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What’s so good about optical compressors?

“What’s the big deal with optical compressors? There seems to be a lot of mystique surrounding them, yet no one really seems to know why?”

Think about what a transducer is: a device that changes one form of energy into another. Anytime you do this unique laws of physics come into play that otherwise aren’t available if energy stays in its original form. In audio we deal with this daily as we work with mics and speakers, but designers discovered many years ago that some interesting things happen when going back and forth between electricity and light. This is the basic principle of the optical compressor: It changes electricity – specifically your audio signal – into light, and then back into electricity again. And in the process there are certain kinds of non-linearities that occur which audio engineers have found produce a very desirable effect when applied to audio signals in a controlled way.

The basic construction of an electro optical attenuator is fairly simple. Like most common compressors the incoming audio signal is split into two parts: the part that passes through the gain stage and is ultimately output, and the part that is used in a detector circuit to control that gain stage. It is this detector circuit that is unique. A light source and sensor are employed to control the gain reduction. The unique properties of these devices cause the gain cell to react in ways that until recently haven’t been very easy to duplicate using conventional components. Most of this has to do with certain time lags that occur, which cause the attack and release characteristics of the compressor to be very pleasing in a way many engineers prefer over other types of compressors.

While a standard incandescent light source can be used in an optical compressor, there is a noticeable delay (read too much delay) between when the input voltage is applied to the filament, and when the filament begins to glow, due to thermal inertia. A fluorescent bulb may also be used, but the light intensity does not vary in strict proportion to the input voltage. Instead, most circuits use an electroluminescent source. Built somewhat like a capacitor, with two conductive plates separated by a small gap, an electroluminescent source reacts very quickly, giving the compressor the possibility of fast attack times, and glows in direct proportion to the input voltage, giving predictable attenuation. Some more modern designs use a special type of LED.

The real magic in an optical compressor as an audio device lies in the photocell, however. The photocell has a gradual and predictable release curve, which is very gentle and musical. The first 50% of its release occurs very rapidly (generally measured in tens of milliseconds), and a complete release that can take several seconds. This type of release curve is generally not found it typical solid state or tube designs. In addition, the cell exhibits a phenomenon known as 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. In the end, however, the differences must be experienced to be fully appreciated.

Also worth noting, some inexpensive compressors on the market now claim to have an optical emulation mode available. In many instances this is just a slower attack and/or decay time and really doesn’t properly emulate the behavior of these optical devices. There are some digital emulations that come pretty close though. Call your Sweetwater Sales Engineer for more info.

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