Steadily growing is the number of MIDI files that result from scanning music rolls and that are posted in the web for free downloading. All those files are characterized as "emulated MIDI" or "e-MIDI". In the same roll scanning context the term "e-Roll" may be found which sounds quite similar. The following chapters try to explain these terms in some detail.
The process of scanning music rolls is explained elsewhere.
In one sentence it can be described in the following way: By means of a high resolution optical sensor a complete graphical image of a music roll is registered and transferred into a personal computer, where, by applying a sophisticated image processing software algorithm, the punched holes in the paper are recognized and converted into corresponding MIDI events.
The track within the music roll results in a definite note pitch; the start of a hole or chain of holes with respect to the direction of paper transport defines the start and duration of the MIDI event.
By employing the fact, that punches normally occur only in a fixed raster, minor errors introduced during the punching and the scanning process can be eliminated in the reconstruction procedure and the MIDI version shows fewer errors than the music rolls!
If we try to play back, e.g. by means of a sound card in a personal computer, the MIDI file as generated in the described procedure, we discover immediately that all notes are playing at the same volume.
There is no reason to wonder, because, during the above described genesis of a MIDI event, terms as loudness or velocity were not mentioned. Information about dynamic, if present at all, is stored at another location of the music roll.
Reproducing pianos are famous for their capabilities to play back differentiated dynamic values of individual notes. The best known systems for reproducing pianos are Welte, Duo-Art and Ampico. As an example we use the Ampico format, being widely spread in the USA.
The following table shows the functions of tracks #1 to 7 on every Ampico note roll, in principle the dynamic of the bass notes, i.e. the notes on the left side of the tracker bar resp. of the piano keyboard. There is a similar list for the treble section on the right side; the separation between bass and treble occurring in the middle of the keyboard, for the Ampico system between notes e and f (MIDI #64 and 65). We are possibly somewhat amazed by the presence of the forte pedal in Track3, as we consider, as told, the left side of the music roll and the piano.
MIDI # | Function | Hole in Tracker Bar |
16 | Slow Bass Crescendo | 1 |
17 | Bass 2 | 2 |
18 | Sustain Pedal | 3 |
19 | Bass 4 | 4 |
20 | Fast Bass Crescendo | 5 |
21 | Bass 6 | 6 |
22 | Bass Cancel | 7 |
The combination of punches in tracks #2, 4, and 6 result in 8 different vacuum levels and thus in 8 different hammer velocities. The weight for track #2 is the lightest one and track #6 has the maximum weight. The graduation is not, as one would normally expect, straight binary - 1 to 2 to 4 - but using the ratio 1 to 2 to 3. In theory, combining tracks #2 and #4 would result in an identical loudness as track #6 alone.
Despite of this irrationality it is common to talk of 7 different velocities. An additional 8th step is present if all three punchings are missing. In this case the velocity is not zero, but a still audible note is played.
Overlaying these fixed levels are the crescendo commands in track #1 (slow) and #5 (fast) that modify the vacuum in a ramping fashion. The presence of a crescendo hole is ramping the vacuum from the present level towards the "louder" direction; the absence of a crescendo command directs the level back into the "softer" direction, until the previous base level is reached. The time constants for the complete ramps are 2 seconds (fast) resp. 11 seconds (slow).
When an Ampico music roll has been scanned, the tracks #1 to 7 represent MIDI events with the corresponding numbers 16 to 22, complying with notes in the very low bass region. If these MIDI events are played back as audible notes, the listening pleasure is also very low, in other words mostly horrifying. Even worse, we still have no velocity variations of the true notes as expected from a real reproducing system. Here a rather simple conversion takes place, which, applying the dynamic information found in tracks #1 to 7, generates a unique velocity for all notes in the bass region. The resulting file, after a similar procedure for the treble region, is called "Emulated MIDI", abbreviated as "e-MIDI", because the function of the reproducing mechanism, found in every Ampico piano, is emulated. This file can be played back on all common MIDI instruments ranging from a simple sound card, present in every standard PC, up to a Disklavier, with satisfying results.
For the sake of completeness one should also mention the requirements of the owners of reproducing pianos, which have been MIDIfied out of various reasons. These "e-Valve Equipped Pneumatic Instruments" are capable to play back MIDI images of music rolls in addition to their genuine paper roll function. These instruments call for MIDI files, that present a true image of the music roll, sometimes described as "e-Rolls". Emulated MIDI will not give satisfactory results, because the capabilities that distinguish these instruments from standard player pianos are not utilized. It is probably needless to mention that a MIDIfied Ampico piano requires the e-roll version of an Ampico roll.
By the pioneering developments in the field of roll scanning it is possible to preserve audio documents that give witness from the beginning of the 20th century and to play them back on a multitude of instruments and devices. Out of technical and practical reasons several MIDI file formats have been established to store and distribute the results of the scanning process.
The "e-Roll" format is almost an image of the music roll itself and thus fits only the requirements of e-valve equipped reproduction pianos. A rather straight forward algorithm converts these files into "emulated MIDI or "e-MIDI" versions, which are compatible to almost any MIDI device. No usable results are known for the transition into the opposite direction. Converting fully qualified MIDI files with individual velocity values into an e-roll format is remaining a challenge for the future.
Walter Tenten
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