Calderal

A pair of ring modulators for drones · Radish · Vulpus Labs

Calderal is a pair of ring modulators designed for drones rather than rhythmic sounds. Each channel multiplies two audio signals together. With sine waves as inputs, the output contains two frequencies: their sum and their difference. All controls change smoothly, allowing very slow modulation.

Each channel can also remove either the sum or the difference, turning it into a frequency shifter. When nothing is plugged into B IN, an internal oscillator provides the second signal. There is also a feedback control that sends the ring-modulated signal back into the input.

A frequency shifter moves every partial in a sound by the same number of hertz, rather than preserving the ratios between them. Shift a harmonic drone by 30 Hz and its partials are no longer harmonically related. Depending on the shift, the result can sound metallic, bell-like or glassy.

To begin: patch an oscillator into channel 1's A IN and connect OUT 1 to your output. Leave B IN empty and keep the controls at their default settings. The channel acts as a VCA, passing the voice through unchanged. Switch SHIFT to WIDE, turn SIDE fully right to SUM, then increase SHIFT slightly. Every partial moves up by the same number of hertz, making the drone increasingly inharmonic.

The two channels

Both channels have the same controls. Channel 2's controls and connections are numbered: OUT 2, A IN 2 and, in the tooltips, Shift 2 and Level 2.

The normal connection. When A IN 2 is empty, channel 2 receives the output of channel 1, after its VCA. The line on the panel between OUT 1 and A IN 2 shows this connection.

This lets you process a drone twice: for example, shift it in channel 1, then ring-modulate it in channel 2. You can also shift it twice. Plugging a cable into A IN 2 disconnects the normal connection, allowing the channels to operate independently.

A channel only runs when its output is needed. Channel 2 runs when OUT 2 is connected. Channel 1 runs when OUT 1 is connected, or when channel 2 receives its signal through the normal connection.

The inputs

A IN and B IN are the two inputs to the ring modulator. They can be swapped without changing the basic operation, but some of the other controls treat them differently. BLEND mixes the ring with the original A signal, while FEEDBACK sends the ring back into A. When B IN is empty, SHIFT supplies its signal.

Both inputs use a ±5 V range. Two full-scale inputs produce a full-scale ring-modulated signal.

DRIVE controls how strongly both inputs are saturated before multiplication. Fully left, the signal remains clean. The default setting is also close to clean. As DRIVE increases, loud peaks become rounded and sine waves acquire a small amount of third harmonic. This adds faint extra sidebands to the output.

DRIVE changes the waveform, not its overall level: a full-scale signal remains full scale. Most of the control's useful range is in its lower half. Hover over the control to see its value, such as Drive: 20%.

SIDE controls which of the two sidebands are heard.

• At the centre, the channel operates as a conventional ring modulator, producing the sum and difference frequencies at half strength.
• Towards SUM, the difference frequency fades out. Fully right, only the sum remains, at full strength.
• Towards DIFF, the sum fades out. Fully left, only the difference remains, at full strength.

The tooltip shows the current setting, for example Side: ring, sum and difference, Side: 75% sum or Side: sum only.

For example, feed a 200 Hz sine into A and a 30 Hz sine into B. At SUM, the output is 230 Hz. At DIFF, it is 170 Hz. Every partial in A is shifted by the frequency of B, making this a frequency shifter.

The unwanted sideband is suppressed by more than 40 dB for input frequencies of 30 Hz and above. Below 30 Hz, a sine wave patched into B is less suitable as a clean shifter; use SHIFT instead.

DRIVE is applied before SIDE removes either sideband. Any extra harmonics generated by DRIVE are therefore processed in the same way, and increasing DRIVE does not restore the rejected sideband.

SHIFT

When B IN is empty, SHIFT supplies a sine wave to the B side of the ring modulator. Its frequency can be set precisely, including very close to zero and on either side of it.

SHIFT sets the frequency in hertz. At the centre, the frequency is exactly zero and the carrier rests at +5 V. The ring therefore passes A unchanged, making the channel a VCA. Around the centre, the control is very fine, much like Littoral's BEAT control. It becomes less fine towards either end.

To the right of centre, SHIFT produces an upward shift when SIDE is set to SUM. To the left, it produces a negative shift, moving the frequencies down.

The small switch beside AMT sets the range:

• FINE, the default: up to ±5 Hz.
• WIDE: up to ±500 Hz.

Hover over SHIFT to see the exact rate, to three decimal places. For example, Shift: +0.200 Hz. At the centre, it reads Shift: off (VCA). Click the tooltip and enter a frequency in hertz to set it precisely. Hover over the range switch to see its setting.

