Thursday, 17 May 2018

Government secrets are a threat to democracy

How PAL television carried color information...

visual = 52e-6;                 % visible line is 52 microseconds
t = linspace(0, visual, 768);   % horizontal
f = 4.43361875e6;               % color carrier

% instead of RGB, intensity Y and color information U and V was transmitted
% coefficients were chosen based on sensitivity of human eye
Y(y, :) = 0.299*R(y, :) + 0.587*G(y, :) + 0.114*B(y, :);
U(y, :) = 0.493*B(y, :) - 0.493*Y(y, :);
V(y, :) = 0.877*R(y, :) - 0.877*Y(y, :);

% Phase Alternating Line
s = (1-2*mod(y,2));

% IQ-modulated color signal was added to the intensity signal
I(y, :) = Y(y, :) + U(y, :).*sin(2*pi*f*t) + s*V(y, :).*cos(2*pi*f*t);

% color could be easily recovered from I
g = sin(2*pi*f*t);
h = cos(2*pi*f*t);
N = 2; % even with N this small the result is rather good
for x = N+1:768-(N+1)
    U(y, 1+x) = mean(g(x-N:x+N).*I(y, x-N:x+N));
    V(y, 1+x) = mean(sig*h(x-N:x+N).*I(y, x-N:x+N));
Color was added on top of the intensity signal using IQ-modulation of 4.43361875 MHz carrier. Minor artifacts were introduced as a side effect of this mechanism. Mainly visible as color distortions such as the one seen here in the cats whiskers. The checker patter was a result of color carrier and phase alternating lines. Phase alternating lines were a mechanism used to combat phase drifts that would have resulted in distortion of colors.

Color TVs mostly removed the pattern carrying the color information on top of the intensity signal by an agressive notch filter (basically removing part of the spectrum), but black and white TVs didn't have such a filter so they suffered from loss of quality in color transmissions. The interesting thing is that black and white recordings which were not designed to record color could still be used to extract the color as all the information was contained in the intensity. The effect is likely a bit exaggerated in this image. Also interlacing has its role to play that isn't captured by this progressive image.

When a cavity forms, the power contained in the sinusoidal signal will get distributed among the available modes of the forming cavity. Depending on when and how fast the cavity formed, the formation could add or remove energy to or from the cavity.
Even without a proper cavity, changing boundary condition like a moving ("pumped") mirror can cause energy transfer from the force responsible for the change of the boundary to the electromagnetic field. In this simulation the source "vnoise" corresponds to quantum vacuum (power proportional to frequency, represented by 0.5 photons in the figure). The bump corresponds to the power of generated entangled photon pairs. It can more typically be observed in a correlation measurement (symmetrically respect to the half of the pump frequency). This shape of the spectrum is characteristic of quantum noise. The effect is know as the dynamical Casimir effect (creation of entangled photon pair out of vacuum by a moving mirror). It is also essentially the same process as spontaneous parametric down-conversion. Cavities like the ones in parametric amplifiers can significantly enhance the number of generated photons.

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