How Analog Plugin Emulations Work (& Are They Better Than Digital?)

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  • There’s much debate around the topic of analog plugin emulations 
  • We delve into how they work and discuss how they are different to purely digital plugins

Is there a single producer out there who doesn’t love vintage analog audio hardware? While so many producers agree that the sound and feel of classic analog hardware is hard to beat, what’s not so great is all the labor involved in setting things up and maintaining the equipment, plus it’s hard to make room for a bulky vintage synth or multitrack tape machine.

Because of this, in recent times we have seen a shift towards virtual analog plugins that combine the best of analog and digital sound. Yes, these plugins are entirely digital, meaning at the end of the day there is only number-crunching going on to generate and process sound, just like any other digital plugin right? Well it’s how the numbers are crunched that matters.

In this article, I’ll look at a topic that has rarely been explored – how do virtual analog plugins and digital emulations of classic audio hardware actually work?

What Are Analog Emulation Plugins?

Analog emulation plugins, or virtual analog plugins, work just like other plugins in your DAW except they are designed to behave and sound like a real audio hardware unit.

This is desirable because analog hardware units are favored primarily for their sound or character.

These type of plugins also play into the nostalgia people have for the analog gear from days gone by, and this is a big market. People want ‘authenticity’ without having the real thing, which seems ironic. But the truth is digital emulations sound very close to the original units most of the time, and offer a lot of advantages to real gear as well.

You’ll find that the approaches vary from company to company and different methods have different effects on how many of the original unit’s characteristics the final plugin manages to capture.

Emulation Plugins Vs Digital Plugins?

There’s a bit of confusion surrounding how emulation plugins differ to other digital plugins. It might be easier to just say that emulations ‘sound more analog’ but what’s actually going on in the code to make that difference?

In emulation plugins, the code will either directly model the circuits of the original unit, or use a bunch of complex equations derived from the original unit to simulate the output response vs the input.

This is opposed to ‘vanilla’ digital plugins such as your DAW’s stock chorus or EQ effect which are much more literal and straightforward in terms of how sound is modified in the code.

Purely digital plugins suffer from problems like aliasing, and though this can be fixed with oversampling, these plugins generally don’t sound as exciting as virtual analog ones.

So if you want to make ‘transparent’ changes to a mix, then a stock or digital plugin is just fine. But if you want to add some character to an otherwise dry recording, emulations are beneficial. Just don’t expect virtual analog plugins to be as CPU efficient as your DAW’s stock plugins.

A Quick History of Audio Plugins

These days audio plugins are a staple of all stages of music production and rarely will any production be all analog.

It still happens, but due to the convenience, power, and affordability that digital music tech gives us, musicians and songwriters are much less likely to take that route with their creations. So let’s look at where plugins came from…

In 1975, despite computers being relatively primitive, Soundstream made the first-ever attempt at a DAW (digital audio workstation) with their Soundstream Digital Editing System.

It involved having a tape recorder hooked to an analog-to-digital converter (ADC) that enabled users to do digital fades and track splicing.

In 1979, Fairlight made their Computer Musical Instrument (CMI) sampler and digital synth. This had a hard disk recorder system and extremely sophisticated software for playing back samples. The CMI was a hit with forward thinking pop artists of the 80s like Kate Bush and Peter Gabriel.

There were further developments in the MIDI technology by Moog and Roland in the early 80s, while personal computers kept getting more and more powerful with each innovation such as the Commodore Amiga and Apple II.

But in 1985, the Atari 520 ST computer broke new ground with its built-in MIDI ports. This made home music production a whole lot easier and more accessible for a whole lot of people.

Another big event in 1985 came when Digidesign released Sound Designer. It was for editing samples for a keyboard, but it would later be further developed into Pro Tools in 1989. In this new iteration, Pro Tools could do 2-track digital recording. 

A company by the name of Waves Audio released the first-ever audio signal processor plugin, the Q10 Paragraphic EQ, in 1992.

Pro Tools would add on the ability for its software to take 3rd party plugins, in addition to its stock ones. With this a bunch of other new companies, as well as established brands, started making their own plugins.

Audio plugins in DAWs became immensely popular and in 2006, Waves Audio released one of the first plugin analog emulations with their SSL 4000 Collection.

Since then, the technology and methods used to emulate analog units have rapidly been developing and expanding.

Some Things To Know First

Analog signals have levels that vary continuously over time. Digital signals can be described as transmitted and/or stored data expressed as bytes made up of 1’s and 0’s.

