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There is no need of any speculation:

Most probably the Welte-Mignon T-100 recording system worked exactly
as Richard Simonton and Jim Crank described it (see, e.g., several
MMD contributions in January and February 2000):

Under each key a carbon rod was fixed by a spring.  The carbon rod
was dipping in mercury, the deeper the harder (and faster) the key
was depressed.  This resulted in a varying resistance and varying
electrical current: the deeper the key was depressed the higher
the current was, which acted on an electromagnet.  Forced by the
electromagnet, a sharp-edged rubber disk printed a line on a running
paper roll during the time when the key was depressed.  The harder
the key was depressed, the broader the line was.

Remember that after the Second World War Richard Simonton was well
befriended with Karl Bockisch and Edwin Welte, both the inventors
of the Welte-Mignon T-100 technology (see, e.g., MMD 2000.04.03 -
2000.04.06).  After the 1930s the T-100 recording system was of no
economical value any more, so it can be understood that the inventors
informed Dick Simonton about the once very secret T-100 technology.

Dick Simonton's friend, Jim Crank, confirmed that he was a proud owner
of some carbon rods, of some feet of an original recording roll with
lines of varying width, and a very old and poor photo of the mechanism
of the mastering machine and of the factory recording piano's trough
under the keyboard.  In addition, Jim Crank explained that the widening
of the lines was taken into account for the evaluation of the dynamics:
a slow key depression gave a gradual widening of the line, a hard key
stroke gave a rapid expansion of the line.

There is a proof that Welte worked with inked discs (or wheels) in
a writing system.  The American Welte Philharmonic Organ recording
system still exists in near-complete state in the Schweizerisches
Landesmuseum (Weiss-Stauffacher collection) at Seewen, Switzerland.
The corresponding Freiburg recording system, which worked somewhat
differently, is described by Kurt Binninger (a former Welte employee)
and published in "Organologica Acta", vol. 19, Berlin, 1987, pp.
179-207.  From the nice drawing on page 200 it can be seen that it
worked also with discs.

For one and a half years I've been working on the reconstruction of the
T-100 recording system strictly based on this historical description.
Recently (20th of September 2003) I presented my very encouraging
results during the annual meeting of the German mechanical music
society GSM in Triberg (Black Forest).  These results will be published
next year in the GSM Journal, "Das mechanische Musikinstrument".
I discussed a lot on this topic with Craig Brougher, Mark Reinhart,
Hans-Wilhelm Schmitz, and many other Welte-Mignon experts who provided
me important information, for which I thank them very much.

There is only one parameter which defines exactly the loudness of
a piano tone: the momentary key velocity at the HLP (HLP: hammer
let-off point), where the hammer leaves the lever in order to move
freely towards the string.  Nothing else!

I found the following points (by own experiments, theoretical
considerations and historical studies):

1.  Based on the historical piano dynamics recording system by Binet
and Courtier ("Recherches Graphiques sur la Musique", in "L'Annee
Psychologique", Paris 1896, available at the Freiburg University
library), an equivalent system to the later "seismographic"
Welte-Mignon-Licensee system was state of the art already in 1896.
Note that the Licensee system developed in USA in the 1920 and the
T-100 system developed 1901-1904 in Freiburg are completely different!

2.  If high quality and authenticity is requested, dynamic dependent
time delays must be taken into account for each tone during the
translation of the original recording roll of the production master
roll.  These time delays cannot be discriminated by systems which
record the dynamic information only summary for bass and treble.
Therefore -- on reason of accuracy -- the dynamics of each key must
be recorded separately, as Edwin Welte and Karl Bockisch did.

3.  The harder a piano key is depressed the deeper it goes into the
felt of the front rail punching.  There is a key distance travel
difference of nearly 3 mm between ff and pp tones.  The progressive
non-linear force-distance relationship is based on the elasticity of
the felt punching below the key.  The "constant key stroke hypothesis"
(see MMD 2000.02.04.07) clearly is wrong.

