A stereo, self-oscillating spectral filter firmware for the Eurorack module Aurora by Qu-Bit Electronix, inspired by the Harmonic Shift Oscillator by New Systems Instruments.
As with the original HSO, a frequency can be set (Warp/Time), along with a stride (Reflect) and level (Atmosphere). Stride indicates a multiplier factor for the distance between harmonics of the base frequency. Level indicates a ratio between the relative amplitudes given to each of these frequencies. Each input signal is filtered to include only the frequency components matching the frequency and stride controls, at levels determined by the level control and the number of the targeted harmonic.
As an example, say the left channel's input is a sum of five sine waves, at 300 Hz, 400 Hz, 600 Hz, 800 Hz, and 1200 Hz. Suppose frequency = 300 Hz, stride = 1, and level = 0.5. The left output signal should then contain the full-amplitude 300 Hz wave plus a half-amplitude 600 Hz wave, plus an eighth-amplitude 1200 Hz wave, summed together. However, if frequency is set to 100 Hz, stride is set to 3, and level is set to 1, the output will be only the full-amplitude 400 Hz wave (100 + 3*100).
Unlike the original HSO, the stride control can go negative, in which case sub-harmonics are targeted, acting as a multiplier of divisions of the base frequency. Given the example above, say frequency = 1200, stride = -1, and level = 0.25. The output for would then be the full-amplitude 1200 Hz wave, a quarter-amplitude 600 Hz wave, and a sixteenth-amplitude 300 Hz wave.
To achieve this processing, each of the input signals is processed with an FFT, to determine the frequencies that make up in the input. Each of the target frequencies (determined by the set frequency and stride) is then scaled (based on the level and harmonic number) and all other frequency content is removed.
The frequency is controlled using coarse (Warp knob) and fine (Time knob) controls. It can be modulated via exponential FM (Warp CV input, which tracks V/oct with a range of +5/-5 octaves), and linear FM (Time CV input, with a range of 20% of the frequency).
The resonance control (Blur), boosts the level of the base frequency components and, if level is above zero, proportionally boosts the following frequencies as well. When the knob is past 75%, "self-oscillation" will occur, outputting waves similar to the original HSO, controlled by stride and level as expected. However, unlike the original HSO which theoretically generates all such harmonics, this firmware only generates a fixed number of partials (currently 8, including the fundamental).
With stride at 0 and level at 1, all frequencies above the frequency are passed, creating a kind of brick-wall high-pass filter.
The reverse button toggles between this standard mode and a reversed mode in which stride becomes negative and targets sub-harmonics instead. With stride at 0 and level at 1, all frequencies below the frequency are passed, creating a brick-wall low-pass filter. This can also be achieved without reverse, by sending a negative CV value to the stride (Reflect) CV input. In general, the knob and CV values are added together. If reverse is active, this value is then negated. Sub-harmonics are targeted whenever this final value is negative.
The freeze button activates a more explicit filter mode, disconnecting the stride and level controls from input processing. Only the base frequency is boosted with resonance, other passed frequencies are at unity gain. However, when resonance is boosted to self-oscillation, the stride and level controls can still affect the generated wave-shape, allowing for a wide range of possible sounds, even without modulating the frequency.
The states of the reverse and freeze buttons can be temporarily toggled
with gates to their respective CV inputs. Pressing the button inverts the way
these gates are interpreted. The user-selected state is the default, and a high
signal to the CV switches to the opposite state until the signal goes low again.
These user-selected reverse and freeze states are also saved to the USB drive
(assuming it is present) to a file called HSO.txt, and loaded on startup.
The mix control works as one might expect, fading from the dry signal (CCW) to the wet signal (CW). Because the processing introduces a delay, the input signal is delayed by the same amount to ensure the signals being mixed are in-sync.
Currently, the shift button does nothing.
Finally the front-panel LEDs show estimates of the signal levels. Along the top, the left input signal level is shown in purple and the output level is shown in green near the bend at the top-right. In cyan between them is the average of these two levels. The same is done for the right input and output signals going down the right side of the module. The reverse and freeze button LEDs are white whenever their respective mode is active, and off when in the default state (e.g. if the user enables filter mode with the freeze button but also sends a high gate signal to the input, the light will be off).
Ensure that the freeze control is off (not lit up). Send a signal to the left (or left and right) inputs. Set stride and level somewhere in the middle of their ranges. Adjust the frequency until your hear output and adjust the parameters until you get something you like.
This can be a great way to turn a noisy source (even white noise) into something more musical. Note, however, that the output level may be fairly low, since the targeted harmonics may already have a low level in the input signal.
It you want to lock onto specific partials of your input, the following process seems to work best. Use the coarse frequency (Warp) to find a rough starting point. Next adjust stride and level until you're getting close to the output you want. For example, the may be some desired partials going in and out. Finally adjust the fine tuning (Time) until you've zeroed in on an output you like.
There will likely always be some subtle movement to the sound, especially if level is fairly high, due to fact that the calculations for higher order harmonics (or sub-harmonics) are very sensitive to the frequency and stride values, which subject to noise from their respective controls. It's best to view this as a kind of subtle chaotic modulation. :)
Without an input signal, set the resonance (Blur) to its maximum value to trigger HSO-style self-oscillation, controlled by the frequency (Warp/Time), stride (Reflect), and level (Atmosphere) controls. The left and right channel outputs are always 90 degrees out of phase with one another, as with the original HSO.
Between the coarse and fine tune (Warp and Time knobs), and the exponential FM (Warp CV, V/oct) and linear FM (Time CV) inputs, a wide array of harmonic and in-harmonic sounds are possible.
Note however, that due to soft clipping on the outputs, setting resonance all the way up may introduce some saturation beyond the expected spectrum. Dialing back the resonance some (~3 o'clock) avoid this if it is not desired.
Send a signal to the left (or left and right) inputs. Press the freeze button to enable filter mode (the button should light up). For a low-pass filter, press the reverse button until it is also lit up. For a high-pass filter, leave reverse disable. Now listen to the left and right outputs and adjust the other controls as desired.
Due to the sharp cutoff, you can often hear individual harmonics from the input signal drop off or appear as the frequency changes.
Download the bin file (see Releases section). If compiling from source,
follow the steps below and find the binary at build/AuroraHSO.bin.
Place the .bin file on your Aurora's USB drive (make sure this is the only
.bin file in the base of the drive) and start the module with the USB
inserted. This should trigger the firmware flashing procedure and once the
white lights go away you should be good to go.
If you would like to make changes to the firmware and build it yourself, you can follow the below instructions to get started.
When testing your changes, you may want to enable performance logging, which
writes timings of various sections of the code at a regular cadence to the USB
drive in a file called HSO.log. To enable this find the line:
#define LOG_ENABLED 0
And change the 0 to a 1. To disable logging again, change it back to a 0
and rebuild the project.
Follow the Daisy Toolchain installation
instructions.
Once you have that installed, run the following commands in a Terminal/Git
window, run ./make.sh. After the first time, you can simply run make to
compile any changes.
Install the packages dfu-util and gcc-arm-none-eabi
(gcc-arm-none-eabi-bin on Arch).
Then within this project run ./make.sh. This will compile the Aurora SDK
components as well as this project. After the first time, you can simply run
make to compile any changes.