Exploring Optical Compressors and Their Unique Characteristics

Optical compressors are a unique type of audio compressor that utilize an optical attenuator and photocell to control gain reduction, serving as a dynamic control tool for the audio signal. With their smooth, musical sound and rich history in classic recordings, optical compressors continue to be prized tools for audio engineers and producers, acting as a cornerstone in dynamic control. This article explores what makes optical compressors special, how they differ from other compressor designs, and techniques for utilizing them to master the art of controlling dynamics in audio production. 

Understanding Optical Compressors

An optical compressor, also known as an opto compressor, uses a light source, usually an electroluminescent panel or LED, and a photocell to control the signal level. The heart of the optical design is the optical attenuator, which sits between the light source and photocell, crucial for dynamic control in the system. As the input signal level increases, the optical attenuator progressively blocks more light to the photocell, creating resistance and controlling gain reduction. Compared to VCA (voltage-controlled amplifier) or FET (field effect transistor) compressors, optical compressors have a smoother, more gradual response to transients that many engineers feel is more musical and less distorted. This helps add warmth and character to recordings. 

Components and Operation of Optical Compressors

The core components of an optical compressor are: 

•  Light Source: Provides light flux to the photocell, a core component in electro-optical compressors. This is usually an LED or electroluminescent panel, serving as a stable light source in devices where accurate audio signal processing is necessary. 
•  Photocell: Converts light energy into electrical resistance to control gain reduction. Common photocells are made from cadmium sulfide or cadmium selenide, acting as resistors in the light path for precise audio signal manipulation. 
•  Optical Attenuator: Situated between the light source and photocell, this attenuates light transmission to control resistance levels. Common mediums used in electro-optical compressors are liquid electrolytes or flexible membranes, which contribute to their unique, non-linear compression characteristics. 
•  Amplifier: Applies the gain reduction generated by the photocell, an essential function of electro-optical compressors, to the signal. This is usually a tube or FET amplifier. 

As the input signal level rises, the optical attenuator reacts by progressively blocking more light to the photocell, increasing resistance and lowering the output gain to compress the signal. 

Benefits of Using Optical Compressors

Some key benefits that make optical compressors a popular dynamics processing choice include: 

•  Smooth, musical compression: The optical attenuator creates a gradual signal response, resulting in warm, smooth compression compared to aggressive VCA/FET designs. 
•  Intuitive controls: Optical compressors generally have simple controls like ratio, attack, release, and make up gain. This makes it easy to dial in rich, musical compression, especially when utilizing a limiter for tighter dynamic control. 
•  Great for mastering: The smooth sound of optical compression makes it a smart choice for mastering applications where transparency and musicality are key. 
•  Vintage appeal: Classic optical compressors like the LA-2A and CL 1B introduced the optical sound to many iconic recordings, giving modern optical emulations vintage vibe and acting as a leveling amplifier in the process. 

Comparing Optical Compressors with Other Types

While all compressors control gain to reduce dynamic range, optical compressors differ from VCA and FET designs in a few important ways: 

Differences Between Optical and VCA Compressors

•  Response: Opticals have a gradual, rounded response while VCAs react more quickly and aggressively to transients. 
•  Sound character: Opticals sound smoother and add warmth while VCAs provide cleaner, punchier compression. 
•  Controls: VCA compressors often have more parameters to shape the compression response. 

Characteristics of Optical vs. FET Compressors

•  Transients: FETs react strongly to transients while opticals have a more relaxed response. 
•  Sound: FET compression, a distinct type of compressor, is known for its punchy, aggressive sound while opticals excel at smooth, rich compression. 
•  Knee: a crucial setting in both tube compressors and bus compressors for smooth transition in amplitude suppression. Opticals often have a softer knee resulting in more gradual compression. 

