the row of black switches seen on the lefthand side of the box to switch between channels and thus have either Virgil or 'er Ladyship 'speak' (which, by the way, makes a very loud 'clacking' noise each time the solenoid is activated – a rather strange phenomenon as this is obviously never heard on television).