INTRODUCTION TO SYNTHESIZERS - hol.gr

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Now we have the minimal requirements to create a synthesizer voice. But we still need to make this sound playable as a music instrument voice. Let's see what's missing! First of all, natural sounds normally don't just instantly switch on and off. Sounds are hardly ever "static" but change their character through time. A real life sound has always a fade in and fade out period. To take an example, a drum hit begins very sharply as the drumstick hits the skin and also fades away quite fast. The sound volume of a note on the piano will also rise rather quickly, but will dampen much more slowly. The sound of some instruments - like for instance the violin - can be made to sustain for a long time, while the sound of a drum inevitably fades away, regardless of if we press the drumstick against the skin after the initial hit or not. This behavior is called the envelope of the sound. Let us illustrate such an envelope: Of course this is just a very simplified envelope curve, and the envelopes of acoustic sounds are a lot more complex than this one. But we can still identify some main parts of the envelope that we will be able to use in our synthesized sound. These parts are: • Attack (the initial onset of the sound) • Decay (the first fading of the sound) • Sustain (the level at which the sound is held as long as the key is depressed) • Release (the fade out of the sound) This kind of envelope is often called an ADSR-envelope (by combining the initial letters of the name of the different phases - Attack, Decay, Sustain and Release). Envelopes for other sounds does not necessary always have to look like in the example above. For drum sounds the sustain phase may for instance be lacking completely, since a drum sound cannot be sustained infinitely. The amplitude envelope for a short, percussive sound with a long reverberant echo may look like the one in the following illustration: This kind of envelope is very well suited for drum-like staccato playing - even if the timbre itself is not a drum sound, but a string or human voice sample. 6
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 <strong>gr</strong>adually 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 7
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INTRODUCTION TO SYNTHESIZERS - hol.gr

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