MODULATION SECTION (Thru-Zero
FM)
One of the features of TZ0 that many
analog VCOs do not have is through-
zero linear FM. A VCO with standard
linear FM, when presented with a
modulating signal that will take it all the
way down to 0Hz frequency, will simply
stop making any sound. This produces
an effective DC offset of the modulating
signal that increases with more
modulation, thus bending the primary
pitch of the oscillator upward as
modulation increases. On the other
hand, a TZFM oscillator core will simply
modulate towards 0Hz, then “start
spinning backwards” and continue to
modulate. The result is natural sounding
modulation, such as that of a bell or a
pipe being struck, without the bending
of the fundamental pitch like regular
linear FM.
The FM section of TZ0 is shown in the
above block diagram. The “0Hz” point
discussed above is simply adjusted with
the “TZ Offset” knob. This has the side-
effect of modulating the primary pitch of
the oscillator, so adjustments in “TZ
Offset” need to be accompanied with
re-tuning via the tuning pots.
One interesting usage of the “TZ Offset”
pot, is to set it completely counter-
clockwise (to zero). This means that the
oscillator core itself will not make any
sound on it’s own, but it can be
modulated with a signal from the “Lin
FM” input. This has the effect of
reversing the role of the tuning pots and
the “Lin FM” input: The “Lin FM” signal
effects the overall pitch heard, and the
tuning knobs (and V/oct input) change
the timbre of the sound.
Finally, a simple VCA has been included
right after the “Lin FM” input. This
allows the amount of TZFM modulation
to be shaped by an envelope or other
signal.
The “Lin FM” input on TZ0 is AC-
coupled.
