Okay, I'm finally writing something technical again, as many
of my assignments are taking me away from the technical.
We all love music. As
a musician (keyboardist), I would like to say something about synthesized music
and synthesizer internals.
Synthesizer - A Sound
Wave Generating Machine
The notion of synthesis takes music back to basically what
it is, patterned sound. So different
instruments generated different sound waves.
Synthesizers take music down to its basics: it is a sound wave generating machine. Because it can generate any sound wave, it is
capable, as many of you know, of simulating the sound of most instruments, as
well as capable of pitch (frequency), sound wave shape, and volume (amplitude
or amplification).
Sound Waves
Sound waves are studied by physics. Sound results from the compressions and
rarefactions of a sound wave through a medium where particles exist and can be
displaced (air, water, solids, etc...).
There is no sound in a vacuum, as in space, as there are no particles to
be displaced. Think of a pure sound wave
as a sine function, which oscillates regularly at a certain frequency. Higher frequencies are perceived as higher
pitch sounds and lower frequencies are perceived as lower pitch sounds. Humans can perceive sound wave with
frequencies as low as 20 cycles per second (think sine wave) and as high as
20,000 cycles per second (the audible range).
We are most sensitive to sounds around 3,000 cycles per second. The amplitude (height) of the sound wave
gives its intensity or volume. Everyone
knows that when you turn up the "amplifier" you get a louder
sound. Loudness is measured in decibels
from a whisper at 30 decibels to a rocket launch at 180 decibels. In synthesizers, one is also concerned with the
waveshape, which gives the quality of the sound (e.g. a violin versus a
drum). By waveshape is meant the
modification to the sine wave which may have a quick rise and slow fall or a
slow rise and quick fall or any other shape (e.g. sawtooth, etc...). As we discuss synthesizer internals, remember
the synthesizer is doing all of its electronic music magic in silence (just by
voltage fluctuations and controls) and actually makes no sound until the
voltages are sent to the amplifier and the amplified signal out to a speaker
which vibrates in a medium like air and creates the compressions and
rarefactions of the air medium which we perceive as a sound.
Synthesizer Internals
Consider the synthesizer internals diagram below. The text that follows will explain it.
Figure 1: Synthesizer Internals (My Diagram).
Central Controller
The heart of the synthesizer is the central controller which
translates the messages from the keyboard panel to signals to operate the
various oscillators, envelope generators, filters, amplifier, and memory access
(to sound patches). The central
controller either receives messages through the MIDI (Musical Instrument
Digital Interface) controller (pathway K3) or from the keyboard panel (pathway
K1). The central controller then
performs the requisite memory access, e.g., for a patch number request, or
signal generation to control the audio production components.
Oscillator (Voltage Controlled Oscillator --- VCO).
The oscillator(s) are the heart of the production of the
audio signal. Oscillators produce a
voltage fluctuation of a specified duration and frequency. Controlling the time it takes the voltage to
rise, the duration for which that voltage level is maintained, and how quickly
the voltage falls determine the wave shape (attack, sustain, decay of the wave)
output of the oscillator. The exact
contour of the waveshape can typically be modified by voltage control signals
received by the oscillator from the central controller (pathway V1 -- voltage
control signal one) and ultimately from a patch editor located on the key panel
or in computer software. The frequency
of the voltage fluctuations (pitch) can likewise be controlled (via pathway
V1), as well as the wave's amplitude.
The output of the oscillator is a complex waveform with harmonic overtones,
some of which are undesirable and need to be filtered. The VCO's output is sent to the VCF (Voltage
Controlled Filter) via pathway A1 (audio signal one).
Filters (VCF --- Voltage Controlled Filters).
The VCF mathematically eliminates various harmonic overtones
of the waveform received from the oscillator, while emphasizing others. It is a well-known mathematical fact that
complex wave forms (ala Fourier analysis) can always be resolved into their
sinusoidal components. I.e., the
frequency of component sine waves are whole number multiples of the frequency
of the complex waveform as a whole.
Along these lines, a filter may identify undesirable overtones and
prevent them from passing to the amplifier while emphasizing others.
Transient Envelope Generator
The voltage control signals which the filter applies to the
signal generated by the oscillator is generated by the transient envelope
generator (pathway V2). These include
cutoff frequency signals, which cuts off certain overtones and will determine
how bright or mute the sound is. Other effects
can be induced, such as tremolo, which is a slow sine wave envelope sent to the
VCF to modify the oscillator audio signal.
Envelope Generator
The function of the envelope generators is to generate the
shape of the voltage signal which is applied by the filers and the amplifier to
the audio signal.
Memory
Most commercial synthesizers have two basic features. One, they come with a library of preset
sounds, and two, they offer some ability to modify these sounds or create new
sounds and store them (via pathway M1).
It is important to understand that what a pre-set sound (or voice) is,
is a 'patch,' i.e., a set of instructions stored in memory which the controller
can retrieve (via pathway M1) to initiate the output of certain audio signals
from certain oscillators, and to apply certain voltage control signals to
certain filters and the amplifier to produce a specific sound. I.e., the patch is a type of program for the
internal synthesizer components to create a certain sound. Many synthesizers allow the sophisticated
user to modify their own waveforms and create their own patches which they may
save to memory (via pathway M1). The
library of preset sounds (voices) or patches, is stored in non-volatile memory
and loaded into memory upon power up.
From the user perspective, the accessibility of these patches appear as
'instruments' arranged in m banks of n instruments. Synthesizers often come with thousands of
preset patches or instruments. Some commercial synthesizers will offer
additional off-line patch storage and keyboard sequence (a song) storage as
disks, cartridges, or even now memory sticks.
Summary
This has been a brief, semi-technical description of
synthesizer internals. The block diagram
indicates the fundamental components of any synthesizer. In reality, a synthesizer is a bit more
complicated in that it will typically have multiple oscillators (6 to 8 are
typical), and multiple filters, and signals may be processed in alternative algorithms
selecting different oscillators and filters using different pathways. Nonetheless, the signal processing always
proceeds in the order I have described.
Synthesizers may simply be thought of as signal processors, where the
output is an amplified audio signal.
Next I will create a blog on MIDI (the Musical Instrument
Digital Interface). Watch for it, and
thanks for reading my blog.
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