ADDITIVE SYNTHS

HARMOR

Harmor is the one additive synth I am least familiar with, but the synth most additive fans are looking for a replacement of
in many ways it is the “serum” of additive synthesis

Harmor has a fixed layout, it allows for spectral resynthesis and has many graphs/vector (mseg style) interfaces for setting partial frequency or filter shapes

These graphs are very large with a capable editor allowing for some incredibly powerful sound design

PARSEC

Parsec is my personal favorite additive synth, it takes many of the features I’d love from Harmor and others, and puts them into a convenient layout

Parsec has a small modular layout, with 2 oscillators that contain over a dozen osc algorithms
and 4 additive processing slots that can load over a dozen different osc fx

RAZOR

Razor is another classic “fan favorite” additive synth. It has a much more narrow set of options, but still manages to create a wide range of sounds

Razor has 2 oscillator slots again, but with fewer algorithms
2 filter slots each with their own set of additive filter algorithms
1 additive algorithm slot for an osc fx
and 1 additive “dsp” style effect slot for additive reverb

SINE MACHINE

Sine Machine is a newer additive synth, it manages to do some very fun things with a limited set of controls, but ends up producing a fairly narrow range of usable sounds.

Sine Machine has a normal layout with a few different tabs for different osc fx
the envelope tab allows for skewing the envelope time per partial
the vibrato and tremolo tabs allow you to apply modulation per partial with different intensities for cascading effects

SUMU

This is probably one of the more experimental additive synths out there, most of the innovation comes from the sample resynthesis.

Sumu resynthesizes audio with an interesting algorithm that differs from traditional spectral resynthesis
Sumu has FM per partial, but this is a rather noisy effect
Sumu also has a cool cluster LFO thing similar to sine machine’s tremolo effect
And finally Sumu has a per partial spatial stereo effect that is subtle but sounds very nic

RADICAL1

This is the newest of these additive synths, it kinda tries to take the phaseplant approach of being fully modular, but lacks many of the experimental modules that make additive fun.

My demo has expired and they never got back to me about an NFR
they made me sign an NDA, then ghosted me, very weird…

ZEBRA 3

A really cool combination of additive and wavetable, I’ll be focussing on the additive effects modes here

Zebra 3’s additive mode offers a small handful of algorithms to play with, but these can be manipulated via transformable MSEG shapes for some very unique processing

ALCHEMY

Alchemy has a fairly obtuse additive engine

It offers a couple algorithm slots with a small handful of algorithms each

PIGMENTS

I can barely justify calling this one an “additive” synth, the harmonic mode lacks some key aspects of additive synthesis leading to a dry and unpleasant sound that lacks variability. I’m providing it as an example of “what not to do”. but there are still a couple cool things here, namely interesting filter shapes that can be blended and transformed together as well as an interesting cluster mode.

One of the reasons Pigment’s engine didn’t sound great at first was because no matter how you spread the partials, they would always be phase locked, creating a buzzing sound, they’ve since allowed you to switch this off

LOOM

Another classic, I love the modular nature, but there’s a lack of interesting modules limiting it’s over all usefulness.

Loom has a modular layout with 8 slots that allow for over a dozen different algorithms
Most algorithms are basic, otherwise this would be one of my favorites, I’d love the parsec/razor algorithms in a system with more slots

COMMON ADDITIVE ALGORITHMS

Partial spread:

Spreading partials away from the harmonic series is an important and powerful aspect of additive synthesis.
This allows you to bring partials into new harmonic structures otherwise impossible with other forms of synthesis.

Most common is an even spread where partial frequencies are multiplied by the spread amount
this can also be used to squeeze partials closer together if the multiplied amount is less than
Another variation of this spread/squeeze algorithm pushes all partials towards a specified frequency
the further a partial is from this frequency, the more influence the depth will have on that partial

There’s other distribution algorithms out there, but I am not capable of reverse engineering each and every one. They all tend to have an atonal influence on the sound.

