Editing Analog television (section)

===Color video signal extraction===
[[File:Burnt-in timecode.jpg|thumb|right|Color bar generator test signal]]

==== U and V signals ====
A color signal conveys picture information for each of the [[RGB color spaces|red, green, and blue components]] of an image.  However, these are not simply transmitted as three separate signals, because: such a signal would not be compatible with monochrome receivers, an important consideration when color broadcasting was first introduced.  It would also occupy three times the bandwidth of existing television, requiring a decrease in the number of television channels available.

Instead, the RGB signals are converted into [[YUV]] form, where the Y signal represents the [[luminance]] of the colors in the image. Because the rendering of colors in this way is the goal of both [[monochrome]] film and television systems, the Y signal is ideal for transmission as the luminance signal. This ensures a monochrome receiver will display a correct picture in black and white, where a given color is reproduced by a shade of gray that correctly reflects how light or dark the original color is.

The U and V signals are ''color difference'' signals. The U signal is the difference between the B signal and the Y signal, also known as B minus Y (B-Y), and the V signal is the difference between the R signal and the Y signal, also known as R minus Y (R-Y). The U signal then represents how purplish-blue or its complementary color, yellowish-green, the color is, and the V signal how purplish-red or its complementary, greenish-cyan, it is. The advantage of this scheme is that the U and V signals are zero when the picture has no color content.  Since the human eye is more sensitive to detail in luminance than in color, the U and V signals can be transmitted with reduced bandwidth with acceptable results.

In the receiver, a single demodulator can extract an additive combination of U plus V.  An example is the X demodulator used in the X/Z demodulation system. In that same system, a second demodulator, the Z demodulator, also extracts an additive combination of U plus V, but in a different ratio. The X and Z color difference signals are further matrixed into three color difference signals, (R-Y), (B-Y), and (G-Y). The combinations of usually two, but sometimes three demodulators were:

{{ordered list
| list-style-type=lower-alpha
| (I) / (Q), (as used in the 1954 RCA CTC-2 and the 1985 RCA "Colortrak" series, and the 1954 Arvin, and some professional color monitors in the 1990s),
| (R-Y) / (Q), as used in the 1955 RCA 21-inch color receiver,
| (R-Y) / (B-Y), used in the first color receiver on the market (Westinghouse, not RCA),
| (R-Y) / (G-Y), (as used in the RCA Victor CTC-4 chassis),
| (R-Y) / (B-Y) / (G-Y),
| (X) / (Z), as used in many receivers of the late '50s and throughout the '60s.
}}

In the end, further matrixing of the above color-difference signals c through f yielded the three color-difference signals, (R-Y), (B-Y), and (G-Y).

The R, G, and B signals in the receiver needed for the display device (CRT, Plasma display, or LCD display) are electronically derived by matrixing as follows: R is the additive combination of (R-Y) with Y, G is the additive combination of (G-Y) with Y, and B is the additive combination of (B-Y) with Y. All of this is accomplished electronically. It can be seen that in the combining process, the low-resolution portion of the Y signals cancel out, leaving R, G, and B signals able to render a low-resolution image in full color. However, the higher resolution portions of the Y signals do not cancel out, and so are equally present in R, G, and B, producing the higher-resolution image detail in monochrome, although it appears to the human eye as a full-color and full-resolution picture.

==== NTSC and PAL systems ====
[[File: Waveform monitor.jpg|thumb|left|Color signals mixed with the video signal (two horizontal lines in sequence)]]
In the NTSC and PAL color systems, U and V are transmitted by using [[quadrature amplitude modulation]] of a subcarrier. This kind of modulation applies two independent signals to one subcarrier, with the idea that both signals will be recovered independently at the receiving end. For NTSC, the subcarrier is at 3.58&nbsp;MHz.{{efn|Their exact frequencies were chosen such that (for NTSC), they are midway between two harmonics of the frame repetition rate, thus ensuring that the majority of the power of the luminance signal does not overlap with the power of the chrominance signal.}} For the PAL system it is at 4.43&nbsp;MHz.{{efn|In the British PAL (D) system, the actual chrominance center frequency, with equal lower and upper sidebands, is 4.43361875&nbsp;MHz, a direct multiple of the scan rate frequency. This frequency was chosen to minimize the chrominance beat interference pattern that would be visible in areas of high color saturation in the transmitted picture.}} The subcarrier itself is not included in the modulated signal ([[Double-sideband suppressed-carrier transmission|suppressed carrier]]), it is the subcarrier sidebands that carry the U and V information. The usual reason for using suppressed carrier is that it saves on transmitter power. In this application a more important advantage is that the color signal disappears entirely in black and white scenes. The subcarrier is within the bandwidth of the main luminance signal and consequently can cause undesirable artifacts on the picture, all the more noticeable in black and white receivers.

