In response to Bill Finch's posting in 021117 MMDigest.  Bill, I have
been a part of development on a player system you described in your
article.  The development company, Overture Music Systems, has two
patents concerning this technology.  The basis of the technology has
it roots in the work done by Charles Stoddard with the Spark-O-Matic.
I will try to describe the theory in short form.

MIDI is the code language used to control the solenoid that plays the
note.  The Boesendorfer SE used a higher resolution proprietary format.

MIDI music is base of today's music libraries.  MIDI has a resolution
using velocities.  Velocity is a unit of speed not related to
acoustical conditions in pianos.  The relationship of volume to MIDI
velocities has not been set to a standard.  Volume or decibel (dB)
levels very between manufactures.  This is why MIDI files vary in the
playback.  The MIDI format is mainly used in electronic music (e.g.,
synth keyboards).  The use of this format is widely supported by music
editing programs which is the reason the player piano companies use
this format.

At Overture Music Systems we set dB level (in a acoustic piano) to
written music notation.  We found most common dB levels used by MIDI
keyboards from different manufacture were in the range of 30-90 MIDI
velocity units, in a decibel relationship to the acoustic piano.

We monitor the key motion with a magnet mounted on the end of the key.
This is monitored by a Hall-effect sensor.  We have set the key speed
to a dB level and a MIDI velocity to the key speed. In general, the
middle of the MIDI velocity range came in at a key event that took
100 ms to complete.

We use pulse width modulation (PWM) at 38 kHz with 40 VDC to drive the
solenoid.  We use a digital signal processor (DSP) to monitor the key
speed and will increase or decrease the PWM during playback to maintain
the key speed.  Creating the lookup tables for the MIDI velocities has
been a time-consuming effort, as the piano action has inertia values
that change as the key speed increases.  Our system is coming to the
market soon and we feel it will bring new experience to player pianos.

Don Dusenbury

 [ Can the Overture solenoid player system accommodate friction changes
 [ in the piano action, Don?  The Ampico B player had an adjustment for
 [ this purpose that was pretty effective.  -- Robbie