4.  The MHRM (minimum height of the repetition mechanism) must be taken
into account.  The best measuring condition is to write lines only in
the zone where the key is at or below this point.  In this case it can
easily be decided whether a tone is repeated (interruption of the line)
or held (continuous line).  The MHRM point is about 2 mm above the pp
and 5 mm above the ff key position, therefore the measurement range of
the mercury-carbon (Hg-C) sensor must be around 5 mm.

5.  Upon dipping the carbon rods into mercury, surface waves are
created which disturb the contact accuracy of the Hg-C sensor.  The
wave amplitude must be minimised by dipping the rod with its sharp edge
exactly perpendicular to the surface into the mercury.  Any rotation
or horizontal movement component gives rise to a much bigger amplitude.
Because the piano key moves in a rotational manner about the balance
rail fulcrum, a direct fixation of the rods at the keys is forbidden.
The only useful method is to transfer the vertical component of the
key movement with a well-guided prolongation device, e.g., a metal rod
moving a metal tube.  To ensure that the carbon rod follows exactly
the key movement, it must be pressed against the key by a spring.
Advantageously, the spring allows to transfer the current from the
moving carbon rod to a fixed point from which it can be transferred by
a connecting cable to the writing apparatus without any problem.  Such
a system fits exactly the description of Richard Simonton.

6.  In addition, the sharp edge of the carbon rods minimise spark
effects due to the inductance of the electromagnets: the conductivity
(inverse electrical resistance) and thus the current varies continuously
from zero to the maximum value.  To avoid oxidation effects (due to
dissolved metal impurities), a very pure grade of mercury has to be
used; following Simonton, the Hg surface was protected with an oil,
a very common technique in that time.

7.  A theoretical calculation of the electrical resistance of a
pyramidal edge of round section shows that it varies very sensitively
with the dipping height.  Special conditions regarding the resistance
of the connecting cables and the electromagnet can be chosen that the
conductance (and the current through the electromagnet) varies linearly
with the dipping height.

8.  This theoretical resistance calculation could experimentally be
proven by a small device using a normal pencil lead of 2 mm diameter.
If a 12-volt DC source is used a very high sensitivity of around 0.8
amperes per millimeter, within a range of 5 mm, can be achieved.
This Hg-C sensor, in the exact form as described by Richard Simonton
and confirmed by Jim Crank, can be used as a very sensitive analogous
key-travel measurement system.

9.  By experimental tests of elastic discs of different types (one
wheel with a round section of the elastic material and one with a
sharp-edged elastic part), I confirmed that the line width varies with
the force with which the wheel is pressed to the paper.  In both cases
there is a non-linear relationship between line width and force.

10.  By small modifications of the Welte Philharmonic Organ writing
system as described by Kurt Binninger, a very useful Welte-Mignon
T-100 recording system can be reconstructed.

11.  The momentary value of the key velocity is given by the first
derivative of the key-travel distance versus time.  With an analogous
key-travel measurement system, the momentary velocity value at the
hammer let-off point can be determined exactly (by graphical derivation
methods).  Other systems, like the Nystroem system or the Ampico-B
recording system, are only able to determine medium values by the
two-point [travel-time] method.  Medium values (which can easily be
determined with the above described system as well) take no account of
acceleration or slowing down effects which a pianist may perform within
the two-point measuring zone.

12.  Therefore the Welte-Mignon T-100 system must be regarded as the
principally best of all known dynamic recording systems.

Best regards to all Welte-Mignon friends!

Ludwig Peetz
Pirmasens, Germany

P.S: Within the next months I plan to build a one-key model of this type.

 [ Dr. Peetz teaches polymer and textile technology at the Pirmasens
 [ campus of Kaiserslautern College, and I believe he is also a master
 [ at chess!  ;-)  Danke sehr, Prof. Peetz, for contributing your
 [ technical article to MMD.  -- Robbie