So, in the audio world, analog audio is the electrical signal that replicates the original sound wave. In a basic analog recording, it involves sound energy being converted from real mechanical sound waves to electrical energy (via microphones and guitar pickups), and then to a magnetic signal for tape storage.

Later on when you play back the tape, the recording is converted back into an electrical signal that is is sent to your amp and speakers which produce mechanical or ‘real’ sound waves that you can hear.

With digital audio, an ADC (Analog to Digital Converter) consecutively captures thousands of tiny samples of that analog signal, creating digital ‘samples’ that can be interpreted by computers. Different settings and components on the ADC determine the accuracy of the converted digital audio.

The samples are taken at a periodic interval we call the sample rate. You can think of samples as like ‘pixels’ but for a waveform instead of an image. The bit depth is the resolution or precision of the sample, and both the bit depth and sample rate will directly affect the quality of a digital signal or waveform. So these qualities in an ADC play a very important role in ensuring the digital recording is an accurate representation of the analog signal it is converting.

(You can read our full guide to sample rate and bit depth in What Sample Rate You Should Use? (+Myths & Misconceptions))

So, How Does Analog Emulation Work?

The two approaches for analog emulation with digital signal processing I’m going to look at are white-box testing and black-box testing.

White-box testing focuses on testing the internal workings and structures of a system while black-box testing focuses on functionality and modeling the circuit without knowing its internal workings.  

White Box Testing

The white-box or circuit component approach involves digitally rebuilding the analog circuit with code. This can also be called the component approach or physical circuit simulation.

The first part of the process is taking apart the hardware device and making extensive notes on how each part and component is configured.

These components and their settings are entered into a software circuit simulator program where their behavior is analyzed and rebuilt via coding. Then the coding modules are put together the way the physical circuit is wired. 

The result of all this coding is that the software generates various I/O (input/output) plots. From these graphs, a transfer function can be derived, allowing the creation of a differential equation (basically an equation that describes a bunch of physical variables) that relates the input levels and control settings vs. output levels.

This differential equation is used to generate digital signal processing (DSP) routings that emulate the hardware.

Some problems that occur with this method is that in order to get an accurate emulation, one needs to know exactly how each component is working which can be difficult, especially with vintage units where sometimes the provided schematics may not match the classic units.

Another problem with this method is that it’s very difficult to emulate the frequency-dependent interactions between components, which is problematic when the final audio output is very frequency-dependent.

So in many ways, it’s very difficult to get every little detail and variable in the emulation that the original unit inherently possesses.

Black Box Testing

The other main approach, the black-box approach, develops the digital signal processor right from the beginning.

It involves sending different test signals through the analog unit. The inputs and outputs are recorded using as many different settings on the device as possible. The more various settings are measured, the more accurate the emulation is.

The aim of each test signal measurement is to measure a specific characteristic. Looking at original schematics and listening for nonlinear behavior can be used to determine which test signals to start with.

After all these measurements of the inputs vs. the outputs are taken, the resulting relationships are used to write a code that simulates the hardware’s effects. 

One of the main issues with this method is that due to more testing being required for better accuracy, developing a plugin this way can be very time-consuming if the original analog unit has continuous controls (and most do). 

How Does This Work In Practice?

When going about developing an audio plugin that emulates an analog unit, the methods are usually not just the white-box vs. black-box approach. Many companies will go with a hybrid method.

For example, Waves Audio uses the white-box method for the most part. They get the schematics, additional technical documentation, and research into any default settings in the time period that the original unit was mainly popular.

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People using white-box texting take the unit apart, figure out the signal flow, then write equations based on how the components act. This gets entered into a program.

Then, they measure the plugin against the original in performance and perform listening tests with people familiar with the original units.

Today, they have expanded their modeling process so that they can emulate the sound of multiple units in a signal chain since each unit interacts with the others.

The very popular Universal Audio plugins are expensive for a reason, especially once their method of emulation is explained. Described as “circuit modeling” instead of circuit simulation, their method involves rebuilding each and every element of the circuit

They start with the schematic in order to figure out how the circuit actually works. They also double-check any differences between the actual unit and the schematics, which as we discussed can occur.

First UA tackle the easier parts of the circuit before diving into the nonlinear elements, then the time-varying sections (such as how heat can affect the performance of tube amps the longer a unit is left on).

Math Attack

If you’ve made it this far, congrats, but explaining the final stages of this process is tricky if you’re not familiar with some of the underlying mathematical principles.

