What Is An Optical Compressor? (With Mixing Tips & Sound Examples)

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COMPRESSION DEFINITION || OPTICAL || FET || VARI-MU (Tube) || VCA || DIGITAL || DIODE-BRIDGE || MASTER BUS COMPRESSION

Compression is the process of reducing the dynamic range, and evening out the loudest and softest elements of an audio signal. However, different types of compressors work in different ways. The five main types of audio compressor circuitries serve as the basis for the majority of the plugin emulations found in today’s DAWs. One of the most widely used types is Optical compressors. Opto compressors use a light source to modify the dynamics of the incoming audio signal. In this Opto compressor article, we will delve deep into the inner workings and applications of Opto compressors showing you how to use them and the situations they are useful for in mixing and mastering. We will use real applicable examples with sound examples to explain this to you.

What Is An Optical Compressor? (TL;DR)

While Opto compressors might sound different, their functioning is quite similar to that of other compressors. 

The incoming signal passes through the amplifier and a control circuit that is controlled by the light source. The intensity of the light source is proportional to the amplitude of the incoming signal.

The louder the incoming signal gets, the brighter the light shines, and vice versa. 

The light bulb projects light through a light-dependent resistor, which monitors the fluctuations of the incoming signal. The light-dependent resistor (LDR) is housed behind the amplifier circuit and monitors the resistance of the output signal. 

The light-dependent resistor works on the principle of semi-conductor photoconductivity. Basically, the photons that reach the semiconductor reduce the resistance of the light-dependent resistor, allowing more current to pass through the resistor.

As the resistance decreases, the amplifier circuit reduces the gain, which causes optical compression. 

For every one dB increase in the input amplitude about the threshold, the output increases by a value just below 1 dB. The exact values depend on the set ratio and the characteristic behaviour of the exact photocell circuit used.

Since brightness and reactivity are directly proportional to the input voltage, repeatable results are achievable in compression. 

The light source that is used in an Opto compressor is generally an electro-luminescent material. In simple terms, electroluminescent materials emit light in response to an electrical current or a powerful electric field.

What Goes On Inside An Opto Compressor?

To put it basically:

Further explanation for the nerds 😉:

In the middle of the optical compressor is housed an optical attenuator, called the T4-B. The T4B comprises two photocells, which are light-dependent resistors. The photocell alters the resistance value in relation to its exposure to light. This change in resistance value results in audio compression in an Opto compressor. 

(You can read more about building your own opto compressor circuitry here).

The first photocell is used to control the audio level. The second photocell monitors the game reduction meter, which displays the amount of compression. 

On the back side of the photocells lies the electro-luminescent panel, whose brightness is proportionate to the voltage it receives.

A resistor is added to the photocell resulting in a voltage divider that is similar to a console’s fader. Using an incandescent bulb is an option. But in an optical compressor, we need a light source to emit rays at the photocell at great speeds in order to alter the levels.  

This optical attenuator, T4B, was primarily designed for the highly renowned LA2A in the Sixties. 

The audio signal passes through an electroluminescent panel that operates at a medium-to-high AC voltage. The audio signal gets amplified and sent directly to the light. 

So basically, the peak reduction dial on the LA 2A compressor and LA 3A models acts as a gain controller for the amplifier which modifies the light intensity of the electro-luminescent panel.

So, the higher the light intensity of the electro-luminescent panel Gets, The  Lower the resistance of the photocell. This eventually results in the gain reduction of the audio signal. 

All these processes described above, occur inside the central T4B. 

How Is Light Used For Audio compression?

Since the photocells used here were calibrated for everyday applications like switching on a street light or turning on a light meter, the reactions to audio are pretty peculiar. 

For instance – if you are walking along the road on a hot summer afternoon, and you suddenly enter a shaded shop that is slightly darker than the outside – it would take a few seconds for your eyes to adjust to the sudden change in light exposure.

The photocell in our limiter reacts in a similar way. If you were to crank the limiter (similar to the hot sun), it would take a while for the compressor to release (similar to our eyes getting adjusted to the shaded shop).

The LA3A’s Optical Gain Reduction Plot:

The impact of the photocells can be better understood with the famous Urei LA 3A optical compressor’s gain reduction plot.

If we look closely, we can see that the initial 10 dB of gain reduction follows a smooth, soft knee curve, which suddenly changes into a limiter as the curve angle changes.

The LA3A limiter control switch should be experimented with because it represents a holistic change in regions rather than an exact ratio change, such as a 2:1 or a 10:1.

