What Is An Optical Compressor & How Does It Work?

The optical compressor is one of the most common types of hardware compressor (or plugin emulation) on the market and should be understood on our way to audio mastery.

In this article, we'll learn about how optical compressors work; have a look at some examples of optical compressors, and consider their strengths, weaknesses and typical applications.

A Primer On Compression

Before we get into detail about optical compressors, it'll be good to learn (or at least brush up on) the basics of compression.

Click here to skip ahead to the section What Is An Optical Compressor?

Dynamic range compression (what we mean when discussing “compression”) is the audio process that decreases the overall dynamic range of an audio signal. Put differently; compression is a tool for reducing the difference in amplitude between the highest and lowest amplitude points of the signal.

Compressors perform by attenuating only the “loudest parts” of the signal (rather than bringing the quiet parts up, which is considered “upward compression“).

The performance of a compressor, then, is defined by two important questions:

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

The threshold and ratio control parameters of a compressor answer each of these questions, 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 ratio of a compressor 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, which passes through the gain reduction circuit and gets compressed.
• The control signal (sidechain) path that reads manipulates the sidechain signal (input or external) and controls the gain reduction circuit.

In the case of optical compressors, the gain reduction circuit is centred around an optical photocell arrangement.

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

What Is An Optical Compressor?

As the name would suggest, an optical compressor utilizes an optical photocell arrangement (a light source and a light detector) at the core of its gain reduction circuit.

More specifically, an opto compressor uses a light-dependent resistor (LDR) to attenuate the program/input signal and achieve compression. Yes, an optical compressor uses light as a means of controlling audio signal levels.

Before we get into the larger optical compressor design, let's focus our attention on the optical photocell arrangement.

An optical compressor's LDR (often referred to as a photocell) is a resistor that changes resistance depending on the amount of light it receives.

LDRs operate on what is called “semiconductor photoconductivity”. The energy of photons reaching the semiconductor decreases the resistance of the LDR and allows for greater current to flow through the resistor.

So then, the brighter the lamp of the opto compressor, the lower the resistance of the LDR.

The type of light source of an opto compression may vary. However, having fast reaction time and brightness in direct proportion to the input voltage is often highly valuable to make the design more predictable. This generally means the use of an electroluminescent source.

However, any of the following light sources could potentially be used in an optical compressor gain reduction circuit:

• Light-emitting diode (LED): a semiconductor diode that glows when a voltage is applied. Opto compressor LEDs are often designed to provide specific attack, release and voltage-to-brightness characteristics.
• Electroluminescent device: two conductive plates separated by a small gap (similar to a capacitor). Electroluminescent devices offer fast attack times and a brightness that is directly proportional to the input voltage.
• Fluorescent bulb: a low-pressure mercury vapour gas-discharge lamp that uses fluorescence to produce light. Fluorescent bulbs are rather non-linear in terms of input voltage and brightness.
• Incandescent bulb: a lamp that emits light via the heating of its filament. Thermal inertia causes slow reaction times between brightness and input voltage.

Although light is incredibly fast, optical compressors are actually rather slow (compared to FET and VCA types, at least) since the light source and LDR are not particularly fast.

Note that the light source is a transducer (it converts one form of energy to another). The light source converts electrical energy (audio signal) into electromagnetic energy (visible light). Transducers will always have some amount of non-linearity in the way they convert energy, which can have both good and bad effects on the overall sound of the compressor.

One notable feature of most light sources is that the input signal frequency will have an effect on how the light source reacts and produces light. In the context of optical compressors, this generally translates to a frequency-dependent attack time.

So how does this work in the context of a compressor? Well, the optical photocell arrangement is effectively controlled by the sidechain and acts as a variable resistor that controls the amount of compressor.

Here's a simple signal flow chart to express the compressor sidechain. As discussed, with optical compressors, the light source and LDR (optical photocell arrangement) would be the main variable-resistance element of the gain reduction circuit.

Note that optical compressors are nearly always designed with feedback topology.

Note that, unless an external sidechain is used, the audio input acts as both the program/audio signal that gets compressed and the sidechain signal that controls the compression.

The level detection circuit will be some sort of rectifier that turns the audio signal into DC voltage. This signal can be manipulated, if need be, to achieve the desired attack and release times and to alter the threshold and ratio of the compressor.

This DC control signal will control the light source, and the LDR will behave according to the amount of light it receives. This will effectively control the attenuation of the gain reduction circuit.

As the audio input level increases, the control voltage increases.

As the control voltage increases, the light source shines brighter.

As the light source shines brighter, the resistance of the LDR drops.

As the resistance of the LDR drops, more of the input signal is sent to ground versus the output. Put differently, as the resistance of the LDR drops, the gain reduction/compression also increases.

The gain reduction circuit of an optical compressor can be simplified (albeit oversimplified) as a voltage divider with a variable resistor (the optical assembly itself):

Where we have the following general equation:

V_{\text{out}} = V_{\text{in}} \cdot \frac{R_2}{R_1 + R_2}

• V_out: Audio Out
• V_in: Audio In
• R₁: resistance of the fixed resistor (audio circuit before the optical assembly)
• R₂: variable resistance of the LDR of the optical assembly

By breaking it down like that, we can see that as the resistance of the LDR increases, more signal transfer occurs. Therefore, as the resistance of the LDR decreases (as the input signal increases), there will be more attenuation/compression of the output audio signal.