CV and AMT control the shift rate with an external control voltage. At the centre of AMT, the CV has no effect. At full AMT, each volt changes the rate by 1 Hz in FINE mode or 100 Hz in WIDE mode. A ±5 V CV can therefore cover the full range of the SHIFT knob. Moving AMT left of centre reverses the CV's effect.

The shift rate passes smoothly through zero and continues into negative values. A slow Glacial lane can therefore move the drone upwards, pass through zero and then move it downwards.

Patching B IN disables SHIFT. When a cable is connected to B IN, SHIFT, AMT and the range switch are disabled. The controls turn grey, and the line from the SHIFT section to B IN indicates the connection. The tooltip reads Shift: off, B is patched. Remove the cable to use SHIFT again.

What SHIFT is for. On WIDE, shifts of a few hertz or more can turn a harmonic drone into an inharmonic one. A very slow shift on its own mainly produces a tremolo. With SHIFT at 0.2 Hz, the ring output swells, fades to silence and returns with its polarity reversed.

With BLEND halfway and SIDE on SUM, the dry signal and a copy shifted up by 0.2 Hz interfere with each other, making the whole drone rise and fall in level.

Slow shifts are particularly useful with FEEDBACK, in a stereo pair, or in a loop through Alluvial. See the starting points below.

Stopping. When SHIFT reaches exactly zero, its carrier returns to its resting level of +5 V over about one second. This ensures that a stopped shift leaves A at its original level and in phase. Other rates, however slow, are unaffected. A CV sweeping through zero does not trigger this behaviour: the carrier continues running as the rate changes sign. Each channel saves the carrier's position with the patch.

BLEND and the bleeds

BLEND crossfades between the original A signal and the ring-modulated signal.

• Fully left, at DRY, the output is A alone.
• Fully right, at RING, the output is ring alone.
• Halfway, at AM, the two are mixed at equal power.

With SIDE at the centre and a sine wave in B, the halfway setting produces amplitude modulation: the level of A rises and falls with B.

Hover over BLEND to see the setting, such as Blend: 50% ring.

The dry signal is kept at its original level at every frequency and is delayed to match the ring path. The two signals can therefore be mixed without comb filtering or phase cancellation. Fully left, BLEND sounds the same as the original A input.

CV and AMT allow a control voltage to change BLEND. At the centre of AMT, the CV has no effect. At full AMT, +5 V moves BLEND through its entire range, while -5 V moves it back through the same range in the opposite direction.

For example, set BLEND fully left and connect a Glacial lane to POS. The lane can then fade the ring-modulated signal in over the dry signal and fade it out again.

BLEED A and BLEED B are small trims that control how much of each input leaks through the ring modulator. Real analogue multipliers have some leakage; these controls let you adjust it deliberately.

At the centre, each bleed is cancelled. To the right, the signal passes through in phase; to the left, it passes through inverted. The controls are particularly fine around the centre, where small changes matter most. At their maximum settings, the leaked signal is 12 dB below its input. At the default position, the residual leakage is about 43 dB below the input.

Hover over either control to see its level, for example Bleed A: -43 dB, Bleed B: -43 dB, inverted or Bleed A: nulled.

BLEED A adds a little of the original sound beneath the sidebands. With B IN empty, BLEED B adds the internal SHIFT carrier itself. At zero frequency, this is a small DC offset, which the ring modulator's DC blocker removes.

FEEDBACK

FEEDBACK sends the ring-modulated signal back into A, before DRIVE.

Fully left, at its default setting, there is no feedback. As feedback increases, each pass produces another set of sidebands, spaced apart by the SHIFT frequency. Each successive set is quieter than the previous one.

At SUM, the sidebands extend upwards in frequency. At DIFF, they extend downwards. With SIDE at the centre, they extend in both directions.

At a shift rate of 0.2 Hz, the result is a cluster of closely spaced frequencies that beat slowly against each other. At 50 Hz on WIDE, feedback produces a more obviously inharmonic spectrum, with copies of each partial spaced 50 Hz apart.

With SHIFT at zero, feedback simply increases the gain.

Fully right, FEEDBACK is set just below the point where the loop sustains itself. The loop has a soft limit at ±5 V, so even maximum feedback with DRIVE fully clean remains bounded.

Two filters are built into the feedback loop. One removes frequencies below 20 Hz, preventing low-frequency rumble when descending sidebands pass through zero. The other rolls off frequencies above about 6 kHz, preventing high-frequency whistling.

Hover over FEEDBACK to see its value, such as Feedback: 60% or Feedback: off.

CV and AMT, on the bottom row beside LEVEL's controls, let an external CV control feedback. At the centre of AMT, the CV has no effect. At full AMT, a ±5 V CV covers the full range of FEEDBACK.

