Logic Pro's Compressor circuit types

Logic's Compressor offers a variety of circuits to choose from, each with different characteristics.

This article will help you understand what those differences are and when to best use each circuit.

Occasionally you will also get an explanation of a general compressor feature to better understand why it matters.

In essence, a compressor is a tool for dynamically controlling or shaping a signal. Often a compressor is considered to reduce dynamics or punch, but set up correctly it can increase punch as well.

When a compressor manipulates the level of the signal it can also impart harmonic distortion. How a compressor does both things varies from compressor to compressor.

First you should download the cheat sheet to get an easy but detailed overview of all the Logic Compressor circuits:

Logic Pro X Compressor Circuit Types Cheat Sheet (PDF)

For the best experience please have the PDF open while reading the remainder of this article.

1. Platinum Digital

History and usage

The Platinum Digital circuit is an original design by Emagic/Apple.

It is the cleanest mode available in Logic’s Compressor and a good choice if you do not want to color the signal. One of the reasons for its clean sound is its relatively slow release speed.

Nominal vs. actual speed

Some characteristics in the circuits are hidden from the user. Identical nominal (i.e. printed) values on the attack and release speeds do not lead to identical results between circuits. Do not expect identical values to sound or behave the same between circuits. To some degree this goes for the ratio too.

Furthermore, non-linear speeds mean that the actual values vary continuously in the compressor. Non-linerarity depends on factors such as the current amount of gain reduction or the type of input signal.

Here is an example of a non-linear release speed: When the compressor does less than say 3 dB of gain reduction, the release increases in speed. This tends to pull up low levels more, e.g. tails and reverb. This is great for smashing an individual drum sound or increasing low level details in a vocal.

The opposite could be the case with a different compressor circuit: With less than say 3 dB of gain reduction the release gradually slows down. This avoids pulling up low levels quite as much, which in turn keeps the bass punchy and avoids a full drum set/drum bus losing its punch and groove.

The idea behind non-linear speeds is usually to make the compressor react more "intelligently" to the material in some way. This underscores why it matters which compressor you choose for each job.

In other words: always rely on your ears rather than the nominal values.

RMS vs. peak detection

The Platinum Digital is the only Logic Compressor circuit with user-selectable RMS or peak detection. By default it is set to RMS detection.

RMS detection averages the input detection slightly over time, in so-called windows. This is useful for smoothing out the input energy and subsequently dealing more with the body or sustained energy of a signal.

In the Platinum Digital you can switch to peak detection by clicking the “Side Chain” button and selecting peak instead.

Peak detection uses the highest points in the input signal to trigger compression. This is useful for detecting and subsequently controlling transients or shaping the signal more aggressively.

Internal vs. external side-chain detection

Notice that the side-chain is the part of a compressor that detects and potentially filters the input signal before feeding it to the control circuit.

By default the side-chain is internal in all compressors. The side-chain simply listens to the input signal coming from the channel on which the compressor is inserted.

However, when you switch the side-chain to an external input, you substitute the internal signal with an external one. This is often used for ducking, i.e. pumping a bass synth with a kick drum.

The external side-chain is enabled only when you switch it to another input source in right corner of the plug-in window header.

The Platinum Digital circuit works well in peak detection mode for clean external side-chain scenarios, such as ducking a bass to the beat of a kick drum.

If you experience audible clicks with the fastest attack speed while ducking, then switch to a third party compressor, such as the FabFilter Pro-C. This allows you to set a longer lookahead time, something that is not adjustable in Logic’s Compressor.

Measurements

This spectrogram shows the harmonic saturation over time, as well as the waveform envelope for the Platinum Digital (peak detection mode):

This spectograph shows the harmonic saturation as peak energy for the Platinum Digital (peak detection mode):

Platinum Digital verdict

Score: 5/5 - Clean and versatile on any material, including synths. RMS mode for averaging, peak mode for shaping and external side-chaining.

Authenticity: N/A

2. Studio VCA

History and usage

The Studio VCA circuit in Logic’s Compressor is inspired by the Focusrite Red 3 hardware compressor, often used on the full mix or on drums.

The Focusrite is based on a voltage-controlled amplification design, which can be clean sounding hardware, but the Studio VCA is not necessarily clean.

In fact, it is one of the best modes for creating intentional intermodulation, i.e. harmonic distortion relative to the speed and amount of compression. In other words: the faster attack and release speed and the more gain reduction - the more obvious saturation you will get.

That being said, it is potentially cleaner than the Vintage VCA, since the Studio VCA has no additional noise artifacts.