Being optical in nature, the attack release and the ratio will change according to how much you crank it. For these reasons, optical compressors like the LA3A or LA2A are a great place to start experimenting with compression. Due to how musical they sound, It’s hard to go wrong with them.

How Does An Optical Compressor Sound?

The various light sources we saw earlier illuminate at different velocities. So changing the material of the light source will change the sound drastically. Changing the material of the resistor also changes the sound.

But since the T4B was so successful with the LA 2A, It is the most commonly used one.

With an attack of 10 milliseconds and a smooth, slow release (ranging up to 5 seconds),  the T4B opto cell is instrumental in creating smooth and musical response times. 

While the initial attack and release response is fast, the delayed response time results in the initial release fading into the upcoming ones. This is referred to as ‘multi-stage release times’ in Opto compressors. 

Being program and frequency-dependent, the compression depends on the amplitude and duration of the input. 

The reaction times in an Opto compressor dictates the sonic character.

For example, if your vocals have a large dynamic range, or they stay over the threshold for an extended period of time, then the characteristic slow release kicks in. If your vocals stay below the threshold, then the release is fast by default.

If your optical compressor’s VU meter is taking a long time to return to zero after some heavy compression, it means that the ‘multi-stage releases’ are at play. 

These characteristics make Opto compressors a great fit for vocal recording. Since the compressor responds to your articulations in such an elaborate way, it helps you focus on the release of your phrases, resulting in a highly inspired vocal take.

Scenarios Where Optical Compression Is Useful

Being so transparent and musical in nature, it’s generally quite difficult to screw up a recording with an Opto compressor. 

While it’s very common to find Opto compressors on vocals, bass, and acoustic instruments, they can be used to round off a kickdrum or smoothen out the mid-range in an electric piano.

The high-frequency side chain dial also works great on brass instruments.

In fact, if you were just to use the transformer and the amplifier without gain reduction, you could add musicality and liven up a lifeless or harsh MIDI file.

Vocals:

In this male vocal example, we wanted to add more power to the frequencies arising from the area between the singer’s chest and the throat. 

So instead of going for a FET compressor, which would have added cream and distortion to the vocals, we decided to go for an Opto compressor, which will provide a multi-stage release to build upon.

With only two knobs to work with, you need to set it up for what you want.  Since we want to add more body to the sound, we begin by tweaking the side chain high-frequency knob. 

Similar to the UA hardware units, moving the knob counter-clockwise lets you use the compressor like a De-Esser when you move away from the flat position.

 Basically, we are filtering or equalizing the sibilant frequencies that are fed to the side chain. By dialing this knob down to 11, we create a dip in the sibilance area.

You can read more about De-Essing here.

Now we can start tweaking the gain and peak reduction knobs, which will take effect on top of this filtered sibilance contour we just created. 

We set the gain at 36 and reduced the peak to 46. Similar to the analog units, the scale is not linear and is calibrated to fit the units of the hardware knob.

We Set the analog hum to 60 Hz and choose the compressor mode, which sets the ratio to approximately 3:1 on the LA2A.

You can notice how there is more low midrange body In the compressed audio example. We used this as a foundation to allow the singer’s voice to flow through the multi-stage release section.

You can hear how the releases are more dynamically defined and bring emotion to the vocals. Since the preceding phrase’s release is well-defined, it feels like the vocalist is taking in more air as he is singing.

While in this example, the LA2A is added later, it is a great idea to keep it on while recording. The compressor’s auditory feedback allows the singer to sing more expressively.

Bass:

Since our bass example here has a lot of harmonic overtones, we wanted to define the attack of the underlying notes further by making them rounder. We’ve made a separate sample for the limiter and compressor to display their versatility on bass.

Bass (Compressor Mode):

Bass (Limiter Mode):

We found the sweet spot on the bass as we hit 50 on the peak reduction dial. We slightly lowered the gain down to 38 from the default value of 42. At this time, the side chain knob was set to ‘flat’ by default where all the frequencies experience equal reduction. While the attack was clear, it could not cut through its harmonic overtones.

Dialing the knob counter-clockwise increases the voltage amplifier gain within the peak reduction circuit. As this affects frequencies about 1KHz, we found it affected the higher harmonics of the bass pluck, which generally happened at around 2KHz and 7KHz. 

At a value of 100 or 11 o’clock on the dial, we found the perfect balance between the bass attack and its overtones.

In the limiter mode, we decided to go for more low and rumble. Pulling the peak reduction to around 46 gave us the right kind of body and width. We pulled the gain knob down to around 35, resulting in a gain reduction of around 4.5 dB.

