Just one thing I would like to add on the subject of Music Roll
In the case of the common player piano, The wind motor and transmission
will compensate for roll acceleration to a certain degree due to the
physics of the motor and transmission.
For example: If a long roll is played. Paper builds up on the take-up
spool. As a result, the ratio changes causing the roll to speed up.
But that is not what always happens. As the ratio from the paper buildup
increases, The load on the motor increases also. In players with small
wind motors, It is more noticeable. The motor slows down because it's
As the ratio increases, The load becomes harder and harder to turn.
That coupled with The drag of the upper spool brake, That motor is
working pretty hard near the end of a long roll. In unrestored pianos
that still play, I have even seen the motor stall while playing a long
IMO, As far as the 4% goes, Any "test" will have to be far more
accurate than a metronome how are you sure that the metronome started
at the first chord? The only way I could conceive an accurate
measurement could be made would be to map out the entire roll, allowing
for perforation steps plus the piano would have to run at the _exact_
tempo every single time, That is easier said than done.
Just my two cents