Editing Analog television (section)

==Receiving signals==
The television system for each country will specify a number of television channels within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using [[amplitude modulation]] on one carrier frequency, and the sound is transmitted with [[frequency modulation]] at a frequency at a fixed offset (typically 4.5 to 6&nbsp;MHz) from the picture signal.

The channel frequencies chosen represent a compromise between allowing enough [[Bandwidth (signal processing)|bandwidth]] for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called [[vestigial sideband]] is used to reduce the channel spacing, which would be nearly twice the video bandwidth if pure AM was used.

Signal reception is invariably done via a [[superheterodyne receiver]]: the first stage is a ''tuner'' which selects a television channel and frequency-shifts it to a fixed [[intermediate frequency]] (IF). The signal [[amplifier]] performs amplification to the IF stages from the microvolt range to fractions of a volt.

===Extracting the sound===
At this point the IF signal consists of a video [[carrier signal]] at one frequency and the sound carrier at a fixed offset in frequency. A [[demodulator]] recovers the video signal. Also at the output of the same demodulator is a new frequency modulated sound carrier at the offset frequency. In some sets made before 1948, this was filtered out, and the sound IF of about 22&nbsp;MHz was sent to an FM demodulator to recover the basic sound signal. In newer sets, this new carrier at the offset frequency was allowed to remain as ''intercarrier sound'', and it was sent to an FM demodulator to recover the basic sound signal. One particular advantage of intercarrier sound is that when the front panel fine tuning knob is adjusted, the sound [[carrier frequency]] does not change with the tuning, but stays at the above-mentioned offset frequency. Consequently, it is easier to tune the picture without losing the sound.

So the FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of the [[NICAM]] and [[Multichannel television sound|MTS]] systems, television sound transmissions were monophonic.

===Structure of a video signal===
The video carrier is demodulated to give a [[composite video]] signal{{efn|The RF signal modulation is inverted compared to the conventional AM{{snd}}the minimum video signal level corresponds to maximum carrier amplitude, and vice versa.}} containing luminance, chrominance and synchronization signals.<ref>{{cite web|url=http://www.tek.com/Measurement/App_Notes/25_7075/eng/25W_7075_3.pdf?wt=480&wtwi=2217&wtla=EN&wtty=TI&wtsty=Primer&wtpt=DETAILS&wtlit=25W-7075-3&wtcat=signal+generators&wtti=PAL+Video+Measurements|title=Pal systems&nbsp;– Television measurements|date=September 1999|publisher=Tektronics Incorporated|access-date=25 November 2010|url-status=dead|archive-url=https://web.archive.org/web/20110718081158/http://www.tek.com/Measurement/App_Notes/25_7075/eng/25W_7075_3.pdf?wt=480&wtwi=2217&wtla=EN&wtty=TI&wtsty=Primer&wtpt=DETAILS&wtlit=25W-7075-3&wtcat=signal+generators&wtti=PAL+Video+Measurements|archive-date=18 July 2011}}</ref> The result is identical to the composite video format used by analog video devices such as [[VCRs]] or [[CCTV camera]]s. To ensure good linearity and thus fidelity, consistent with affordable manufacturing costs of [[Television transmitter|transmitters]] and receivers, the video carrier is never modulated to the extent that it is shut off altogether. When intercarrier sound was introduced later in 1948, not completely shutting off the carrier had the side effect of allowing intercarrier sound to be economically implemented.

[[File:Video-line.svg|center|alt=Diagram showing video signal amplitude against time.]]
[[File:NTSC Signal.png|thumb|400px|[[NTSC]] [[composite video]] signal (analog)]]
[[File:An interlaced PAL frame with 20ms duration.jpg|thumb|400px|right|A waterfall display showing a 20&nbsp;ms long interlaced PAL frame with high FFT resolution]]
Each line of the displayed image is transmitted using a signal as shown above. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for PAL, [[NTSC]], and SECAM television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the [[colorburst]], and the chrominance signal) are not present.

[[File:Videosignal porch.jpg|right|400px|thumb|Portion of a PAL video signal. From left to right: end of a video [[scan line]], front porch, horizontal [[sync pulse]], back porch with [[colorburst]], and beginning of next line]]
The ''front porch'' is a brief (about 1.5 [[microsecond]]) period inserted between the end of each transmitted line of picture and the leading edge of the next line's [[sync pulse]]. Its purpose was to allow [[voltage]] levels to stabilise in older televisions, preventing interference between picture lines. The ''front porch'' is the first component of the [[horizontal blanking interval]] which also contains the horizontal sync pulse and the ''back porch''.<ref name="tveng" /><ref name="analogtv" /><ref name="Maxim">{{cite web |title=Basics of Analog Video |url=https://www.maximintegrated.com/en/design/technical-documents/tutorials/7/734.html |website=maximintegrated.com |publisher=Maxim |access-date=21 May 2021}}</ref>

The ''back porch'' is the portion of each scan line between the end (rising edge) of the horizontal sync pulse and the start of active video. It is used to restore the black level (300&nbsp;mV) reference in analog video. In signal processing terms, it compensates for the [[fall time]] and [[settling time]] following the sync pulse.<ref name="tveng" /><ref name="analogtv" />

In color television systems such as PAL and NTSC, this period also includes the [[colorburst]] signal. In the SECAM system, it contains the reference subcarrier for each consecutive color difference signal in order to set the zero-color reference.

In some professional systems, particularly [[satellite link]]s between locations, the digital audio is embedded within the line sync pulses of the video signal, to save the cost of renting a second channel. The name for this proprietary system is [[Sound-in-Syncs]].

===Monochrome video signal extraction===
The luminance component of a composite video signal varies between 0&nbsp;V and approximately 0.7&nbsp;V above the ''black'' level. In the NTSC system, there is a ''blanking'' signal level used during the front porch and back porch, and a ''black'' signal level 75&nbsp;mV above it; in PAL and SECAM these are identical.

In a monochrome receiver, the luminance signal is amplified to drive the [[control grid]] in the [[electron gun]] of the CRT. This changes the intensity of the electron beam and therefore the brightness of the spot being scanned. Brightness and contrast controls determine the DC shift and amplification, respectively.

===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.