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Synthesis - Part 3

There are many different types of synthesis and with varying names and sometimes their names can get very confusing as, quite often, writers like to use their own titles for a type of synthesis that is already well known. So, I am going to describe the most important and most commonly used types of synthesis and I will use their most common names as opposed to some of the fancy names that other writers like to adopt.

BUT, before we get deep into synthesis you need to learn one of the most important aspects of sound….HARMONICS

First, the emotional definition or description:

All musical tones have a complex waveform, made up from loads of different frequencies. That’s easy as we have already covered these topics. All sounds are formed using a combination of sine waves at varying frequencies and amplitudes. This, you know as well, so you see how easy it is so far. Now it gets a touch more involved. If we look at the frequencies of a complex waveform, then the lowest frequency is called the fundamental frequency. The fundamental frequency determines the pitch of the sound. The higher frequencies are called overtones. If the overtones are multiples of (x1, x2, x3 etc) the fundamental frequency then they are called harmonics. The overtones or upperpartials as some people like to refer to them as, must be multiples of the fundamental to be known as harmonics. These frequencies and their amplitudes determine the timbre of a sound.

Now, the simpler explanation:

If you have a waveform that has a fundamental frequency of 100 kHz, then the second harmonic will be 200 kHz and the third harmonic will be 300 kHz and so on……

If you think about the irregular waveform of noise then you will understand that it has no harmonics. Noise, as we discussed earlier, contains a wide band of frequencies and it is generally accepted that, at waveform level, there are no harmonics as the waveform is non-repeating.

Harmonics will keep creeping into these tutorials and they are essential when it comes to synthesis.

Now let us look at the different types of synthesis.

ADDITIVE

Does what it says on the tin. By adding one or more basic and simple waveforms together and their harmonics you create a complex waveform. However, you need to add an enormous amount of harmonics to create the simplest of sounds and this type of synthesis can be complicated to create in the form of a synthesizer but the Kawai K5000 does exactly that. You can create extremely rich textures or wild and crazy sounds on this beast. Personally, I love additive synthesis but then again I am receiving very intense therapy. The process of additive synthesis is also referred to as summing the waveforms and harmonics. This method adopts Fourier analysis. Described as the representation of a sound's frequency components as a sum of pure sinusoidal waves . An analysis of a sound's frequency components is taken at a steady state to give an approximation of that sounds spectrum. As most natural sounds are spectrally dynamic, one single Fourier analysis could not possibly represent a sound in sine waves. By 'windowing', a Fast Fourier Transform (FFT) takes several of these approximations and strings them together to better predict a sound's spectrum over time. Although this is daunting to take in it is crucial to know and I don’t expect you to understand Fourier analysis but just thought I would bring it in now as we will come back to this at the advanced stages of these tutorials.

SUBTRACTIVE

This process involves the generating of complex waveforms and then filtering the frequencies so that you are then left with the sound you want. You take away the frequencies. Obviously the filters are crucial in subtractive synthesis and the better the filters and the wider the choice of filters available, the better the end result will be.

FREQUENCY MODULATION (FM)

The output of one oscillator (modulator) is used to modulate the frequency of another oscillator (carrier). These oscillators are called operators. FM synthesizers usually have 4 or 6 operators. Algorithms are predetermined combinations of routings of modulators and carriers. To really explain this I would have to go into harmonics, sidebands, non-coincident and coincident series and the relationships between modulators and carriers. So I won’t go there. What I will say is that FM synthesis can create lovely digital type of sounds, from brittle to lush. A little bit of info for DX owners, is that the oscillators on these synthesizers were all sine waves.

PHYSICAL MODELLING (PM or PHM)

This form of synthesis simulates the physical properties of natural instruments, or any sound, by using complex mathematical equations in real-time. This requires huge processing power. You are not actually creating the sound but, you are creating and controlling the process that produces that sound. Waveguides and algorithms come into this process heavily but, again, I won’t go into that. What I do find interesting is that the Nord Lead uses PM synthesis to emulate an analogue synthesizer. Weird huh?

LINEAR ARITHMETIC SYNTHESIS

This type of synthesis takes short attack sampled waveforms called PCM, or Pulse Code Modulation and combines them with synthesized sounds that form the body and tail of the new sound. By layering these and combining them with the synthesized portion of the sound you arrive at the new sound. The final sound is processed by using filters, envelope generators etc. This is one of the most common forms of synthesis used in the 90s and even today. Roland were the most famous for adopting this type of synthesis and the D50 was one of the most common of the synthesizers that used LA synthesis. By the way, a great synthesizer and still used today.

