Analog synthesis
Words by H.M Sowards
In the last few decades, the world has seen the synthesizer rise from an experimental oddity to a mainstay in today's music world. This electronic instrument crosses over into almost every style of contemporary, popular music from country to jazz to techno. The synthesizer has brought electronic music a long way from its archaic beginnings when electronic musicians played with vacuum-tube circuitry and tedious tape splicing. The invention of the synthesizer has made it easy for the electronic musician to create new and imitate familiar sounds with relative ease. But just how does the analog synthesizer create electronic sound and further more how is it amplified in recording situations? If you want to know the answers to these questions... keep reading... we're about to find out.

Synthesizer technology is broken up into two categories "analog" and "digital". However, with the addition of the digital analog modeling synth, things aren't as cut and dry as it used to be. But... In order to get a good basic understanding of how sound in a synthesizer is produced, the old fashioned analog system will be discussed here.

A typical analog synthesizer consists of a keyboard, usually at least 2 oscillators, a low frequency oscillator (LFO), usually 2 envelope generators (EG), a filter and a voltage control amplifier (VCA). Playing the synthesizer involves routing audio and control signals from the keyboard through the various components listed above (Yelton 49).

When a key is depressed on the keyboard a pitch control signal is sent to the oscillators (VCO- Voltage Control Oscillator). An oscillator is a circuit that creates a single periodic wave form at a desired frequency (Moog 17). The oscillato generates the desired frequency and wave form and then routes an audio signal to the Voltage Control Filter (VCF)(49) . Before we contrinue on the route the audio signals thake within the synthesizer, let's discuss waveforms for a moment

Analog oscillators usually offer a limited variety of waveforms. The most common are the Sine, the Triangle, the Pulse, and the Sawtooth. Sine waves are the simplest of waves. They are pure and contain no harmonics. Sine waves are known for their hollow and woodwind-like sound. Triangle waves rise linearly and fall linearly at the same rate (17). Triangle waves are brighter sounding than Sine waves because of the sharp edges in their wave form, but they are not as sharp sounding as the Pulse and Sawtooth waves. Pulse waves rise abruptly and stay level for a period of time and then fall abrubptly and stay level. The percentage of time it takes for it to complete its cycle is called its "pulse width". The Pulse wave is known for creating a variety of tone colors. Since it has periods where the wave is flat, there are spots where certain harmonics will be stronger than others (17). Sawtooth waves rise linearly anddrop abruptly every cycle. These waves have high harmonic content and are known for their bright, "buzzy" full sound.
Meanwhile, the keyboard has also sent control signals to another part of the synthesizer, the envelope generators. There are usually 2 envelope generators in a synthesizer. One controls the Voltage Control Filter (VCF) and the other the Voltage Control Amplifier (VCA). I''ll explain what these two devices do in a moment, but first lets talk a minute about the the type of control signals that the keyboard is sending to these devices. The first control signal that is sent to the envelope generators is called a "trigger" (Yelton 35). The "trigger" sends a message to the generators telling them a key has been pressed. This begins the envelope generator's process of creating an envelope for the wave form being generated by the oscillators. As long as the key on the keyboard is held down, another control signal, the "gate" is sent to the envelope generators. The "gate" signal tells the generators that the note is still being played and the envelope being generated will stay open until this "gate" signal ceases. When the "gate" signal stops, the envelope will go into it's final stage (29). In some analog synths you will find that the "gate" signal will double as the "trigger" signal as well.

Envelope generators are used to help the synthesizer produce expressive sounds (Moog 27). They allow you to shape sounds by sending a varying control signal to the synth's VCA and VCF. Like I mentioned earlier there are usually two envelope generators in a typical analog synth. The output of one generator is hardwired tp the Control Voltage (CV) input of the VCF, and the output is wired to the CV input of the VCA. Having the generators set up like this allows you to have control over the brightness and loudness of the sounds that the synth produces. The envelope connected to the VCA tells the amplifier when to open, how far to open, and when to close (Yelton29). The other sends a control signal to the VCF telling it how to filter the incoming audio signal. The most common type of envelope generators are the ADSR type (Attack, Decay, Sustain, Release). The Attack of the envelope determines how much time it will take for an audio signal level to go from zero to maximum level when a key is struck (29). For the envelope that is controlling the filter, the Attack controls how long it will take the filter to open to it's maximum brightness (35). The Decay controls how long it will take for the envelope to fall to it's sustain level and determines how far to close the cut off frequency in the filter before it falls to it's sustain level (29). The Sustain of the envelope controls how much signal can pass through the VCA when the note is being held and tells the VCF to hold brightness at a steady level until the key is released. The Release stage of the envelope starts when the gate signal to the envelope generator ceases. The amplifier's output is dropped down to zero at the rate determined by the Release parameter (29) and the VCF returns to it's original cut off frequency (35).Other types of envelope generators found in analog synthesizers are the AR (Attack & Release), DADSR (Delay, Attack, Decay, Sustain, Release) and ADS (Attack, Decay, & Sustain) found in the Minimoog.

