Negative frequency

[terry michaels]

Message text written by "Goddard, Duncan"
>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? the most you'll see is a phase change, surely?
and don't "real" rotary algorithms use a combination of vibrato (for
doppler
effect) and pan?
sorry for being stupid, I've never come across this phenomenon before.

d. <

Hi Duncan:

I dealt with the concept of negative frequency a few years back when I
designed am image rotator for laser graphics displays.  It consisted of a
matrix of 4 analog multipliers that manipulate the X and Y values of a
raster scanned image, the goal was to be able to rotate an image projected
on a screen in the Z axis.  Another was to describe is is if you had a
photo laying on a desk, and you rotated the photo in the horizontal plane,
leaving it laying flat on the desk.  In order to do this I built a sine
wave oscillator with sine / cosine (quadrature) outputs.  It was commanded
with a single control voltage pot, turning the pot to the right caused the
image to rotate to the right faster and faster, turning the pot to the left
caused the image to rotate to the left.  When the pot was centered, the
image would more or less hold in one orientation, you can smoothly start
rotation either way by turning the pot one way or the other.  The
quadrature oscillator went smoothly through zero, there were no glitches or
switching points.  This was a good visual example of through zero
oscillation.

The oscillator circuit was fairly simple, it was your basic OTA switch
driving an integrator with 2 comparators setting the triangle peak levels,
except it used two CA3080's with the inputs cross connected, supplied by
differential currents.  The sine / cosine function was done with two Analog
Devices AD639 trig function chips, they work great, but are rather
expensive.

Terry Michaels