WAVETABLE SYNTHESIS

This form of synthesis incorporates the use of pre recorded digitized audio waveforms of real or synthetic instruments. The waveforms are then stored in memory and played back at varying speeds for the corresponding notes played. These waveforms usually have a looped segment which allows for a sustained note to be played. Using envelopes and modulators, these waveforms can be processed and layered to form complex sounds that can often be lush and interesting. The processes are algorithmic and memory is crucial to house the waveforms. I could get into linear crossfading sequentially, quasi-periodic and sine functions etc. But I won’t.

GRANULAR SYNTHESIS

This is the method by which tiny events of sounds (grains or clouds) are manipulated to form new complex sounds. By using varying frequencies and amplitudes of the sonic components, and by processing varying sequences and durations of these grains, a new complex sound is formed. Simply put, this form of synthesis creates some crazy and interesting sounds.

ADVANCED VECTOR SYNTHESIS ( AVS )

This method of synthesis incorporates the combining and processing of digital waveforms. Using PCM samples, effects and filtering this method of synthesis can create stunning sounds, from lush and evolving pads to strange stepped sequences. Korg made the famous Wavestation range of synthesizers and these were based around the Sequential Circuits Prohet VS. Working off a 2 dimensional envelope using an X and Y axis (joystick) and 4 voices, this synthesizer also had wave sequencing, playing a loopable sequence of PCM samples in a rhythmic and/or crossfaded fashion. The idea was to be able to crossfade 2 or more waveforms using the joystick. Freaky but incredible.

So, there you have it! Of course there are other types of synthesis but for the purposes of these tutorials it is best to concentrate on one type in particular, Subtractive, simply because this is the most common of all analogue synthesis, in fact, it is even referred to as analogue synthesis. It is also an extremely good way to understand the components and functions of all the parts that go to make up an analogue synthesizer and how best to utilise them in programming. This form of synthesis crosses over extremely well for modern programming techniques.

Synthesizers used to be categorized into 2 categories, analogue and digital, but in the modern world we have the hybrid digital analogue synthesizer. Whether the processing is digital or analogue, the sounds are produced in much the same way, albeit with different terminology and some different routings. A good example of the terminology being adapted is: VCO, voltage controlled oscillator becomes a DCO, digitally controlled oscillator.

A little background and information first, before we launch into the meat of this tutorial.

Let’s look at a typical analogue synthesizer’s parts and features and for the sake of this tutorial I am going to keep it as simple as possible.

As I mentioned earlier, Subtractive synthesis involves the filtering of frequencies from a complex waveform to then be left with the desired sound. This is, of course, a very simplistic definition and it can best be shown with a simple flow chart. I knew you were starting to miss graphs and charts so I thought I would make you happy with another one here.

OSCILLATOR ------ > EG------ > FILTER ------ > AMPLIFIER

Let us look at what each one of these components means and what they do.

Osc - Oscillator.

Usually an analogue synthesizer will have 2 or more Oscillators and a Low Frequency Oscillator.

An oscillator creates a single periodic waveform at a certain frequency. In other words, the Osc is an electronic device used to generate a tone. Remember our earlier tutorial about waveforms? Well, in analogue synthesis, the Osc generates a sound or waveform and they are usually, a sine, saw, triangle, pulse etc. The Osc generates this waveform continuously. The rate at which it generates one cycle is what we regard as pitch and this is measured in Hz. I expect you to know about pitch and Hz. If you are unclear about this then go back and reread the tutorials 1 and 2 again as it is crucial you understand this before you continue with this section of the tutorials. Back to Oscs: On analogue synthesizers the Oscs are referred to as VCO, Voltage Controlled Oscillator, on modern synthesizers that incorporate digital processing, DCO, Digitally Controlled Oscillator, on samplers they can be referred to as samples, voices, waveforms etc…

On our synthesizer, when you press down on a note, the Osc will generate a waveform (sound) and the pitch. Which note you press determines the pitch, so, if you press A3 (using the chart in part1) it will generate a pitch (rate) of 220Hz. This is also a good time to throw in the LFO, Low Frequency Oscillator, also know as the Modulation Generator. This is an Osc separate to the other standard Osc and usually operates below 10Hz, well below our hearing range. The beauty of having this Osc is that it is commonly used to modulate the sound, and since we can’t hear it, we use it for other purposes. Modulation is the process by which you use one waveform, or controller, to alter or shape the properties of another waveform. This can have a dramatic effect on the sound. It is a great ‘shaping’ tool. I am sure you have come across this before. If you route the LFO, using a sine or triangle waveform, to another Osc’s pitch then you get a vibrato type of effect and this is sometimes what takes place in your synthesizer when you move the mod-wheel up. You modulate the sound, giving it a vibrato type of effect. Using different waveforms on the LFO and routing this to the other Oscs will give you so many different timbres that it really is a useful tool.

