Subharmonikorg: Subharmonic Oscillators for the Korg NTS-1

I have been using the Korg NTS-1 for about 5 months, but it was just recently that I was able to spend some time trying to understand how to program it. Korg published its logue-sdk which is written in C/C++ and it provides useful templates and make files in order tu build custom oscillators, delays and modulation effects for your logue based synths. I have only tried programming the NTS-1 since I believe one of the main purposes of this little synth is to experiment and understand the logue-sdk and how to digitally process the signals, which is another way of saying that if you mess up and brick your synth it will only cost you $99 to replace it instead of bricking the Minilogue XD which retails at $549.

For a complete review of the NTS-1, I recommend my favorite synth reviewer Loopop. Watch the video below:

The Subharmonikorg¶

With the introductions out of the way, lets talk about the Subharmonikorg. I was inspired to program this oscillator after I saw what Moog did with its analog synth, The Subharmonicon which is sort of a mix of two previous instruments: the Tratonium and the Rythmicon. I wanted to emulate the subharmonic sound generation and found out that the subharmonics are derived from the Undertone Series which is basically a mathematics formula that is used to calculate the subharmonics of a given note based on its frequency. So if it is mathematically derived then it could be programmed.

The formula is easy: just divide the note frequency by an integer and you will get the undertone frequency related to the original note. This can be illustrated in the code hosted in the Logue-Hub Repository:

const float w1 = osc_w0f_for_note(
    ((params->pitch)>>8) + oscillator.semitone, 
    params->pitch & 0xFF
);

...

sw1 = w1 / oscillator.undertone1;
sw2 = w1 / oscillator.undertone2;

Where the function osc_w0f_for_note returns the frequency value that corresponds to the pitch (note value) passed to the OSC_CYCLE function exposed in the logue-sdk API. Then w1 is divided by the undertones values taken from the editing params of the NTS-1, which are integer numbers greater than zero. I limited the max possible value of subharmonics to the 7th undertone.

Features¶

The features for version 1.1-0 are as follows:

• One main harmonic oscillator with semitone calibration up to one octave higher and one octave lower.
• Two subharmonic oscillators that divide the main oscillator tone up to the 7th undertone.
• Choose different undertones for each suboscillator.
• Mix between the main oscillator and the suboscillators with the shape and alt knobs respectively.
• Choose between square, saw and sine waveforms.
• Modulate the main oscillator mix with the LFO.
• Subharmonic oscillators arpeggiator with clock adjustment.

The arpeggiator is a separate implementation from the native NTS-1 arpeggiator. In the current version 1.1-0, there are only two patterns available which only affect the suboscillators: down and up-down. This means that if you hold a note, the main oscillator will stay on it while both suboscillators will cycle from the 2nd through the 7th subharmonic. Having separate arpeggiators allows you to play with interesting rhythm patterns.

Parameters¶

The parameters you can edit are listed below.

The Showcase¶

I did some noise experiment where I played with the NTS-1 Subharmonikorg and Moog's Werkstatt-01. The Subharmonikorg is playing its arpeggio while I hit some lead notes with the Werkstatt. I tried to keep the mood creepy.

Here is the video of the weird experiment:

• For the whole 7 minutes noise, listen to it in bandcamp here.

• Get the latest version of the Subharmonikorg in github here.

There are some other oscillator ideas I have and some new features I want to test out for the NTS-1 for this oscillator. Hopefully I will also try this out with my newest adquisition, the Minilogue XD.