GreenXenon wrote:

GreenXenon wrote:

I'm thinking of a theoretical device using analog audio recording on
VD optical tracks containing the oldest film chemical composition.

Blue takes up less space than green [for the same amount of data], so
I'm guessing blue would make more efficient use of the film's length.
This is because blue light is of shorter wavelength than green light.
Per area, blue can represent more info than green.

What is the best wavelength of blue to use?

You're confusing two different sorts of recording here. It's true that
in digital recording to optical discs the shorter wavelength of blue
light is an advantage (as in Blu-Ray vs. standard DVD), because that
style of recording looks at the presence or absence of individual "pits"
representing binary data, and blue light can be focused more tightly
onto smaller pits on the disc.

This has almost nothing to do with variable-density audio recording on
film, in which analog audio signals are photographed as varying gray
tones on the track.

Black-and-white film is inherently grainy; the various levels of gray in
a VD track are made up of varying densities of exposed-and-developed
grains -- starting out as crystals of a silver halide, then developed to
bits of metallic silver in the emulsion. On a microscopic scale, b/w
film is just that: opaque silver bits in a clear emulsion. It only
looks gray on a broader view, just as a halftoned newspaper photograph
made up of dots of black ink on paper simulates grayscales at ordinary
reading distance.

So it's necessary for the soundtrack to be wide enough (usually at least
a couple of millimeters) that the reading process "looks at" a broad
enough area to average out the film grain. If you used a laser spot
pickup similar to the one in a DVD player to read a VD film soundtrack,
the output would be full of random noise as the individual grains passed
by the reader.

And of course, to expose this wide a track, the sound recorder must
project the flickering light through a slit that's as wide as the track.

In other words, the area you're looking at is so much larger than the
wavelength of light that the color of the light for recording or
playback doesn't matter.

-Neil Midkiff