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A Guide to Analog Subtractive Synthesis with the Moog Phatty Keyboard


What is a synthesizer?

The advent of the synthesizer and its subsequent popularity in the late 1960s and throughout the ’70s forever changed the landscape of music and sound design. The concept of imitative synthesis, where a device is used to emulate a particular acoustic instrument, has been a huge boon for the advancement of the technology of synthesis. For instance, if we wanted the sound of a violin or oboe added into a musical piece, but neither do we play nor have the ability to hire someone who can play said instrument, having a synthesizer available makes it an invaluable tool for re-creating the appropriate-sounding part. Synthesizers, however, have gone well beyond the mere “synthesizing” of acoustic instruments. And with the vast variety of types of synthesis available today, we can now create sounds previously unheard in the natural world.  

Developing a solid understanding of how synthesis works will not only help you to fully unlock the true potential of the Moog Sub Phatty, but can be applied to any hardware or software synthesizer, as well as some FX processors. However, before we can begin understanding the components of a synthesizer, we must first understand the basic elements that compose sound. 

Basic Elements of Sound

The basic elements of sound are identified as Frequency, Timbre, and Amplitude.

Elements of sound

Frequency  Sound waves travel in a consecutive series of peaks and troughs, with the number of cycles per second measured in units described as Hertz (Hz). For example, a waveform that has three cycles per second would be translated into written form as 3 Hz. A waveform of 1200 cycles would be written as 1200 Hz or 1.2 kilohertz or kHz. Human hearing is generally thought to be in the range that is identified between 20 Hz and 20 kHz.  

Timbre  Most sounds can be broken up to reveal a composite of sine waves at different frequencies. The largest and typically lowest-frequency tone is referred to as the fundamental, or the first harmonic. If the sound has additional harmonics that are a whole multiple for the first, they are called overtones. A waveform with a lot of harmonic overtones might be described as sounding overly bright, buzz-like, clangorous, smooth, round, or any number of similar descriptions. These terms are indicative of the quality of the sound. Often referred to as Timbre (pronounced TAM-ber), this is also known as “the character” of the sound.   

Amplitude: Amplitude represents the distance between the high point of a peak and the low point of a trough within the waveform. The unit of measurement used to describe amplitude in the audio realm is called decibels or dB, and is commonly referred to as loudness. Because human hearing is logarithmic, meaning we don’t hear all frequencies at the same dB, we are most sensitive to frequencies between the ranges of 250 Hz and 2 kHz. However, note that lower frequencies are required to have higher amplitude for our ears to perceive them at the same loudness as higher frequencies. 

The Basic Components of Synthesis

Most synthesizers use the same terminology and design to create and shape sounds. We’ll be focusing on Analog Subtractive Synthesis as the basis for understanding the components and how they reflect the three basic elements of sound. The following circuits are used to create, shape, and even modify sounds. In addition, all the circuits are voltage controlled (CV), which allows for multiple differing circuits to manipulate audio signals. 

Don’t worry if you find these descriptions even the least bit confusing, as we’ll be looking at examples of each circuit in the following sections to provide further clarification.

Voltage-Controlled Oscillator (VCO)

If we were to zoom in on a guitar string after it’s been struck or plucked, we’d find that the string oscillates, or vibrates visibly. The oscillation of the string forces the air molecules around it to move, generating the waves that we perceive aurally as sound. The mass or thickness of the string will determine at what frequency the oscillator vibrates. Thicker strings will have a lower frequency of oscillation, while thinner strings will oscillate at a faster rate.  

All synthesizers use an oscillator circuit to generate sound waves. The “Voltage Control” aspect is what determines the frequency or pitch of the oscillator, with the higher voltage settings resulting in higher frequencies. Most synthesizers are capable of generating at least four basic waveforms: Sawtooth, Square, Triangle, and Sine Waves. In the following section, I’ve included an example of each waveform. As the file plays, you will hear me adjust the octave switch to hear each waveform at different octaves.

Basic waveforms

Sawtooth Wave  The Sawtooth waveform is one that is rich in overtones and harmonics and is generally perceived and described as buzzy or bright. It is arguably the most popular of the basic waveforms and is sometimes considered the all-purpose waveform, rendering it most applicable.  