Developers determine what domain the audio will be represented in as it travels through the circuit, time domain or frequency domain. Using this information, they develop an initial differential equation. 

Then they develop a nonlinear differential equation that reflects the modeled system’s behavior in a finite number of states (basically they’re looking at how each state will change as a function of all values and their derivatives, taking into account the components’ behaviors and interactions). This is sort of like ‘sampling’ the machine so that its behavior can be mapped to code.

Finally, devs use algorithms to simplify the system and filter out redundancies, making it easier for the computer to work with.

This is done by reducing the order of systems, therefore reducing the number of possible states as well. Obviously there can be an element of compromise here, but the goal is to make the plugin as accurate as possible without sacrificing too much just to make it manageable.

With the initial differential equation, first it is converted into the discrete-time domain. Linear time-invariant sections are isolated and the solution to these is approximated.

Any leftover elements of the system must be solved at the same time due to all of the subsystems’ interactions reflecting the interaction of the components inside the original unit. 

Probably how a lot of you are feeling right now.

After all this math, plugin developers will run listening tests to evaluate the accuracy of the code, using test tones such as sweeps, noise bursts, steady-state tones, samples, solo instruments, full mixes, and multitrack sessions. These tests will ideally involve engineers who are deeply familiar with the original equipment.

They determine whether the aesthetic of the plugin matches the original hardware, paying special attention to the ends of the control ranges since it will expose the discrepancies due to having the most extreme behavior.

After the first listening tests, they go back to the DSP code and perform any needed revisions, and repeat the listening tests and revisions until they reach the point where they have achieved the best result possible with the provided money and time.

Are Analog Units More Superior Than Their Digital Emulations?

On any audio engineering, music production, or related topic forum, there will always be the one question posed: analog vs. digital? This will inevitably lead to the mention of plugins vs. analog hardware. There will be different sections of the peanut gallery that will bring up flame wars over this so-called ‘issue’.

The answer isn’t exactly a simple yes or no

There are outside variables to consider such as:

  • What field of music is being considered
  • The production value one is working with
  • The experience level of the user

Obviously, the digital emulation will be a much better bargain for someone who works entirely on a computer.

Even for producers and engineers that can afford the highly prized vintage gear, some just prefer the digital plugins more than the analog units, and that’s perfectly fine.

Advantages Of Analog Hardware

Analog units are able to provide a sonic characteristic with non-linearity and quirks that are very difficult to capture in the digital realm, as mentioned in the advanced emulation process by Universal Audio themselves.

Distortion, tape hysteresis, and random noise are elements that are harder to quantize in a mathematical equation. For example, tube preamps impart a sonic character onto a signal that can be difficult to quantify or otherwise replicate with software in a convincing way.

Analog units are also easier to tweak, as actually using the physical unit is more intuitive than using a mouse. This is especially true for synthesizers which can have a lot more parameters than  compressors or equalizers.

The analog realm has an infinite amount of output level values within whatever range an analog unit can possibly produce. This isn’t possible in digital audio, so the developers have to approximate as much as they can without compromising on the actual behavior of the plugin.

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Advantages Of Digital Plugins

Having said that, digital plugins do have many advantages over their analog counterparts, one obvious one being cost. Vintage gear is so much more expensive than the digital plugins that emulate it. 

A single piece of analog gear (that doesn’t even have to be vintage) can be priced about the equivalent of an advanced ‘everything bundle’ on a good portion of the plugin manufacturer sites.

Another huge advantage to digital plugins is their convenience – you can download them in an instant, they don’t take up any space in your studio, you can run dozens of copies simultaneously, they have presets, the settings save and load with your project, and so on.

While yes, digital plugins sometimes can cause a DAW to crash and you need to have a pretty powerful computer to have a lot going at once, going into a menu to create a signal chain of multiple plugins is much faster than manually patching a signal chain with cables.

Also, emulation plugins can expand upon the original unit in various ways, adding new features that don’t betray the core character of the hardware.

Like many industries, money is time, and plugins allow engineers and producers to be quicker at recording, mixing, and mastering. 

As mentioned before, there are advantages and disadvantages to analog devices and plugin emulations of them. The preference of one over the other depends on the user, what they have to work with, and what they’re using them for.

Wrapping Up

The market for analog emulation plugins has led to multiple companies developing their own methods, derived from the black-box and white-box methods, but these are certainly not the only approaches one can take.

In return, this has provided users with the experience of getting to interact with digital versions of these analog devices, bridging the gap between audio engineers and producers of the older and new generations.