Novation SUMMIT - Page 18

18
The Mixer
To extend the range of sounds that may be produced, typical analogue synthesisers have
more than one Oscillator (Summit has three for Part A and three for Part B). By using
multiple Oscillators to create a sound, it is possible to achieve very interesting harmonic
mixes. It is also possible to slightly detune individual Oscillators against each other, which
creates a very warm, ‘fat’ sound. Summit’s Mixer allows you to create a sound consisting of
the waveforms of Oscillators 1, 2 and 3, a Noise source and the Ring Modulator output, all
mixed together as required.
OSC 1
OSC 1 VOLUME
OSC 2 VOLUME
OSC 3 VOLUME
COMPLEX
WAVEFORM
MIX OF
OSC1, 2, 3,
NOISE AND
RING
MODULATOR
MIXER
INPUT TO
FILTER
OSC 2
OSC 3
NOISE
RING MOD
NOISE
RING MOD
The Filter
Summit is a subtractive music synthesiser. Subtractive implies that part of the sound is
subtracted somewhere in the synthesis process.
The Oscillators provide the raw waveforms with plenty of harmonic content and the Filter
section subtracts some of the harmonics in a controlled manner.
There are three basic filter types, all of which are available in Summit: low-pass, band-pass
and high-pass. The type of filter most commonly used on synthesisers is low-pass. In a
low-pass filter, a “cut-off frequency” is chosen and any frequencies below this are passed,
while frequencies above are filtered out, or removed. The setting of the Filter Frequency
parameter dictates the point above which frequencies are removed. This process of
removing harmonics from the waveforms has the effect of changing the sound’s character
or timbre. When the Frequency parameter is at maximum, the filter is completely “open” and
no frequencies are removed from the raw Oscillator waveforms.
In practice, there is a gradual (rather than a sudden) reduction in the volume of the
harmonics above the cut-off point of a low-pass filter. How rapidly these harmonics reduce
in volume as frequency increases above the cut-off point is determined by the filter’s Slope
parameter. The slope is measured in ‘volume units per octave’. Since volume is measured in
decibels, this slope is usually quoted as so many decibels per octave (dB/oct). The higher
the number, the greater the rejection of harmonics above the cut-off point, and the more
pronounced the filtering effect. Each of Summit’s filter sections has a 12 dB/oct slope, but
two of the same type can be cascaded (placed in series) to produce a slope of 24 dB/oct.
Summit also allows two different types of filter to be cascaded, or even to be placed “in
parallel”, so that the mixer output is treated by both.
A further important parameter of the filter is Resonance. Frequencies at the cut-off point
may be increased in volume by advancing the filter’s Resonance control. This is useful for
emphasising certain harmonics of the sound.
As Resonance is increased, a whistling-like quality will be introduced to the sound passing
through the filter. When set to very high levels, Resonance actually causes the filter to
self-oscillate whenever a signal is being passed through it. The resulting whistling tone
being produced is actually a pure sine wave, the pitch of which depends on the setting of
the Frequency control (the filter’s cut-off point). This resonance-produced sine wave can
actually be used for some sounds as an additional sound source if wished.
The diagram below shows the response of a typical low pass filter. Frequencies above the
cut-off point are reduced in volume.
Volume
Volume
Frequency
Cut-off
Frequency
Volume
Volume
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
When resonance is added, the frequencies around the cut off point are boosted in volume.
Volume
Volume
Frequency
Cut-off
Frequency
Volume
Volume
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
In addition to the traditional low-pass filter type, there are also high-pass and band-pass
types. On Summit, the filter type is selected with the Shape switch
58
.
A high-pass filter is similar to a low-pass filter, but works in the “opposite sense”, so that it
is frequencies below the cut-off point which are removed. Frequencies above the cut-off
point are passed. When the Filter Frequency parameter is set to minimum, the filter is
completely open and no frequencies are removed from the raw Oscillator waveforms.
Volume
Volume
Frequency
Cut-off
Frequency
Volume
Volume
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
With a band-pass filter, just a narrow band of frequencies centered around the cut-off point
is passed. Frequencies above and below the band are removed. It is not possible to fully
open this type of filter and allow all frequencies to pass.
Volume
Volume
Frequency
Cut-off
Frequency
Volume
Volume
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
Frequency
Cut-off
Frequency
More complex relationships between volume and frequency can be obtained by using
simple filters of the types described above in combination. Summit allows you to “cascade”
two filters of different types, creating a “series” combination. Such a combination will