Explain the usage of the 5 most important synthesis modules: Oscillator, Filter, Amplifier, Envelope, and LFO.

The goal of this tutorial is to explain how the 5 more important synthesis modules work, these would be:

Oscillator, Filter, Amplifier, Envelope, and LFO.

Oscilator
An Oscillator generates a sound. Its task is to create a waveform which will produce a different sound depending on the shape of the waveform.

The oscillator does this continuously. The rate at which it generates each cycle of the waveform is what we hear as pitch. Pitch is measured in Hertz (Hz)

The most common oscilators are:

1. Saw Wave - shaped like the teeth on a saw blade, this produces a very common sharp, biting tone.
2. Square Wave - looks like a (near) perfect square, produces a reedy, hollow sound.
3. Pulse Wave - a variation on the above, the pulse wave is half as wide as a square wave, and has the unique ability to have its width modulated (called ‘Pulse Width Modulation').
4. Triangle Wave - unsurprisingly shaped like a triangle, this sounds somewhere in between a saw wave and a sine wave.
5. Sine Wave - a smooth rising and falling shape (like a horizontal ‘S'), this produces a mild, soft tone.
6. Noise - not exactly a waveform, but a source of sound produced by a certain colour of noise.

Filter
A Filter is used to remove frequencies from the waveform so as to alter the timbre.The filters can be labeled VCF (Voltage Controlled Filter) or DCF (Digitally Controlled Filter) in our synthesizer.

After being filtered, a brilliant-sounding sawtooth wave can become a smooth, warm sound without sharp treble. 
 
The most common filters are:

1. Lowpass filter: Low frequencies are passed; high frequencies are attenuated.
2. Highpass filter: High frequencies are passed; low frequencies are attenuated.
3. Bandpass filter: Only frequencies within a frequency band are passed.
4. Band Reject filter: Only frequencies within a frequency band are attenuated.
5. Allpass filter: All frequencies in the spectrum are passed, but the phase of the output is modified.

Filter Parameters

Cutoff Frequency

The cutoff frequency, or cutoff, as the name suggests, determines where the signal is cut off. For instance if a signal contains frequencies that range from 20 to 4000 Hz, and a cutoff frequency is set at 2500 Hz, frequencies above 2500 Hz are filtered. The lowpass filter allows frequencies below the cutoff point of 2500 Hz to pass through, unaffected.

Resonance

The resonance control emphasizes or suppresses signals around the cutoff frequency. Resonance is where the frequencies at the Cut-Off position are boosted (made louder).

• When a little resonance is introduced, the frequencies around the Cut-Off are boosted a little. 
• With a lot of resonance, the frequencies around the Cut-Off are boosted a lot while the lower frequencies will start to diminish a little. 
• When too much resonance is applied, the frequencies around the Cut-Off are so pronounced that the filter will itself start to oscillate and "howl" (like feedback, but in a tuned way).

The overall result of using filter resonance is a change in the basic waveform’s shape and, therefore, its timbre.

Drive

Drive adds an amount of gain to the waveform as it enters the filter an input gain control thus overdriving the filter and distorting the waveform. This waveform distortion changes the timbre of the sound, making it much harsher.


Filter Slope

As you now know , the purpose of a  filter is to cut off the signal at the set cutoff frequency. This cutoff doesn’t happen abruptly but rather at a given slope, which is measured in decibels (dB) of gain reduction per octave.

Put another way, you can define how steep the “cliff” is at the cutoff point by choosing a relatively severe or more gentle slope.





Amplifier


The amplifier module of a synthesizer is responsible for controlling the level—or loudness—of the signal over time. It is the  part of the synthesizer that amplifies the sound that is generated by the oscillator. It normally has a control input which affects the level of amplification and which is normally be driven by an envelope or an LFO  which controls the overall loudness (or amplitude) of the sound over time.

The amplifier section on a synthesizer may be labled as VCA (Voltage Controlled Amplifier) or DCA (Digitally Controlled Amplifier).


