What Is A Pulse Width Modulation Compressor & How Does It Work?

Among the many different types of compressors, the pulse width modulation compressor is often overlooked. That being said, this powerful type of compression is worth knowing about and understanding.

In this article, we'll discuss PWM compression in detail, covering the technology and theory involved and also go over some characteristics and applications for this type of compressor.

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A Primer On Compression

Before we get to the main discussion on PWM compressors, I thought it would be helpful to start with a more general explanation of compression.

Click here to skip ahead to the section What Is A Pulse Width Modulation Compressor?

Compression (that is, dynamic range compression) is the process of reducing the dynamic range of an audio signal. In other words, it's the process of reducing the difference in amplitude between the highest and lowest points of the audio signal.

Compressors work by attenuating only the loudest parts of the signal.

In order to attenuate the “loudest parts” of a signal, two key questions must be answered:

• What constituted the loudest parts?
• By how much should the loudest parts be attenuated?

A compressor answers these questions with the threshold and ratio parameters, respectively.

What is the threshold of a compressor? The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage. As the input exceeds the threshold, the compressor kicks in (with its given attack time). As the input drops back down below the threshold, the compressor disengages (according to its release time).

What is the ratio of a compressor? The compressor ratio compares the number of decibels the input signal is above the threshold to the number of decibels the output signal is above the threshold. In other words, it is the relative amount of attenuation the compressor will apply to the signal.

Other compressor parameters worth mentioning are the following (I've added links to in-depth articles on each parameter):

• Attack Time: the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
• Release Time: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.
• Knee: the transition point around the threshold of the compressor where the output becomes attenuated versus the input.
• Makeup Gain: the gain applied to the signal after the compression takes place (typically used to bring the peaks of the compressed signal up to the same level as the peaks pre-compression).

All compressors work with a gain reduction circuit that effectively compresses the audio signal in response to a control signal. This control signal (also referred to as the sidechain) is derived from the input audio signal (common) or via an external audio signal (less common). It is manipulated via the aforementioned compressor parameters.

So every compressor will have two critical signal paths:

• The audio signal path that passes the program signal through the gain reduction circuit.
• The control signal (sidechain) path that reads manipulates the sidechain signal (input or external) and controls the gain reduction circuit.

In the case of PWM compressors, the gain reduction circuit is centred around a pulse width modulator.

With that primer, let's get into PWM compressors and how they act to compress the dynamic range of audio signals!

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What Is A Pulse Width Modulation Compressor?

A PWM compressor, as the name would suggest, is a compressor that utilizes pulse width modulation to control its compression/gain reduction circuit.

This begs the question, “what is pulse width modulation?”

PWM (pulse width modulation) is a signal processing method that utilizes high-frequency pulse signals with “on” and “off” values to control the amplitude/power of a signal. By splitting the signal into discrete parts and “muting” some parts, PWM will reduce the average amplitude/power of a signal.

As the name suggests, the width of the pulses is modulated over time. The longer the “on” values of the pulse wave are compared to the “off” values, the louder the resulting average amplitude will be.

The width of each pulse is largely defined by the duty cycle, which is noted by the percentage of “on time”. This is shown in the image below:

Note that the pulsing rate must be very fast to avoid any auditory changes in amplitude. The PWM chips of PWM compressors should operate at frequencies in the hundreds of kiloHertz or even faster (be able to switch on and off in well under 10 µs).

In theory, the faster, the better when it comes to PWM compressor switching frequencies. This is to avoid potential sonic artifacts. However, the entire design must work well with whatever PWM chip frequency is used.

At frequencies this high, we won't hear and audible “on” and “off” or (as we would in a choppy square wave tremolo effect, for example). Rather, as discussed, the pulse width modulation will bring down the average amplitude of the signal it's affecting.

The gain reduction of a pulse width modulator “switch control” is fairly straightforward when done correctly. The percentage of time the pulse width modulation signal is on is equal to the percentage of the initial input level that gets outputted by the gain reduction circuit. Let's consider a few examples:

• If the switch is on 50% of the time and off for 50% of the time, then the average output level would be 50% that of the average input level.
• If the switch is on 75% of the time and off for 25% of the time, then the average output level would be 75% that of the average input level.
• If the switch is on 25% of the time and off for 75% of the time, then the average output level would be 25% that of the average input level.

So we can easily see how PWM switching is a tool for reducing the signal amplitude and, therefore, could be used as the gain reduction element of a compressor.

That being said, the design of a PWM compressor is deeply complex.

A pulse width modulator circuit must convert the control voltage/sidechain into this variable-width on-off switch to control the amount of compression.

On top of that, the pulse width modulator must be designed to read the control voltage/sidechain according to the typical compressor parameters. These parameters include the following (I've provided links to in-depth articles on each control):

• Threshold: the amplitude limit that dictates when the compressor will engage and disengage.
• Ratio: the ratio of input signal amplitude above the set threshold to the output signal amplitude above the threshold.
• Attack: the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
• Release: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.

For example, the duty cycle must only drop below a full 100% (always on) as the control voltage/sidechain surpasses a defined threshold. The PWM must also be designed with a variable rate at which the duty cycle will drop below 100% as the threshold is surpassed (as defined by the ratio control). Time constants must also be taken into account in the pulse width modulator.

