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Logic Pro X Compressor Circuit Type Cheat Sheet


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This post and my subsequent replies have been distilled into an article:

Click on the above preview link to read the article and download the cheat sheet PDF.

October 30, 2023 - Continually updated

 

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Original post, March 2, 2018:

I spent some time measuring and experimenting with the different circuit types in the Logic Pro X Compressor.

I did this by measuring harmonics using a spectrograph and a spectrogram as well as analyzing the waveform envelope. A lot of experimenting, null testing and comparing.

It's a fairly technical "cheat sheet", more of an overview of what goes on in the algorithms behind the scenes, really.

- Now with practical usage tips as well!

As far as I know there are no resources anywhere on the Internet (or in the manual) that contain these specific details about the LPX Compressor. This PDF won't replace your ears, but if you wonder what's going on in the different circuit types in the Compressor then you might find a clue in this PDF.

Please don't upload this PDF anywhere else, but feel free to link (directly) to it on my server or to this thread. This way I can always keep things updated, if necessary.

I'm very interested in comments, especially critical comments, i.e. if you find anything wrong, difficult to understand, unclear or a typo.

DOWNLOAD CHEAT SHEET HERE - UPDATED LINK

See article link at the top of this post for both the full article and updated cheat sheet PDF link.

Edited by Holger Lagerfeldt
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Thanks!

 

Some people requested graphs. I'm working on the graphs, but that could take some time. For now, here are some screen grabs that could help you understand the info in the PDF better.

 

Spectrograms = Show the waveform envelope + a spectrogram reading which shows the harmonic saturation (over time) and its relation to the amount of compression.

 

Spectrographs = Show the harmonic saturation in a different way (peak energy) and where it isn't partly obscured by the waveform in the spectrogram.

 

General comments about the tests

These test have been done by using a baseline and by measuring null test residue. The compressors have been set up to match each other as closely as possible, both by sound and especially by measurement, not by nominal values, which can be misleading due to offsets.

 

My initial interest in doing this was become someone claimed there were no discernible differences between the algorithms.

 

While I did test the RMS mode in the Platinum Digital it's quite different and has a large offset which means it needs to be examined separately.

 

The first one below is a good example of why I've included both the spectrogram (1 kHz) and spectrograph (50 Hz). The spectrogram shows a short initial burst of distortion which is not surprising on the transient of the test bleep when the compressor is doing a fair amount of quick compression. However, there's sustained (and again, expected) 3rd harmonic saturation, but it's obscured by the waveform in the spectrogram. This is clearly visible on the spectrograph instead. Of course I could have done separate waveforms for all the spectrograms, but I was lazy...

 

Platinum Digital (Peak mode)

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/01-platinum-digital-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/01-platinum-digital-spectrograph.png

 

What's the Platinum Digital good for

Overall, the Platinum Digital is very clean and obviously a good choice if you want things un-colored. For the same reason it can work well in peak mode for external side-chain applications where you don't want to hear the sound of the compressor itself. However, a the lack of an adjustable look-ahead parameter in the Logic Compressor can be problematic when doing instant external side-chain ducking. In that case you'll need something like the FabFilter Pro-C2.

 

The Platinum Digital appears to have a non-linear release time that lets go more quickly of the signal at lower levels of gain reduction. Something similar can be seen with the Waves Renaissance Compressor in Electro mode. This means it's good at pulling up low level information under certain circumstances, mainly in peak mode since the RMS mode is offset quite a lot.

 

Platinum Digital is the only circuit with selectable RMS/peak detection. By default it's in RMS mode. RMS uses an averaged window in the detection circuit which means it's potentially a good choice for smoothing out overall levels rather than shape or control peak energy. The only other circuit type that has this function is the Vintage Opto, which has a (sort of) RMS approach to detection.

 

Studio VCA

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/02-studio-vca-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/02-studio-vca-spectrograph.png

 

What's the Studio VCA good for

First of all, this is an example of why it's prudent to examine an algorithm and not make too many assumptions. Some places on the Internet label this as a very clean algorithm, but that's not quite true.

 

The Studio VCA has potential to do a lot of useful saturation when you push it, but adds no additional noise artifacts. The saturation is proportional to the amount and speed of the gain change performed by the compressor. It's got peak detection, and for those three reasons it does hard parallel compression (New York style smashing) well. A lot of the parallel sound comes from the saturation, not just the increase in average level as the smashed and saturated signal is blended with the uncompressed signal, using the wet/dry mix knob in the Compressor.