Square Wave  The Square waveform is often described as having a hollow quality. Like the Sawtooth waveform, it contains a lot of harmonic content, yet it sounds quite different in comparison. 

Triangle Wave: The Triangle waveform has fewer harmonics and overtones when compared to the Sawtooth and Square waveforms. It’s an ideal waveform for recreating bass sounds, as well as flute tones.  

Sine Wave: The Sine waveform has no harmonics or overtones and is described as soft in comparison to the others. Sine waves are excellent for creating sub-bass sounds and this is generally how they are applied. 

Voltage-Controlled Filter (VCF)

The filter circuit of an analog synthesizer is arguably the most important circuit of all because it defines the sound of the synthesizer and helps differentiate one manufacturer’s sound from another. The Moog ladder filter sounds very different from an Arp Odyssey, Oberhiem, Yamaha, Sequential Circuit, and so on. These filters are used to remove overtones, as well as harmonic content. 

In the following section, I’ve recorded a sawtooth wave through different filter types. The Sub Phatty does not have a multi-mode filter, so I’ve used a Doepfer multi-mode filter to produce the example. The recordings are just filter sweeps with no resonance, emphasis, or overdrive. There are four filter types, including Low Pass, High Pass, Band Pass, and Notch filters.

Types of Filters 

Types of filters

Low Pass  A low pass filter (also known as a High Cut Filter) removes the higher frequencies and allows for the low frequencies to “pass” through.

High Pass  A high pass filter (also known as a Low Cut Filter) removes the low frequencies and allows for the higher frequencies to pass through.

Band Pass  A Band Pass filter combines both Low and High pass filters to allow for a central band of frequencies to pass through.

Notch  A Notch filter works in the opposite manner of a Band Pass filter. It notches out the central frequencies instead, allowing both the high and low frequencies to pass through.

Filter Slope

The slope of the filter defines how steeply the filter rolls off the frequencies. Slopes are often referred to as “poles,” with each pole representing a 6 dB increment. For instance, the classic Moog filter contains four poles or 24 dB per octave. The most common filter slopes are two-pole (12 dB) and four-pole (24 dB). In this section, I’ve recorded the Sub Phatty’s most excellent low pass filter with a one-pole filter sweep and a four-pole filter sweep. You will hear the four-pole completely cut out once the filter is at its lowest frequency setting, while the one-pole will still be heard.

Filter poles

Filter controls  The basic controls of a filter are the Cutoff and Resonance.

Filter Cutoff  The filter cutoff control is the point at which the filter starts to remove frequencies. For instance, a low pass filter with the cutoff set to 450 Hz will start rolling off at frequencies higher than 450 Hz.

Filter Resonance: The resonance control is essentially a frequency-dependent feedback circuit. When increased, it creates an emphasis at the filter cutoff frequency. Some filters will launch into self-oscillation when the resonance is increased enough. When using the Filter Cutoff control, this feature becomes tunable feedback. Increasing resonance on a high or low pass filter generally results in a thinner sound, but affords a sharper, more defined shape. When using the notch and band pass filter, the resonance controls the width of the frequency band so that the higher the setting, the narrower the frequency band will become. In the following recording, I’ve added resonance to the four-pole filter sweep and set the resonance very high to hear the self-oscillation. Notice how the sound begins to break up and distort.


Filter Resonance

Voltage-Controlled Amplifier (VCA)

The Voltage-Controlled Amplifier, in essence, is a pre-amplifier acting as a volume control. The voltage control aspect allows for dynamic control over loudness.  

Low-Frequency Oscillator (LFO)

The Low-Frequency Oscillator produces various waveforms like a VCO, but the frequencies are well below human hearing (.1 Hz to 10 Hz). It is used to create modulation effects as if an invisible hand were physically moving a particular parameter at a given rate and pattern.

 LFO Controls  The parameters of an LFO control the rate or frequency and depth or amplitude.

Rate  The Rate controls the frequency of the LFO so that the higher the rate, the faster the resulting oscillation of the LFO.