The following is a simple signal flow chart to express the compressor sidechain. The audio input is used as the program/input signal for the gain reduction circuit and as the control signal that feeds the pulse width modulator (which, in turn, compresses the program/input signal):

When designed correctly, a PWM compressor has the potential to be the fastest-acting compression type with the lowest distortion. These somewhat rare compressors require intricate design details to sound great but offer beautiful compression when done properly.

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Characteristics Of Pulse Width Modulation Compressors

In this section, we’ll consider a few of the typical characteristics of PWM compressors:

• Low distortion
• Very fast attack and release times
• Very transparent
• Flexible parameter controls

Before moving on to the PWM compressor examples, I'd like to point you toward my video on the seven different compressor circuit types. You can check it out below:

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PWM Compressor Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s have a look at 4 different pulse width modulation compressors.

In this section, we’ll discuss:

• 500 series PWM compressor: Great River Electronics PWM-501
• 19″ rack unit PWM compressor: ART Dual Limiter
• 19″ rack unit PWM compressor: Crane Song SDC8
• PWM compressor plugin: Kramer PIE

Great River Electronics PWM-501

The Great River Electronics PWM-501 is an outstanding pulse width modulation compressor in the 500 series format. The performance of this compressor is incredibly versatile and excels at compressing virtually any audio source.

In addition to the standard attack, release, threshold, ratio and makeup gain controls, this superb PWM compressor also has the option to switch between feedback/feedforward circuitry options for a more rounded and warm response or a more aggressive compression style, respectively.

This compressor also features a second-order (12dB/octave) variable high-pass filter immediately after the feedback/forward circuit to reduce low-end in the output.

Multiple PWM-501s can be linked together in interesting ways with the link control.

ART Dual Limiter

The ART Dual Limiter is a dual-channel PWM compressor/limiter in a rock-mountable build.

The channels can act independently of one another or be stereo-linked by the press of a button.

Each channel has a ratio button to switch from a 4:1 (compressor) ratio to a limiter (∞:1). Input and output levels can be controlled as well as the attack and release times of each channel's compression/limiting circuit. Each channel also has its own gain reduction meter.

Crane Song STC-8

I'd be remiss if I didn't mention the Crane Song SDC8 in an article about pulse width modulation compressors. This expertly designed class A unit is capable of simultaneous compression and limiting, which is made possible, largely, by pulse width modulation compressions and the STC-8's sophisticated side-chain.

The STC-8 offers two channels for stereo processing (or simply processing two signals at once). These two channels run independently of one another but can be linked via the Stereo Link button. The Color button engages an “enhancement circuit,” which adds a tube-like warmth to the audio.

Each channel has a rotary switch with 4 groups of 4 preset each, which allows for an easy and predictable setup of the compressor limiter. The four groups are made up of combinations of two advanced functions (PDR and A-MOD), or the lack thereof. The groups can be defined as:

• PDR and A-MOD disengaged
• PDR engaged and A-MOD disengaged
• PDR disengaged and A-MOD engaged
• PDR and A-MOD engaged

Program Dependent Release (PDR) is a superb choice for levelling the volume of the audio without bringing up the background noise. Without PDR, the compressor will offer constant release times and excels at enhancing the sustain or ambience of the audio signal.

Attack Modification (A-MOD) causes the peak limiter to dynamically modify the attack time of the compressor function. When a slower attack time (3 or higher) is selected on the compressor, any signal triggering the peak limiter automatically reduces the attack time of the compressor. This effectively allows the compressor and limiter to act simultaneously.

Each of the 4 groups has three presets (A, B and C) along with one Variable Mode (V) setting. In Variable Mode, the attack and release times can be dialled in with the attack and release knobs, as can the knee/compressor shape via the Shape knob.

Waves Kramer PIE

The Kramer PIE is a PWM compressor plugin based on the legendary Pye compressor from the 1960s.

This precision-modelled plugin offers the sound on a PWM compressor in a simple digital plugin. The VU meter is super-accurate and can be set to monitor input level, output level or gain reduction. The Analog switch toggles between different analog characteristics caused by noise floor and hum, based on the power supplies of 50 Hz or 60 Hz.

The other controls include knobs for threshold, ratio, decay time (release) and output (makeup gain).

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Related Questions

What are the different types of audio compressors? The term “type” can have a few meanings so let's look at a few different “types of compressors.

In terms of circuit topology, compressors will generally fall into one of the following types:

• Variable-Mu (Tube) Compressor
• FET Compressor
• Optical Compressor
• VCA Compressor
• Diode Bridge Compressor
• Pulse Width Modulation Compressor
• Digital Compressor
• Compressor Plugin

In terms of how a compressor will perform when compressing an audio signal (and the typical tasks it will be set to do), we can think of the following types of compression:

• Multiband Compression
• Peak-Metering Compressoion
• RMS-Metering Compression
• Feedback Compression
• Feed-forward Compression
• Upward Compression
• Limiting Compression
• Parallel Compression
• Bus Compression

Should compression be used on every track? As a general rule, compression should be used with intent and, therefore, only be used on every track in the case that every track would require it. More often than not, there will be certain tracks in a mix that sound perfectly fine (and better) without dynamic range compression.

Once again, the typical benefits of using compression on a track include (but are not limited to) the following:

• Maintaining a more consistent level across the entirety of the audio signal/track
• Preventing overloading/clipping
• Sidechaining elements together
• Enhancing sustain
• Enhancing transients
• Adding “movement” to a signal
• Adding depth to a mix
• Uncovering nuanced information in an audio signal
• De-essing
• “Gluing” a mix together (making it more cohesive)