So then, the input signal will feed the lamps, and as the input amplitude increases, the LDR will cause an increase in gain reduction to the signal.

Note that much of the character of optical compressors come from the natural non-linearities of such a system.

The attack and release time are defined by both the level detection/rectifier circuit and the optical photocell arrangement itself. The specifications will vary according to the physical components used. That being said, the attack and release times along with the ratio and threshold controls can be adjustable depending on the level of complexity in the optical compressor circuit.

So the sidechain path will manipulate these parameters. In more detail, they include any of 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.

Due to the nature of the optical level detection path, optical compressors generally exhibit what is known as a soft knee in their compression graphs. That is an input/output graph; whereas the input signal approaches and exceeds the threshold, the amount of compression will increase as the signal level increases.

Relative to many other compressor types, optical compressors have a simple design and are rather transparent due to the low distortion characteristics of most opto circuits. Colour can be achieved by adding transformers and tubes into the circuit, which is often the case with older hardware models.

Opto compressors are often chosen to help smooth out audio with fewer transients, helping to fit tracks into the mix without colouring them too much. They are typically avoided for bus compression and mastering.

Characteristics Of Optical Compressors

In this section, we'll consider a few of the typical characteristics of optical compressors:

• Generally low distortion due to LDR
• Relatively slow attack and release times
• Non-linear attack and release controls
• Frequency-dependent attack
• Transparent sound
• Naturally soft knee

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

Optical Compressor Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 4 different optical compressors to help solidify our understanding of this type of compression.

In this section, we’ll discuss:

• 500 Series optical compressor: Atlas Pro Audio Leviathan 500
• 19″ rack unit optical compressor: Universal Audio Teletronix LA-2A
• Optical compressor effect pedal: Ampeg Opto Comp
• Optical compressor plugin: Tone Empire OptoRed

Other noteworthy optical compressors include:

• Tube-Tech CL 1B
• Golden Age Comp-2A

Atlas Pro Audio Leviathan 500

The Atlas Pro Audio Leviathan 500 is a 500 Series optical compressor inspired by the Universal Audio LA-2A and LA-3A that marries vintage and modern technology together.

APA's Leviathan 500 is a class-A opto compressor with a fully discrete design. It features a Vintage Mode whereby the attack falls between that of the LA-2A and LA-3A.

Modern features include variable attack (2 to 40 ms) and release (100 ms to 2 s) times along with variable ratio (2:1 to 20:1). There's also a low-frequency roll-off (high-pass filter) sidechain option called “Punch” for keeping the low-end well-represented.

In addition to these controls, the Leviathan 500 offers gain and peak reduction (threshold) controls and LED metering. These units can easily be bypassed, and two units can be linked together in a stereo pair for bus and master compression applications.

Universal Audio Teletronix LA-2A

The Universal Audio Teletronix LA-2A is an iconic optical tube compressor that has certainly reached “legendary” status since its inception in the early 1960s.

Today, Universal Audio Teletronix LA-2A units are still produced according to original specifications with carefully selected components and put together with hand-wired point-to-point wiring.

The optical gain reduction circuit provides near-instant gain reduction with zero increase in harmonic distortion.

On top of superb sonic performance, the LA-2A is also very straightforward to use with only two controls. The first control is Peak Reduction, which effectively dials in the amount of compression the signal will experience. The other is Gain, which applies makeup gain to the signal post-compression.

The LA-2A has a Limit/Compress switch that effectively alters the compressor's input/output curve (the ratio and knee). A VU meter can be set to monitor either the output level (at +4 or +10 dB) or the amount of gain reduction.

Ampeg Opto Comp

The Ampeg Opto Comp is a smooth vintage-style optical compressor in the form factor of a stompbox.

This rugged compressor pedal is equally at home compressing bass guitar and guitar signals and works wonders in adding sustain and headroom along with a unique analog character.

The Opto Comp offers three controls. The Compression control alters the amount of compression applied to the signal while the Output Level control applies makeup gain. Compression release time is controlled via the Release knob.

Tone Empire OptoRed

The Tone Empire OptoRed is a transparent optical-style compressor plugin programmed to emulate the classic sound of optical compression while offering advanced functionality.

Like many opto compressors, the OptoRed has a VU meter to visually represent the amount of gain reduction. It also has a virtual toggle switch to go between compression and limiting.

In terms of control parameters, this plugin has both input and makeup gain controls with the option for auto-gain. A simple “Compress” control can be dialled between 0 and 100% (+10 dB to -30 dB threshold control), and a Wet/Dry mix control allows for parallel processing.

This Tone Empire plugin offers a Lo-Cut knob that engages a -12 dB low-shelf filter with a variable cutoff frequency up to 100 Hz.

A dedicated Sidechain control allows users to adjust the sidechain signal independently from the program signal. Users can adjust the gain of the sidechain signal along with the cutoff frequency and Q factor of a low-pass filter.

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
• Feedforward 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)