The favorite solution is a partial slope, Where the tuning of each partial can be defined by a curve
this is usually done where a flat line is the normal harmonic series
X axis is partial
and Y axis is tuning
some synths used a free draw mode, where every partial could be addressed individually
while others use a vector drawing system (like an mseg) to apply clean transformations across partials

this partial “remapping” system is very powerful requiring only a single MSEG style curve window
various curves can be loaded instead of algorithms

Another common oscillator type is typically called “string”
this is a more subtle partial spread that seems to really disperse partial phase in an interesting way

Additive Filters:

The filters in additive synths are what give them a lot of color and power
you are not restrained to traditional filter shapes, you can create any series of notches and ripples

popular filters include phaser style shapes

again, a free draw filter curve is very powerful here, allowing you to draw a filter curve as you would draw an LFO, this opens up loads of filter shapes

Partial decay and envelope offset

one of the more musical additive features is independent partial decay where each partial has its own envelope and decay at different rates.

This is usually done by making higher partials decay faster, giving an incredibly realistic physical modeling quality.

other more advanced systems allow you to choose which partials decay longest or even influence over attack and other envelope stages.

Random drops:

A fun and common additive effect algorithm is the “random drops” style effect
this effect will silence partials and randomly “ping” individual partials with a brief decay for a glimmering granular quality

there is tones of room to explore with this type of algorithm, such as density, rate, range from a focus frequency, “ping” shape or even pinging groups of partials

Some variations to this are a strum or “arp” mode that pings through partials in a more predictable pattern.

Additive Reverb

one of the prettiest additive effects is to use the partials as the reverb instead of reverb dsp.
I have no clue how this is done from a design perspective, just that it tends to sound amazing

Noise

Applying subtle noise modulation to each partial can really make the oscillators sound lush and beautiful. I’m not sure if there’s any trick to this, like having slower noise on lower partials, probably more subtle influence on higher partials to soften it up.

Modular Layout

A few additive synths take a modular approach to their layout
these modules contain a variety of additive algorithms, from filters, to distortions, and other various additive processes that shape the sound
this lets the user create a unique set of additive modules for their sound

this approach leaves a lot of mystery to the user as additive processes are often times quite unlike typical dsp, their effect can range from very intense to incredibly subtle with new terminology that doesn’t exist outside of additive synthesis

This is my favorite system, especially when a wide range of algorithms are available to mix and match, but I could imagine a fixed layout with the right set of modules is closer to what you are after.

POWERFUL FOR MUSICAL INSTRUMENTS

I would say that all of the algorithms I listed are powerful for musical instruments
Partial spread allows for unique timbres, the creative filter shapes make for very unique tones
Partial decay makes amazingly convincing acoustic emulation
And random drops/noise give texture and life

COMBINES WELL

Again, I’d say all of these combine well, I think I’d love to see a combination of “random drops” and “partial decay” there’s room to combine these into something new.
Partial spread makes all of these unique in a new way
And additive filters and reverbs bring new motion to the table

INTUITIVE PARAMETERS

This is a little tricky, I’m not sure any of this is “intuitive”
additive filters are likely the most intuitive, but once you start using creative shapes, they aren’t as familiar
and creative shapes are what make them interesting

“Intuitive” comes down to the design, some additive synths give you a big ol’ matrix of hundreds of sliders for each partial so you can manually set phase, frequency, and volume per partial, THIS would be the least intuitive

Taking any of these effects/algorithms and baking them down into 1-4 knobs will be fairly intuitive
I think the problem comes from some of the more subtle effects that I didn’t really give much attention to here

MUSICAL MODULATION

I’d say the most obvious effect for musical modulation would be the creative additive filters
w/o motion they just kinda sound like a tone shape, but with motion the various partials coming in and out sound incredible

Partial spread kinda doesn’t benefit from motion, but is one of the more important controls still.
A little motion can make it sound amazing, but too much becomes extreme sound design.

One of the more musical effects, “partial decay”, already IS motion, so applying modulation doesn’t do too much obvious to it

Then there’s “random drops” which is also motion already, but when more modulation is applied to it, you can get interesting cascades that slow down or speed up and some really musical “arpeggiation” of the partials