Novation A-Station - Mixer and Filter

10Synthesis Tutorial
To extend the range of sounds that may be reproduced, a typi-
cal Analogue synthesizer has more than one Oscillator. The A-
Station has three independent Oscillators and a separate Noise
Oscillator.
For flexibility, a mixer section is included so that the amplitude
(volume level) of each of the Oscillators may be adjusted inde-
pendently and mixed together to form a harmonically complex
waveform.
Filter
The A-Station is an Analogue subtractive type of music synthe-
sizer. Subtractive implies that part of the sound is subtracted
somewhere in the synthesis process.
The Oscillators provide the raw waveforms with plenty of har-
monic content and it is the Filter that subtracts harmonics in a
controllable manner.
The Filter in the A-Station is a Low Pass type. A cut-off point is
chosen and any frequencies below the point are passed and
any above are filtered out. This process of removing harmonics
from the waveforms has the effect of changing the sounds char-
acter or timbre.
In practice there is a gradual reduction in the volume of the har-
monics above the cut-off point. How quickly these harmonics
are reduced in volume above the cut-off frequency is deter-
mined by the Filter’s slope. This slope is measured in ‘volume
units per octave’ Since Volume is measured in decibels this
slope is quoted in number of decibels per octave (dB). Typical
values are 12dB or 24 dB per Octave. The higher the number,
the faster the harmonics are cut and the more pronounced the
filtering effect.
A further important feature of the Filter is the Resonance con-
trol. Frequencies at the cut-off point may be increased in vol-
ume by this control. This is useful for emphasizing certain har-
monics of the sound.
Mixer and Filter
Response of a typical low
pass filter. Frequencies
above the cut off point are
reduced in volume
When resonance is added,
frequencies at the cut off
point are boosted in volume
OSC1
OSC1 VOLUME
MIXER
INPUT TO
FILTER
COMPLEX
WAVEFORM
MIX OF
OSC1,2 AND 3
OSC2 VOLUME
OSC3 VOLUME
OSC2
OSC3
Volume
Volume
Frequency
Frequency
Cut off
Frequency
Cut off
Frequency
Mixer