F
or Part 4 of the MIDI Ultimate
Synthesiser build, we are going
to add the circuits for the mod-
ulation sources. Control voltages can
also enact many of the manual po-
tentiometer controls used to modify
the characteristics of the sound being
produced. These control voltages take
various forms, including cyclic wave-
forms, random voltage sequences and
envelope generators. Envelope gener-
ators can be single shot or continually
re-triggered to make more complex
cyclic waveforms.
We briefly discussed each of the
modulation sections of the MIDI Ulti-
mate in Part 1. The patching diagram
(Fig.5, Part 1) shows how the modu-
lation sources can be routed to the
oscillators (VCOs), fi lter (VCF) and the
voltage-controlled amplifi er (VCA).
Sounds that have a constant frequency,
amplitude and harmonic content can
be rather bland. Being able to change
some or all of these characteristics
over time using modulation sources
is what brings the synth to life.
No synth will be complete without
two main modulations sources, the
envelope generator (ADSR) and the
low-frequency oscillator (LFO). Rec-
ognising how important these are, the
MIDI Ultimate includes two of each of
them. Another modulation source at
our disposal is the sample and hold
(S&H) generator that outputs a snap
shot or sample of a waveform period-
ically. The waveform being sampled
by the S&H can be any of the wave
shapes from LFO 2 or from the noise
source we built in Part 3. The rate of
sampling can be manually adjusted,
as can the glide, or portamento, from
one note to the next.
Those familiar with analogue syn-
thesis can skip this next section onPart 4 | by Paul Cooper | soundtronics.co.uk
SynthesiserSynthesiserSynthesiserSynthesiserSynthesiserSynthesiser
modulation, but for those new to an-
alogue synths the following will aid
in understanding the synthesiser’s op-
eration and use.Modulation
The waveforms from the LFOs can be
patched to the VCOs to vary their fre-
quency – this is know as ‘vibrato’. As
the LFOs have four choices of wave
shape, the vibrato effects can be quite
different. A square wave will cause
the VCO to alternate between two
frequencies, whereas a triangle wave
shape will cause the VCO frequency
to rise and fall linearly between two
frequencies. The difference between
the two frequencies is determined by
the amount of modulation depth set
on the VCO(s). The starting frequen-
cy can vary with what key is being
pressed on the MIDI keyboard or other
modulation sources.