The Basics of an Analog Synthesizer

The process that I’m following to put my modular together is similar to the process I recommend to folks new to buying motorcycles. For your first one, get a good, simple synth that covers all the basic functionality. There’s a good chance that you’ll eventually grow out of it or want to change it. You should not and cannot purchase the perfect synth, so don’t try.

“My goodness! You say that with such authority. How do you know that I can’t purchase the perfect synth?” you might be thinking. Well, because it doesn’t exist. The perfect synth (or motorcycle) is a transient thing. You will change your mind, your likes, and your dislikes, and your taste will evolve. This isn’t something to worry about, it’s something to be enjoyed.

With all that out of the way, let’s look at what makes up a basic synthesizer.

Just the Facts, Ma’am

In a basic synthesizer you have a few, simple pieces:

  • Tone generation
  • Frequency control
  • Mixing/Volume control
  • Controls (for controlling all of the above)

Each of these items is just as important as the others. Even if the controls don’t technically make sound, without them, you’re dead in the water. Also, as I go through each of these, I’ll point out typical, basic configurations and the types of control they require. Once we get to the control section, all of it will come together in a neat, little package…I hope. ūüėÄ

Tone Generation – Oscillators

Tone generation typically (but not always) starts with the oscillators. These little modules pump out a particular waveform at a particular frequency. Basic analog synthesizers typically have 2 or more of these. Usually, a single oscillator will pump out a sine, triangle, saw, and square wave all at the same frequency. By having two oscillators, you can mix them together to get more complex tones, particularly when they’re a little out of tune with each other.

The minimum typical set of controls for oscillators is:

  • Frequency – Sets the frequency of the oscillator
  • Fine tuning – Adjusts the frequency at a much more fine-grained level

Frequency control – Filters

Once the audio comes out of the oscillators, it’s pretty cool, but after a while, you’ll find that it gets a little boring since all you can do is put a couple of waves together. You don’t have any volume control, yet, so it’s quite literally two tones just smashed together with no movement other than the offset of the tuning which can cause subtle pulsing. One really cool thing that using different waveforms gets you, though, is¬†harmonics. Harmonics are multiples of the fundamental frequency. So, if you are playing a square wave, for instance, it will have a ton of harmonics, in addition to the base frequency that you’ve set on the oscillator. So, you can play with those harmonics to further change the sound.

Now, we know we want to change those frequencies around a little bit, and for that, we need filters. Filters are like an extreme version of the equalizers that you may be familiar with on your stereo (though, as I write this, the notion of a “stereo” seems a little dated). Think¬†Bass and¬†Treble. Except in this case it’s quite a bit more extreme. Go watch a blaxploitation film and listen for the wakkachikkawakkachikka of the guitar playing through a wah pedal or listen to Bootsy Collins and Bernie Worrell playing through Mu-Tron III’s on the older P/Funk records. Wah pedals are great examples of filters. They diminish certain frequencies while boosting others.

Filters are typically one or more of the following: Low-pass, high-pass, or band-pass. “What do these mean?” you may ask. And I may reply, “A low-pass filter passes low frequencies while filtering out high frequencies; a high-pass filter passes high frequencies while filtering out the low; and a band-pass passes through a specific band of frequencies while filtering out frequencies higher and lower than that band.” Simple as that. On basic analog synthesizers, like my Prodigy, there is only one filter and it is of the low-pass type.

The minimum typical set of controls for filters is:

  • Cutoff Frequency –¬†Sets the cutoff frequency at which the filter “rolls off” or roughly begins filtering
  • Depth – Sets the amount of filtering applied to the signal
  • Q – In the case of band-pass filters, this sets how many (or few) frequencies around the cutoff frequency to pass

Mixing and Volume Control

Mixing and volume control are just what you expect. These mix multiple signals, change the level of the audio, and may appear at multiple places in the signal chain. There will often be a mixer just after the oscillators and before the filter, and there will be an amplifier at the end of the chain controlling the outgoing volume.

The minimum typical set of controls is:

  • Level – Pretty self explanatory, huh? Sets the level of the signal

Controls

So, lets take a look at what we have so far…

We have oscillators making frequency noises at as many frequencies as we have oscillators (two in the case above). These are getting mixed together and filtered and finally having their level set. Okay, that’s really cool. Buuuut, after playing this way for a little while, you start getting bored. “Hey, man…where’s the motion? Violins and guitars and drums don’t just come on and go off. They start loud and get soft. The play different notes, too,” you’d probably say. And you’d be correct. So, what can we do to put some motion in the ocean? How can we wiggle those waves?

