Electrokinetica The Electro-mechanical Museum

The combination action


In performing light music on a cinema organ, an organist must often make registration changes while playing (i.e. turn stops on or off). One or two stops at a time can be dealt with by hand, but a sweeping change involving dozens of stops would be impractical without some form of automation. What the organist needs is a choice of pre-programmed combinations ready to recall automatically with one touch of a control. In classical organs with mechanical action, ‘composition pedals’ were sometimes provided to enable the organist to select predetermined registrations. These worked directly on the drawstops by means of sturdy mechanical linkages that were necessarily large, few in number and cumbersome to set up. Such a mechanical scheme would be quite unsuitable for a theatre organ with hundreds of stops in a horseshoe console. Various forms of power-assisted combination action were devised, including pneumatic, electropneumatic and electric types. Of the two principal cinema-organ suppliers to the UK market, Wurlitzer favoured the electropneumatic variety whilst Compton chose the direct electric, relying exclusively on electromagnets for all motorised functions within the console. Being independent of the wind supply, these consoles were highly mobile and compact, tethered only by a multicore cable to the organ itself. We will look at the Compton system

The controls

Each stop-key is first and foremost a switch that closes an electrical circuit to the main organ relay when the stop is on (down). To enable a combination system to change the position of the stop-key, its mechanism is built as a unitary assembly incorporating two electromagnets, one arranged to tilt the pivoted armature forwards and one to tilt it back, moving the key with it and closing or opening the contacts. Briefly energising either of these magnets will snap the armature into its proper position where the toggle-spring will hold it until changed again or moved by hand. The organist operates the combination action by pressing a button termed a 'piston', either a 'thumb piston' located in the key slip between the manuals (keyboards), or a 'toe piston' mounted over the pedalboard. Naturally each piston is arranged to operate many stops at once. Most pistons (called 'divisional pistons') control all the stops used on a specific manual, along with certain auxiliary stops such as tremulants. Some organs also include 'general pistons' which control every stop on the console.

Double touch

Many theatre organs have pistons arranged for 'double touch' operation. Gentle pressure on a double-touch piston operates the combination for its own division, but pressing it firmly selects a combination for another division also. Normally the manual pistons will select a pedal combination at second touch enabling the organist to select pedal stops that complement those chosen on the manuals, i.e. to provide a suitable bass. It might seem that every double touch divisional thumb-piston must be able to control the entire pedal division (on second touch) as well as its own manual division (on first touch). In practice this is excessive and and would lead to needless duplication of equipment and tedious setting up of identical combinations for many pistons. It is customary instead to allow each thumb-piston's second touch to select one of the pedal division's own piston combinations. Supposing the organist assigns three quiet combinations on the Swell to pistons 1-3; these pistons could all be set to operate pedal combination No.1 at second-touch, on which a correspondingly quiet bass would be configured.

Storing the combinations

The heart of a combination action is the means by which the organist can preset, for each piston, which stops are to be put on and which taken off. In terms familiar to us in the age of digital computers, each stop that a piston can control requires one binary bit of information (on or off) for that piston. Let us consider the number of bits required to define the combinations in a large organ. Suppose the organ has ten divisional pistons on each of four manual divisions, and six toe-pistons for the pedals. If the manual divisions average 40 stops each and the pedal division has 30, with a further six tremulant stops accessible on all manual pistons, there will be (4 x 10 x 46) + (6 x 30) = 2020 bits. There will also be the pedal combination assignments for the second touches (4 x 10 x 6) = 240 bits. Then there might be six general pistons controlling all 196 stops (6 x 196) = 1176 bits. In total, we need 2020 + 240 + 1176 = 3436 bits. Arranging them arbitrarily into familiar 8-bit bytes we need to store 430 bytes. A very large instrument might need yet more, perhaps over half a kilobyte. This data must be preserved when the organ is switched off, yet should be easy to change (preferably by the organist) when preparing to play a particular piece or programme. Compton used two mechanisms for defining the combinations; the setter-board and the capture system.

The setter-board

The setter-board consists of a matrix of changeover switches. Each piston controls one row of switches and each stop responds to one column. A switch at every intersection selects the state of that stop for that piston, there being as many switches as bits of data required to define all the combinations. Each switch can be set to put the stop either 'on' or 'off', although both positions actually complete a circuit. On many setter-boards it is also possible to put the switches in a 'neutral' position where no circuit is made; in this case the piston will always leave the position of that stop unchanged. When the piston is pressed it energises a multi-contact relay, energising the common terminals of all the setter switches in its row. The switches then route the current to either the 'on' or 'off' magnets in the stop-key mechanisms, or neither if a switch is in neutral. It is essential that the piston relay makes an individual contact for every setter switch, so that there is no incidental cross-connection of the various stop-key magnets when the relay is idle (remember that no semiconductor diodes are used!) Advantages of a setterboard include low cost, reliability and availability of a neutral position. The major disadvantage is the time taken to adjust hundreds or thousands of switches. The organist might sit at the console selecting various stops until the desired sound is produced. He or she might then need to leave the bench, go to the back of the console and set the switches from memory, or alternatively call the stops out to an assistant poised at the setter-board. The setter boards of very large organs were often located in the relay room, being too large to fit within the console, hence very awkward to set. Returning to the computer data analogy, as far as the seated organist is concerned the setter board represents EPROM (Eraseable Programmable Read-Only Memory) because he is unable to change it himself; an additional programmer (the assistant!) is required.

