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MMD > Archives > March 1996 > 1996.03.08 > 05Prev  Next

Re: Stahnke Roll Archival Methodology
By Wayne Stahnke


I am always happy to answer your questions, because the questions indicate that I did not do my job of making everything clear the first time around.

> First of all, let me say that I endorse your method of initially sensing
> the holes pneumatically. Call it a "gut feeling" but I have always been
> somewhat leery of optical based methods to do this. Granted they may be
> capable of finer absolute resolution than a pneumatically based sensor, but
> such resolution can sometimes get you in trouble and has to be
> reverse-compensated for in order not to skew the data. I maintain the most
> important aspect in sensing when and whether a hole is open (and for how
> long) is how the PIANO mechanism REACTS to the hole and therefore you need to
> read the hole the same way the piano does, pneumatically.
> Having said that, I'm a little curious about your pneumatic switch. Do you
> literally have discrete pouches operating discrete microswitches or do you
> use a commercial, integrated differential pressure switch? The distinction
> may be important. With discrete components I imagine you could allow for
> some method of calibration so that the reaction time of individual switches
> can be adjusted to be the same (within some tolerance level) which would be
> absolute necessary to achieve a 0.1mm sensing accuracy as you stated
> previously.

My scanner has discrete pouches operating discrete microswitches and must be calibrated to achieve the accuracy required, exactly as you surmised.

> Next, I note you use a take-up spool as the basic motive force for the
> paper, but as this could introduce timing (or positional) errors, you use a
> "paper follower" (roller/shaft encoder) to provide your ordinate axis data.
> (I use a similar device in my reader and view it as ESSENTIAL if your goal is
> to replicate rolls.) However, in looking at the data in 68283B.MID I see no
> data that appears as if it would be a measure of this, a "sync" pulse if you
> will. I would expect to see one MIDI note, outside the range of the nominal
> 100 MIDI notes which define the compass of the tracker bar, and that this
> "note" would repeat continuously throughout the length of the roll. (I'm
> looking at the data with the "Note View" option of Cakewalk. Is this data
> not visible (or has it been removed) for some reason?

The scanner operates in a manner different from yours and all other scanners that I know about. The switches are sampled (not scanned) by latching their states in a register in response to the output of the shaft encoder. Thus, sampling is initiated not periodically in time, but rather periodically in displacement. This removes time from the sampling process altogether, with the favorable consequence that the paper speed does not have to be precisely controlled. The "sync pulse" is a single tick in the file.

> If your reader does indeed capture the chain bridging, how does it do this
> since normal pneumatic reading of the holes, by design, ignores the bridges?
> Does this happen "naturally" at this (relatively slow) reading speed
> (10mm/sec. =3D approx. 2'/min.) or have you adjusted the "height" dimension
> of the holes in the tracker bar, so that they can "resolve" the bridge?
> (This in itself would introduce note duration errors.)

The tracker bar ports have been narrowed to resolve the webbing. Thus, the resulting roll image is not suitable for operating a pneumatic instrument, but must be processed first to model the hole diameter in the roll, the punch advance, and the port size of the tracker bar.

> One minor point you did not address was the polling of the microswitches.
> Intuitively, if you are scanning the switches sequentially at a high enough
> rate, and the paper is moving slowly enough, any errors introduced by this
> method would be negligible, but I don't have a good feel for what this ratio
> should be. If I recall correctly, your design in the Bosendorfer systems is
> (about) 800 full keyboard scans per second. (Is MIDI better or worse?)i One
> eight hundredth of 10mm is 1/80th of a mm paper movement between adjacent
> switches. Which, when said another way, means only eight switches can be
> polled before your claimed accuracy of 0.1mm is exceeded. What then does
> this imply about the relative positional accuracy of two notes struck
> "simultaneously" at opposite ends of the keyboard? Does your algorithm
> anticipate this and automatically "de-skew" the data? I would think this
> necessary since rolls are read (and perforators punch) in a parallel domain,
> not serial.

The switches are sampled (not polled) within 50 microseconds of the command from the shaft encoder. This corresponds to a paper travel of 0.0005 mm at a paper speed of 10 mm/s.

> Finally, Jim Heyworth (in Digest 96.03.06) basically questioned how we can
> judge the intrinsic accuracy of the data in the 68283B.MID file given the
> normal tools at our disposal (even if we happen to have a copy of the
> production roll.) The resolution of such display/editing/ sequencing programs
> as Cakewalk appear unsuited for this. It seems to me to only way to do this
> would be to do a side-by-side physical comparison of a master roll produced
> by your method with the original master roll artifact. Does Keystone have
> this particular master? Has this been done (or considered?) Please
> understand that I am not expressing doubt in your methods, only trying to
> better understand (and have confidence in) them. And please let me know if
> any of the necessary (but unstated) assumptions I have made in the foregoing
> are faulty. BTW, I too, would be interested in knowing more technical detail
> about your data format.

I do not know if the master roll for this selection survives, but whether it does or not is irrelevant. Keep in mind the purpose of the master roll: to provide the hole pattern for the perforator. What is important in a master roll is this pattern, not its embodiment in a particular piece of paper.

The punch advance of rolls is large enough to be visible to the unaided eye, although a magnifying glass or jeweler's loupe makes for more comfortable viewing. With a straightedge across the roll, we can see very clearly when holes have been perforated in the same punch step and when they have not By counting the scallops in the slots, we can move the straightedge one punch step at a time. The webbing is also of great help in counting punch steps by eye. In the note field, the webbing in Ampico rolls from the period in which roll 68283 was manufactured is punch-skip-skip-punch; thus the spacing from one hole to the next is exactly 3 punch steps. For the 1B, 3B, 5B, 5T, 3T, and 1T ports the webbing is punch-skip-skip-skip-punch; thus the spacing from one hole to the next is exactly 4 punch steps. White space can be converted to punch steps by dividing its length by the punch advance, 0.857 mm for the roll in question.

When I suggested you examine the recreated master roll in detail, I meant to carefully examine a portion of the roll by eye, comparing it punch for punch with the image on the computer screen. If you do this, you will find that the patterns are the same. Thus a new roll perforated on the right equipment (that is, on a perforator with the correct punch diameter and correct punch advance) would be identical to an original one.

I hope this answers all of your questions. With best regards, I remain

Sincerely Yours,

Wayne Stahnke

Key Words in Subject:  Archival, Methodology, Roll, Stahnke

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