Chelys Lyra

UBZR1 Modulation

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*", is used to control the amount of modulation, when 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 though 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 and the VCOs and the Noise source, 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

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: VCO3 and VCO2 go together and ADSR2 and Noise go together. Note that for the first pair, there are two unused states. The multiplexers are controlled by 3-bit logic signals.

Modulation Matrix Concept

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

Destination VCO1 VCO2 VCO1+2 VCO1 VCO2 VCO3 Filter
Source PW PW Lin.Freq. Exp.Freq. Exp.Freq. Exp.Freq. Freq.
VCO3 O1 break 1 2 break 3, 5 - 4
  sym.     sym.      
VCO2 O1 ? - - ? - 5 ?
ADSR2 1 break not 2 3 4 5
    sym. useful        
Noise OP/R break 6 7 not not not not
  sym.     avail. avail. avail. avail.

Last change: 2019-09-19
© 2002-2020 Dr. Thomas Redelberger