INTRODUCTION TO SYNTHESIZERS - hol.gr

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The volume of the sound the same as the amplitude of the waveform. The amplitude is the "height" of the waveform, or the height difference between the lowest and the highest part of the wave. The higher the amplitude, the louder the sound. The roar of a jet engine has for instance a much higher amplitude than a whispering voice. It's just like the difference between a pebble that your through in the pond, and a boulder. Trust me, the boulder will make much higher ripples! The amplitude of a sound is also a measurement of its energy state - the louder the sound is, the more energy it carries. It is probably hardly a surprise that sound waves with very high energy can damage your ears. And not just your ears: the roar of a jet engine from at close range can destroy the blood vessels in the human body. Extremely high-energy sounds can even be used to shatter rocks. But since our goal is not rock shattering, let us instead look at how a synthesizer works to create artificial sounds! The oscillators Now we understand the principle behind our sonic environment. Let's see how we can create artificial sounds with a synthesizer. First of all we need something to generate a soundwave with. A device to create a wave with is usually called an oscillator. The first synthesizers used analog electronic oscillator circuits to create waveforms. These units are called VCO's (Voltage Controlled Oscillator). More modern digital synthesizers use DCO's instead (Digitally Controlled Oscillators). A simple oscillator can create one or two basic waveforms - most often a sawtooth-wave - which we have already looked at in the previous chapter - and a squarewave. Most synthesizers can also create a completely random waveform - a noise wave. Sawtooth-wave Square-wave Noise These waveforms are very simple and completely artificial - they hardly ever appear in the nature. But you would be surprised to know how many different sounds that can be achieved by only using and combining these waves. A synthesizer has a function to "map" the wave over the entire keyboard range, and make the frequency of the wave shift from one key to another. Since the frequency of a soundwave is the same as its pitch, we can use the keyboard to play the wave as an instrument. Filters To be able to vary the basic waveforms to some extent, most synthesizers use filters. A filter is an electronic circuit, which works by smoothing out the "edges" of the original waveform. If you listen to a sawtooth wave, you will find it rather "harsh" - because the waveform has a lot of sharp "edges". These edges are in the acoustic terminology called overtones. Now, if we apply a filter to the waveform, some overtones will be removed - and the waveform will become 4
more "rounded". The more overtones are being removed, the more "rounded" or "dull" the sound will become. The filter in other words control the brilliance of the sound. You decide how the filter will operate and how much of the overtones will be removed. The point where the filter will start cutting off overtones is called the cutoff frequency. You can say that overtones higher than the cutoff frequency will not be let though the filter. As the filter is opened up, the cutoff frequency is shifted higher up, and more overtones are let to pass through. The effect is the sound gradually getting brighter. Click on the icon to the left to hear a synthesizer pattern with a gradually opening filter. Most synthesizer filters can not only remove overtones, but also create completely new ones by distorting the original waveform near the cutoff frequency. This feature is called resonance and can create very interesting sounding results. The amplifier Click on the icon to the left to hear a synthesizer pattern with a gradually increasing resonance. To control the volume of a sound in the synthesizer, the signal is passed through an amplifier circuit. The amplifier can raise, or lower the height of the waveform, thereby raising or lowering the volume of the sound. It is very important to keep in mind that when we speak about amplifiers in synthesizer terminology, we don't mean the same kind of amplifier that is required to drive a pair of speakers. Synthesizer amplifier circuits only affect the waveform, and not the sound as it emerges from the Line Out jacks of the instrument. If we want to connect loudspeakers to the output of the synthesizer, we still must use some kind of power amplifier. We can conclude this chapter in the following diagram: The oscillator creates a waveform The filter rounds down the wave to the requested amount of brilliance Now, who said sound synthesis was complicated? Envelopes The wave is amplified by the amplifier to the requested sound level 5
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INTRODUCTION TO SYNTHESIZERS - hol.gr

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