|
9.26.05
Tuning
for Perfect Pitch Linearity
by Christopher & Don Parrish-Bell
The
ideal pitch response from a theremin antenna occurs when the pitch
oscillator is tuned slightly "above" the LC
resonance of a coiled antenna. This is tuning to the sweet spot for the maximum transfer
of
energy into the pitch field. The antenna LC resonance is below the
oscillator frequency to avoid frequency cross-over.
Modern
day pitch field thinking is fixated on passive hand capacitance; my research
points to the active influence of current flow in the theremin antenna.
Lev Sergeevich Termen referred to the pitch field as electro-magnetic
not capacitive, the antenna was also referred to as an electrode not a
capacitor plate.
The Lev antenna coil spring has parasitic capacitance between the close
coil turns. The overall spring stretch should be at a maximum of 1/2".
Stretching is what tunes it. IMHO the closely gathered spring coil turns
cause the entire
antenna coil to self resonant around 900 kHz driven by
energy from the pitch oscillator. The pitch oscillator should have
the highest Q coil while the antenna responds with a broad band
behavior.
Moving your hand closer to the
coiled antenna begins
to de-tune it, this also begins to attenuate the transfer of
energy from the pitch oscillator
driving the antenna's pitch field. Your
hand has a loading effect on the pitch oscillator which changes the
pitch oscillators capacitance reactance Xc and shifts it
frequency ever so slightly lower along with the influence of hand
capacitance.
Amazingly these two affects together generate a perfectly linear
pitch response relative to a musical scale!
The highest notes would be
attenuated the most. This is why with proper "Lev" LC tuning,
compression of the higher notes cannot occur next to the antenna. It is
at this point next to the antenna that the pitch field energy has been
reduced to a minimum by the off tuning of the LC
caused by the loading effect of your hand in the pitch field.
I originally
saw this effect while studying the LC qualities of a
coiled spring
antenna back in 2004. The tests I used made it obvious that
something unusual was happening.
I reviewed
the schematics of other theremin designer after my own personal journey
into theremin behavior. I
found that most theremin designers either connect the antenna direct, through a
capacitor or they throw some chokes in line, all this with a straight rod
antenna. These methods give you a pitch field with a wide spread of
lower musical notes and the jammed up notes near the antenna.
"The true
phenomenon found in Lev
Sergeevich's heterodyne theremin antenna design is that the notes of the pitch field
are spread out to match a piano keyboard perfectly."
The theremin's
linear note spread will match the repeat distance of each piano key for over five
octaves, note for note. This happens all the way up next to the antenna!
Christopher
|
