THE MUSIC
In previous centuries to enjoy music either you played an instrument yourself or had enough money to go hear somebody else play at a concert or to hire yourself a musician. The quest was to devise a mechanism that would play music on a musical instrument without any musician. Nothing was mass produced and it was all an extremely expensive and crude affair.
Crude revolving pinned barrel mechanisms triggering notes were perfected by the mid 19th century. This barrel and cylinder technology limited song duration to one revolution of the barrel. By staggering pins and spacing notes you could get 10 or 12 tunes on one barrel and one inventor arranged spiraling music to increase playing duration but the music barrels were costyly and cumbersome.
The system of cheap individual tunes was looked at. Music box manufacturers arranged the music a metal disc which revolved in the music box. Cheaper than a whole barrel, it provided cope for easily changed tunes and an unlimited repertoire. But, the tune was still only one revolution long.
By punching the music from a paper sheet music could be as long as you liked. A robust system of a long strip of card folded up neatly into a portable "book" was utilized based on the principle of the Jacquard weaving loom. The holes in the musical version operated the music mechanism instead of creating a weaving pattern. However, paper wound on a spool was cheap and mass-producable and so became the choice for manufacturers.
THE MECHANISMS
In barrel operated mechanisms the protuding pins pushed the mechanism which played the note. Fine control of musical dynamics wwas impossible.
Music for paper operated systems was unsuitable for mechanical readering systems and so pneumatic reading systems were developed. Using suction or pressure directly through the music to operate the mechanism was unsatisfactory as it required the complete operating pressure to pass through the paper. A low pressure system to read the holes triggering a higher pressure mechanism was thus devised. This valve system was adopted in many variations. Suction power was used in preference to pressure power as the mechanisms were easier to construct.
In any typical player piano when the treadles are operated the hole system is emptied of air. Atmospheric pressure air enters through holes in the music uncovering the reading bar (called the "tracker bar"). This pressure change triggers a switching valve opening an internal channel allowing full suction power to operate a pneumatic motor (like a minature bellows) which collapses pushing upwards and playing the note. Some systems have double sets of switching valves and other variations but the principles are the same.
The suction generated by foot treadles pumping out the mechanism (or electric pumps where fitted) powers all the other units from driving the music over the reading bar and rewinding it to operating the sustain and soft pedals and operating mechanisms to vary the dynamics of the music.
Special systems were introduced where the mechanism contains music roll operated unit for modifying volume etc units and thus can performance which plays back the music correctly as would a real pianist. These are known as "reproducing pianos" and produce performances with high degrees of realism. Famous composers and pianists recorded special rolls for these instruments to reproduce their performances in the days before gramophone technology was developed and was still far too primitive to capture an equivalent recording.
For a more in-depth version than this potted history why not check out the links page on this site and go thru to Ian McLaughlin's great Pianola Page or the Player Piano Group's Page!