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The Player Piano Mechanism

Materials

The brown and yellow parts are wood. Where wood grain is shown the part is sectioned (effectively sawn in two) for purposes of illustration. The sectioned parts extend the length of the valve chest (2).

All grey or black  items are steel or plastic except for the tracker bar which is brass.

The pouch (2-D) is made of leather.

The pneumatic tubes (1-F) are made of rubber.

The pneumatic cloth (3-B) is a rubberized air-tight fabric.

A piano roll (1-A) consists of a perforated paper scroll wound upon a plastic spool and is 11.5 inches (292 millimetres) wide.

Notes

The animated diagram is not necessarily true to scale.

Each  tracker duct (1-G) terminates as a hole in the tracker bar (1-D) and there is a hole for every key in the piano - eighty eight notes.

The pneumatic tubes (1-F - one for every key) are flexible and do not go around sharp bends as shown.

There is actually more than one valve chest. The valve chests are located one above the other in a staggered (offset) arrangement. This allows each pneumatic and associated valve greater width than would otherwise be the case.

Pneumatics and the player piano

There are very many examples of machinery where power is transmitted by air pressure - either above or below atmospheric pressure (suction). The air pressure in suction-assisted machines is only 5 percent or less below atmospheric pressure. The milking machine provides a good example of a suction-assisted machine as does the player piano. 

With the player piano suction is typically produced by bellows driven by two foot pedals although in up-market player pianos an electrically driven air pump is used. Where foot pedals are used, the suction is evened out by means of a large collapsing storage bellows that is equipped with a spring that pushes it open. The spring tension maintains constant suction.

Operation of the player piano player mechanism

In an upright piano the parts of the player mechanism accessible by the user are shown in (1) above. The piano roll (1-A) is placed by the user in the position shown and the paper scroll (1-C) is threaded over the tracker bar and is hooked onto the take-up spool (1-C).

A suction-driven motor drives the take-up spool at a speed that is controllable by the user via a lever under the piano's keyboard. Tension between the piano roll and the take-up spool keeps the scroll paper held tight against the tracker bar thus sealing off all of the tracker ducts (1-G).

The following describes what happens when a hole (1-E) in the scroll paper moves over a tracker duct (1-G).

No Hole

In the normal situation the hole in the tracker bar (1-D) is blocked by the paper in the roll and the valve (2-E), pouch (2-D), and pneumatic (3-A) are in the at-rest state.

Hole Appears

When a hole in the paper moves over a tracker duct (1-G), a connection is established via the pneumatic tube (1-F) whereby air from the atmosphere can flow into the lower chamber (2-I). The cross-sectional area of the tracker duct is greater than that of the bleed (2-F). This means that  the air flowing into the lower chamber (2-I) via the open tracker duct overwhelms the air flowing out through the bleed. This results in the air pressure in the lower chamber being higher than the pressure in the middle chamber (2-H). This, in turn, causes the pouch (2-D) to expand upwards and push the valve (2-E) upwards as well.

When the valve (2-E) reaches its uppermost position it connects the pneumatic (3-A) to the middle chamber (2-H) and the air pressure in the pneumatic then drops. The result is that the pneumatic collapses under atmospheric pressure and forces the push-rod up, causing the piano action to operate.

Hole Disappears

When the roll paper (1-C) moves on and the hole in the paper (1-E) is no longer over the tracker duct (1-G), the opening to the atmosphere is blocked. This means that the only opening to the lower chamber is via the bleed. The result of this is that the pressure in the lower chamber equalizes with the pressure in the middle chamber (2-H). When this happens, the valve (2-E) drops back down to its normal position and the passage from the pneumatic to the atmosphere is re-established. As a result, the pneumatic (3-A), no longer under atmospheric pressure, re-opens and the push-rod (3-C) drops causing the piano action to return to its normal state. 

A point to be made regarding the underlying principle of the player piano mechanism is that a weak signal  (appearance of a hole over a tracker duct) is amplified with the result that a note is struck by the piano action. This principle is applied through the use of  the clever pneumatic circuitry described above. A modern-day equivalent might be the touch-sensitive electric lamp.

Extensions to the basic player piano mechanism

Development of the player piano extended from the late 1890s into the 1930s.

With the basic player piano loudness and rate of play have to be controlled by the user through the use of control levers. Instructions for operating these levers are printed on the piano roll. The master copies of basic piano rolls are cut using special machines by operators reading sheet music.  

An important development of the player piano was the reproducing piano. A piano roll for a reproducing piano has several extra tracks in addition to the usual eighty eight - one for each piano key. These additional tracks contain control information that regulate the degree of suction in the valve chest at the times notes are played. This means that, at the time a reproducing piano roll is played, expression control is automatic and not manual as is the case with the ordinary player piano.

The master copy of a reproducing piano roll is cut on a specially adapted piano being played by a human pianist. Many famous pianists of the early 20th century made recordings for the reproducing piano - many years before electronic recording. When played on a reproducing piano such rolls yield a quality of playing indistinguishable from that of the pianists as they made the original recordings.

After the era of the player piano

In its day player and reproducing pianos were very popular and appeared in parlours and sitting rooms all over the world. They were, of course, driven out by acoustic then electronic recording. However, they have not completely disappeared - there are many in museums and there are many enthusiasts who restore and maintain them. One can still obtain player piano rolls - there are plenty on eBay. There have also been many electronic recordings made of music played on player pianos written or even played by early 20th century artists such as Scott Joplin. 

An interesting development in recent years is the development of optical scanners for player piano rolls that can generate MIDI files that can be played on computers and electronic instruments.