User Manual

13
SYNTHESIS TUTORIAL
This section covers the general principles of electronic sound generation and processing
in more detail, including references to Peak’s facilities where relevant. It is recommended
that this chapter is read carefully if analogue sound synthesis is an unfamiliar subject.
Users familiar with this subject can skip this section and move on to the next.
To gain an understanding of how a synthesiser generates sound it is helpful to have an
appreciation of the components that make up a sound, both musical and non-musical.
The only way that a sound may be detected is by air vibrating the eardrum in a regular,
periodic manner. The brain interprets these vibrations (very accurately) into one of an
infinite number of different types of sound.
Remarkably, any sound may be described in terms of just three properties, and all sounds
always have them. They are:
• Pitch
• Tone
• Volume
What makes one sound different from another is the relative magnitudes of the three
properties as initially present in the sound, and how the properties change over the
duration of the sound.
With a musical synthesiser, we deliberately set out to have precise control over these three
properties and, in particular, how they can be changed during the “lifetime” of the sound.
The properties are often given different names: Volume may be referred to as Amplitude,
Loudness or Level, Pitch as Frequency and Tone as Timbre.
Pitch
As stated, sound is perceived by air vibrating the eardrum. The pitch of the sound is
determined by how fast the vibrations are. For an adult human, the slowest vibration
perceived as sound is about twenty times a second, which the brain interprets as a bass
sound; the fastest is many thousands of times a second, which the brain interprets as a
high treble sound.
Time Time
A B
If the number of peaks in the two waveforms (vibrations) are counted, it will be seen that
there are exactly twice as many peaks in Wave B as in Wave A. (Wave B is actually an
octave higher in pitch than Wave A.) It is the number of vibrations in a given period that
determines the pitch of a sound. This is the reason that pitch is sometimes referred to as
frequency. It is the number of waveform peaks counted during a given period of time which
defines the pitch, or frequency.
Tone
Musical sounds consist of several different, related pitches occurring simultaneously. The
lowest is referred to as the ‘fundamental’ pitch and corresponds to the perceived note of
the sound. Other pitches making up the sound which are related to the fundamental in
simple mathematical ratios are called harmonics. The relative loudness of each harmonic
as compared to the loudness of the fundamental determines the overall tone or ‘timbre’ of
the sound.
Consider two instruments such as a harpsichord and a piano playing the same note on the
keyboard and at equal volume. Despite having the same volume and pitch, the instruments
still sound distinctly different. This is because the different note-making mechanisms of
the two instruments generate different sets of harmonics; the harmonics present in a piano
sound are different to those found in a harpsichord sound.
Volume
Volume, which is often referred to as the amplitude or loudness of the sound, is determined
by how large the vibrations are. Very simply, listening to a piano from a metre away would
sound louder than if it were fifty metres away.
Volume
A B
Having shown that just three elements may define any sound, these elements now have to
be realised in a musical synthesiser. It is logical that different sections of the synthesiser
‘synthesize’ (or create) each of these different elements.
One section of the synthesiser, the Oscillators, provide raw waveform signals which
define the pitch of the sound along with its raw harmonic content (tone). These signals
are then mixed together in a section called the Mixer, and the resulting mixture is then fed
into a section called the Filter. This makes further alterations to the tone of the sound, by
removing (filtering) or enhancing certain of the harmonics. Lastly, the filtered signal is fed
into the Amplifier, which determines the final volume of the sound.
Oscillators Mixer Filter Amplifier
Additional synthesiser sections - LFOs and Envelopes - provide further ways of altering
the pitch, tone and volume of a sound by interacting with the Oscillators, Filter and
Amplifier, providing changes in the character of the sound which can evolve over
time. Because LFOs’ and Envelopes’ only purpose is to control (modulate) the other
synthesiser sections, they are commonly known as ‘modulators’.
These various synthesiser sections will now be covered in more detail.
The Oscillators And Mixer
The Oscillator section is really the heartbeat of the synthesiser. It generates an electronic
wave (which creates the vibrations when eventually fed to a loudspeaker). This waveform
is produced at a controllable musical pitch, initially determined by the note played on the
keyboard or contained in a received MIDI note message. The distinctive tone or timbre of
the waveform is actually determined by the waveform’s shape.
Many years ago, pioneers of musical synthesis discovered that just a few distinctive
waveforms contained many of the most useful harmonics for making musical sounds. The
names of these waves reflect their actual shape when viewed on an instrument called an
oscilloscope, and they are: Sine waves, Square waves, Sawtooth waves, Triangle waves
and Noise. Each of Peak’s Oscillator sections can generate all these waveforms, and can
generate non-traditional synth waveforms as well. (Note that Noise is actually generated
independently and mixed in with the other waveforms in the Mixer section.)
Each waveform (except Noise) has a specific set of musically-related harmonics which can
be manipulated by further sections of the synthesiser.
The diagrams below show how these waveforms look on an oscilloscope, and illustrate
the relative levels of their harmonics. Remember, it is the relative levels of the various
harmonics present in a waveform which determine the tonal character of the final sound.
Sine Waves
Volume
Harmonic
1
Sine Wave
Sawtooth Wave
Volume
Volume
Harmonic
Square Wave
Volume
Harmonic
1 2 3 4 5
Volume
Harmonic
1 3 5 7
Triangle Wave
1 2 3 4 5
Harmonic
1 2 3 4 5
Noise
These possess just one harmonic. A sine waveform produces the “purest” sound because
it only has this single pitch (frequency).
Triangle Waves
Volume
Harmonic
1
Sine Wave
Sawtooth Wave
Volume
Volume
Harmonic
Square Wave
Volume
Harmonic
1 2 3 4 5
Volume
Harmonic
1 3 5 7
Triangle Wave
1 2 3 4 5
Harmonic
1 2 3 4 5
Noise
These contain only odd harmonics. The volume of each decreases as the square of its
position in the harmonic series. For example, the 5th harmonic has a volume 1/25th of the
volume of the fundamental.