03-12-2020, 11:07 PM
ThePanrock Wrote:Cue discussion of the difference between modulation and mixing. Without checking first, is mixing where each component waveform modulates the other?
No... Mixing, (at least, linear mixing) is where the signals are added together. Such as takes place at audio frequencies in a mixing desk.
If you apply higher frequency sawtooth to the horizontal deflection thingies (coils or deflection plates) and lower frequency sawtooth to the vertical plates, then (ignoring the trapezium effect of the scanning-at-an-angle), you'd get a rectangular raster.
Now, if you add a bit of vertical sawtooth to the horizontal sawtooth and apply to the horizontal deflectors, you'd get a skewed raster, slanted slightly from (say) top left to bottom right. But the horizontal width would still be constant: each line would be the same length. It would just start at a slightly different position and finish at a slightly different position, from one line to the next.
That's not what is needed to create the reverse trapezium. What is wanted is a raster with short lines at the top, and longer lines at the bottom. This means that the slow, vertical sawtooth must modulate the amplitude of the horizontal sawtooth scan waveform, so that the lines get progressively longer as the frame scan progresses. When the target is scanned at an angle, the geometrical distortion and the intentionally-introduced amplitude modulation cancel out to give the rectangular scanning of the target.
Modulation is what also takes place in a superhet 'mixer,' if it was true linear mixing, no sum or difference frequencies would be produced. But the unfortunate choice of terminology does indeed cause confusion!