For the above reasons, and because the Studio VCA has faster and more linear attack and release speeds, it is a good choice for New York style parallel compression.

Focusrite Red 3 hardware compressor:

Parallel compression

With any type of parallel compression you blend the uncompressed input signal (called the dry signal) with the compressed signal (called the wet signal).

While parallel compression can be used in a subtle manner it is often used to compress the wet part heavy-handedly to the point of smashing it, but only blending a bit of the wet signal with the dry one.

This lets you maintain the clarity, dynamics and transients of the dry signal, but increase the low level signals, perceived fullness and width via the over-compressed wet signal.

The downside to any type of parallel compression can be audible breathing, where low level parts suddenly pop up too much. Pumping is usually less of a problem with parallel compression.

A significant part of parallel smashing is the added harmonic distortion, not just the average increase in level.

For New York parallel smashing with the Studio VCA try using the settings shown in the screenshot above of the Studio VCA.

Lower the threshold until you get severe compression, e.g. 20 dB, then use the mix knob to find the right blend between the two signals.

Use a faster release speed for even more breathing and distortion. Use a slower release for less breathing and a cleaner sound.

If you use a slower attack speed with New York compression you are likely to get clicking artifacts and overloads, but it can increase the punch of certain signals.

With auto gain set to 0 you get automatic make-up gain compensation, which is particularly handy when blending.

It is quite safe to use auto gain when doing extremely fast parallel compression, but in other cases auto gain is best left off, as it can cause output overloads with hot input signals.

Measurements

Studio VCA spectrogram:

Studio VCA spectrograph:

Studio VCA verdict

Score: 5/5 - Great for parallel compression and drums

Authenticity: 3/5 - Somewhat grittier than its inspiration

3. Studio FET

History and usage

The Studio FET is inspired by the Urei 1176LN blackface hardware compressor. A lot of the sound in the hardware compressor comes from the Cinemag transformers and the way the FET circuit imparts harmonics.

This includes a combination of odd and even harmonics, but most emulations, including the Studio FET, do not get the even harmonics right. The same goes for UAD’s 1176 emulations, so it is a common problem with plug-ins.

While the harmonics may not be right, the envelope characteristics are closer to the source of inspiration with the Studio FET. The Urei 1176 is often used for peak control on vocals, controlling an electric guitar, beefing up a bass guitar or controlling a live snare drum.

Urei 1176LN blackface:

Testing and comparisons

For my testing I used a couple of my own hardware 1176s, including the Warm Audio clone with Cinemag transformers and a DIY Gyraf 1176 with Lundahl tranformers, as well as the various UAD software emulations.

For a more aggressive and gritty sound try the Vintage FET circuit in Logic’s Compressor instead.

There is a famous all-buttons-in trick with the 1176, which makes it sound completely squashed, but this is not possible with the Studio or Vintage FET circuits.

Measurements

Studio FET spectrogram:

Studio FET spectrograph:

Studio FET verdict

Score 4/5 - Does a decent job on vocals, guitar, bass, and individual drums in reasonable doses

Authenticity: 3/5 - Lacks some of the magic of the source inspiration

4. Classic VCA

History and usage

The imposed auto speeds of the Classic VCA seem inspired by the dbx 160 compressor, but the Classic VCA also includes the "Over-Easy" soft knee from later dbx compressors, such as the dbx 165.

The Classic VCA circuit is intentionally not very tweakable for a deceptively simple fire & forget approach. It is also the only Logic Compressor circuit with full auto attack and release.

With this approach, and its RMS detection, the Classic VCA compressor can work perfectly on tracks that do not require fine-tuned compression, but it is less suitable for shaping and precise control.

I do not currently own any dbx 16X compressors today, but I used them in mixing sessions in the late 1990s on drum busses and bass, tasks for which they are well-suited.

dbx 165 hardware compressor:

dbx 160 hardware compressor:

Measurements

Classic VCA spectrogram:

Classic VCA spectrograph:

Classic VCA verdict

Score: 4/5 - Fire & forget on a drum bus or for keeping a bass synth/guitar controlled but punchy

Authenticity: 4/5 - A pretty good representation of the dbx 16X concept

5. Vintage VCA

History and usage

The Vintage VCA is inspired by the SSL 4000 G mixing console bus compressor from the late 1980s, but unfortunately there is no full auto release in the Vintage VCA. The full auto release was an important feature of the hardware compressor due to its natural sound and easy setup. 

