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What Is an Optical Compressor? (Answered)

Are you looking to add some punch and clarity to your mix? Look no further than the optical compressor! With its smooth, transparent compression and range of adjustable controls, this dynamic range compressor is a go-to choice for recording and live sound engineers. From taming explosive drums to adding sustain to vocals, an optical compressor is a versatile tool that can help bring your music to the next level. But what is an optical compressor?

In this article, we will give you an overview of what an optical compressor is and explain how it can help you with your audio. So if you’re looking for a tool to reduce file sizes, this post is for you!

Image of an optical compressor and other audio equipments. Source: cottonbro studio, pexels
Image of an optical compressor and other audio equipments. Source: Cottonbro Studio, Pexels

What is an optical compressor? An optical compressor is a type of analog compressor that utilizes a light element and optical cell to shape the dynamics of an audio signal. When the volume of the audio signal increases, the light element emits more light, which in turn causes the optical cell to reduce the amplitude of the output signal.

What is an optical compressor?

An optical compressor is a gain-reduction circuit that relies on an optical photocell (a light source and a light detector). To reduce the strength of the input/programming signal and compress it, an Opto compressor uses a light-dependent resistor (LDR). So yes, light is used in an optical compressor to adjust the volume of an audio stream.

Let’s analyze the optical photocell setup before moving on to the bigger optical compressor design. The LDR (also known as a photocell) in an optical compressor is a light-dependent resistor that adapts its resistance to its surroundings. Semiconductor photoconductivity is the basis for LDR operation. The LDR’s resistance drops as more energy from the photons passes through the semiconductor, allowing more current to flow.

Therefore, the lower the resistance of the LDR, the higher the lamp of the Opto compressor. Opto compression may use a variety of light sources. However, having a quick response time and brightness that scale directly with the input voltage is frequently useful. It is common practice to use an electroluminescent light source for this purpose.

An optical compressor gain reduction circuit may, however, make use of any of the following light sources:

Light-emitting diode (LED)

A semiconductor diode emits light when a current is sent across it. In many cases, the attack, release, and voltage-to-brightness properties of an Opto-compressor LED are purposely tailored to the application.

Electroluminescent device

In this case, we have two conducting plates with a little space between them (similar to a capacitor). As a result, electroluminescent devices have a quick on-time and a brightness directly proportional to the applied voltage.

Fluorescent bulb

A low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce light. Fluorescent bulbs are rather non-linear in terms of input voltage and brightness.

Incandescent bulb

The light source that produces illumination utilizes the heating of a filament. As a result, light’s response to changes in input voltage is sluggish due to thermal inertia.

Even though light travels at an incredible speed, optical compressors are quite sluggish (at least when compared to FET and VCA kinds) due to the slowness of the light source and the light-dependent resistor (LDR).

Light is a transducer, so keep that in mind (it converts one form of energy to another). The auditory signal is transformed into electromagnetic energy, which is then used to power the light source (visible light). Some degree of non-linearity in energy conversion is inherent in all transducers; this can have beneficial or detrimental consequences on the compressor’s sound.

To a certain extent, the input signal’s frequency will affect the light source’s behavior and output. As a result, the attack time of optical compressors is often proportional to the operating frequency.

How does an optical compressor work?

The optical attenuator, known as the T4B, is a key component of an optical compressor. This device contains two photocells, which are light-dependent resistors that alter their resistance value based on the amount of light they are exposed to.

The resulting change in resistance value causes the audio signal to be compressed, resulting in the distinctive sound of an optical compressor. The amount of gain reduction applied is determined by the ratio setting of the compressor, which specifies the amount of compression applied for every decibel the signal exceeds the threshold.

Optical compressors also have attack and release controls, which determine how quickly the compressor responds to the incoming audio signal and how quickly it releases the gain reduction once the signal falls below the threshold.

In addition, the make-up gain control allows the user to boost the overall level of the compressed signal to compensate for any loss of volume that may have occurred during the compression process.

Overall, optical compressors work by continuously monitoring the audio signal level and adjusting the gain accordingly to achieve the desired level of compression. This process helps to even out the dynamic range of the audio and adds sustain, clarity, and punch to the sound.

Characteristics of Optical Compressors

Optical compressors are known for a few key characteristics, including:

  • Low distortion due to the use of a light-dependent resistor
  • Relatively slow attack and release times
  • Non-linear attack and release controls
  • Frequency-dependent attack
  • Transparent, natural sound
  • A soft knee allows for smooth transitions between compressed and uncompressed signal levels.

Where should you use an optical compressor?

