Broadcast television systems

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Broadcast television systems (or terrestrial television systems outside the US and Canada) are the encoding or formatting systems for the transmission and reception of terrestrial television signals.

Analog television systems were standardized by the International Telecommunication Union (ITU) in 1961,[1] with each system designated by a letter (A-N) in combination with the color standard used (NTSC, PAL or SECAM) - for example PAL-B, NTSC-M, etc.). These analog systems for TV broadcasting dominated until the 2010s.

With the introduction of

DTMB
.

Analog television systems

Analog television system by nation
Analog color television encoding standards by nation

Every analog television system bar one began as a

luminance
with the symbol Y. Monochrome television receivers only display the luminance, while color receivers process both signals. Though in theory any monochrome system could be adopted to a color system, in practice some of the original monochrome systems proved impractical to adapt to color and were abandoned when the switch to color broadcasting was made. All countries used one of three color standards: NTSC, PAL, or SECAM. For example, CCIR System M was often used in conjunction with NTSC standard, to provide color analog television and the two together were known as NTSC-M.

Pre–World War II systems

A number of experimental and broadcast pre-WW2 systems were tested. The first ones were mechanically based and of very low resolution, sometimes with no sound. Later TV systems were electronic, and usually mentioned by their line number:

System A
, remaining in operation until 1985.

ITU standards

On an international conference in Stockholm in 1961, the International Telecommunication Union designated standards for broadcast television systems (ITU System Letter Designation).[1] Each standard is designated by a letter (A-M).

On VHF bands I, II and III the 405, 625 and 819-line systems could be used:

  • A – 405-line system
  • B – 625-line system
  • C – Belgian 625-line system
  • DI.B.T.O. 625-line system
  • E – French 819-line system
  • F
    – Belgian 819-line system

On

UHF bands Bands IV and V
only 625-line systems were adopted, with the difference being transmission parameters like channel bandwidth.

  • G – 625-line system, 5 MHz video bandwidth
  • H – 625-line system, 5 MHz video bandwidth
  • I – 625-line system, 5.5 MHz video bandwidth
  • K – 625-line system, 6 MHz video bandwidth
  • L – 625-line system, 6 MHz video bandwidth

Following further conferences and the introduction of color television, by 1966[2] each standard was designated by a letter (A-M) in combination with a color standard (NTSC, PAL, SECAM). This completely specifies all of the monaural analog television systems in the world (for example, PAL-B, NTSC-M, etc.).

The following table gives the principal characteristics of each standard.

megahertz
(MHz).

World analog television systems[2]
System Introduced Lines Frame rate (fps) Channel bandwidth

(MHz)

Video bandwidth (MHz) Vision/sound carrier separation (MHz) Vestigial sideband (MHz) Vision modulation (+, -) Sound modulation (AM, FM) Chrominance subcarrier frequency (MHz) Vision/sound power ratio Usual color standard Assumed display device gamma[3][2]
A 1936 405 25 5 3 −3.5 0.75 + AM none 4:1 none 2.5 - 2.0
B 1950 625 25 7 5 +5.5 0.75 - FM 4.43 PAL/SECAM 2.8
C 1953 625 25 7 5 +5.5 0.75 + AM none none 2.0
D 1948 625 25 8 6 +6.5 0.75 - FM 4.43 SECAM/PAL 2.8
E 1949 819 25 14 10 ±11.15 2.00 + AM none none 1.7
F 1953 819 25 7 5 +5.5 0.75 + AM none none 2.0
G 1961 625 25 8 5 +5.5 0.75 - FM 4.43 5:1 PAL/SECAM 2.8
H 1961 625 25 8 5 +5.5 1.25 - FM 4.43 5:1 PAL 2.8
I 1962 625 25 8 5.5 +5.9996 1.25 - FM 4.43 5:1 PAL 2.8
J 1953 525 30 6 4.2 +4.5 0.75 - FM 3.58 NTSC 2.2
K 1961 625 25 8 6 +6.5 0.75 - FM 4.43 5:1 SECAM/PAL 2.8
K1 1964 625 25 8 6 +6.5 1.25 - FM 4.43 SECAM 2.8
L 1961 625 25 8 6 -6.5 1.25 + AM 4.43 8:1 SECAM 2.8
M 1941 525 30 6 4.2 +4.5 0.75 - FM 3.58/3.56 NTSC/PAL 2.2
N 1951 625 25 6 4.2 +4.5 0.75 - FM 3.58 PAL 2.8

