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UBZR1 Modulation

Chelys Lyra

Modulation "Wheel"

UBZR1 Modulation Wheel Processing

To control the musical performance, UBZR1 is using an analog wheel at the analog keyboard (well it is actually a "slider" type potentiometer, because that I had available). The keyboard interface is combining this "Wheel" analog voltage with another voltage coming from the "Digital-In" module. It generates the larger of the two. The combined signal, called "Wheel*", can be used to control the amount of modulation in real-time (provided modulation is enabled).

This combination of keyboard and Digital-In levels allows to force a "minimum" modulation via the Digital Interface and potentially increase it via the keyboard.

Modulation Overview

UBZR1 Modulation Matrix Overview

Modulation in UBZR1 is implemented using a concept of a "modulation matrix": a set of modulation sources can be "patched" to a set of modulation destinations.

A full matrix would imply all n:m combinations are available. However, there are some considerations, that limit the number of useful combinations:

  1. One source usually modulates one destination only. Exception: vibrato using VCO3, to modulate the pitch of both VCO1 and VCO2
  2. More than one source could modulate the same destination simultaneously, although not likely more than two
  3. If there is overlap between sources and destinations, then some combinations do not make sense. E.g. if VCO3 is source, it does not make sense to modulate VCO3 pitch.

The implementation specifics:

Modulation Sources

UBZR1 Modulation Sources

UBZR1 has the following modulation sources:

  1. VCO3 current output O1. As VCO3 can be used as a low frequency oscillator (LFO), this source is important for vibrato type modulation
  2. VCO2 current output O1. VCO2 can be used for occasional special modulation situations, because VCO2 usually runs at audio frequencies
  3. ADSR2. ADSR2 timings and contour can be set independently from ADSR1. UBZR1's implementation is specific because ADSR2's voltage contour output is not used directly to modulate. Rather, ADSR2 steers an exponential current source. This allows both for very smooth and also very strong modulation.
    In fact, the exp. current source can be thought of an oscillator with zero frequency. A polarity switching logic signal determines the "phase". This allows to deliver also an inverted contour.
    Above diagram shows the symmetry of the exp. current source behind ADSR2 and the VCOs and the Noise sources, whose VCA output currents are also driven by exponential current sources
  4. Noise current output OP/R, which offers the pink or red "wave forms". This is useful for random type modulation.

The overall amount of modulation can be controlled by the Wheel* signal, which can be controlled both by the keyboard and by the Digital-In module.

Note that VCO1's O1 current output is not available as modulation source, because it is used for the ring-modulator functionality (together with VCO2).

Modulation Destinations

The following diagram shows the four audio generating modules (VCO1-3 + Noise), the voltage controlled filter and their modulation capabilities:

UBZR1 Modulation Destinations

UBZR1 has the following modulation destinations:

  1. VCO1 exponential (i.e. Volt/Octave) frequency modulation
  2. VCO1 and VCO2 linear frequency modulation. This does not change the average pitch and is well suited for vibrato
  3. VCO1 rectangle pulse width modulation
  4. VCO2 exponential frequency modulation
  5. VCO2 rectangle pulse width modulation
  6. VCO3 exponential frequency modulation
  7. Filter exponential frequency modulation

Modulation Matrix Detail

UBZR1 Modulation Matrix Detail

The chosen implementation of the matrix is a compromise. There are multiplexer switches per source and a fixed "wired" matrix, with choices of "useful" combinations. The four multiplexers can not be switched independently, but are rather coupled in pairs:

Note that for pair A, there are two unused states.

The four multiplexers are controlled by 3-bit logic signals, three bits per pair.

Modulation Matrix Concept

The following table shows another representation of the chosen matrix. Entries:

Destination Source VCO1 PW VCO2 PW VCO1+2 Lin.Freq. VCO1 Exp.Freq. VCO2 Exp.Freq. VCO3 Exp.Freq. Filter Freq.
VCO3 O1 sym. A1 A2 sym. A3, A5 - A4
VCO2 O1 ? - - ? - A5 ?
ADSR2 B1 sym. not useful B2 B3 B4 B5
Noise OP/R sym. B6 B7 n.a. n.a. n.a. n.a.

Please note, that VCO3 can run both as low frequency oscillator (LFO) and at audio frequencies. Thus, VCO3 could be used for vibrato effects, when run in LFO mode.

VCO2 can only run at audio frequencies. When used as modulation source in combination with VCO3 (state A5) , there can be chaotic sound effects because of non-linear feed back.

Last change: 2022-05-31
© 2002-2022 Dr. Thomas Redelberger