What is an Optical Compressor?

I'm not an electrical engineer, when people start getting super electrically geeky regarding audio I usually get lost rather quickly.  The gritty important details fly over my head before I can even think about absorbing & understanding them.  Keeping this in mind I'll make this simple enough so that anyone who understands basic compression can understand how an opto-compressor works on an electrical level.  After all, it was the simple explanations that taught me what I know about them.  Once you know how it works, you'll know more about compression in general.  That way when you do go analog compressor shopping and you'll be better informed when making your audio investment.

An optical compressor (also known as a leveling amplifier) is a compressor that uses an electro optical attenuator & a photo conductive cell to monitor and control the dynamics & amplitude of processed program material (audio).  Already lost?  Wondering what the hell an electro optical attenuator & photo conductive cell is?  Keep reading you don't need to know what they are, only what they do. 

An electro optical attenuator features a light source. (in newer products these are LEDs)  This light source's intensity is directly proportional to the input signal of the material you're compressing.  So, as input level rises the LED shines brighter & with more intensity.  Conversely, as input level drops the LED shines with less intensity.  Well that's all fine and dandy but...  How does a flashing light source compress audio?  Here's how: The amplitude dependent light source is just one part of an optical compressor, we also need something to monitor this light source.  This is where the photo conductive cell comes in.  It monitors the lights source so the unit knows how loud the incoming signal is via the light's intensity.  Remember, I said earlier that the intensity of the light is directly affected by the level of input signal.  That's really important for understanding the larger concept. 

A photo conductive cell's job is to watch over the light source and then adjust resistance based on the intensity of the light.  When a louder signal is pushed through the optical compression unit the light shines brighter, the photconductive cell picks up on this brightness and in turn reduces its resistance being applied to the given signal.  This then tells the amplifier to reduce it's gain thereby creating a form of compression.



Unlike other compressors that allow you to choose attack & release times, an opto's release time is program dependent.  You can't choose the attack & release, they will change based off the material they are processing. (this is what makes optical compressors desirable)   Since a light doesn't just shut off and instead dims as it goes out, the release time is directly proportional to the level of the incoming signal which is simply being represented as light in an opto-compressor.  The same is also true for the attack times of compression when using a opto-compressor. This is why many people believe opto-compressors sound better.  They're said to be more musical because their response is non-linear, just like music is.  Other compressors have set response times (you adjust the attack & release) that are linear and exact.  With optical compressors the response (attack & release times) is set internally.  This response as I stated before is based of the properties & responsiveness and quality of the light source & photo conductive cell being used in the optical compressor.

Most Compressors you've seen also usually have a makeup gain control.  We need this control because when we apply compression to something we're working on the amplitude decreases.  We use makeup gain to return the material to it's previous amplitude with the compression applied.  Opto-compressors do not have a makeup gain control.  Their makeup gain is set and built into the circuit.  For every dB above the chosen threshold the signal hits the gain will increase by just less than a dB.  This can make using an optical compressor tricky because a lot of people aren't used to the gain adjusting as they adjust compression.  However this also means there are less controls to adjust and it's just a matter of getting used to this new way of manipulating compression.

Hopefully this all made sense to the folks in audio layman's term land.  Like I said before you don't have to know exactly how each part functions, only what each electrical component does to understand how it works.  Now that you know a little bit more about optical compressors when you're ready to buy a nice analog compressor to fatten up your own sound you'll be more educated about the wider process of compression! 

Best,

Nick

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