Replicating Punched Paper
By Robbie Rhodes
Spencer Chase wrote in Digest 961128 that he is designing a roll scanning and punching system, and that he plans to punch the new rolls "at the same step size as the originals to maximize speed and accuracy." In Digest 961204 Spencer wrote:
> I would like to scan rolls with a degree of precision that
> captures the information as accurately as it was punched.
That's great, Spencer, and as Robin Pratt would say, "Let's do it right!"
Several people have made music roll copying systems in the last 40 years that I've been following this topic. Most of the time the recuts were okay for orchestrions, and passable for 88-note pianos, but the crude methods just couldn't maintain the precision needed to copy reproducing rolls. The Ampico recuts by Orville Cooper, using a single punch with a single-channel "reader", were miserable! Then Larry Givens started producing "microfine" recuts (more than 60 steps per inch, asynchronous) and the results were generally okay. _But not perfect._
A couple of years ago I visited Siegfried Wendel's museum in Rudesheim am Rhein, and he also showed me his music roll copying machine. Siegfried has a problem: because the instruments in the museum are played several times daily, he cannot keep original rolls on the machines, and so he must make copies. His semi-automatic single-punch machine works not much better than Cooper's did: it's okay for orchestrion rolls, but not at all satisfactory for the reproducing piano rolls.
Siegfried is a fine engineer, and a canny businessman. He might decide that it isn't worth spending _his_ time to fix the process, and instead he might engage a smart machine builder to get the job done. So he goes to a clever machinist friend, who has worked with Siegfried previously, and who says, "Ja, Siegfried, but you are a difficult customer to satisfy. How shall we both be satisfied that I have done the job properly?"
These guys, the machinist and the customer, aren't making machines and copying rolls for fun -- they are both spending time and money on a serious job. They know very well that the task must be completely specified, and that both parties must agree upon the conditions which define that the job has been done acceptably. You can be sure that they will "do it right".
My illustration above is fictional, but it is probably so close to the truth that Siegfried will wince when hears of it! My illustration following is also fictional, but I think it illustrates how an engineer/businessman like Siegfried would proceed.
The task:
The task is to replicate a standard 88-note music roll, or perhaps the "98-channel" versions such as Ampico or Welte Licensee. That the data is music is inconsequential: the task is only to duplicate the pattern of holes which are punched in a paper roll. A skilled machinist and machine builder, who has no knowledge about the _use_ of the paper roll, will need the information discussed below.
The most important aspect is the definition of an acceptable product, so that the machinist will be paid by the satisfied customer! The customer describes the problem to the machinist, and they write a job specification which is agreeable to both.
Acceptance Criteria: The replica is declared acceptable if
1. The pattern and number of punched (overlapping) holes matches the original; this will be verified by laying the copy over the original and inspecting visually.
2. The dimensions are within the specified tolerance.
Facts:
1. The spacing of the holes on the short axis is constant, 9 per inch, and the centers of the holes lie upon a straight line which is perpendicular to the long axis.
2. The spacing of the holes on the long axis can be from 240 to 400 or more holes per foot, but it is constant for a given roll.
3. All holes are the same diameter.
4. The paper is 11.25 inches wide; the length is indeterminate.
After some discussion these inspection tolerances are agreed upon:
1. Hole diameter: +/- 0.002 inch
2. Paper width and short axis (note axis) dimensions: +/- 0.012 inch non-cumulative
3. Long axis: 0.030 inch in 10 inches, therefore 0.3 percent.
4. Skew: The hole centers will lie within +/- 0.004 inches of the true perpendicular line.
The customer leaves and the machinist begins designing. It will be easy to make a paper transport for his computer-controlled punching machine -- no problem. But how shall he "read the data" of the original old paper roll?
After some thinking, the machinist builds a simple roll transport with a "reader head" consisting of 98 photodiodes which read the short-axis data. The paper roll wraps around a precision measuring roller which registers the distance along the long-axis each 0.001 inch. (For this, the machinist uses a shaft encoder transducer similar to the devices on his numerical-control milling machines.)
The on/off signals from the photodiodes are sampled every 0.001 inch (0.25 mm); each _change_ in the state of the channel is sent to the computer, accompanied by the distance measurement output from the measuring roller transducer.
The computer program is told beforehand that the original roll was punched at approximately 360 steps per foot, and so it looks for changes at roughly these intervals. "Clusters" of change events which occur together are grouped together (a "chord"). After grouping the events in this fashion the computer processes the data again, to determine the actual steps per foot of the original roll, and finally the computer assigns the clusters to discrete output distances at 360 steps per foot. It is a _synchronized_ copying process.
The machinist punches a copy of the roll from the data file created by the reader machine. He and the customer verify that the new copy meets the Acceptance Criteria, and everyone is happy. No musical parameters were needed -- they simply copied the holes.
Epilogue -- Questions and Answers:
Q: "Why did you require hole-for-hole replication?"
A: "Because we know that if it matches hole-for-hole, we did the job perfectly."
Q: "Is there an easier method?"
A: "We couldn't think of another method which would yield acceptable results and also be _measureable_. We had to have a simple and conclusive acceptance method."
Q: "What was the most difficult task?"
A: "Defining the acceptance criteria!"
Robbie Rhodes |
(Message sent Fri 6 Dec 1996, 01:53:31 GMT, from time zone GMT-0800.) |
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