Negative frequency

[Uros Milicevic]

"Goddard, Duncan" wrote:
> 
> >>>Negative frequency is when the wavefrom goes just as fast in the
> > opposite direction<<<
> >
> >>>The interesting stuff happens with the transistion from positive
> > frequency to negative frequency.  A "thru-zero VCO" will have a linear
> > control voltage that will let you take the waveform down to zero Hz
> > and then backwards.<<<
> >
> >>>You can PM modulate a low frequency sine wave through zero Hz for a
> >   nice chorus animation effect.  (The best DX-7 organ patches do this
> >   to emulate a rotating speaker.)
> >  And DJ scratching is effectively thru-zero FM.
> >   "Wheekwho-wheekwho-wheekwho." <<<
> >
> well, I don't know if I get it or not. why is it called "negative frequency"
> when all that happens is you turn the frequency down to zero and them back
> up again? 
There is very nice graphic on Juergen site
(http://www.synthfool.com/diy/hj.html  
 freq. shifter section ) that shows
triangle wave going thru zero . Frequency goes negative in " mathematic
domain " .
For example if you have sine that is modulated by "bipolar" heaviside (
did I spelled
this right ?)  in moment t1, you will have sin(wt) for t<t1 and sin(-wt)
for t>t1
in graph it will look like part of the function right of t1 is like
mirror image of
left part . This is what you see , and what you hear is sidebands you
usually get
with fm/pm .
Anyway , the graphic will tell you all .
cheers
uros