Paia 9308 Assembly And Using Manual page 24

What would an analog synth be without a GLIDE control
to grab and twist for really expressive portamento?
FatMan uses the common approach of charging a
capacitor (C12) through a variable resistor (R32). |C10:a
buffers the voltage on the capacitor and drives the Master
Pitch control R34 which is used to transpose both
oscillators over slightly more than an octave range.
The two VCOQs are identical except for the Offset control
(R40) which allows the pitch of VCO #1 to be raised and
lowered an octave relative to VCO #2.
VCO #1 also has a
trimmer that allows it's zero intercept to be adjusted to
match that of VCO #2.
The result of this relatively slow charging and quick
discharging is a ramp (sawtooth) waveform and in the
interest of simplicity this is the only oscillator waveform
available. A ramp is the most harmonically rich of the
common waveforms, having both the even harmonics of a
triangle and the odd harmonics of a pulse. The filter can
be used to track the pitch of the oscillators and reject all
harmonics in the ramp leaving only the fundamental sine
wave.
Potentiometer R56, the Osc1/Osc2 Mix control, allows the
VCF to be driven by either VCO1 or VCO2 or a mix of the
two. The VCF design is a State Variable Filter which has
been configured to give a low-pass response with reso-
nance, adjustable with R114, at the corner frequency.
The filter is built around C17, an LM13600 type Dual
Operational Transconductance Amplifiers (OTA) with C20
and C21 as the tuning capacitors. Two control currents for
setting the gain of the two OTAs in 1C17 are produced by
the VA consisting of |C13:d, Q8, Q9 and associated
resistors. Four separate voltages are summed to set the
corner frequency of the filter; a static voltage that sets the
initial frequency is adjustable with R74, Velocity CV
adjustable by R69, Pitch CV adjustable by R71, and
finally the output of the filter's dedicated transient genera-
tor adjustable with R115.
The filters AR transient generator works by charging C22
through R83 and R84 for the Attack portion of the cycle
and discharging it through R81 and R82 for the Release
section, Charging and discharging currents are steered by
D3 and D4 as Q7 is switched on and off by the TxD fine of
the uP. Voltage on the capacitor is buffered by 1C12:c and
the comparator IC8:c monitors the buffers voltage and
switches the processor's INT1 input when the peak
voltage is reached. The firmware's response to this is fo
24 FatMan MIDI Synth
switch from Attack to Release. Closing the Sustain switch
S3 prevents this "peak reached" signal from getting back
to the uP so that the Release portion of the cycle won't
happen until the key that initiated the response is re-
leased. The result is to switch the transient from a non-
sustaining AR to an Attack / Sustain / Release (ASR)
response.
The Attack/Decay/Sustain/Release (ADSR) transient
generator dedicated to the VCA is similar to the filter's
A(S)R. Under control of a pair of the uP's output lines
(P12 & P13), capacitor C19 charges and discharges
though steering diodes D6-D8 at rates set by R92, R94
and R96. The Sustain control R90 sets the voltage level
to which the Decay portion of the cycle falls. 1C12:d
buffers the voltage on the capacitor and comparator IC8:d
signals the processor when the peak of the Attack is
reached.
When the Punch switch S1 is closed the combination of
C34 and R98 add a slight delay (about 20 ms.) between
the time that the ADSR reaches its Attack peak and the
time that this information reaches the uP. The result is a
short Sustain interval that adds punch to sounds with fast
Attack and Decay dynamics. When S1 is open, the ADSR
behaves in the normal, technically correct way.
FIRMWARE
The FatMan firmware is responsible for recognizing MIDI
Note On and Off messages and breaking them down into
Note number and Velocity values. Note number is
checked for being in the range of 36-84 and then con-
verted into octave ranges by division and the data
required to drive the DAC by look-up table.
The Velocity data from Note On and Off messages are
handled in much the same way, except that the 0 to 127
step range of this data is first scaled to range from
36-84.
Pitch Wheel messages are also supported. In the FatMan,
Whee! data modulates the Pitch data before it gets to the
DAC. This is possible because only 12 of the 256 possible
values of the DAC are used for pitch and the space
between these values is available for modulation. Musical
range of FatMan's Pitch Wheel is +/- a semitone. Since
there are no pitches available above the highest C or
below the lowest, wheel data is ignored on these bends.
The firmware is also responsible for turning on and off the
proper sample and hold at the proper time to produce
Pitch and Velocity CVs. It manages the A(S)R and ADSR
transient generators, turning on their Attack cycle when a
note is played and managing Decay, Sustain and Release
as appropriate for the status of the transient and any Note
Off messages which may be received.
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