Again, the most important knob turned out to be the side chain dial. Setting it to 36 or around 10 o’clock gave us a powerful low and rumble, along with a widening influence on the harmonic overtones as the frequencies got higher. You can notice how rich and well-separated the higher harmonics sound. 

It’s sometimes hard to believe such results can be achieved with a simple two-knob compressor.

Mix Bus:

In this mix bus example, we have used the LA3A, which is a solid-state optical compressor. Unlike other compressors, we can crank up the compression way more due to the LA3A’s highly transparent compression curve.

This gets rid of a lot of the problems with low-end pumping and unwanted artefacts while reducing the gain in a musical way.

We started off by tweaking the peak reduction knob, which controls the attack and release times along with the compression. As we are dealing with wide-ranging harmonic material in the mix bus, we just focused on getting an average peak reduction value that works best.

At this stage, we bypassed the compression to check for changes in attack and overall width. While it sounded wider than the dry sample, we decided to keep increasing it till the bass distorted. We found a sweet spot at around 3.6 for peak reduction.

Mix Bus (Before):

Mix Bus (After):

After that, we proceeded by using the gain control to match the original signal. We went with the more aggressive limiter mode as the higher ratio was bringing out more harmonic richness throughout the frequency spectrum.

Dialing the high-frequency side chain to around 72 or 2 o’clock brought out the richest parts of the guitar’s high and high mid frequencies. We left the analog switch unchanged at 60 Hz.

Notice how the LA3A’s attack on the bass on the 6th and 9th sec of the track propels the track ahead, adding energy to the track. 
This defined low end brings focus to the Intricate note separation that we hear in the higher frequencies, especially between the 13th and 18th-sec mark on the track.

1176 FET Compressor And LA3A Opto compressor In Series:

Putting a FET 1176 in front of an LA 2A or LA3A has been an industry standard trick for decades. Because of the peculiar and minimalist nature of Opto compressors, it is difficult to tame large transients without causing noticeable anomalies.

Dry Signal:

Only Opto enabled:

FET + Opto enabled:

In this sound example, we were going for 2 to 3 dB of gain reduction with an LA3A on our drum bus. Initially, we just had the LA3A on, and the loud cymbal crash of 5dB in the 1st measure of the 2nd bar completely disrupted our compression range. 

The Opto responded in time for the cymbal crash, but due to its multi-stage release structure, it was unable to return in time for the second measure. The compressor started pumping, and the smoothness was disrupted.

This is a very common problem if you are running an Opto-compressor through a track with sudden transients. Engineers generally stick an 1176 FET before an Opto compressor to counter this problem.  

We did the same by inserting an 1176 with the ratio set at 4:1, attack at 1, and release at 7. Since the 1176 doesn’t have a threshold, increasing the gain has the effect of reducing the threshold. The 1176 managed to comfortably tame the 3dB rise in amplitude during the cymbal crash. 

Running this signal into the LA3A gave us the right kind of leveling that we wanted. 

Popular Optical Compressors:

Here’s a short list of popular Optical Compressors that have become a studio staple over the years:

1. Universal Audio Teletronix LA-2A Classic Leveling Amplifier
2. Universal Audio LA-3A
3. Tube-Tech CL 1B Tube Optical Compressor

1. Universal Audio Teletronix LA-2A Classic Leveling Amplifier:

Compatibility: macOS 10.14+ Mojave onwards. UAD, UADx, LUNA, UA Connect compatible with M1/M2 processors. Windows 11+. Windows 10 Anniversary Update necessary for Thunderbolt connections with UA devices. 64bit only. 
Price: Universal Audio Teletronix LA-2A Classic Leveling Amplifier $4699 (for hardware)Teletronix LA-2A Classic Leveler Plug-In Collection $149 (for emulation)

2. Universal Audio LA-3A:

Compatibility: macOS 10.15.7+ Windows 10+. Windows 21H2. 64bit only. AAX Native, AudioSuite, VST, AU, SoundGrid.
Price: Universal Audio LA-3A $1895 (for hardware) Waves CLA3ACompressor / Limiter: $24 (for emulation)

3. Tube-Tech CL 1B Tube Optical Compressor:

Compatibility: macOS 10.14+ Mojave onwards. UAD, UADx, LUNA and UA Connect are compatible with M1/M2 processors. Windows 11+. Windows 10 Anniversary Update necessary for Thunderbolt connections with UA devices. 64bit only. 
Price: Tube-Tech CL 1B Tube Optical Compressor $3920 (for hardware) UA Tube-Tech CL 1B MkII: $149 (for emulation)

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