volt.contr.ideas

[Martin Czech]

Some ideas about Voltage Control (VC).
1. 
Everthing that can thoreticaly be VC, should be made
controllable (e.g. A, D, R -times, S- level, Waveform symmetry etc.).
In the past and even today many machines do not allow overall VC.

2. 
If we finally have verything VC, what are the appropriate voltage levels ?

a) Modulation: 
The 1V/oct. standard is widely accepted. A frequency sweep over the entire
audio range of 20-20000 Hz should be possible, so a range of 1:1024
would be required.  This is accomplished with a modulation source level of 
10Vpp. Are there other musical implications ? 
Yes : Take for example the question of musical pitch change,
or more specific, voltage controlled pitch change (EG->VCA->VCO).
In this signal chain the VCA controlls the amount of pitch fluctuation
There are three main cases :
-"String type vibrato" : the pitch always goes up from the center pitch,
 never down. In this case the modulation source should supply 0 to 10V.
-"Bagpipes howl" : The pitch raises from begining (bag empty) to the center pitch.
 This requires a -10 to 0 V modulation source.
-"Mellotron fluctuation" : The pitch changes symmetrical, this requires
 a -5 to 5V modulation source.
This little example shows :
It would be sufficient for the modulation source to have a -5 to 5V Signal
which can be shiftet by a -5 to 5 Offset Voltage, like in old Tektronix
lab hardware. Note that the dc offset has to be added before the VCA to
to things right.
Of course, we don`t always need the 10Vpp full scale modulation.
But if gives the best accuracy if we do the voltage computations (VCA)
in full scale and turn the amplitude down at the modulation target 
(input pot vco) , thus reducing errors as well.
So , starting at the 1V/oct. standard , we can derive a modulation range
of -10V to +10V by musical necessity, and we can also derive a modulation
source output level of 10Vpp with adjustable offset from -5V to 5V.
Every Module should handle this voltage range, which makes some means for 
voltage reduction (pot) necessary for some devices (analog mutiplier :
10x10V=100 V if Unity=1V !)
There are also other reasons for a -10V to 10V range :
-historical : analog computation machines used 10V as unity
-OP-AMPs with +-15V supply can`t manage voltages that are much higher

b)
Audio voltages:
Generally I don't want to make a sharp decision between audio or subaudio
devices or voltages since it is the great advantage of a modular system
that everything can communicate with everything. (So, DC blocking caps are
generally a bad idea ! Modules like VCAs should be able to handle DC as well
as AC. Maybe we need two different inputs (ac, dc) if we want to keep
dc offset out in the ac case.)
Thus it would be advantageous to have a 10Vpp output voltage for audio sources.
But we need some "headroom" in the audio chain to avoid slew-rate or even
clipping distortion (e.g. filters with high q-values).
So this would lead to attenuation, processing and again amplification
of the audio signal inside the modules and this is undesireable since
repeated amplification will dramatically raise the noise floor. 
To avoid all this and to be compatible to standarts, i'd recomend
an audio level of about 2Vpp or 1V amplitude.
This would provide sufficient headroom and noise immunity if
modern devices are used (e.g. AD 2018 OVCE as VCA).
Again , voltage reduction (pots) is needed at the inputs to allow
for larger voltage swings (e.g. LFO as input, or filter resonance).

 
m.c.

comments invited !