The feedback loop is one sample long, so all the successive sidebands are heard together. To make each repetition arrive later than the previous one, put a delay in the loop. Alluvial can provide this when patched around the channel.

The VCA

Each channel has its own VCA after the ring modulator. FEEDBACK is taken before the VCA, so changing LEVEL does not affect the feedback loop.

LEVEL sets the channel's output level. Its default setting is 80%. Hover over it to see the value, such as Level: 80%.

CV and AMT allow a control voltage to change the level. At the centre of AMT, the CV has no effect. At full AMT, a ±5 V CV covers the full travel of LEVEL.

LEVEL and AMT move smoothly when adjusted, avoiding zipper noise. The CV is smoothed just enough to prevent clicks when the input voltage changes suddenly, while still allowing audio-rate amplitude modulation.

The level follows a square-law curve, as on Littoral and Fluvial. This gives a more even-feeling response during slow fades, rather than making most of the change happen near the end.

OUT, on the vermilion block below FEEDBACK, carries the channel's signal after its VCA. When A IN 2 is empty, it is also the signal received by channel 2.

Jacks

Each channel has its own set of jacks. Channel 2's are identical.

All inputs receive 0 V when nothing is connected, except B IN, which receives the SHIFT carrier, and A IN 2, which receives OUT 1.

Notes

Ring-modulating two voices of the same drone does not necessarily produce beats. If two Littoral voices are tuned a fraction of a hertz apart, with one in A and one in B, their fundamental frequencies produce a steady tone an octave above and a difference frequency too low to hear. The DC blocker removes the latter.

Beating can still occur in three ways:

• The third and fifth harmonics of the two voices produce additional frequencies around the octave, beating at twice and four times the original rate.
• Two beating voices in A can be ring-modulated against a third voice in B. The resulting sidebands preserve the beat while shifting it in pitch.
• The dry signal mixed in by BLEND retains the original beating.

Calderal does not tune notes. There is no V/OCT input or set of ratio presets. Pitch relationships are set at the sources, using Littoral's COARSE and BEAT controls or Glacial lanes patched into them.

SHIFT and B IN. A sine wave patched into B works as a frequency shifter at frequencies above about 30 Hz. Below that, use SHIFT, which works accurately at any rate, down to a few millionths of a hertz and through zero.

The ring is clean, and the dry signal stays in phase. With SHIFT at zero, SIDE fully to either end and BLEND halfway, the dry signal and the ring output are identical. They add together without frequency cancellations.

Starting points

Beats moved in pitch. Set up two Littoral voices that beat against each other and connect both outputs to A IN 1. Connect a third voice, tuned a just third above them (a 5:4 ratio), to B IN 1. Set SIDE to the centre and BLEND to RING. The beating appears in the sidebands, shifted up and down by the third. Move BLEND towards AM to mix the original drone back in.

A composed blend. Connect a Glacial lane to both BLEND's CV and a Littoral voice's BEAT CV. Set both AMT controls slightly right of centre. As the beat speeds up, the ring-modulated signal fades in over it.

A VCA. Leave B IN empty, set SHIFT to the centre and BLEND to RING. Set LEVEL to zero, then connect a Glacial lane to LEVEL's CV with AMT fully right. The lane now controls a slow fade using the square-law level curve.

The bleed. Connect a drone to A and a second source to B, with BLEND set to RING. Turn BLEED A up from its null to mix the original drone beneath the sidebands. Turn it left of the null to invert the leaked signal, which can reduce frequencies where it overlaps with the ring output.

An inharmonic drone. Connect a Littoral drone to A IN 1 and leave B IN empty. Set SIDE fully right to SUM and SHIFT to 10–50 Hz on WIDE. Slowly increase FEEDBACK. Each partial develops a series of quieter copies above it. Switch SIDE to DIFF to make the copies extend downwards instead.

A stereo pair. Use a mult to connect the same source to A IN 1 and A IN 2. This is necessary because A IN 2 otherwise receives OUT 1. Set both SIDEs to SUM. Set SHIFT 1 to +0.1 Hz and SHIFT 2 to -0.1 Hz, both on FINE. Send OUT 1 left and OUT 2 right. The stereo image will move slowly between the speakers.

Shifted, then ringed. Connect a drone to A IN 1. Set channel 1 to SHIFT on WIDE with SIDE on SUM. Leave A IN 2 empty so channel 2 receives OUT 1. Connect a second Littoral voice to B IN 2, set SIDE 2 to the centre, and take the output from OUT 2.

A spiral. Mix a drone with Alluvial's OUT L and connect the mix to A IN 1. Connect OUT 1 to Alluvial's IN L. Set SIDE to SUM, SHIFT to a few hertz on WIDE and FEEDBACK to a low setting. Each time the sound passes through the tape loop, it returns shifted a little further.