Exact Scanning of Rolls
By Wayne Stahnke
|Hi, John! I was pleased to hear from you, and of course I am happy to provide you with a description of my roll scanning process.|
I sense the presence or absence of holes in the roll pneumatically, using pouches and microswitches, but no valves. The paper is advanced continuously at a speed of about 10 mm/s by a take-up spool. The units along the length of the roll are displacement, sensed by a measuring roller coupled to an optical shaft encoder. This arrangement captures the webbing, but no visual effects. The end result is a roll image in my own data format. I would be happy to provide to you with a description of the data format on request. The data represents the x-y location of each hole; all perforations are treated in the same manner, with no attempt at interpretation of the data.
It should be clear from this description that I have not broken any new ground in the methodology or implementation of roll scanning. What is new about my work is its accuracy. The location of each hole is determined precisely enough to allow further processing, in particular the reconstruction of the master roll.
Traditionally, we have all thought of a roll as having the holes positioned in discrete locations across its width, but continuously along its length. This is an error, of course. The rolls were manufactured by punching a row of holes with the paper at a standstill, followed by a discrete paper advance before stopping the paper and repeating the process. This advance, which I call the "punch advance," is typically on the order of 1 mm. It varies from manufacturer to manufacturer, and was sometimes upgraded by a given manufacturer, apparently to increase accuracy.
Thus, a rolls should properly be viewed as a bit map of an x-y matrix of pixels, each "pixel" consisting of the presence or absence of a hole. The fact that the pixels are larger than the punch advance, yielding a slot with scalloped edges, is not material to this discussion.
With this picture in mind, it is clear the the goal of scanning a roll is to reconstruct the x-y matrix and the (on-off) value of each pixel. Stated differently, our goal is to restore the master roll from which the production roll was made. When we have done this, we have scanned the roll exactly.
To illustrate this point, I have attached the file _68283B.MID to this message. The file consists of the recreated master roll of Ampico recording 68283, Paderewski's Menuet, Op. 14, No. 1, played by Sergei Rachmaninoff. I chose this roll because it apparently sold in large quantities, and is available in almost every large Ampico roll collection. I suggest that you take the time to pull the file up on your computer screen and examine it carefully with a copy of the roll (and a straightedge) at hand. You will find that all of the perforations are at their correct places, as determined by counting punch steps. A new roll made from this file would be identical to a roll made in the 1920s.
Thus, my contribution to roll scanning technology has not been in developing a new method or apparatus. Rather I have contributed two things: (1) Setting as a goal exact, rather than approximate, scanning of the roll; (2) Demonstrating that this goal can be achieved.
There has been a lot of discussion lately about scanning of roll collections on a large scale. To my knowledge, no one has yet asked the question, "How good will the scans be?" I would like to suggest that we should consider anything less than exact scanning to be inadequate. We have been entrusted with a precious legacy, and we have a duty to preserve and protect it. Preservation of the performances on these rolls must not be corrupted by scanner errors, or we will have failed to live up to our responsibility to the artists and engineers who created them, and to future generations of listeners.
With best regards, I remain
[ Editor's Note: I have compressed the MIDI file (68283B.MID) with "zip"
[ and named it 68283B.ZIP. I have further UUENCODED that file and have
[ enclosed it at the end of the digest. Those of you with MIME-compliant
[ mailers can probably open it as an attachment.
[ As Robbie explained it to me, the MIDI file is really an image of a
[ Master Roll, intended to control a perforator, and therefore the timing
[ is different from a MIDI file to control a synthesizer or a solenoid
[ piano -- "the slots in the master roll are "lengthened", so to speak!
(Message sent Wed 6 Mar 1996, 01:39:31 GMT, from time zone GMT-0500.)