I think now is as good a time as any to introduce you to Pulse Width Modulation (PWM) since we’re on the subject of modulation. If you select a pulse for an Osc, then what happens is this. The pulse wave goes through it’s cycles (remember cycles of waveforms?) , each cycle being a sharp climb, staying at that level, dropping quickly and staying at that level and so on. The time it takes to complete one cycle is measured as a percentage and is called the Pulse Width. Modulating the Pulse Width gives dramatic results. It is the best of the best when it comes to ‘having a weapon’. You can get so many different timbres by just using this that it is the most important waveform selection on an Osc, on a synthesizer along with the LFO. A quick tip while we’re here. The PWM can be used to create the Unison mode on a synthesizer. Unison means all together and stacked. If you take 10 sounds and stack them on top of each other and play them all together with one note it is called Unison. So you can imagine the sounds we can create using PWM.

EG -Envelope Generator

The Oscs then run through Envelope Generators.

Synthesizers usually have 2 envelope generators. One will control the VCF and the other the VCA. When used with the VCA, the EG is used to vary the volume of a sound to create the natural dynamic movement of a sound. When used with the VCF, it can change the timbre of a sound over time by controlling the cut-off frequency of the VCF. Ok, don’t let this scare you. I am taking you through this a step at a time and giving you the necessary information as we go along instead of listing all the parts and defining them then expecting you to understand what I am talking about.

Remember earlier in parts 1 and 2 we discussed ADSR, Attack, Decay, Sustain and Release? Well the most common EGs are ADSR generators. Now, let me explain how it works as this will be useful to you. One EG is routed through to the CV (Control Voltage) input of the VCF (Voltage Control Filter). The next EG is routed through to the CV input of the VCA (Voltage Control Amplifier).

TRIGGERS AND GATES

To understand triggers and gates, it is important that I explain how the signal generated behaves with the envelope generators.

When you press a note on the synthesizer’s keyboard it Triggers a signal (Osc) that is sent to the EGs (Envelope Generators) and all this signal does is to tell the generators that a key is being pressed. This then begins the envelope’s generators to start to shape the signal that is being sent by the oscillators (ADSR). The Gate simply keeps the generators open as long as your finger is still pressing the note on the keyboard.

Using today’s synthesizers, let’s look at what happens when you press a note on the keyboard with a pad sound loaded. The pad starts to build (VCA) and then maybe starts to get brighter (VCF) and when you let go the sound either stops or dies away. You triggered the note (Oscs), VCA and VCF and the gate kept the build and filter going on and when you let go the gate stopped. Now that is way too easy.

Now, let’s look at these stages and what they mean. Once you have the hang of this, the rest starts to slot in nicely. Briefly explained, we use an oscillator that creates a tone, run it through a filter to remove the frequencies we want to remove and then adjust the volume of the sound over a period of time using the amplifier (shape). Why it starts to get complicated or, more accurately, looks complicated is because of the terminology and the fact that the word Voltage is used everywhere. I don’t want to get into the electronics of the signal path of an analogue synthesizer or how a synthesizer works in terms of electronics as this would just confuse you. But what I will do is give you a very brief explanation of why we use these terms and how a synthesizer works. You have to remember that when synthesizers were invented they worked off a voltage path and the voltage was controlled throughout the components. Pressing a note on the synthesizer’s keyboard (or using any external controller) sent a pitch signal and triggered the oscillators which, in turn, created a tone at that pitch. Remember we talked about the note being pressed at A3 (220 Hz)? The tone was then sent through an envelope generator (EG), which was then controlled using our ADSR tool, so you now had a shape to the tone. If the tone was sent to the filter, the ADSR shaped the filter. If it was sent to the amplifier, the ADSR shaped the volume. The hard wiring and circuits of these synthesizers were made so that one EG went to the filter, which was controlled by voltage of course, and the second was sent to the amplifier, which was controlled by voltage as well. By using the CV input of the filter meant that the voltage was controlled at the input stage. The same CV input was used on the amplifier. By varying the voltages at any of these input stages meant we altered the shape or filter characteristics of the sound. Today we use digital signal processing and that makes it appear simpler.

Because we are still on the beginner’s stage of these tutorials, I have kept these descriptions as simple as possible. What happens later and how we use these tools will blow your mind.

Next, we will be exploring the huge subject of FILTERS. However, I would like you to try to use this tutorial in playing with the oscillators, filter and amp on your synthesizer, analogue, digital or hybrids. Zero all the parts on the synthesizer, and only use the first 2 oscillators and the low frequency oscillator, the filter ADSR and the amp ADSR on the synthesizer. Now adjust the settings for the oscillators and play with the ADSR of the filter and amp and note what happens. Enjoy yourself but also try to take into account what is actually happening every time you make an adjustment. Make sure your ears get accustomed to the changes you make and, most important of all, have FUN!!!!!!