[ A regenerative controller is one which over-compensates,
 [ either by design or accidentally.  Richard describes
 [ a regenerative suction regulator which evidently increases
 [ the suction in the action stack as more notes are played.
 [ This behavior is in stark contrast with the Duo-Art stack
 [ regulator which supplies less suction as more notes are
 [ struck together in a chord.  -- Robbie

Dear MMD,  I once owned a coin operated Hupfeld cafe piano which had
an intriguing label on the pump.  It read: "The pump must make 70
revolutions per minute.  More revolutions will cause disturbing noises
and less revolutions will impair the expressions"

The latter comment was interesting because the piano was no more than
an electrically pumped 88-note player with a simple stack regulator and
a pair of theme valves.  The reference to "the expressions", therefore
seemed a little optimistic.  When it was given to me in 1960, the
instrument was untouched internally, the motor having burned out in
the early 1930s.

All it needed was a new motor and for the heavy hoses to be replaced,
whereupon it sprang to life, playing well and, to my astonishment,
producing quite considerable expression.  Doubting my ears, I put a
gauge on and the vacuum was indeed varying between about 8 and 24
inches water column.  This turned out to be due to deliberate
over-compensation in the stack regulator, achieved by using a weak
spring.

The effect is familiar with wind motor governors which, if the spring
is too weak, actually slow down as you pump harder.  Applied to the
stack regulator the effect causes the piano to play louder as the
demand increases.  Therefore, crashing chords really do crash and
single notes play softly.  This is entirely appropriate for the "Bier
Keller" style of music this piano was designed to play.

I still have a roll which came with it which I recently scanned
and then wrote a simple expression program to emulate the effect.
(Hupfeld.mid attached).

To most ears, 1920s German pop was not great music but the result is
interesting, especially when viewed in Cakewalk or a similar program
which shows the note velocities.  It was obviously arranged with the
deliberate intention of creating 'the expressions'.  There is no soft
pedal, all the expression comes from the 'velocity versus demand'
effect described, enhanced only by the theme perforations.

The technique is very clever by virtue of its simplicity,
characteristically Teutonic, but it could easily be missed in
restoration.  I've always wondered if it is widely known about
and used in other instruments ?

Richard Stibbons
Cromer, England

 [ I will place the MIDI files at the MMD Sounds site,
 [ http://mmd.foxtail.com/Sounds/  Another article by Richard
 [ on this topic will appear in MMD tomorrow.  -- Robbie