Re: Perforator Advance Distances
By Craig Brougher
|Spencer Chase is curious as to the resolution a perforator should have to copy all kinds of old rolls and to turn out an accurate reproduction. I would like to enter this thread and add my two cents for what it's worth.|
I'm not a roll cutting guru by any means, and there may be some others here who would like to correct this opinion, which is why I am writing it. Here is my opinion about roll accuracy, as it relates to the most critical of all masters, that of the Ampico reproducing piano, whose masters were actually cut 3 to 1 in length to be as precise as possible (most other rolls were cut 2 to 1).
The most single important measurement to maintain in a high quality reproducer roll is the perforation width. A wide hole triggers each note or intensity with minimum time variations and greatly narrows the natural differences between valves and sensitivities created by a host of uncontrollable factors. The best rolls had holes 5/64" or perhaps a bit wider, from my measurements. (I can slide a .081 drill bit through the holes of an original Ampico roll, for example, even though the trackerbar hole measures only .072 wide. That leaves about .039" for each web between the notes). The reason I stress punch diameter is because perforators with smaller punches do not perform well when the most sensitive notes are to be played. For regular player pianos, this is not a problem. But when trying to play consistently at 5" of vacuum or below, it becomes a very big problem. Unless the holes are correctly sized, the roll won't play as well regardless of stepping increments.
Now, for resolution, let's say your punches are .081" diam. (#46) and you wish to make a trial run with a perforation every .081" on the note sheet. That gives you a perf chain without any distance between them. You would get 148.148 holes/foot. Granted, your resolution has to be better than that, because your punch is .081 and you will need to be able to make at least three steps of the perforator in that distance. Four steps seems to me to be adequate for anything you would ever do in paper. That means that each step equals about .020.
Here is one way to arrive at this number: The height of a trackerbar hole is .060. The time it would take a .020 increment to pass the hole at a tempo of 70 is: .020+.081~ .100 , .100 divided by 1.4 in/sec = .07 seconds. That is the minimum length of time, or time increment that you are able to differentiate between in your music at this tempo. Of course, this allows for step differences between different makes of rolls too because the player itself could not possibly resolve a chain of increments like that with only .020 bridge between them, but you are also interested in precisely copying a roll that didn't use a multiple of your steps. (The difference between four-steps per diameter and three steps per diameter is .02 seconds, or .027 inches per step difference.)
Frankly, for a number of reasons, I prefer to divide the diameter of the punch by an odd number of steps. It would seem to me that if you were to divide the .081 diameter by 5, or .0162"/step, giving 61.7 steps/ inch, this would seem to me to be workable, leaving .4 step over at the end of each foot. That seems to me you can never miss a punch by much over .2 step increment (about .0003), worst case. So about three times your postulated basic increment of .005" would be about perfect, and easier to do. If you want to use even numbers, you could double your .005 to .010 and do it just as well, I would think.
The wisdom of step increments which are not even numbers per foot seems to me to allow a perforator to be more accurate by seldom falling into a "slot" in which the start or end of a hole varies across its own step lines more than 25%, and so is resolved to the closest step line. Sometimes, over-punching or missing the first punch by a tiny fraction is caused whenever music is laid out by the foot and then changes a little over time.
The problems of perforator error, as I have understood it, came in trying to do it with even number increments/foot and /or even number of step lengths/ft. Since the perforator mechanisms copied technology developed first in the printing trade, one might go back to the incrementing used historically (if such a thing existed for one reason or another) to see if he could find some common scale. It's a sure thing that the 88 note trackerbar for player pianos was developed from the pica scale used by printers, and scale engines could be set up to graduate just about anything, English, Metric, or Timbuktu.
[ I, too, discovered the 9/inch printers scale when I worked at a print
[ shop. (I think it was named "Agate".) But Craig, is it really a "sure
[ thing" that the music roll industry specifically adopted the printers
[ standard? After all, in that era the decimal rule was very uncommon,
[ and many industries defined their own independent standards. Can you
[ offer evidence for your statement? -- Robbie
(Message sent Thu 5 Dec 1996, 16:20:46 GMT, from time zone GMT.)