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

However, given your philosophy on interface and instrument design, I think it might be wise to avoid trying to replicate the things that make Harmor great
which tend to be graphs, numbers, and drawable curves

That said, I think there are still some simple features from Harmor you should take a look at

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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

Layout: modular system with 4 additive “modules” and 2 additive oscillators

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

Layout: modular system with 5 additive modules

2 osc modules and 2 filter modules and an effect

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.

Layout: 

SUMU

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

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

ZEBRA 3

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

ALCHEMY

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

LOOM

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

additive processing

modular layout

many of these additive synths take a somewhat modular approach to the layout

this comes in the form of several module slots where different additive algorithms can be loaded

in some cases there will be special sections for oscillator modules

there’s usually a wide range of algorithms to chose from, with a very wide range of sonic impact

with some modules being incredibly subtle and others being completely transforming

Partial spread

various partial spread shapes

random spread

partial envelopes

unique filter shapes

“random droplets”

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 1

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.