Depth  The Depth controls the amplitude of the LFO, which in turn effects the amount of modulation sent to the desired circuit. With a lower setting, the resulting modulation is subtle, while a higher depth will result in a much more extreme effect.

LFO Effects  Routing the LFO signal to the voltage control of each basic circuit results in three classic effects. In the following recordings, I’m creating these three classic effects by adjusting the depth first, followed by the rate, and then change the different waveforms of the LFO (Triangle, Square, Ramp Down, Ramp Up, Sample and Hold).

Vibrato  LFO to VCO 

Wah Wah  LFO to VCF  

Tremolo  LFO to VCA 

Envelope Generator (ADSR)                       

Using an Envelope Generator circuit allows for the shaping of sound over a period of time. There are different stages of the envelope that allow for the slowing or speeding up of various signals. Some synthesizers will contain a dedicated envelope generator for the VCA and VCF circuits but, at the very least, there will be one envelope routed to the VCA that controls how a sound is triggered. At its fastest settings, the sound is triggered instantly, while slower settings allow for sounds to slowly fade in and out.  

The basic stages of an Envelope Generator are called Attack, Decay, Sustain, and Release. When a key is pressed, the level is controlled over time as the sound passes through each stage of the envelope.  

Stages of Envelope Generator (ADSR)

Attack  The Attack stage commences when a key is pressed and reflects the time it takes to run-upwards from the level of 0 to the Decay Stage.

Decay  The Decay stage starts at the peak of the Attack and continues into the Sustain stage. If the Decay and Sustain are set at the same level, there is no discernible difference between the two.  

 Sustain  The Sustain stage is the level after the Decay and continues until the key being pressed is released.  

 Release  The Release stage is triggered upon release of the key and is the time taken from the Sustain stage to the level of 0. 

VCA Envelope

The following recordings are of different envelope settings that will illustrate how the different ADSR components work.

VCA Envelope 1

VCA Envelope 1: This recording has a short attack, which causes the sound to trigger instantly, while the decay has a slow fade into sustain, which is at a lower level. The release is short, which causes the sound to cut off abruptly. This is a typical setting for percussive, bass, and lead sounds.

VCA Envelope 2

VCA Envelope 2: In this recording, there is a long attack and a long decay with no sustain or release. The envelope causes the sound to rise and fall and is great for shorter one-shot sounds.

VCA Envelope 3

VCA Envelope 3: This recording is a classic example of how a pad might evolve. The long attack, decay, and release create a slow fade-up and fade-out of the tone.

VCF Envelope

The following three recordings use the same-shaped envelope as above, only routed to the filter section. I’ve also included the knob settings that reflect the same envelope shapes shown above.

VCF Envelope 1

VCF Envelope 1: In this recording, the filter is being shaped. It’s subtle, but you can hear the initial attack of the filter is open, but quickly closes during the decay portion. Again, this is a great setting for percussive, bass, and lead sounds.

VCF Envelope 2

VCF Envelope 2: In this recording, you hear the filter open slowly from the longer attack and then slowly close as the decay moves into sustain, which has a much lower setting.

VCF Envelope 3

VCF Envelope 3: In this recording, take note of the long attack, decay, and release, which creates a slow fade in and out of the filter.

The Moog Sub Phatty Synthesizer

Most synthesizers share many of the same features and functions, so you can apply what we’ve covered to almost any software or hardware synthesizer. In the next section, let’s delve a little deeper into the Sub Phatty and see how to interface with the different circuits we’ve discussed so far.

Signal flow path

Signal Flow

In the block diagram above, we can see the signal flow of the audio, control voltage, and gate signals. Most analog subtractive synthesizers follow the same signal path. The two voltage-controlled oscillators (VCO), sub oscillator, and noise source combine into a mixer and the signal is then passed into the Voltage-Controlled Filter (VCF). From there the signal passes through to the amplifier, which feeds into the physical outputs (master and headphones). There are additional control voltage circuits that act as modifiers. The two envelope generators (ADSR) help shape the sound over time, while the LFO acts as an invisible hand that modulates the VCO’s pitch and waveform, as well as the filter.