 

I generally don't use auto gain, but when the Studio VCA is set up for internal parallel compression with the fastest attack time it makes sense from a workflow point of view, since you're unlikely to get output overloads. Activating the output limiter and output clipper (labelled Distortion) will contain incidental overloads, just in case.

 

Studio FET

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/03-studio-fet-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/03-studio-fet-spectrograph.png

 

What's the Studio FET good for

A lot of the sound in the 1176 LN Blackface hardware comes from the Cinemag transformers and the way the FET circuit imparts a sonic signature, which includes a mix of odd and even order harmonics. As you can see, there are only odd harmonics in all of the Logic Compressor circuit types. But it's the same with the UAD 1176 emulation: while it's a lot closer to the response of the real 1176, it's not when it comes to harmonics.

 

This is one of the benefits of using analog hardware: you get a mix of even and odd harmonics. I cover this in my video here:

.

 

I've had a couple of hardware 1176 LNs and I've built one myself, based on the Gyraf schematics. The original and the Warm Audio clone have Cinemags, my DIY has Lundahl trafos. I also have the UAD plug-ins.

 

The 1176 LN is often and with good reason used for peak control of vocals, smashing a snare drum, controlling an electric guitar, beefing up bass, etc.

 

Classic VCA

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/04-classic-vca-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/04-classic-vca-spectrograph.png

 

What's the Classic VCA good for

The Classic VCA circuit isn't particularly tweakable since it's got full auto attack and release and a non-adjustable knee. While the GUI looks like a 160A model, the algorithm is more likely inspired by the dbx 165 or the later 160XT model, since those have the dBX Over Easy® soft-knee, which is present in this algorithm.

 

I've never owned the hardware, but I've used it while mixing an album years ago, 1997 I think. We used it on kick and snare to add some snap. To me this algo is a bit of a hit and miss thing and despite its easy fire & forget look, it could lead to pumping instead. As always: YMMV.

 

Vintage VCA

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/05-vintage-vca-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/05-vintage-vca-spectrograph.png

 

What's the Vintage VCA good for

The Vintage VCA is Inspired by the SSL 4000 G Bus compressor, although there's no full auto release in this circuit, which is a big draw of the hardware. The semi-auto option available for all circuits in the Logic Compressor (except the Classic VCA) can get you only a bit closer. If you want the real SSL G Bus settings then you can more or less look at the hardware and type in the fixed settings in the Vintage VCA to get in the ballpark or download my (now very old) SSL presets here which are based on a slightly modified mastering version of the DIY SSL Type 4000 comp. I've had both the hardware SSL XLogic G-Series Bus compressor, the Gyraf SSL Type 4000 G comp and the Danfield 726 SSL "clone".

 

Typically this one is used for a bit of mix bus glue or similar. It's prone to distortion with fast attack and release settings, so be careful. When used on a full mix consider activating the internal side-chain filter in the Compressor with a HP filter @ 60 Hz or so to reduce pumping caused by the kick and bass. This is especially important when using the Vintage VCA on a full mix since it's got built-in frequency dependent compression that triggers more on bass content.

 

A more advanced trick on a full mix is to make the Vintage VCA not clamp down too hard on a wide chorus (e.g. one with hard panned guitars, panned vox or a very wide synth) by switching the internal side-chain detection mode from Max to Sum. This means the detection signal is no longer based on the highest level of either stereo side, but based on a summed input signal. Since such a summed input signal means that any anti-phase signal will be removed or reduced (since the sum is a mono signal) it results in less compression than it would have done otherwise.

 

It's easy to test exactly how this works sending a Utility > Test Oscillator sine tone with a Utility > Gain plug-in where only one side is polarity inverted. In Max mode it will react to the sound like expect, in Sum it will complete ignore the input signal - there will be 0% compression regardless of your other settings. This is an extreme example of course.

 

Vintage FET

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/06-vintage-fet-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/06-vintage-fet-spectrograph.png

 

What's the Vintage FET good for

This Vintage FET is the Silverface 1176 LN which has a more aggressive sound than the Studio FET Blackface version. This one is more grabby and it's a lot more noisy, which can work well on some types of sounds, like a snare drum that needs smash and grit.

 

And no, you can't do the all-buttons-in trick on this FET either ;-).