Notes by system

A
Early United Kingdom and Ireland VHF system (B&W only). First electronic TV system, introduced in 1936. Vestigial sideband filtering introduced in 1949. Discontinued on 23 November 1982 in Ireland and on 2 January 1985 in the UK.[4][5]
B
VHF-only in most Western European countries (combined with system G and H on UHF); VHF and UHF in Australia. Originally known as the Gerber standard.[6]
C
Early VHF system; used only in Belgium, Italy, the Netherlands and Luxembourg, as a compromise between Systems B and L. Discontinued in 1977.[5]
D
The first 625-line system. Used on VHF only in most countries (combined with system K on UHF); VHF and UHF in China.
E
Early
HDTV) picture quality but uneconomical use of bandwidth. Sound carrier separation +11.15 MHz on odd numbered channels, -11.15 MHz on even numbered channels. Discontinued in 1984 (France) and 1985 (Monaco).[7]
F
Early VHF system used only in Belgium, Italy, the Netherlands[
television programming to be broadcast on the 7 MHz VHF channels used in those countries, at a substantial cost in horizontal resolution. Discontinued in 1969.[5]
G
UHF only; used in countries with System B on VHF, except Australia.
H
UHF only; used only in Belgium, Luxembourg and Netherlands. Similar to System G with a 1.25 MHz vestigal sideband.
I
Used in the UK, Ireland, Southern Africa, Macau, Hong Kong and Falkland Islands.
J
Used in Japan (see system M below). Identical to system M except that a different black level of 0 IRE is used instead of 7.5 IRE. Although the ITU specified a frame rate of 30 fields, 29.97 was adopted with the introduction of NTSC color to minimize visual artifacts. Discontinued in 2012, when Japan transitioned to digital.
K
UHF only; used in countries with system D on VHF, except China, and identical to it in most respects.
K1
Used only in
French overseas departments and territories
.
L
Used only in France. On VHF Band 1 only, the audio is at −6.5 MHz. Discontinued in 2011, when France transitioned to digital. It was the last system to use positive video modulation and AM sound.
M
Used in most of the
Taiwan, Philippines (all NTSC-M), Brazil (PAL-M), Vietnam, Cambodia and Laos
(SECAM-M). Although the ITU specified a frame rate of 30 fields, 29.97 was adopted with the introduction of NTSC color to minimize visual artifacts.
N
Originally developed for Japan but not taken up. Adopted by Argentina, Paraguay and Uruguay (since 1980) (all PAL-N), and used briefly in Brazil and Venezuela. Allows 625-line, 50-frame/s video to be broadcast in a 6-MHz channel, at some cost in horizontal resolution.

Evolution

For historical reasons, some countries use a different video system on

405-line
system A, unlike all the other systems, suppressed the upper sideband rather than the lower—befitting its status as the oldest operating television system to survive into the color era (although was never officially broadcast with color encoding). System A was tested with all three color standards, and production equipment was designed and ready to be built; System A might have survived, as NTSC-A, had the British government not decided to harmonize with the rest of Europe on a 625-line video system, implemented in Britain as PAL-I on UHF only.

The French 819 line system E was a post-war effort to advance France's standing in television technology. Its 819 lines were almost high definition even by today's standards. Like the British system A, it was VHF only and remained black & white until its shutdown in 1984 in France and 1973 in Monaco. It was tested with SECAM standard in the early stages, but later the decision was made to adopt color in 625-lines L system only. Thus, France adopted system L both on UHF and VHF networks and abandoned system E.

Japan had the earliest working HDTV system (MUSE), with design efforts going back to 1979. The country began broadcasting wideband analog high-definition video signals in the late 1980s using an interlaced resolution of 1125 lines, supported by the Sony HDVS line of equipment.

