The first mechanical player piano was a device which was placed in front of an ordinary piano, playing it by means of hammers which struck the keys. Later the mechanism was installed inside an enlarged piano cabinet, and the instrument became known as a player piano.
The earliest players were operated mechanically by planchettes, small wooden boards studded with projecting pins which played the notes by means of a system of levers. Some instruments were operated by means of slotted and folded cards, as used today in fairground organs. The slots in fact comprise the program for playing the instrument.
The modern player piano, which reached its peak of popularity in the 1920s, is a pneumatic device that uses a roll of paper with perforations. The roll is 11.75 inches (300 mm) wide and the perforations are spaced nine per inch (25 mm). The spool box, with a tracker bar, is placed in front of the operator, behind the keyboard; the drive mechanism is placed on the right-hand side. Directly underneath these is the value chest, and underneath the keyboard are the suction bellows, or exhausters, operated by pedals. The expression controlling devices are also located here: the sustaining (loud) pedal and the half-blow (soft) pedal, together with any accentuating mechanisms. The pedals of the piano are operated by two large pneumatics activated by perforations in the margin of the roll.
The performer, having placed the roll in the spool box and hitched the leader to the take-up spool, starts pedaling. This exhausts the main pneumatic chest, and the drive motor moves the roll forward. The speed is controlled by the tempo lever on the front of the keyboard. Usually the tempo is printed on the paper: 40 means 4 ft (1220 mm) per minute, and so on. Sometimes a red line is printed on the roll, and speed changes are accomplished by following this line with a pointer attached to the tempo lever. Underneath the right-hand end of the keyboard, a suction governor is connected in a tube from the motor to the exhausters. Moving the tempo lever also moves a slide valve which, in conjunction with a spring-loaded pneumatic coupled to another slide valve in the governor, regulates the suction on the motor, controlling the speed independently of the pedaling.
As the first perforation opens a hole in the tracker bar, air passes in. This lifts a soft leather diaphragm, which in turn lifts a poppet valve which opens a port to the small note pneumatic connecting it to the suction chest. This pneumatic closes quickly and operates the piano hammer in the same manner as if the key were struck. When the perforation has gone past, air is cut off from the diaphragm, which returns to its original position. The poppet valve drops, cutting off suction from the pneumatic, which opens, allowing the hammer to fall back and the note to be damped. Equalization of air pressure takes place through a tiny hole by-passing the diaphragm.
The degree of suction determines the loudness of the music. Expression is achieved mainly by pedaling lightly or heavily but many player pianos have alternate means, making it possible to accentuate chords or single notes; it is in these devices that the various makes of player pianos differ. In the case of the Pianola (trade name of the Aeolian Company), the pneumatics are divided – 44 notes in each half. The loudness of each can be subdued with respect to the other by means of levers operating sliding valves connected to section governors, in an arrangement similar to that controlling the speed of the driving motor. This makes it possible, for example, for the melody (right-hand piano part) to be played more loudly than the accompaniment (left-hand or bass part).