[sdiy] Re: linear FM

[jhaible at debitel.net]

> > And given a certain ambiguity of triangle oscillator states (unlike a
> > saw oscillator, a triangle oscillator has no clear mapping of a voltage
> > to a phase),
> 
> It does. 
[...]
> My point was that you need to view both the voltage of the cap and the state
> of
> the schmitt-trigger to get the full phase.

Exactly what I tried to say. Looking just at the capacitor voltage leaves
two possibilities for phase (phase in the traditional sense), and only
the history (as stored in the schmitt trigger etc.) will make the mapping
clear.

If I understood Achim's mails right, he proposes an (alternative) definition
of phase, where the mapping between current & voltage and phase & frequency
is *always* clear (without looking at attitional information), and he does
this by introducing a switching between pos and neg frequency (and alternating
increase and decrease of phase). This is clearly different from the classic
definition of phase (or so I thought), but I wanted to find out if it
is a consistent alternative definition.



> So, would changing the signs of a sine/cosine core be equalent to running it
> backwards in time/at negative frequency?
> Yes.

Certainly, but that's not the point, as in practical life you cannot decide
between positive and negative frequency - you'll only notice if the
sign *changes*. And I don't see something like that in any unmodulated
oscillator - you must *force* the "natural" succession of states and
voltages to *reverse* by an external modulation to achieve that.
IMO this applies to any oscillator (it's only most clear for a sine
oscillator). 

If you look at the triangle oscillator again, the schmitt trigger controlling
the charge / discharge of the capacitor is mandatory to create an oscillator
at all. Without it, you just have an infinite ramp, which will hardly
count for "oscillation" at all. No oscillation, no frequeny. No oscillation,
no phase. So the whole contraption that creates the oscillation in the
first place is the black box that creates a _fixed_ frequency, regardless
what internal switching takes place to achieve this goal. 
Only if you add some *extra* contraption for modulation - thru-zero
modulation even! - it makes sense to talk about a reversal of frequency
or decreasing phase as well as increasing it. Unless you make entirely
different definitions for "phase". IMO.

JH.


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