Roll Playback Speed Change
By Robbie Rhodes
Janosy and Macsai seek Welte-Mignon player acceleration/deceleration data for their project of reproducing the music from the transcribed rolls, using a Welte-to-Midi emulator and a synthsizer or Disklavier. Virtually all classical rolls include inherent time correction which attempts to compensate for the increasing paper velocity as the diameter increases on the take-up spool. But playing the transcribed roll via Midi fails to duplicate the action of the spoolbox, and so "the music slows down." I can't think of any reliable method to obtain the data they need by direct measurement of the paper speed: every airmotor system has too many uncontrolled variables.
Rick Pargeter, in digest 950614, said that he must continually move the Tempo lever as he plays a long roll on his piano, in order to keep the music speed constant. I have the same problem with my airmotor piano, and I do the same "manual deceleration" during playback. I am aware that the piano would do better if there were no leaks and the bearings were perfect, et cetera. It has been 5 years since the airmotor and governor were re-covered, and I don't know how long since new felt was fitted in the bearings. The piano is almost 80 years old, and just "typical", I suppose.
My Ampico B piano with governed electric drive motor also requires "manual deceleration", but not as much as the airmotor piano with the smaller diameter spool. Not all rolls demand playback speed adjustment, but most of my favorite pop tunes *do*. It seems, with these songs, that the mind is as sensitive to tempo change as it is to pitch change: one percent is generally discernable. (And everyone knows the seasickness that accompanies the pitch change as a phonograph spring-motor slows down, or the battery in a cassette player dies!)
I have adequate data from Ampico roll analysis and literature which suggests that a take-up spool of 4 or 5 inches diameter, as I recall, would provide the proper acceleration of the roll for a spool turning at constant revolutions per minute. This data assumes that the average thickness of the paper on the spool is about .003 inch (.075 mm). From this, we could derive the optimum airmotor deceleration (only for Ampico) due to the constantly increasing torque. (I will have to find my notes about this for you, Zoli.)
Notice that I say "optimum". The speed-versus-torque characteristic of the airmotor and companion pressure regulator ("governor") are very dependent upon system friction, and upon air leaking into the airmotor and governor pneumatic. I don't know of any standards for airmotor systems regarding the recommended friction of the airmotor bearings, and there is no way to compensate for air leaks while retaining the same speed-torque characteristics. There will be very big differences between pianos.
It seems to me that we must rely upon our ears and good musical judgement to re-create the proper "tempi" throughout as the music is played.
-- Robbie Rhodes, 15 July 1995
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(Message sent Sun 16 Jul 1995, 00:56:21 GMT, from time zone GMT-0700.) |
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