INTRODUCTION TO SYNTHESIZERS - hol.gr

Of course not just the loudness of a sound changes over time, but both the timbre, and the pitch might change
between the onset and the fade-out - like how the pitch of a train whistle drops as the train passes by.
To simulate the different envelopes on a synthesizer, we must be able to control the oscillator, the filter and the
amplifier in a much more detailed way than just our basic on/off function. We want to be able to control the
envelope for each synthesizer circuit. This is done by using envelope generator circuits.
The envelope generator of the oscillator controls how the pitch changes through the duration of the sound, while
the envelope generator of the amplifier controls how the volume changes over time. The filter also has its own
envelope generator, which controls the changes in the "brilliance" of the sound.
The envelope generators on most synthesizers use the ADSR model we have looked at in this chapter. But on
some other synthesizers we have more detailed control over the envelopes of the sounds. The Casio CZ-series
synthesizers have as much as eight different envelope stages, which makes it possible to create some very
complex envelopes.
Modulation
Rapid periodic changes in a sustaining sound's pitch is called vibrato. If the periodic changes affect the loudness
of the sound instead of the pitch, it is called tremolo. Vibrato and tremolo are almost always an important part
of the sound of acoustic instruments - for instance, a sustained violin sound without vibrato will sound
surprisingly raw and unmusical. Let's see how we can achieve these effects on our synthesizer voice!
If you have read the previous chapter, you should now at least have a basic understanding of which part of the
synthesizer that controls the pitch and the volume of the sound. (Come on folks, of course I am referring to the
oscillator and the amplifier!)
Since vibrato affects the pitch of the instrument, it shouldn't be a surprise that the effect can be simulated by
applying a slight, periodic change to the oscillator wave. This is called modulating the oscillator. Tremolo on the
other hand affects the amplitude of a sound, so for this effect the modulation should instead be applied to the
amplifier.
Both vibrato and tremolo effects require a rather slow wave with only around two or three cycles every second.
To create this modulation wave, we must now introduce a new synthesizer component: the Low Frequency
Oscillator (LFO). Some synthesizer manufacturers use the term Modulation Generator instead.
Natural vibrato and tremolo effects tend to increase in strength as a note is sustained. For example, a flute
sound might have a tremolo effect, which begins at some point after the note has sounded, and the tremolo
effect gradually increases to some maximum level, where it remains until the note stops sounding. This is
accomplished in synthesizers by applying an envelope generator to the LFO.
Not all synthesizers allow this kind of precise control over the LFO, and quite often we can only set a delay time
before the modulation begins.
However, on most synthesizers we can select the waveform of the LFO. An LFO with a triangle- or sine-shaped
waveform applied to the pitch will give a normal vibrato, but if we change the waveform of the LFO to a
squarewave, we will achieve a different musical effect, called a trill.
Voices
Well, now we have a fully usable synthesizer voice. But we still have only one voice - so we can't play chords
yet. Even if we play five keys at once on the keyboard, we will still only hear one tone.
The first synthesizers were all like this - they are monophonic synthesizers. If you wanted to play chords, you
would have to buy more than one synthesizer - one for each tone, or use a multitrack tape recorder and
overdub the takes.
But wait a minute - why can't we put more than one oscillator, one filter and one amplifier in one instrument?
Then we could play more than one sound at once. Right?
Absolutely right! It didn't take long until synthesizer manufacturers realized this possibility, and soon four-voice
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