Applications of Optical Compression in Music Production

With their ability to transparently glue mixes together and add cohesion, optical compressors excel in the following applications: 

Utilizing Optical Compressors in Different Settings

•  Mix bus compression: Opticals add warmth and balance to full mixes without compromising clarity. 
•  Mastering: The smooth sound makes optical compression perfect for mastering applications where transparency is key. 
•  Vocals: Optical compression helps vocals sit nicely in the mix, adding richness and body. 
•  Acoustic instruments benefit significantly from optical tube compressor technology for natural, non-linear dynamic control. The soft knee and smooth response of opticals works nicely on acoustic guitars, pianos, and other acoustic sounds. 

Optical Compression for Acoustic Guitar Recording

When recording acoustic guitars, optical compressors: 

• Use a FET compressor to help control transient peaks for an even level.
• Add warmth and character to the acoustic tone
• Enable the guitar to cut through a mix nicely without sounding harsh

Mastering Techniques with Optical Compressors

For mastering engineers, optical compression is a secret weapon. Useful techniques when mastering with optical compressors include: 

Using Optical Compressors in Stereo Applications

• Set linked stereo compression to glue the left/right channels together.
• Use the compressor's sidechain filters for frequency conscious compression.
• Drive the compressor harder for smooth peak limiting.

Emulating Classic Optical Compressors with Plugins

Many plugin developers meticulously model legendary optical compressors: 

• LA-2A: Warm, smooth compression perfect for vocals and acoustic instruments.
• Teletronix LA-3A: Adds rich harmonic texture with fast, aggressive compression.
• Fairchild 670: The original mastering compressor, often seen as a type of compressor, offers vintage tone-shaping.

Popular Optical Compressor Emulations in Plugin Formats

Top analog emulations for mastering optical compression in-the-box include: Universal Audio LA-2A Collection - a premier example of tube compressors enhancing the audio signal., renowned for its electro-optical leveling amplifier capabilities. 

• Universal Audio LA-2A Collection
• Waves CLA Classic Compressors, an essential compressor plugin for any audio engineer's toolkit.
• Softube Tube-Tech CL 1B
• Plugin Alliance AMEK EQ 200, a discrete choice for mastering.
• Acustica Audio Cream2, which models the behavior of optical tube compressors for authentic dynamic control.
• Slate Digital FG-Stress, a revered compressor plugin known for its versatility in dynamic control.

Adjusting Attack and Release Times in Optical Compressor Emulations

Since the optical element inherently slows down transient response, attack and release behave differently than fast VCA/FET compressors, offering a unique form of dynamic control. Tips for adjustment include: 

• Attack: Faster settings over 20ms may sound more natural and transparent, mimicking the non-linear response of optical compressors.
• Release: Optical release times often start around 100ms. Adjust to track tempo.
• Listen for artifacts: If compression sounds distorted, lengthen attack and release times.

Tips for Optimal Use of Optical Compressors

To master the subtleties of optical compression, one must understand the role of the leveling amplifier. 

Setting Compression Ratio and Gain Reduction Levels

• Lower ratios from 2:1 to 4:1 often sound most natural when applying amplitude management via a limiter.
• Moderate GR between 3 to 6 dB tends to sound best for a subtle audio signal enhancement. More sounds unnatural.
• Higher ratios have uses but must be handled with care.

Implementing Sidechain Processing with Optical Compressors

• Insert an EQ before the compressor to filter compression frequencies.
• Use a send/return loop for parallel compression techniques.
• Tailor the sidechain response to fit the audio material.

Applying Parallel Compression Techniques with Optical Compressors

• Create an aux send path to blend compressed and dry signals.
• Heavy parallel compression adds energy and character, often achieved through the 1176 or acme audio opticom xla-3 for their distinct non-linear characteristics.
• Mix to taste with the dry signal for thickness and punch.

With their distinct sound and design, optical compressors are indispensable for smooth, rich compression and tone-shaping, often employing dynamic control techniques. By mastering the unique response and behavior of these units, audio engineers can learn the art of taming dynamics with musicality and vintage character.