Optical compressors are commonly used in recording and live sound settings to even out the dynamic range of an audio signal and add sustain, clarity, and punch to the sound. They are often used on vocals, drums, bass, and other instruments to achieve these effects.

In the recording studio, optical compressors can be used on individual tracks during the tracking or mixing stages or on the mix bus to add overall glue and punch to a mix. They are particularly useful for taming transient-heavy sources such as drums and percussion and can also add sustain and clarity to vocals and other instruments.

In live sound settings, optical compressors can be used on the front-of-house mix or on individual instruments in the monitor mix to help control the dynamic range and improve the overall clarity and punch of the sound. They are also commonly used on vocals to help the vocals cut through the mix and be heard clearly by the audience.

Optical compressors are versatile tools that can be used in various recording and live sound situations to shape the dynamics of an audio signal and improve the overall sound quality.

Image of an optical compressor with red and blue lights. Source: andreu marquès , pexels
Image of an optical compressor with red and blue lights. Source: Andreu Marquès , Pexels

Benefits of using an optical compressor

There are several advantages to using an optical compressor:

Smooth, transparent sound

Optical compressors are known for their smooth, transparent compression that preserves the integrity of the original audio signal. This makes them a good choice for recording engineers who want to achieve a polished, professional sound without introducing unwanted distortion or coloration.

Versatility

Optical compressors are highly versatile and can be used on various instruments and vocals to achieve various effects. They are particularly useful for taming transient-heavy sources such as drums and percussion and can also add sustain and clarity to vocals and other instruments.

Easy to use

Optical compressors often have a simple, straightforward design with a limited number of controls, making them easy to use even for those new to compression.

Durability

Optical compressors are generally very reliable and durable, making them a good choice for use in live sound settings where dependability is important.

Classic sound

Many classic compressors, such as the Urei 1176, are optical and have contributed to the sound of numerous hit records over the years. An optical compressor can help you achieve a classic, timeless sound in your recordings.

If you want even more tips and insights, watch this video called “What are Optical Compressors? | Too Afraid To Ask” from the CSGuitars YouTube channel.

A video called “What are Optical Compressors? | Too Afraid To Ask” from the CSGuitars YouTube channel.
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Frequently asked questions (FAQ)

Do you still have questions? Below are some of the most commonly asked questions about an optical compressor.

What is the difference between Opto and FET compressors?

FET compressors are known for adding punch and character to a sound and can be particularly effective on percussive sounds or those with distinct transients. One well-known example of a FET compressor is the Urei 1176. On the other hand, optical compressors utilize a photocell and light bulb to control the gain reduction applied to the audio signal.

Is 1176 an optical compressor?

The Florence 1176 is an optical compressor that takes inspiration from the classic 1176 compressor but features a modern optocoupler. This allows it to offer three different modes of operation: compressor, limiter, and sustainer. It is an evolution of the original 1176 design.

Should vocals always be compressed?

Compression can be a useful tool for controlling the dynamics of vocals or other elements in a mix. The amount of compression applied will depend on the performance style and the music genre. In pop music, for example, vocals are often heavily compressed with a minimal dynamic variation.

Conclusion

So there you have it – a brief overview of what an optical compressor is and how it works. Whether you’re a seasoned audio engineer or just starting, an optical compressor is a valuable tool that can help you achieve the professional-quality sound in your recordings and live performances.

However, just as you can’t become an expert cyclist by reading a book, you can’t become a great music producer by reading articles alone. It’s time to take action! Go and put what you have learned into practice.

This article covered an optical compressor, how it works, and its characteristics. Here are some key takeaways:

Key takeaways

  • An optical compressor is a gain-reduction circuit that relies on an optical photocell (a light source and a light detector). 
  • The optical attenuator, known as the T4B, is a key component of an optical compressor. This device contains two photocells, which are light-dependent resistors that alter their resistance value based on the amount of light they are exposed to. 
  • Optical compressors are known for a few key characteristics.
  • Compared to via or fet-based compression, the personality of optical compression is said to be higher.
  • If you want to dampen volume spikes and transients as much as possible, a “voltage-regulated amplifier” is your best bet.
  • To become an audio expert, we need to be familiar with the optical compressor, one of the most popular forms of hardware compressor (or plugin emulation).

So, do you use an optical compressor? And did I cover everything you wanted to know? Let me know in the comments section below (I read and reply to every comment). If you found this article helpful, share it with a friend, and check out my full blog for more tips and tricks on music production. Thanks for reading, and never stop making music.

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Audio Apartment Author
Written By Andrew Ash
Hey there! My name is Andrew, and I've been making music since I was a kid. I now run this blog all about home studios and music production. If you want to improve your home studio setup, this is the place for you!

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