Optical Compressor

Opto-Compressor 101

An optical compressor, often an opto-compressor, is a dynamic range attenuator. First, a light source in a photosensitive cell controls the gain reduction in an optical compressor. A transducer lies in the core of the mentioned compressor. This device is capable of transforming energy from one form to another. Engineers discovered that interesting things occur years ago after playing back and forth between light and electricity. Hence, this approach can be used to process the audio signal. The basic principle within opto-compressors could be described by saying that electricity (an audio signal) is transformed into light and then into electricity again. Certain nonlinearities occur during this process, creating an appealing effect on the human ear.  

Electro-Optical Attenuator

The construction of the electro-optical attenuator is relatively simple. The audio signal splits into two parts. The first one goes through the gain stage, while the other part is a detector. It controls the mentioned gain stage. Moving forward, the light sensor and the detector control the gain reduction. The unique characteristics of these devices are causing the gain cell to react in ways that could not be recreated until recent times by using modern components. This influences attack and release parameters within the compressor and creates certain time lags. This is preferred by the mix and mastering engineers compared to the other compressors.  

The History Behind Optical Compression

One of the first devices that used an optical attenuator was now the famous LA-2A Limiting Amplified by Teletronix. Jim Lawrence, an electrical engineer at the University of Southern California, is the mastermind behind this device. Primarily, he was working on the Titan Missile Program, but his true passion was radio and broadcasting. During a stint at KMGM in Los Angeles, he became frustrated with riding the gain to maintain a constant signal level on the air. The idea of a "leveling amplifier" was born from that frustration. His revolutionary idea was to create the world's first leveling amplifier utilizing optical sensors. Lawrence drew from his background with military optical sensors to design a circuit that would "level" the incoming audio signal. He combined a luminescent panel with photo-resistors and sealed them in a vacuum-tube-sized metal canister. This optical attenuator, known as the T4, gives the LA-2A its gentle, program-dependent optical compression, revered to this day by audio professionals worldwide.  

Final Analysis

To summarize, the foremost notable feature of the opto-compressor lies in its photocell. However, the photocell has a gradual and soft release curve that sounds natural and musical. Compared to the tube or solid-state compressors, in an opto compressor, the first 50 % of the release time occurs rapidly. In addition, the complete release range can take up to several seconds. Furthermore, the photocell creates a phenomenon called memory. The cell's release time depends on the duration and intensity of the light to which it has been exposed. This means the release time will be longer when the input signal is loud, and the compressor works harder. This interactive release automatically smoothes out the kind of uneven volume fluctuations that can result from other forms of compressors.