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MMD > Archives > November 1998 > 1998.11.05 > 17Prev  Next

Motor Speed Regulation & Music Roll Acceleration
By Robbie Rhodes

Following Craig Brougher's article in MMDigest 981104 I commented:

>[ Some pop songs bother me when the music accelerates, and some
>[ songs don't.  Both my Ampico B and the Duo-Art pumper have excellent
>[ motor-speed regulation, and the effect is the same with either piano
>[ -- I find that I slowly push the Tempo down with my thumb or finger
>[ so that the music doesn't accelerate.  A 3-minute pop music roll
>[ which begins at Tempo 100 is slowly decelerated, ending at Tempo 95,
>[ for example.  Some songs bug me, some don't.  -- Robbie

Craig wrote to me later with these questions and observations:

> If both the Ampico B and Duo-Art have excellent regulation, then
> why slowly push the tempo lever down as it plays?  When you say
> "a three-minute pop song" are you talking about an 88 note roll
> or an Ampico roll?  There is no way an Ampico B can give you an
> acceleration in the music in only 30 feet of paper, so I'm
> wondering what you're speaking of?  On the other hand, 88-note
> rolls all speed up toward the end.

I apologize that I seemed to confuse different aspects of
"acceleration", so let me define these terms:

   Music Roll Acceleration -- the paper velocity gradually increases
as the roll is played, because the take-up spool diameter increases
although its axle rpm (revolutions per minute) is more-or-less

   Music Acceleration -- the musical beats per minute increases as
the music is played.  Most popular music -- fox-trots, waltzes, marches,
etc. -- is performed with constant beats per minute.  In fact, a
pianist or conductor is ashamed if the song speeds up during
performance !

   Motor Speed Regulation -- Ideally the motor governor holds the note
sheet (spool drive) motor speed constant under any load condition.  In
reality, the motor speed always decreases to some extent as the load
(torque demand) increases.

Now to answer Craig's questions.  The note sheet motor of the Ampico B
is tightly controlled by a fly-weight governor; it has excellent regu-
lation and the take-up axle speed is virtually constant rpm.  The
air-motor and governor of the Duo-Art is excellent too.  My Themodist
pumper piano uses the same components.

Both the Ampico B and the Themodist Pianola have take-up spools turning
at constant rpm and slowly increasing in diameter, and so the paper
speed across the tracker bar slowly increases.  The pop songs are
arranged with constant beats per foot of paper, therefore if the paper
speeds up during playing, so does the musical tempo.

Virtually all 88-note rolls, and most reproducing piano rolls _of
popular songs_ (like fox-trots), are punched with constant beats per
foot.  So, when I play these rolls on the Ampico B or the Pianola,
the music slowly gets faster.

The acceleration isn't much in 3 minutes, or a 30-foot roll, but it's
there, about 4 or 5 percent.  If the take-up axle speed is constant,
then the percentage speed change is the difference of the starting and
ending diameters divided by the starting diameter.

Since the Ampico B displays the same _paper acceleration_ as the Ampico
A -- a necessity for compatibility -- we conclude that the air-motor
and governor and supply-spool hold-back torque are all uniquely
balanced to provide the correct paper acceleration.  This is in contrast
with the Duo-Art paper drive, in which the apparent goal is constant
take-up axle speed.

It's only about 4% speed-up over three minutes ... sometimes it bugs me,
sometimes it doesn't.  But a metronome confirms that it's real.

Robbie Rhodes

(Message sent Fri 6 Nov 1998, 04:42:56 GMT, from time zone GMT-0800.)

Key Words in Subject:  Acceleration, Motor, Music, Regulation, Roll, Speed

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