 

Vintage Opto

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/07-vintage-opto-spectrogram.png

http://www.centerforlydteknik.dk/download/logic/compressor-circuit/07-vintage-opto-spectrograph.png

 

What's the Vintage Opto good for

The Vintage Opto is inspired by the Teletronix LA-2A, a tube based optocoupler design. Although it's got a small amount of noise, it's actually quite clean during sustained compression. The spectrograph is a bit misleading here, the spectrogram above it paints a better picture.

 

An interesting thing about the Vintage Opto is the detection and timing circuit which is RMS-style. I say "style" since the hardware inspiration's light-based detection is unique compared to a binary RMS/peak classification.

 

Often used on vocals, bass, and with slow gentle settings on piano. It's especially well suited for levelling a vocal. Many times it's used in series with an 1176 LN to obtain serial compression, e.g. the 1176 LN catches the peaks while the LA-2A follows up by smoothing out the average levels. Not a bad algorithm at all.

Edited by lagerfeldt
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I've updated the PDF today with a new interesting measurement.

 

This is the "Hidden frequency dependency" parameter, aka. "if bass frequencies can result in more compression".

 

It's not a simple parameter to measure since it also depends on the attack and release time and of course the total amount of gain reduction.

 

I've 'quantized' my measurements into "None, Small, Medium and Strong" classifications.

 

If you want to see it in action, open a Test Oscillator with a sine tone. Then insert the Vintage Opto, set a high ratio, use a slow attack and fairly quick release, then adjust threshold for 5-10 dB of gain reduction.

 

Now sweep the oscillator from 20 Hz to 20 kHz and you will see a big difference (i.e. several dBs) depending on the frequency. Try adjusting the attack and release parameter.

 

Do the same with the Platinum Digital and you will see virtually no difference, no matter the input signal frequency or attack/release setting.

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That's really interesting, have you considered putting a 'Most suitable for..', or 'Traditionally used for...' row on there too?

 

If you had that on there it'd be worth printing and putting up on a wall for reference as it helps techies and newbies 'less-techies' too?

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In the upper right corner of the Compressor plug-in there's a button named "Side Chain". Click this and you'll see some new options under "Detection". There's a "Peak" and "RMS" option, only available in the Platinum Digital circuit.

 

All the other circuits have fixed peak detection, except the Vintage Opto which has fixed RMS-style detection.

 

It's also under these "Side Chain" options you'll find the ability to switch between Max and Sum detection, as well as activate the built-in side-chain filter.

 

Max detection means that the loudest level of either stereo side will trigger compression. Sum means that the two sides of the stereo image is folded down to mono and this summed signal is used for detection. The main difference between Max and Sum is that Sum tends to result in less detection/compression with wide stereo sounds, e.g. a wide stereo synth or dubbed and hard panned instruments or vocals. This is especially useful when a compressor is strapped across the full mix and you don't want it to over-compress during a loud, wide chorus.

 

The side-chain filter can be used to manipulate the compressor into reacting less or more to certain frequency content. Filtering out frequencies will make the compressor less sensitive to those frequencies (HP or LP or cutting with a parametric or shelf) while boosting with a parametric or shelf will make the compressor react more to those frequencies. It's important to understand that the compressor is still compressing the full frequency (broadband) signal - there's no multiband compression going on. You're only manipulating the detection circuit.

Edited by lagerfeldt
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They are.

 

It's a common misunderstanding about the side-chain. The side-chain is simply the signal chain that's feeding the control circuit in the compressor (or at least in a feed-forward compressor design).

 

An external side-chain is when you substitute the regular input signal and feed the compressor an external signal instead. This is called external side-chain compression or ducking.

 

It's a lazy use of the words when people refer to external side-chain compression as side-chain compression. This leads a lot of people to misunderstand what the side-chain really is.

 

So when you manipulate the side-chain you're manipulating the input signal being fed to the control circuit - whether it's internal or external.

Edited by lagerfeldt
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Couldn't fit it on one page! :-D

It's ok, i can verify my age as the tape browns over time ;)

 

Many thanks again for spending the time on this, compression is such a huge part of everyone's workflow and i've never spent the time (Mainly cause i can't) to understand fully what's involved with each compressor type, so it's great to see them objectively presented in that way by someone far more knowledgeable - it will definitely make a positive influence on me, so again, big thanks.

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Most excellent! Thank you Holger - what a fantastic resource.

 

BTW - fantastic videos on YouTube as well. Really enjoyed the two that are posted and hoping you continue to post more. Great stuff.

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