Newsgroups: rec.music.synth

In rec.music.synth, eirikur@ranger.enet.dec.com (Eirikur Hallgrimsson) writes:


    In article <1680272@hpsad.sad.hp.com>, smithj@hpsad.sad.hp.com (Jim Smith) writes...
    >  One problem with linear VCO's is that the vibrato can get pretty gnarly
    >  at the low end.

    Linear VCO's are basically misfeatured for scale or interval-oriented
    music, because ANY modulation that is applied to them has linear
    consequences which aren't *musically* linear, because music is
    exponential...  The difference in frequency between one note and another
    (say one octave up) is different in each octave.  Linear VCO's operate on
    frequency, not semitones. Hence, a pitch envelope that applies an up-sweep
    from a third below the note will only work in one place on the keyboard--at
    all other keys it will be off musically.  There are valid timbral reasons
    for having linear FM inputs on VCO's, though.

I knew this, of course.  In fact, if you really want to add uniform vibrato
to (say) your Paia 4700, the place to sum the modulation into is the voltage
reference for the keyboard.  But, then, that's a hack.

The problem becomes even worse if your modulation has a DC component, since
then, not only will the pitch detune, but that detuning amount will vary
with key number.

As you say, linear VCO inputs are good for FM.  In fact, Electronotes 
had some designs for "through-zero" linear-modulatable exponential VCOs.
By "through-zero,"  I mean that the FM input would allow the frequency to
become negative, which is necessary for Chowning-style FM.

    Divider architectures (electronic organs) have the same problem, according to
    my memory, but I can't prove it to myself on the back of a PostIt right now.
    Maybe I'm wrong.  I do recall very unpleasant effects in the bass pedal
    register from switching on vibrato in my (dear, departed) Farfisa CC23 and
    CC26 (Combo Compact and Compact Deluxe).

Hmm.  Actually, the modulation on a divider organ should be okay, since the
amount of frequency deviation would be divided right along with the pitch.

    I once tried to design a system that allowed PWM (pulse width modulation)
    in a divider system.  Dates me, eh?

THAT may be where the problem comes in.  The same percentage of PWM at the
same rate will sound like a greater effect at lower frequencies.

    ObChroma: 

Uh oh!  What have I started?  Next thing you know, Metlay's gonna start
posting some ObOberheim! :->

    The Chroma, being slightly pre-MIDI, has a very interesting scale
    of modulation levels and attenuation.  It's based on semitones, which was
    necessary because some control signals are real analog, and some are
    software.  I think it better than the everything in the world goes from
    0-127 that manufacturers have gone to to make MIDI implementations simpler.

I really liked the fact that DX-7 fixed frequencies were in Hertz, and ratios
were in true ratio amounts.  I think it would really be nice (and would
even be practical in digital synths) to have physical units for parameters
for which that made sense.  That would also make it easier, perhaps, to port
patches, for those who think that's a thang ta do.

    Hmmmm.  I should check to see if the KMX MIDI adaptor for the Chroma sends
    or receives both bytes of the pitch-bend message.

Speaking of MIDI bytes, I only just last week noticed that the ESQ-1 sends
release velocity!  I wonder just how many instruments do this, besides the
ones that respond to it.

     |Jim Smith      Patch Variation #5:       baby on back         |
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