The signal path is constantly generating sound; however, we won’t hear it until the amplifier receives a gate trigger. Think of the gate/trigger as a light switch. If we flip the switch, the lights are instantly illuminated. The envelopes then act as sophisticated “dimmers” that enable us to shape the light’s intensity over time. 

When we strike a key on the keyboard, two things happen. A control voltage is generated and sent to the VCO that determines the frequency or pitch of the oscillators. Simultaneously, a gate/trigger is generated and sent to the voltage-controlled amplifier by way of the envelope, which can then either open the sound instantly, or over a period of time. Additionally, the gate/trigger is also sent to the filter by way of the envelope, which again shapes how the filter reacts over time.

Sub Phatty Oscillators and Mixer

Oscillators and Mixer settings

The Sub Phatty offers two oscillators, each with a variable waveform selector and four-way octave switches. The octave switches allow each oscillator to play higher “2” or lower “16.” Oscillator 2 offers an additional frequency selector, which allows for detuning or slightly offsetting the pitch when mixing both oscillators together. To hear the oscillators, you must first turn up the Osc1 knob on the mixer.

In the audio recording above, I have recorded Oscillator 1 while manipulating the variable waveform selector and the octave switch. I’ve started on the lowest octave, with the waveform at the far left, which is a triangle wave. The triangle wave has very few harmonics and is great for basses. As I move the waveform dial to the right, we hear it morph into a saw wave, followed by a square wave, and then to a pulse wave. 

In this recording, I’ve got a saw wave mixed with the Sub Osc, which is a square wave that tracks one octave below Oscillator 1. I turned the oscillator and the sub oscillator down, so you can hear each individually. Notice how it really does fatten up the sound.

In this recording I’m mixing both Oscillator 1 and 2 together and I’ve got both set to the same waveform and octave. Notice how the sound starts to modulate as I adjust the Freq knob of Oscillator 2. I’m also switching between different octaves and adjusting the waveform of Oscillator 2.

In this example, I’ve recorded the two oscillators with Hard Sync engaged, which causes Oscillator 2 to lock to Oscillator 1’s phase. Every time a new note is played, Oscillator 2 resets, even if the waveform hasn’t completed. Adjusting the frequency of Oscillator 2 while synced causes the waveform to become distorted and twisted, with lots of additional harmonics.

The Sub Phatty Filter

We’ve already spent some time listening to filters and filter sweeps, so in this section I’m going to focus on some of the other capabilities offered by the low pass filter. 

Filter settings

MultiDrive  The MultiDrive is an analog distortion processor that offers effects ranging from an asymmetrical tube-like warmth to an aggressive hard clipping, with a smooth continuous transition in between. Dialing-in the MultiDrive can help add a distinct edge to the tone, while increasing the responsiveness to filter resonance, waveform, and oscillator level.

This recording demonstrates how the MultiDrive affects the timbre of the oscillators. In the first part, I’m adjusting the Frequency of Oscillator 2, while increasing and decreasing the MultiDrive knob. In the second half of the recording, I’m again adjusting the Oscillator 2 Frequency knob, except with the Hard Sync on, which offers huge sound with lots of harmonics, especially when pushing the MultiDrive to 100%.

EG Amount  The envelope generator (EG) amount controls how much the ADSR envelope circuit affects the filter. At 12:00, the knob reads “0” and is not affecting the filter. The recordings of the filter envelope were with the EG amount knob turned clock-wise (0 to +5) and resulted in the filter opening. If we move the EG amount knob counter-clockwise, the envelope is inverted and subsequently causes the filter to close. Be aware, the filter must be set appropriately, since opening the filter cutoff will have no effect if the knob is placed fully clockwise. Inversely, the filter cutoff will have no effect if the knob is placed fully counter-clockwise.

In this example, I have the ADSR set with a longer attack and decay setting and a shorter sustain release. In the first half of the recording, I’ve got the EG amount set fully counter-clockwise at -5, causing the filter to close or move from right to left. In the second half, the EG amount is set fully clockwise at +5, thereby causing the filter cutoff to open or move from left to right.

