5
Synthesis Tutorial
Elements of a sound - Oscillators and Waveforms
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 chapter and move on to the chapter - MIDI Tutorial on Page
10.
To gain an understanding of how a Synthesizer generates sound it is
helpful to have an understanding of the components that make up a
sound, be it musical or 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 vibra-
tions (very accurately) into one of an infinite number of different types
of sound.
Remarkably, any sound may be described by just three terms, and all
sounds always* have them. They are :
* Volume
* Pitch
* Tone
What makes one sound different to another is the proportion of these
three qualities initially present in the sound and how these three terms
change throughout the duration of the sound.
With a musical synthesizer we deliberately set out to have precise con-
trol over these three terms and, in particular, how they can be changed
throughout the duration of the sound. These terms are often given dif-
ferent names, Volume is referred to as Amplitude, Pitch as Frequency
and Tone as Timbre.
Pitch
Taking the example of air vibrating the ear drum, the pitch is deter-
mined by how fast the vibrations are. For an adult human the lowest
vibration perceived as sound is about twenty times a second, which the
brain interprets as a bass type sound, and the highest is many thou-
sands of times a second, which the brain interprets as an extreme tre-
ble type sound.
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 frequency of the waveform peaks which are counted
during a given period of time.
Tone
Musical sounds consist of several different related pitches occurring
simultaneously. The loudest is referred to as the ‘Fundamental’ pitch
and corresponds to the perceived note of the sound. Pitches related to
the fundamental are called harmonics. The relative loudness of these
harmonics compared to the loudness of all the other harmonics (includ-
ing the fundamental) determines the 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 would still sound distinctly
different. This is because the harmonics present in a piano sound are
different to those found in a harpsichord sound.
Volume
Volume, which is 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.
Having shown that just three elements make up any sound, these ele-
ments now have to be related to a Musical synthesizer. It is logical that
a different section of the Synthesizer ‘Synthesizes’ (or creates) these
different elements.
One section of the synthesizer, the Oscillators, can generate multiple
waveforms which provide the pitch of the sound along with its raw har-
monic content (tone). These waveforms may be mixed together in a
section called the Mixer. The mixed signal is then fed into a section
named the Filter which is responsible for further altering the tone of
the sound. It does this by removing (filtering) certain undesired har-
monic frequencies. Lastly, the filtered signal is fed into a final section,
the Amplifier which determines the final volume of the sound.
Additional synthesizer sections; LFOs and Envelopes provide ways of
altering the pitch, tone and volume of a sound by interacting with the
Oscillators, Filter and Amplifier. They introduce changes in the char-
acter of a sound that evolve thoughout the duration of the sound.
Because the LFOs and Envelopes only purpose is to control (modu-
late) the other synthesizer sections, they are commonly known as
‘modulators’.
These various synthesizer sections will now be covered in more detail.
The Oscillator is really the heartbeat of the Synthesizer. It generates
an electronic wave (which creates the vibrations). This Waveform is
produced at a controllable musical pitch, initially determined by the
note played on the K-Station keyboard or a received MIDI note mes-
sage. The initial distinctive tone or timbre of the waveform is actually
determined by the wave’s shape.
Many years ago, pioneers of musical synthesis discovered that just a
few distinctive waves contained most of the useful harmonics for musi-
cal synthesis. The names of these waves reflect their actual shape
when viewed on an instrument known as an Oscilloscope, and are
known as, Sine waves, Square waves, Sawtooth waves, Triangle
waves and Noise Waves.
Each one has a specific fixed amount of musically related harmonics
(except noise waves) which can be manipulated by further sections of
the Synthesizer.
*Noise is a special case since it contains all frequencies.
Wave B
Wave A
Wave B is twice the pitch of Wave A
Time
Wave BWave A
Volume
Wave A is louder than Wave B but is the same pitch
Oscillators
Mixer Filter
Amplifier
Audio path of the main Synthesizer blocks
Elements of a sound
Oscillators and waveforms
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