> I don't think that the comparison between sound and colour is at all
> valid.
>   Simultaneous sounds interact with each other in a way that colours don'=
t
> and there is no visual equivalent of the beat frequency.
>

Both sound and light add linearly, so there is not heterodyne between them
in our usual environment. For sound, if we have two frequencies of 900Hz
and 1.1kHz, the individual signals remain separate, but this is also the
same waveform that would be generated by modulating a 1kHz tone with 100Hz
using double sideband suppressed carrier amplitude modulation. The
envelope of a DSPSC signal is the same as would appear if we were to full
wave rectify the modulating frequency, which creates a doubling of the
modulating frequency (along with some other components). We don't know if
they are individual tones or a single tone with tremolo.

With color, our eyes have three bandpass filters (X, Y, and Z) similar to
RGB except that the X filter is similar to red but has a second peak in
the blue region.

We detect monochromatic colors by "slope detecting" between the filters.
Yellow hits both the X and Y filters. This yellow can be either
monochromatic yellow or any other spectrum that causes the same output of
these filters (such as monochromatic light at the X peak and Y peak).

Detection of violet is interesting. As we move to shorter wavelengths
above blue, the output of the Z filter starts going up, but the X filter
(similar to red) starts going up. We determine this is violet. We can
simulate violet by driving the X and Z filters with red and blue light.

It's interesting that we have these three filters and the shapes of the
filters. It seems that we'd be able to do good color perception with just
two filters, and I think some animals have two. I guess the three filters
gives us better color perception.

Harold






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