"GreenXenon" wrote in message
...
On May 13, 8:43 am, Peter wrote:
On May 12, 10:42 pm, GreenXenon
wrote:
On May 12, 1:23 pm, Neil Midkiff
wrote:
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
What determines the highest-frequency sound that can be
recorded onto
a VD optical track?- Hide quoted text -
- Show quoted text -
The speed at which the film moves, the bandwidth of the
transducer and
the intensity of the illumination, are important factors.
Near the
limit of these factors, the performance of the film might
be a factor.
You say intensity of illumination. Does that mean that a
more intense
light can allow for a higher-frequency than a less intense
light?
By transducer, I assume your talking about the
photoelectric cells
that convert the optical signal playback from the film
into an
electric signal to be amplified and sent to a loudspeaker.
Am I on the
right track?
As for the tape speed, does the nyquist theorem apply? If
so, what is
the minimum tape speed required to record a sound of 1 Hz?
IOW, how
high of a frequency is allowed per speed? In devices where
sampling-
rates are used, the sample-rate must be at least 2x that
maximum
frequency of the input signal.
Ultimately the frequency limit if the resolution of the
system. Film is probably the limiting factor here but there
are other contributors. Plus, film resolution is not a
simple matter.
As far as transducers are concerned the limit is
probably the slit length of the reproducer, assuming a
conventional system. Most film resolution is given as a
contrast limit, usually as the 50% point. This may not work
for sound recording. BTW, the limit is the same whether the
record is variable density or variable width. The
_practical_ limit for threatrical motion pictures was around
6000 Hz but that was a compromise taking into account a
great many factors. The Bell Telephone Laboratories did
experiments in the late 1930s using especially constructed
modulators and reproducers that were flat to about 15,000
but his has never been practical for normal release prints.
Experimentally, its certainly possible to obtain a limit
of 15Khz on 35mm film moving at 90 feet/minute (24 FPS) and,
in fact, on 16mm film at 60 feet/minute (24 FPS). The use
of a laser recorder to write directly on the film would
eliminate losses due to the mecanical modulator, the
recording lens, etc. All this has been researched and
reported. Such a system would probably have better
performance as a pulse modulated system rather than direct
analogue recording of whatever type.
I think you need to start doing some research on your
own because the questions you ask are all answered in the
literature. I recommended the Journal of the SMPTE and its
still the best place to start.
I will also say that I take most posts seriously. This one
may be serious or a troll. The fact that its cross-posted to
six different news groups tends to indicate troll but I give
it the benefit of my doubt.
--
--
Richard Knoppow
Los Angeles
WB6KBL