The capture system

To overcome the inconvenience of the setter board, the combination capture system was devised to enable the organist to program the pistons directly from the console. By pressing the 'setter piston' and holding it while pressing any combination piston, the current combination of stop-keys is stored to that piston. This requires a memory technology equivalent to EEPROM (Electrically Eraseable Programmable Read-Only Memory) that can be not only 'read out' by pressing the combination piston but also electrically 'written to' when the setter piston is used. Using the capture system, the organist is empowered not only to set up all the combinations simply by registering the organ and pressing a few pistons, but to edit the combinations on-the-fly, by recalling a piston, making any desired changes, then storing the new combination back to the same piston all within a few seconds. But how did an organ-builder implement four kilobits of electromechanical EEPROM in 1936?

Compton's sliding-busbar setter mechanism

The Compton electromechanical setter mechanism stores the state of each bit by engaging a springy wire contact into a notch in one of two busbars. Each wire imitates the common terminal of a setter-board switch, and each notch either the 'on' or 'off' contact likewise. The memory unit is arranged as a matrix with stops occupying the rows and pistons the columns, one wire contact being located at each intersection. To the left of the matrix there is a 'select' electromagnet for every stop, arranged to pull a horizontal busbar assembly leftwards against a spring when energised. Each busbar assembly consists of three layers riveted together; an 'on' busbar with one notch per column, an insulating spine, and an 'off' busbar also with one notch per column. The busbars and insulator are cut away in such a a pattern that when the wire contact is engaged in the 'on' notch, it contacts only the 'on' busbar, and likewise for the 'off' notches. The two busbars are connected, via the return springs, to the 'on' and 'off' magnets of the stop-key units. When the 'select' magnet is on, the 'on' notches lie directly above the wire contacts' free positions; when off, the 'off' notches are positioned likewise. Being springy, the wires follow the movement of the busbars, remaining engaged in their notches unless released by being pulled downwards. Below the matrix there is a 'set' electromagnet for every piston, arranged to pull a vertical insulating tracer downwards against a return spring when energised; its function is to release the wire contacts from the notches in the busbars by gathering them in triangular cutouts in the tracers.

Setting a combination

  1. The organist sets up the registration on the stoptabs as normal
  2. He then presses and holds the setter piston which energises several multi-contact relays
  3. One group of relays isolates the 'recall' electromagnets and prepares circuits for the 'set' magnets instead
  4. Another group of relays completes the circuits of the 'select' magnets, energising all magnets belonging to stops that are currently 'on'
  5. The energised 'select' magnets pull their busbar assemblies to the left; the wire contacts remain in their notches and move with the busbars
  6. The organist presses a combination piston, which energises the corresponding 'set' magnet
  7. The 'set' magnet pulls its tracer down, releasing all the wire contacts for that piston from their notches and holding them in the normal position
  8. These contacts will now be lying beneath either the 'on' or 'off' notches in their busbars as appropriate for the present registration
  9. The organist releases the combination piston, de-energising the 'set' magnet
  10. The 'set' magnet releases the tracer, engaging the wire contacts into the notches, whether 'on' or 'off' depending on the busbar position
  11. The organist releases the 'setter piston' switching off all the relays and magnets

Recalling a combination

On the back of the memory unit is almost a mirror-image of the piston 'set' magnets and vertical tracers, in this case wired to provide the 'recall' function. The rear ends of the wire contacts project through holes in the 'recall' tracers, which although normally (and necessarily) isolated, are brought into contact with a live metal grille when the piston 'recall' magnet is energised. When the organist wishes to recall a combination during performance, the following occurs:

  1. The organist presses the combination piston
  2. Because the 'setter piston' is not already pressed, the combination piston energises the corresponding 'recall' magnet
  3. The piston 'recall' magnet pulls down its tracer and makes all wire contacts for that piston live (one per stop)
  4. The other ends of the wire contacts enliven either the 'on' or 'off' busbar for each stop according to which notch they occupy
  5. The current is delivered to the stop-key magnets which move the stops into the correct combination
  6. The organist releases the piston, switching off all magnets.

We are making a demonstration video of the rare capture system fitted to the Southampton Guildhall organ. Only a few instruments were equipped with this system so we are unlikely ever to own one at Electrokinetica but there will be a surprising twist coming soon: The very same Compton mechanism will be found lurking inside something completely different...

Further reading

  • Whitworth, Reginald. The Electric Organ. London: Musical Opinion, 1930.
  • Bonavia-Hunt, Noel A. The Modern British Organ. London: A. Weekes & Co., 1947.

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