Well, let me tell you how we’re gonna put a little pep in your step! We’re gonna control the hell out of all this stuff. This is where synths really get fun.

The Envelope Generator

Let’s start with volume and dynamics control. Everything you hear has a characteristic called a volume envelope. Think about when a gunshot goes off (Pew! Pew!) What you hear is the attack of the shot as it rises rather quickly to its maximum volume (the ‘P’ in Pew) and the decay as it dies out (the ‘ew’ in Pew). Now, think about a violin being aggressively bowed. You hear the attack of bowed string as it climbs to maximum volume, then the decay from this initial bowing to the sustain of the continued bowing, and finally, when the bow is taken away, you hear the¬†release of the volume back down to its initial silent state. This is what we refer to as a simple ADSR (Attack, Decay, Sustain, Release) envelope. And the controller we use for this is called an Envelope Generator.

An envelope generator, when triggered, generates a control voltage that changes over a period of time to match the envelope that you set. You feed this control voltage to the amplifier at the end of the audio chain to control loudness automatically. In this way, you can create that Pew! envelope if you want to create a percussive sound. It also lets you set a sustain level for the sound to decay to when you keep the generator triggered (think, the continued bowing of a string). As long as the envelope is sending a control voltage to the amplifier, you will hear something at its respective level.

Okay, about this¬†trigger thing…Well, since many of our controls are temporal, they need to know when to start. Therefore, we need to send them a control signal of some sort. For our purposes, we send a¬†gate signal. Let’s picture a keyboard (the piano, ebony and ivory sort) with one key. You press that one key, it turns on the sound. You release it, the sound stops. When you use this with an envelope generator (EG or ADSR for short), the ¬†EG will start the cycle of attack, decay, sustain, release. This gets slightly trickier at this point. If you release that key at any time, the EG will immediately skip to the release phase. If the EG gets through attack and decay and the key is still pressed, it will go to the sustain level until you release the key, then it will enter the release cycle.

The EG is a¬†very powerful tool for adding motion to your sound. Along with being used for volume control, it is also used to control filters. Let’s go back and pull out those P/Funk records again. You hear Bootsy’s bass? It’s kinda freaky how it sometimes makes this WOWMP! sort of sound. Well, that’s an EG controlling a filter. It just happens to be triggered by the bass reaching a certain threshold level.

The Keyboard

“Hey, wait a minute. My keyboard has, like, a bunch of keys on it. I want to play different pitches!” Well, good for you, because your keyboard takes care of at least two crucial things. Not only does it tell the EG when to start, by sending a gate signal, it also generates a¬†control voltage¬†that tells the oscillators the frequency that they need tune to. In this case, your keyboard is like the conductor of a very small, electronic orchestra. “You, oscillator, over there…play at 440Hz. And you, envelope generator over there, you turn up the volume.”

The LFO (Low Frequency Oscillator)

The LFO is simply another type of oscillator that runs at frequencies below the threshold of hearing (<20Hz). They use a waveform (triangle, saw, square, random, just like the regular oscillator) to modulate a control voltage (CV, for short) that you can send to whatever you have that needs controlling. Technically, you could use an LFO to control your volume instead of an EG. Or, you could control your filter’s cutoff frequency. Or¬†you could even control the level of the sustain of the EG which is in-turn controlling the volume of the amplifier.¬†Which brings us to…

Controls Can Control Anything that Can Be Controlled

This is where modular synthesis breaks away from the typical, all-in-one synthesizer. In an all-in-one synth, you typically have a set of oscillators feeding a mixer, which then goes through an envelope controlled filter, which finally goes through an envelope controlled amplifier. There will often be an LFO controlling one or more presettable parameters.

With modular synths, you can have anything control anything else, assuming that the thing you want to control has a gate or CV input. You could have an EG controlling the relative level of an LFO, which is in-turn controlling the fine tuning of one of the oscillators and the cutoff frequency of your filter. We even have  modules that simply count off time and send gate signals or CV on the beat. Those are called sequencers, and even they can be controlled by CV from something else.

It’s¬†totally badass!!!

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