The SSL 4000 bus compressor is often referred to as a “glue” compressor, probably the most famous of its kind. The hardware is often used on full mixes and on piano. The Vintage VCA, however, is more prone to distortion with fast attack and release speeds, so be careful.

SSL Type 4000 G series bus compressor, here shown in the modern XLogic rack version:

Tips to match the hardware

To more closely match the hardware, you should use a soft knee for ratio 2:1, a medium knee for ratio 4:1 and a hard knee for ratio 10:1.

Filtering the side-chain

When used on a full mix you can activate the Vintage VCA’s high-pass filter in the internal side-chain, and set it to somewhere between 40 and 80 Hz. This will reduce pumping from the kick drum or deep bass sounds.

This is especially important with the Vintage VCA, since it is disproportionally triggered by sub frequencies as mentioned in the PDF.

Max vs. Sum detection

When used on a full mix, another common trick with the SSL 4000 bus compressor is to switch the level detection mode in the side-chain from max to sum.

This addresses the anticlimactic issue of a compressor clamping down too much when the chorus comes in, primarily if a lot of additional stereo information kicks in.

When level detection is set to max (the default), the loudest signal in either side of the stereo signal will trigger compression.

With summed level detection both sides are folded or summed down to mono in the side-chain. This causes the difference signal to collapse during detection. In other words, the compressor does not detect any out of phase signal.

While it can lead to unpredictable results in special cases, it usually leads to a more dynamic, less compressed or louder sound during choruses.

Testing and comparisons

For the testing I have use my original SSL XLogic G Series hardware compressor, my Gyraf SSL Type 4000 G and Danfield 726 SSL hardware clones.

Measurements

Vintage VCA spectrogram:

Vintage VCA spectrograph:

Vintage VCA verdict

Score: 3/5 - Needs careful adjustment to sound right

Authenticity: 2/5 - No full auto release is a missed opportunity

6. Vintage FET

History and usage

Based on the Urei 1176 silverface compressor/peak limiter, the Vintage FET is more gritty and noisy than the 1176LN (Low Noise) blackface successor.

The Vintage FET works better than the Studio FET when you need very aggressive and saturated compression or limiting, like a snare drum that needs to be smashed or a dull bass that needs more grit.

Use extreme settings for extreme drum shaping, e.g. to smash a snare and pull up its tail: ratio 20:1, fastest attack, very fast release, and a hard knee.

The added harmonic distortion can make up for a perceived high frequency loss during compression, which can work well in some cases, but less so in other cases.

Urei 1176 silverface hardware compressor:

Measurements

Vintage FET spectrogram:

Vintage FET spectrograph:

Vintage FET verdict

Score: 4/5 - Fast and furious with lots of harmonics for smashing individual drums or controlling peaks in an aggressive vocal or plucky sound

Authenticity: 4/5 - Captures some of the original's smashing mojo

7. Vintage Opto

History and usage

Inspired by the Teletronix LA-2A tube based optocoupler design, the Vintage Opto circuit is peak-like in its detection.

While the source of inspiration is warmer or more colored, the Vintage Opto is quite clean during sustained compression. With the Vintage Opto, harmonic saturation tends to happen more on initial gain reduction, which is why the spectrograph (peak) appears somewhat distorted, but the spectrogram (sustained) does not.

The Vintage Opto can work well on vocals, bass, and with slower settings on piano. It is especially well suited to vocal levelling or acoustic guitars.

Teletronix LA-2A hardware compressor:

Serial compression

For vocals it is often used in a serial compression chain: a very fast Studio FET or Vintage FET peak limiter first in line to control the peaks, then followed by a slightly slower and deeper digging Vintage Opto to average out levels and control sustained notes.

Measurements

Vintage Opto spectrogram:

Vintage Opto spectrograph:

Vintage Opto verdict

Score: 5/5 - A unique and useful mode for smoothing vocals, guitar, and more

Authenticity: 4/5 - No actual multi stage release, but not bad at all

8. Frequently asked questions

This section is relevant if you wish to do your own prodding of the various circuits or simply wish to understand more about the various compressor functions.

It also deals with some of the follow-up questions people have asked me about this article.

Do I still need to set the release manually when auto release is on?

Yes, the Compressor’s auto release is not a full auto release, so you still need to set the release manually. The only exception is the Classic VCA circuit which has a fully automated attack and release.

Most auto releases work by comparing the peak energy (transients/loudest parts) to the RMS energy (average energy). This relationship between peak and RMS energy is also referred to as the crest factor.

When the average signal is low but the peak energy is high, such as successive percussive sounds, the release is shortened to accommodate the transient nature of these sounds.