The basic concept of analog synthesizers is a technique called "subtractive synthesis". Subtractive synthesis is the act of manipulating tone color by filtering complex wave forms (Strange 33). Basically, it means taking all of the audio signal that the oscillators produce and manipulating them to sound the way you want by "subtracting" frequenceies that are undesirable and "re-shaping" the wave form. Almost all analog synthesizers have atleast one VCF (Voltage Control Filter), and they most often are low pass filters (Yelton 29).

Low pass filters do exactly as the name implies-- they let the low frequenciespass. If you turn a lowpass filter's frequency all the way down.. you close the filter and nothing can ps. When you manually open it up a little at a time, you sou slowly raise the cut off frequency, and will hear the lowest frequencies first (29). If the filter is already wide open, it will have no effect (35). Other types of filters sometimes found on analog synths are highpass filters that filter out low frequencies, bandpass filters that let a certain band width pass and filters out low and high unwanted frequency bands, and bad reject filters that stop a narrow band width from coming through.

It would seem at this point that the audio signal that the synth has created is ready to be passed out of the machine and on to human ears, but there is still one more important step. That is the use of the LFO (Low Frequency Oscillator), also called the sweep or modulation generator, to modulate the signal. Modulation is the act of using one waveform to shape or texture a property of another waveform (Moog 31). The LFO produces sine and triangle wavesin the sub-audio range, usually below 9Jz, that contour the shape of the waveform they are sitting underneath. The LFO can be routed to modulate almost any module of the synth (Leonard 36). An LFO routed to the VCO creates a change in pitch (vibrato). A LFO routed through the VCF will alter the tone color of the sound (trill). Modulating the VCA will change the amplitude(tremolo) (36). Other types of modulation modules that aren't available in every synthesizer are: the ring modulator, which acts on the principle of amplitude modulation, and the random note generator, also called sample & hold (58).

Once the waveform has been created by the oscillator, run through the envelope generators, VCF, VCA and LFO, the signal is ready to leace the synth and become an actual heard sound. Due to the electronic nature of the synthesizer, microphones are not used in recording it's sound. Generally, synthesizers will plug directly into the mixing console for recording. Synthesizers are high impedance devices, and while it is possible to plug them into the mixing desk using 1/4" phone plugs, it is generally advisable to change the signal to low impedance through a device called a "direct box" and then plug the synth into the console with XLR cables. The reason

for this is that low impedance outputs provide a cleaner signal and are less likely to pick up extraneous noise. This is especially important for live performance situations.

In summary, we have seen how sound is created in an analog synthesizer. When a key on the keyboard is depressed, control signals are sent to the VCO, and the two envelope generators. The VCO sends out a specific waveform that is modulated by the LFO and it goes to the VCF that is controlled by the envelope of one of the envelope generators. The VCF controlls the timbre of the wave. The audio signal is then sent to the VCA which is controlled by the envelope of tahe second envelope generator. The VCA controls the amplification of the sound. Once the sound is created it can be plugged into an external amplifier or console using 1/4" phone jacks or the preferred low impedance XLR cables by running the output through a "direct box".

By understanding the basic principles behind the analog synthesizer, how sound is produced and how to record it, the electronic musician can better manipulate electronic sound. It also gives him/her a solid base in which to expand on his knowledge of other synthesizer concepts, such as digital synthesis. The more familiar a synthesist is with how his/her instrument works, the more creative he/she can be, giving the synthesizer a chance to be a popular instrument (read world domination here!) in modern music for many years to come.

Category | TechTalk: Analog synthesis Added: 09/02/07
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