Novation Bass Station II - Manual (Page 10)

10
The Mixer
To extend the range of sounds that may be produced, typical analogue synthesizers have
more than one Oscillator. 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. Bass Station II’s
Mixer allows you create a sound consisting of the waveforms of Oscillators 1 and 2, the
separate sub-octave oscillator, a Noise source, the Ring Modulator output and an external
signal, all mixed together as required.
OSC 1
OSC 1 VOLUME
OSC 2 VOLUME
SUB OSC VOLUME
COMPLEX
WAVEFORM
MIX OF
OSC1, 2 AND 3
MIXER
INPUT TO
FILTER
OSC 2
SUB OSC
The Filter
Bass Station II is a subtractive music synthesizer. 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.
7 types of Filter are available on Bass Station II; they are all variations of the three basic
filter types: Low Pass, Band Pass and High Pass. The type of Filter most commonly used
on synthesizers is Low Pass. On 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. 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. Bass Station II’s filter section provides two
slopes, 12 dB/oct and 24 dB/oct.
A further important parameter of the Filter is its Resonance. Frequencies at the cut-off point
may be increased in volume by the Filter 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 knob (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 Bass Station II, the Filter type is selected with the Shape switch 32 .
A High Pass Filter is similar to a Low Pass Filter, but works in the “opposite sense”, so that
frequencies below the cut-off point are removed. Frequencies above the cut-off point are
passed. When the Filter Frequency parameter is set to zero, 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
When a Band Pass Filter is used, only a narrow band of frequencies centered around the
cut- off point are 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
Envelopes And Amplifier
In earlier paragraphs, the synthesis of the pitch and the timbre of a sound was described.
The next part of the Synthesis Tutorial describes how the volume of the sound is controlled.
The volume of a note created by a musical instrument often varies greatly over the duration
of the note, according to the type of instrument.
For example, a note played on an Organ quickly attains full volume when a key is pressed.
It stays at full volume until the key is released, at which point the volume level falls instantly
to zero.
TIME
KEY "ON" KEY "OFF"
VOLUME
ATTACK DECAY RELEASE
SUSTAIN
TIME
KEY "ON" KEY "OFF"
VOLUME
TIME
KEY "ON" KEY "OFF"
VOLUME
TIME
KEY "ON" KEY "OFF"
VOLUME
ATTACK DECAY
RELEASE
SUSTAIN
SUSTAIN
RATE
TIME
KEY "ON" KEY "OFF"
VOLUME
ATTACK DECAY RELEASE
SUSTAIN
SUSTAIN TIME
TIME
KEY "ON" KEY "OFF"
VOLUME
ATTACK DECAY RELEASE
SUSTAIN
TIME
KEY "ON" KEY "OFF"
VOLUME
ATTACK DECAY RELEASE
SUSTAIN
TIME
KEY "ON" KEY "OFF"
FILTER
CUT-OFF