When there is a lot of average energy but not a lot of peak energy, such as a sustained string chord, the release gets longer for a steadier sound.

With any combination of peak and RMS energy, the auto release will try to adjust the release behind the scenes. The manual setting is still required since it serves as the starting point for any automatic adjustment.

Except in rare cases, auto release in Logic's Compressor is best kept on.

I thought side-chain compression meant ducking one sound with another?

This is a common misunderstanding that comes from many people shortening “external side-chain compression” to the superfluous and meaningless “side-chain compression”.

The side-chain is merely the internal part of the compressor used for detecting and manipulating the control signal. The control circuit then tells the compressor how fast and how much compression needs to take place.

By default the side-chain listens to the incoming signal on the channel where the compressor is inserted.

External side-chain compression is when you substitute the internal side-chain with an external signal. This is often used for ducking purposes.

Is side-chain filtering the same as or a type of multi-band compression?

No, filtering the side-chain simply manipulates the detection circuit in the compressor. It does not dynamically equalize the output signal in any way, which is what a multi-band compressor does in effect.

Filtering the side-chain can both lead to less compression, such as filtering out the sub bass frequencies, or it can lead to more compression by e.g. boosting a parametric/bell band.

However, any compression still takes place on the full, broadband output signal - not on discrete frequency bands.

How can I check the hidden frequency dependency?

Insert a Utility > Test Oscillator plug-in before the Compressor. Set a Compressor circuit for about 10 dB of gain reduction and sweep the Test Oscillator from 20 Hz to 20 kHz.

Notice how deeper frequencies lead to more compression in varying degrees, depending on the circuit type. The only completely frequency linear circuit is the Platinum Digital.

How can I check sum versus max side-chain detection mode?

Insert a Utility > Test Oscillator plug-in. Next open a Utility > Gain plug-in. In the Gain plug-in click one of the two “phase invert” (technically “polarity invert”) buttons to make the signal out of phase. Then insert a Compressor and set it for e.g. 10 dB of gain reduction.

Now switch between sum and max in the side-chain to see that no compression takes place with out of phase material. While this is an extreme example it illustrates how the compressor will compress less with wide material.

How can I check the hidden ratio dependent range?

Range in a compressor means the maximum allowed gain reduction, regardless of the other settings in the compressor. It is also referred to as a gain reduction cap or limit (not to be confused with "peak limiting").

First, send a test tone into the Studio VCA. Set the threshold for -50 dBFS and the ratio to 1.3:1. No matter how hard you push the test tone into the Studio VCA it will cap the gain reduction at around -5 dB. Adjust the ratio up and you will see more gain reduction, but it will still be capped. This is the ratio dependent range.

Second, do the exact same thing with the Vintage VCA and you will notice there is no capping of the range.

What is the Distortion knob in Logic’s Compressor?

The name “distortion” is a bit misleading, since in reality the knob is a clipper. Clipping can lead to audible distortion, which is probably why this name was chosen.

Apart from the default “Off” setting, there are three modes:

• “Soft” clipping simply means there is a knee or transition where the signal is rounded rather than squarely chopped off. In practice this leads to smoother transients and causes distortion to happen earlier dynamically speaking. Soft mode also adds +3 dB of gain into the clipper.
• “Hard” clipping in Logic’s Compressor still has a small transition or knee, but is slightly more rounded and less harsh than regular digital overload. Hard mode also adds +1.5 dB of gain into the clipper.
• “Clip” mode in Logic’s Compressor simply means digital overload clipping. No additional gain is applied into the clipper.

The Distortion parameter is not related to the harmonic distortion or other noise artifacts presented in the cheat sheet, but is a complete stand-alone feature.

Where can I learn more about compression, attack and release speeds, etc.?

I have shot a detailed video about dynamic compression. The video uses the TDR Kotelnikov compressor to explain things, but many settings are relevant to how any compressor works, including the various Logic Pro Compressor circuits.

The video deals with more advanced subjects such as program dependent compression, threshold, ratio, hard knee vs. soft knee, attack & release misconceptions, gain reduction limit / range, peak vs. RMS, peak crest, dual release paths, inertia / non-linear speed, electro / opto / variable auto-release, low frequency relaxation / side-chain filtering, basic feed-forward compressor design, internal vs. external side-chain, frequency dependent ratio, yin & yang / harmonics / asymmetrical shaping, processing targets, stereo sensitivity, processing quality / aliasing, make-up gain, parallel blend, equal loudness trim, equal loudness bypass, and delta eavesdropping.