KB Amount  This knob controls how the low pass filter cutoff tracks across the keyboard. For instance, if the knob is set to “0”, the filter cutoff is unaffected by the keyboard. Placing the knob at “10” will cause the filter cutoff to open, the higher the keyboard is played. This is extremely useful for recreating realistic sounding instruments, where the timbre is brighter in the higher registers.

In this recording, I’m playing a single note while adjusting the octave switch. The first pass is set to “0”, which demonstrates the static filter at any octave, while the second pass is set to “10”, which causes the filer to open.

The Sub Phatty Envelopes

The Sub Phatty has two dedicated envelopes for the filter and the amplifier. We covered the operation of both envelopes in a previous section, but will now focus on how different articulations change the way the envelopes react.

Envelopes settings

Articulations  The Sub Phatty will respond depending on how we play the keyboard. Legato joins notes together without any silence or space between each note, whereas Staccato has space or silence between each note.

In this recording, I’m playing a series of staccato notes, followed by the same notes but in legato. Notice how the envelope attack and decay retriggers each time for the staccato notes, while the legato notes only play the sustain values. By altering your playing style, you can inject a lot of expressiveness into sounds you are creating.

The Sub Phatty Modulation

The LFO is like an extra set of hands dedicated to moving a particular knob or parameter. In the previous section on modulation, “LFO,” we saw how routing the LFO to the Pitch and Filter recreates vibrato and a wah-wah effect. But remember, to hear the modulation effect on the Sub Phatty, you must engage the modulation wheel located next to the pitch bend wheel. If you don’t move the modulation wheel, you won’t hear any modulation. In this section, let’s look at some of the other modulation-routing possibilities

Modulation settings

Wave Amount  This parameter controls the depth or how much of the LFO signal is sent to the variable waveform section of the oscillators. This circuit is an excellent way to add motion to a patch without adjusting the filter or pitch.

In this sample, the LFO is routed to the Oscillator 1’s variable waveform. I have the Wave Amt set to 4 and the Oscillator’s waveform is set to the Square wave setting. As I trigger the note, the Mod wheel is all the way down. When I turn up the Mod wheel, you can hear the waveform start to modulate. As I bring the Mod wheel all the way up, you’ll notice the sound becomes choppy as the waveform moves beyond an audible range. I’ve also started increasing the LFO rate, which creates a broad, distorted and somewhat unpredictable sound.

In this recording, I’ve set up a patch with Oscillator 2 hard sync’d to Oscillator 1 and I’m routing a square wave from the LFO to the pitch while engaging the “Pitch AMT Osc 2 Only” only. As I bring up the Mod wheel, you will hear the LFO only modulate Osc 2.

If you look at the LFO “Source”, which has all of the waveforms to select from, you will notice, fully clockwise, there is a FILT EG setting. This allows the Filter Envelope to be routed to any destination in the modulation section. In this recording, I’ve created an 808-style kick-drum patch and routed the Filter Envelope to the pitch to create the classic downward pitch drum hits found in Bass music such as Drum ’n Bass and Dubstep.

Sub Phatty Pitch

The Pitch section offers the ability to fine-tune the pitch of the instrument, which is especially handy when using other analog synthesizers that might not be in tune. This is also where you can set the octave of the keyboard at the bottom. 

Pitch settings

Glide Rate  Glide, also called portamento or glissando, causes smooth pitch changes between notes. This knob is used to specify how much time it takes to transition from one pitch to the next when you play the keyboard.

In this example, I’m playing legato notes first, followed by altering between staccato and legato. This adds another level of expressiveness to the synthesizer and can be a lot of fun play.

As you can see, the Moog Sub Phatty packs a sizeable punch for its ultra-portable size. I hope you’ve enjoyed this synthesizer overview with examples demonstrating the power and potential of the Moog Sub Phatty. If you are in the market for a synthesizer, the Sub Phatty runs deep and features many additional settings beyond what we’ve explored here. In fact, almost every parameter can be further edited via the software editor or from the keyboard’s shift mode, which allows for a whole other level of control and sound exploration.

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Thanks. Great Overview!

Very nice and complete overview !!