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Re: [microsound] [ot] RGB freq?



> I don't really expect an answer to this, but does anyone know if there
> are particular specific frequencies assigned to the colours RGB in
> digital video?

The short answer is, what do you mean by video?

Just looking at the *light* of course, there're frequencies of EMR we
perceive as particular colors (in a non-trivial way btw, there's a great
book delving into this called "The Perception of Color" I think...), eg:

 http://imagers.gsfc.nasa.gov/ems/visible.html

You could grab the frequency ratios and transpose them down X octaves into
the audible range... :)

If it's oldschool video...

NTSC (US/Japan TVs) encodes color information on a high-frequency carrier
on *top* of the low frequency luminosity channel -- a testament to the
history of television; and a very elegant solution to the question of how
black and white TVs handle color signals: they just low-pass filter out
all the color info!

This is also why you're not supposed to wear herringbone on TV -- the
high-frequency encoding of the black/white pattern starts to slip through
the high-pass color path and you get "chroma creep" -- rainbow washes --
where you don't want them.

This is also why you want "component" video in/outputs on your DVD player
and TV -- no crosstalk!

There are specific frequencies in NTSC that correspond to specific colors
as I recall -- here's an interesting tutorial google coughs up:

 http://www.ntsc-tv.com/ntsc-main-10.htm

-- 

If by digital video you mean VGA, it's encoded in a much more
straightforward way: there are dedicated "lines" (wires in the physical
world typically) for each color R, G and B; you simply scan the entire
screen at a specific horizontal and verticle frequency and resolution
(hope your source and display devices match ~ there're some trick going on
on the 'other' wires on your 15-pin VGA connector to handshake) and at
each pixel-point, the R, G, and B lines are obligated to be at the
appropriate level.

"At," meaning VGA is an *analog* transmission, even though the content is
quantized into pixels.  A low voltage means dim blue, a high voltage means
bright blue.  On CRT monitors I'm pretty sure once it's out on the VGA
cord the signal never re-enters the digital domain, all the way to the
raster gun exciting the phosphors on your monitor (a little, or a lot).

(DVI connectors actually transmit the same information *digitally*, which
requires a much greater bandwidth -- instead of letting a signal float
across 255 values (sampled by A/D convertors say), it's encoded as eight
or so bits and transmitted serially... one reason they don't go very far!)

Hence ~ in VGA, there are no set relationship between color and frequency
-- the horizontal and verticle synchs ("refresh") set the clock, not the
color.

 --

NB VGA is going to be final form of any computer digital video, but the
actual encodings of color in various digital video formats I expect are
all direct RGB encodings -- ie, three bytes (24 bits) or so encoding a
particular color in RGB space.

If you're at that level (RGB) you might assign R, G and B particular
frequencies ... but then changes in color would simply be realized in
shifting "spellings" of the same 3-frequency chord.... :/

One approach that might be fruitful would be to simply map the colorspace
into pitch-space according to some arbitrary but interesting way -- or
even a dynamic one, responsive to the properties of the image unfolding in
time...

 ---

All this being said -- you might want to talk to Scott Arford, a
video/sound artist here in SF who runs the now-shutting-down 7hz venue --
he's done a *lot* of work involving feeding NTSC video signals directly
into mixing boards, ie realizing the visible in sound.

To these ends he's done a huge amount of work specifically composing
images and video which create specific audible textures / rhythms /
events, etc., a lot of painstakingly in aftereffects.  I've asked him
about his process -- which comes first, the sound (then realized as
video?), or the video (selected for by the way it sounds?)? -- and gather
it's been a back-and-forth thing.

He has a fixation on static, glitches, noise in the visible domain -- his
performances tend to integrate "snow" and whitenoise, sculpted
progressively away to create shifting polyrhythms occasionally punctuated
by plasts of square-wave or sawtooth like tones, created by layering
specific high-contrast patterns on the screen.

*Highly* recommended!  I guess the best link would be:

 http://www.7hz.org

 b  e st
  aaron

  ghede@xxxxxxxx
  http://www.quietamerican.org

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