Perforated Music Rolls, Part 1.
By Robbie Rhodes
In Digest 960321 David Wasson described his music roll perforator and why he wants to modify it for synchronous control. Among his reasons are the need for precise control of the musical Tempo, irrespective of the paper speed and distance (his 10-tune rolls are quite large and play for at least 30 minutes), and also to permit "chain" patterns. He has already confirmed that re-sampling his original song file for a new paper speed (recording the MIDI data from one computer to another) causes data corruption.
Here's a fast answer for you, David, and then I'll discuss "why"!
You must substitute a perforator crankshaft signal for the built-in timebase clock of your computer/sequencer, so that the data to your perforator are synchronized with the paper motion. There are at least two methods for external control or synchronization:
(1) Use a switch on the crankshaft to generate the "MIDI Sync" signal which enters the computer via the Printer port or a secondary MIDI port. I think this will cause the sequencer to advance one "tick" for each crankshaft revolution. Check your sequencer documentation.
(2) Use a tach disk on the crankshaft to phase-lock an oscillator at roughly 1 MHz, and feed this signal into the external clock pin of the modem port (substituting for the 1 MHz crystal oscillator of the MIDI adapter hardware.)
These methods, or equivalent, will assure that your perforator and the computer data file are locked together, synchronously, just as the perforator crankshaft moves the sprocket drive of a Master Roll. You will need an intermediate computer or "off-line" processing of your original MIDI files to apply the Tempo Map data to make a Master Roll file, and also to generate the chaining.
Now I'll talk about the theory...
- - - Perforated Music Rolls, Part 1 - - - - - - - - -
The process of synchronously duplicating copies of music rolls was developed in order to assure that each and every production roll is identical, and contains the all the data of the Master Roll, without corruption.
The Master Roll is, by definition, the template which controls the perforator machine. It is nothing more than a binary data file composed of 100-bit "words" which are processed sequentially, one word at a time. Numerical-control (NC) machine tools of the 1960s were equipped with similar punched paper tape controls; the "Master Roll" consisted of 8-bit words, processed sequentially. In both systems the master roll or tape is advanced by a sprocket wheel, with peripheral pins which engage the sprocket holes. Each cycle of the machine advances the master roll one step, or one "word".
It may have been Richard Tonnesen who observed something like, "People tend to think of time and distance as being interchangeable." The movement of the music roll is somehow equated with the movement of the magnetic tape in a cassette. However, when one examines the matter, it is apparent that the speed of the music roll isn't constant, and there are only periodic samples of music data -- the music has been quantized.
[ Part 2 will follow soon... -- Robbie Rhodes
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(Message sent Sat 23 Mar 1996, 00:25:03 GMT, from time zone GMT-0800.) |
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