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Scalability and voltage offsets

In a past column, I deplored the lack of suitable attenuators and inverters in most commercial synthesizers. The aim of this column is to show the importance of CV Mixers and inverting/non-inverting scalable processors in a patch.

In a recent article, I stressed the need for tuning all oscillators properly before any performance. Indeed, in an ideal setting, all oscillator's fundamental frequency should only be changed by using dedicated scalable processors and their DC offsets. There is an advantage to that : you can quickly detune the fundamental frequency of your oscillator without touching the VCO potentiometer's frequency. When needed, you can then quickly return to the original settings.

Non-sequential sequencer
Generally, the best sequence is the one which is perceived as
not being of a sequential nature. Too often, sequences appear to be lifeless : this is often due to their systematic repetitive characteristics.

Ideally, a good sequence should be subtle and constantly evolve in a flowing process. There are various means that can be used to that effect : complex gatings generated by Up/down and divided by n counters, along with the usual HOLD, Up/down, Random and Reset functions available in most commercial sequencers.

In general, mixing of sequential material is best achieved, in the audio domain, just before the filtering process.
For example, let's assume that two VCO's are controlled by two different sequencer DC voltages. In that case, both VCO's individual outputs can either be mixed manually into a passive audio mixer or sent to the audio input of VCA's and Crossfaders. You can then, select the appearance/disappearance of a given audio sequence by voltage control.

*Note :If you are interested in the theory of musical sequences behaviors, look for the article called "Musical sequences and auditory perceptions").

DC Voltage summing and mixing process
In the above illustration, all the sequencer CV outputs A+B+C+D are processed into a CV mixer. Contrary to an audio mixer which sums and amplify high/low level audio signals, the CV mixer is specifically designed to mix DC voltages proportionally : all four inputs voltages are adjustable by attenuating potentiometers.
There are also four DPST switches to quickly kick-in/kick-out individual ABCD sequences into the circuit : in that case, all input potentiometers should be fully clockwise. In case of need, there is also a -/+ Volt Bias potentiometer delivering an offset voltage : this is often used to transpose the mixed sequence output upwards or downwards.

The CV mixer has two mixed outputs : non-inverting (+) and inverting (-). In this example, these two outputs are further processed into four inverters/non-inverters scalable processors : they provide another -/+ voltage offsets to specific modules in the patch.

How to use it
The use of a CV mixer is not as straightforward as an audio mixer : you need, first, to get acquainted to its peculiar behavior!

For example, in the above illustration, sequencer CV output A is patched to CV mixer input 1. Let's assume, that our master sequence is an upwards eight steps diatonic sequence, from C0 to C1. This sequence will appear at the non-inverted output as it is,
while the inverted output will deliver a downwards mirror image of these voltages : i.e C1 to C0!

But, in order to keep our master sequence DC voltages values at 1 Volt/oct, you need to turn the CV mixer input 1 attenuation potentiometer fully clockwise. Evidently, with a setting at 12 o'clock position, the potentiometer would reduce the DC voltages by 50 per cent : i.e half-a-volt per octave!

When sequencer CV output B is sent to input 3 of the CV mixer, the new DC voltages are added up to the existing voltages. On your CV mixer, set input 3 attenuating potentiometer fully clockwise. On your sequencer, set all the potentiometer's values of row B to O Volt. Then, open a few pots to a given value : some steps of sequence A will now be transposed by the sequencer voltages of row B. Proceed in the same manner, as above, to connect your sequencer CV output C and D to input 4 and 2 of your CV mixer, respectively.
If needed, you can use the -/+ BIAS pot to transpose the combined sequences upwards (right) or downwards (left).

Evidently, CV Mixers can also be used to mix other voltages together: sequences DC outputs
can be mixed with smooth and stepped generators, keyboard CV's, sample and holds, X/Y DC joysticks and, even, envelope generators!
In this last case, the individually gated envelopes are mixed together to provide a complex composite envelope at the output : i.e a master envelope (non-inverting output) and an inverted master envelope (inverting output)...

Finally, the use of inverters/non inverters scalable processors completes the patch. These interesting modules can be put to good use in weird FM and Ring modulated clangorous patches : the oscillators fundamental frequency can be quickly detuned, downwards or upwards, by turning the -/+ scalable potemtiometer counter-clockwise or clockwise.

Again, it has to be deplored that there are only few synthesizers manufacturers currently commercializing these kinds of products (I have included the schematics for DIY afficionados interested in building these indispensable and versatile modules).

André Stordeur