Editing Analog television (section)

==Other technical information==
{{Further|Broadcast engineering|Electronic engineering}}

===Components of a television system===
[[File:TV-block-diagram.svg|thumb|500px|Block diagram for a typical analog monochrome [[television receiver]]|alt=block diagram of a television receiver showing tuner, intermediate frequency amplifier.  A demodulator separates sound from video. Video is directed to the CRT and to the synchronizing circuits.]]

The tuner is the object which, with the aid of an antenna, isolates the television signals received over the air. There are two types of tuners in analog television, [[VHF]] and [[UHF]] tuners. The VHF tuner selects the VHF television frequency. This consists of a 4&nbsp;MHz video bandwidth and about 100&nbsp;kHz audio bandwidth. It then amplifies the signal and converts it to a 45.75&nbsp;MHz [[Intermediate Frequency]] (IF) amplitude-modulated video and a 41.25&nbsp;MHz IF frequency-modulated audio carrier.

The IF amplifiers are centered at 44&nbsp;MHz for optimal frequency transference of the audio and video carriers.{{efn|Most of the early television sets (1939–45) used 4 stage IF amplifiers with specially designed video amplifier tubes (the type 1852/6AC7). In 1946 the RCA presented a new innovation in television; the RCA 630TS. Instead of using the 1852 octal tube, it uses the 6AG5 7-pin miniature tube. It still had 4 stages, but it was 1/2 the size. Soon all of the manufactures followed RCA and designed better IF stages. They developed higher amplification tubes, and lower stage counts with more amplification. When the tube era came to an end in the mid-70s, they had shrunk the IF stages down to 1–2 (depending on the set) and with the same amplification as the 4 stage, 1852 tube sets.}} Like radio, television has [[automatic gain control]] (AGC). This controls the gain of the IF amplifier stages and the tuner.

The video amp and output amplifier is implemented using a [[pentode]] or a [[power transistor]]. The filter and demodulator separates the 45.75&nbsp;MHz video from the 41.25&nbsp;MHz audio then it simply uses a diode to detect the video signal. After the video detector, the video is amplified and sent to the sync separator and then to the picture tube.

The audio signal goes to a 4.5&nbsp;MHz amplifier. This amplifier prepares the signal for the 4.5&nbsp;MHz detector. It then goes through a 4.5&nbsp;MHz IF transformer to the detector. In television, there are 2 ways of detecting FM signals. One way is by the [[ratio detector]]. This is simple but very hard to align. The next is a relatively simple detector. This is the [[quadrature detector]]. It was invented in 1954. The first tube designed for this purpose was the 6BN6 type. It is easy to align and simple in circuitry. It was such a good design that it is still being used today in the Integrated circuit form. After the detector, it goes to the audio amplifier.

Image synchronization is achieved by transmitting negative-going pulses.{{efn|In a composite video signal of 1-volt amplitude, these are approximately 0.3&nbsp;V below the [[black level]].}} The ''horizontal sync'' signal is a single short pulse that indicates the start of every line. Two-timing intervals are defined&nbsp;– the ''front porch'' between the end of the displayed video and the start of the sync pulse, and the ''back porch'' after the sync pulse and before the displayed video. These and the sync pulse itself are called the ''horizontal blanking'' (or ''retrace'') ''interval'' and represent the time that the electron beam in the CRT is returning to the start of the next display line.

The ''vertical sync'' signal is a series of much longer pulses, indicating the start of a new field. The vertical sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace.{{efn|The pattern of vertical sync pulses also indicates whether each field represents even or odd lines in interlaced systems depending on whether a pulse begins at the start of a horizontal line, or midway through.}}

A ''sync separator'' circuit detects the sync voltage levels and extracts and conditions signals that the horizontal and vertical oscillators can use to keep in sync with the video. It also forms the AGC voltage.

The horizontal and vertical oscillators form the raster on the CRT. They are driven by the sync separator. There are many ways to create these oscillators. The earliest is the [[thyratron]] oscillator. Although it is known to drift, it makes a perfect sawtooth wave. This sawtooth wave is so good that no linearity control is needed. This oscillator was designed for the electrostatic deflection CRTs but also found some use in electromagnetically deflected CRTs. The next oscillator developed was the blocking oscillator which uses a transformer to create a sawtooth wave. This was only used for a brief time period and never was very popular. Finally the [[multivibrator]] was probably the most successful. It needed more adjustment than the other oscillators, but it is very simple and effective. This oscillator was so popular that it was used from the early 1950s until today.

Two oscillator amplifiers are needed. The vertical amplifier directly drives the yoke. Since it operates at 50 or 60&nbsp;Hz and drives an electromagnet, it is similar to an audio amplifier. Because of the rapid deflection required, the horizontal oscillator requires a high-power flyback transformer driven by a high-powered tube or transistor. Additional windings on this flyback transformer typically power other parts of the system.

[[File:Videosignal porch.jpg|right|250px|thumb|Portion of a [[PAL]] videosignal. From left to right: end of a video line, front porch, horizontal sync pulse, back porch with [[colorburst]], and beginning of next line]]
[[File:Videosignal vsync.jpg|right|250px|thumb|Beginning of the frame, showing several scan lines; the terminal part of the vertical sync pulse is at the left]]
[[File:Videosignal frame.jpg|right|250px|thumb|PAL video signal frames. Left to right: frame with scan lines (overlapping together, horizontal sync pulses show as the doubled straight horizontal lines), vertical blanking interval with vertical sync (shows as brightness increase of the bottom part of the signal in almost the leftmost part of the vertical blanking interval), entire frame, another VBI with VSYNC, beginning of the third frame]]
[[File:PAL signal frame (20ms) and line (64µs) decoding.jpg|thumb|250px|right|Analyzing a PAL signal and decoding the 20&nbsp;ms frame and 64&nbsp;μs lines]]

Loss of horizontal synchronization usually results in a scrambled and unwatchable picture; loss of vertical synchronization produces an image rolling up or down the screen.