A small sample of the subcarrier, the [[colorburst]], is included in the horizontal blanking portion, which is not visible on the screen. This is necessary to give the receiver a phase reference for the modulated signal.  Under quadrature amplitude modulation the modulated chrominance signal changes phase as compared to its subcarrier and also changes amplitude. The chrominance amplitude (when considered together with the Y signal) represents the approximate saturation of a color, and the chrominance phase against the subcarrier reference approximately represents the hue of the color. For particular test colors found in the test color bar pattern, exact amplitudes and phases are sometimes defined for test and troubleshooting purposes only.

Due to the nature of the quadrature amplitude modulation process that created the chrominance signal, at certain times, the signal represents only the U signal, and 70 nanoseconds (NTSC) later, it represents only the V signal. About 70 nanoseconds later still, -U, and another 70 nanoseconds, -V.  So to extract U, a synchronous demodulator is utilized, which uses the subcarrier to briefly gate the chroma every 280 nanoseconds, so that the output is only a train of discrete pulses, each having an amplitude that is the same as the original U signal at the corresponding time. In effect, these pulses are discrete-time analog samples of the U signal. The pulses are then low-pass filtered so that the original analog continuous-time U signal is recovered. For V, a 90-degree shifted subcarrier briefly gates the chroma signal every 280 nanoseconds, and the rest of the process is identical to that used for the U signal.

Gating at any other time than those times mentioned above will yield an additive mixture of any two of U, V, -U, or -V. One of these ''off-axis'' (that is, of the U and V axis) gating methods is called I/Q demodulation.  Another much more popular off-axis scheme was the X/Z demodulation system. Further matrixing{{clarify|reason=matrixing of what?|date=October 2022}} recovered the original U and V signals. This scheme was actually the most popular demodulator scheme throughout the 1960s.{{clarify|reason=Which scheme I/Q, X/Y, matrixing.|date=October 2022}}

The above process uses the subcarrier. But as previously mentioned, it was deleted before transmission, and only the chroma is transmitted. Therefore, the receiver must reconstitute the subcarrier. For this purpose, a short burst of the subcarrier, known as the colorburst, is transmitted during the back porch (re-trace blanking period) of each scan line. A subcarrier oscillator in the receiver locks onto this signal (see [[phase-locked loop]]) to achieve a phase reference, resulting in the oscillator producing the reconstituted subcarrier.{{efn|A second use of the colorburst in more expensive or newer receiver models is a reference to an AGC system to compensate for chroma gain imperfections in reception.}}

[[File:Hanoverbars without PAL delay.png|thumb|right|[[Test card]] showing "[[Hanover bars]]" (color banding phase effect) in the PAL-S (simple) signal mode of transmission.]]

NTSC uses this process unmodified.  Unfortunately, this often results in poor color reproduction due to phase errors in the received signal, sometimes caused by multipath but mostly by poor implementation at the studio end.  With the advent of solid-state receivers, cable TV, and digital studio equipment for conversion to an over-the-air analog signal, these NTSC problems have been largely fixed, leaving operator error at the studio end as the sole color rendition weakness of the NTSC system.{{citation needed|date=November 2022}} In any case, the PAL D (delay) system mostly corrects these kinds of errors by reversing the phase of the signal on each successive line, and averaging the results over pairs of lines. This process is achieved by the use of a 1H (where H = horizontal scan frequency) duration delay line.{{efn|A typical circuit used with this device converts the low-frequency color signal to [[ultrasound]] and back again.}} Phase shift errors between successive lines are therefore canceled out and the wanted signal amplitude is increased when the two in-phase signals are re-combined.

NTSC is more spectrum efficient than PAL, giving more picture detail for a given bandwidth.  This is because sophisticated comb filters in receivers are more effective with NTSC's 4 [[color frame sequence]] compared to PAL's 8-field sequence.  However, in the end, the larger channel width of most PAL systems in Europe still gives PAL systems the edge in transmitting more picture detail.

==== SECAM system ====
In the [[SECAM]] television system, U and V are transmitted on ''alternate'' lines, using simple [[frequency modulation]] of two different color subcarriers.

In some analog color CRT displays, starting in 1956, the brightness control signal ([[luminance]]) is fed to the [[cathode]] connections of the electron guns, and the color difference signals ([[chrominance]] signals) are fed to the control grids connections.  This simple CRT matrix mixing technique was replaced in later [[solid state (electronics)|solid state]] designs of signal processing with the original matrixing method used in the 1954 and 1955 color TV receivers.