Envelope

An envelope generator generates a signal that changes through the length of a sound, normally to control the loudness of that sound. An example of its use is to control the volume of a piano sound. It creates a signal that goes from low to high very quickly (the loud sound happens when the key is pressed), then goes down slowly as the sounds gets quieter.

They are also used to drive other parts of the synthesizer, for example the filter.

The most common envelope has 4 parameters usually   called ADSR Envelope.

 Let us look at what happens to an ADSR envelope when a key is pressed on the keyboard (Before any key of pressed, the envelope is at zero and so there is no sound).

1. Attack - Immediately upon pressing a key, the envelope "opens" from zero to full. This is controlled by Attack time. An Attack time of zero means the envelope goes from zero to full instantly (ie sharp attack). Increasing the Attack time means that this will happen more slowly.
2. Decay - While the key is still pressed, the envelope then drops from full to the Sustain level. This is controlled by Decay time. A Decay time of zero means the envelope goes from full to sustain instantly. Increasing the Decay time means that this will happen more slowly.
    
3. Sustain - While the key is still being pressed, the continuing envelope just remains at the Sustain level. This is set by Sustain level.
   
4. Release - When the key is released, the envelope then drops from the Sustain level to zero. This is controlled by Release time. A Release time of zero means that this will happen instantly. Increasing the Release time means that this will happen more slowly. 

If a key is released during the attack or decay stage, the sustain phase is usually skipped. A sustain level of zero will produce a piano like or percussive envelope, with no continuous steady level, even when a key is held.

Envelope generators are not limited to controlling signal amplitude. They can also control the rise and fall of the filter cutoff frequency or modulate other parameters. In other words, envelope generators can be used as a modulation source or as a “remote control” for a given parameter.


LFO

A Low Frequency Oscillator. It is similar to a normal oscillator except that it outputs very low frequencies (very slow waves) and is used as a control input into another part of the synth. Examples of its use are to create vibrato or tremolo effects.

This oscillator is used only as a modulation source and does not generate any audible signals that form part of your actual synthesizer sound, because it’s too low to be heard. It can, however, affect the main signal by adding vibrato, filter sweeps, and so on.

Without modulation, sounds tend to be boring and fatiguing to the ear. They also sound synthetic, rather than natural, in the absence of some type of sonic modulation. The most obvious type of modulation is vibrato, which is used by orchestral string players to add animation to an instrument’s pitch.



LFO Controls

The LFO generally offers the following controls:

1. Waveform: Allows you to choose the type of waveform (triangle waves and square waves are seen most often). Triangle waves are useful for filter sweeps (slow changes to the filter cutoff frequency) or when simulating an ambulance siren (slow changes to the oscillator frequency). The square waveform is useful for rapid switches between two different pitches (vibratos or octaving, for example).
2. Frequency/Rate: Determines the speed of the waveform cycles produced by the LFO. When set to low values, very slow ramps are produced, making it easy to create sounds such as ocean waves rolling in (when white noise is chosen as the waveform in the main oscillator).
    
3. Sync mode: Allows you to choose between free running (a user-defined LFO rate) or synchronization with an external tempo source (such as a host application).

The LFO can also be controlled with an envelope generator in some synthesizers. As an example of where this might be useful, imagine a sustained string section sound where vibrato is introduced a second or so into the sustained portion of the sound. If this can happen automatically, it allows you to keep both hands on the keyboard.

In certain synthesizers, a simple envelope generator is included for this precise purpose. Often, this envelope consists only of an attack parameter, or occasionally it includes decay or release options. These parameters perform in the same fashion as the amplitude envelope parameters , but they are limited to control of LFO modulations.



LFO Types




In closing

By working on this assignment many doubts I had about the Synthesis modules were clarified, this post is a bit longer that usuall, but i hope this info is usefull for you as it was for me

Thanks a lot for reviwing this tutotial.