===Timebase circuits===
{{Further|Oscilloscope}}
In an analog receiver with a [[cathode-ray tube|CRT]] display sync pulses are fed to horizontal and vertical ''timebase'' circuits (commonly called ''sweep circuits'' in the United States), each consisting of an oscillator and an amplifier.  These generate modified [[sawtooth wave|sawtooth]] and [[parabola]] current waveforms to scan the electron beam.  Engineered waveform shapes are necessary to make up for the distance variations from the electron beam source and the screen surface.  The oscillators are designed to free-run at frequencies very close to the field and line rates, but the sync pulses cause them to reset at the beginning of each scan line or field, resulting in the necessary synchronization of the beam sweep with the originating signal. The output waveforms from the timebase amplifiers are fed to the horizontal and vertical ''deflection coils'' wrapped around the CRT tube. These coils produce [[magnetic field]]s proportional to the changing current, and these deflect the electron beam across the screen.

In the 1950s, the power for these circuits was derived directly from the mains supply. A simple circuit consisted of a [[Series circuit|series]] voltage dropper [[Electrical resistance|resistance]] and a [[rectifier]]. This avoided the cost of a large high-voltage mains supply (50 or 60&nbsp;Hz) [[transformer]]. It was inefficient and produced a lot of heat.

In the 1960s, [[semiconductor]] technology was introduced into timebase circuits.  During the late 1960s in the UK, [[Synchronization (alternating current)|synchronous]] (with the scan line rate) power generation was introduced into [[Solid state (electronics)|solid state]] receiver designs.<ref>{{cite web|url=http://www.oldtellys.co.uk/otpsupps.html|title=TACKLING THE POWER SUPPLY|work=Publication date&nbsp;– unknown|publisher=Old Tellys.co.uk|access-date=24 November 2010|url-status=live|archive-url=https://web.archive.org/web/20120303043022/http://www.oldtellys.co.uk/otpsupps.html|archive-date=3 March 2012}}</ref>

In the UK use of the simple (50&nbsp;Hz) types of power, circuits were discontinued as [[thyristor]] based switching circuits were introduced. The reason for design changes arose from the electricity supply contamination problems arising from [[Electromagnetic interference|EMI]], and supply loading issues due to energy being taken from only the positive half cycle of the mains supply waveform.<ref>{{cite web|url=http://www.yorkemc.co.uk/conferences/emcYork2003/potm/2004-01_SMPS-and-SELC.pdf|title=An Investigation into the EMC Emissions From Switched Mode Power Supplies and Similar Switched Electronic Load Controllers Operating at Various Loading Conditions|publisher=York EMC.co.uk|access-date=24 November 2010|archive-url=https://web.archive.org/web/20120315084947/http://www.yorkemc.co.uk/conferences/emcYork2003/potm/2004-01_SMPS-and-SELC.pdf|archive-date=15 March 2012}}</ref>

===CRT flyback power supply===
Most of the receiver's circuitry (at least in [[transistor]]- or [[integrated circuit|IC]]-based designs) operates from a comparatively low-voltage DC [[power supply]].  However, the [[anode]] connection for a [[cathode-ray tube]] requires a very [[high voltage]] (typically 10–30&nbsp;kV) for correct operation.

This voltage is not directly produced by the main power supply circuitry; instead, the receiver makes use of the circuitry used for horizontal scanning. [[Direct current]] (DC), is switched through the line output transformer, and [[alternating current]] (AC) is induced into the scan coils. At the end of each horizontal scan line the [[magnetic field]], which has built up in both transformer and scan coils by the current, is a source of latent electromagnetic energy. This stored collapsing magnetic field energy can be captured. The reverse flow, short duration, (about 10% of the line scan time) current from both the line output transformer and the horizontal scan coil is discharged again into the primary winding of the [[flyback transformer]] by the use of a rectifier which blocks this [[counter-electromotive force]]. A small value [[capacitor]] is connected across the scan-switching device. This tunes the circuit [[inductance]]s to [[Electrical resonance|resonate]] at a much higher frequency. This lengthens the flyback time from the extremely rapid decay rate that would result if they were electrically isolated during this short period. One of the secondary windings on the flyback transformer then feeds this brief high-voltage pulse to a [[Cockcroft–Walton generator]] design [[voltage multiplier]]. This produces the required high-voltage supply. A [[flyback converter]] is a power supply circuit operating on similar principles.

A typical modern design incorporates the flyback transformer and rectifier circuitry into a single unit with a captive output lead, known as a diode split line output transformer or an Integrated High Voltage Transformer (IHVT),<ref>{{cite web|url=http://www.miniwatt.info/mullard_tn77.pdf|title=Technical note 77&nbsp;– Diode Split for E.H.T. generation|work=Publication date&nbsp;– 1976|publisher=Mullard|access-date=24 November 2010|url-status=dead|archive-url=https://web.archive.org/web/20110721224854/http://www.miniwatt.info/mullard_tn77.pdf|archive-date=21 July 2011}}</ref> so that all high-voltage parts are enclosed. Earlier designs used a separate line output transformer and a well-insulated high-voltage multiplier unit. The high frequency (15&nbsp;kHz or so) of the horizontal scanning allows reasonably small components to be used.