Opto-Compressor 101


An optical compressor, often called opto-compressor, is a dynamic range attenuator. In the first place, a light source in a photosensitive cell controls the gain reduction in a optical compressor. In the core of the mentioned compressor lies a transducer. This device is capable of transforming energy from one form to another. Years ago engineers have discovered that interesting things occur after playing back and forth between light and electricity. Hence, this approach can be used in processing of the audio signal. The basic principle within opto-compressors could be described by saying that electricity (in this case an audio signal) is transformed into light and then into electricity again. There are certain nonlinearities occurring during this process which create an effect that sounds pleasing to the human ear.


Electro Optical Attenuator


The construction of the electro optical attenuator is rather simple. The audio signal splits into two parts. The first one goes thru 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 is influencing the attack and release parameters within compressor and creating certain time lags. This is preferred by the mix and mastering engineers in comparison to the other types of compressors.


The History Behind Optical Compression


One of the first devices that used optical attenuator was now famous LA-2A Limiting Amplified by Teletronix. Jim Lawrence, an electrical engineer at the University of Southern California at that time, is the mastermind behind this device. Primarily he was working on Titan Missile Program but his true passion was the realm of radio and broadcasting. During a stint at KMGM in Los Angeles, he became frustrated with having to ride the gain to maintain a constant signal level on the air.

From that frustration an idea of a “leveling amplifier” was born. 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 which 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, is what gives the LA-2A its gentle, program dependent optical compression, revered to this day by audio professionals worldwide.


Final Analysis


To summarize, the main notable feature in the opto-compressor lies in its photocell. However, the photocell has a gradual and soft release curve that sounds very natural and musical. Comparing to the tube or solid state compressors, in 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 release time of the cell is dependent on the duration and intensity of the light to which it has been exposed. What this means is that when the input signal has been quite loud, and the compressor has been working harder, the release time will be longer. This interactive release automatically smoothes out the kind of uneven volume fluctuations that can result from other forms of compressors.


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