In many parts of the world, analog television broadcasting has been shut down completely, or in process of shutdown; see Digital television transition for a timeline of the analog shutdown.

Technical aspects

Frames

Ignoring color, all television systems work in essentially the same manner. The monochrome image seen by a camera (later, the

LCD
, etc.).

All

3:2 pulldown
" is used for 30 frame/s formats (North America among other countries with 60 Hz mains supply) to match the film frame rate to the video frame rate without speeding up the play back.

Viewing technology

Analog television signal standards are designed to be displayed on a

cathode ray tube (CRT), and so the physics of these devices necessarily controls the format of the video signal. The image on a CRT is painted by a moving beam of electrons which hits a phosphor coating on the front of the tube. This electron beam is steered by a magnetic field generated by powerful electromagnets
close to the source of the electron beam.

In order to reorient this magnetic steering mechanism, a certain amount of time is required due to the inductance of the magnets; the greater the change, the greater the time it takes for the electron beam to settle in the new spot.

For this reason, it is necessary to shut off the electron beam (corresponding to a video signal of zero luminance) during the time it takes to reorient the beam from the end of one line to the beginning of the next (horizontal retrace) and from the bottom of the screen to the top (vertical retrace or vertical blanking interval). The horizontal retrace is accounted for in the time allotted to each scan line, but the vertical retrace is accounted for as phantom lines which are never displayed but which are included in the number of lines per frame defined for each video system. Since the electron beam must be turned off in any case, the result is gaps in the television signal, which can be used to transmit other information, such as test signals or color identification signals.

The temporal gaps translate into a comb-like

frequency spectrum
for the signal, where the teeth are spaced at line frequency and concentrate most of the energy; the space between the teeth can be used to insert a color subcarrier.

Hidden signaling

Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for teletext and closed captioning:

Overscan

Television images are unique in that they must incorporate regions of the picture with reasonable-quality content, that will never be seen by some viewers.[vague]

Interlacing

In a purely analog system, field order is merely a matter of convention. For digitally recorded material it becomes necessary to rearrange the field order when conversion takes place from one standard to another.

Image signal polarization

Another parameter of analog television systems, minor by comparison, is the choice of whether vision modulation is positive or negative. Some of the earliest electronic television systems such as the British 405-line (System A) used positive modulation. It was also used in the two Belgian systems (System C, 625 lines, and System F, 819 lines) and the two French systems (System E, 819 lines, and System L, 625 lines). In positive modulation systems, as in the earlier white facsimile transmission standard, the maximum luminance value is represented by the maximum carrier power; in negative modulation, the maximum luminance value is represented by zero carrier power. All newer analog video systems use negative modulation with the exception of the French System L.

Impulsive noise, especially from older automotive ignition systems, caused white spots to appear on the screens of television receivers using positive modulation but they could use simple synchronization circuits. Impulsive noise in negative modulation systems appears as dark spots that are less visible, but picture synchronization was seriously degraded when using simple synchronization. The synchronization problem was overcome with the invention of

phase-locked synchronization circuits
. When these first appeared in Britain in the early 1950s one name used to describe them was "flywheel synchronisation."

Older televisions for positive modulation systems were sometimes equipped with a peak video signal inverter that would turn the white interference spots dark. This was usually user-adjustable with a control on the rear of the television labeled "White Spot Limiter" in Britain or "Antiparasite" in France. If adjusted incorrectly it would turn bright white picture content dark. Most of the positive modulation television systems ceased operation by the mid-1980s. The French System L continued on up to the transition to digital broadcasting. Positive modulation was one of several unique technical features that originally protected the French electronics and broadcasting industry from foreign competition and rendered French TV sets incapable of receiving broadcasts from neighboring countries.

Another advantage of negative modulation is that, since the synchronizing pulses represent maximum carrier power, it is relatively easy to arrange the receiver automatic gain control to only operate during sync pulses and thus get a constant amplitude video signal to drive the rest of the TV set. This was not possible for many years with positive modulation as the peak carrier power varied depending on picture content. Modern digital processing circuits have achieved a similar effect but using the front porch of the video signal.

Modulation

Given all of these parameters, the result is a mostly-continuous

vestigial sideband modulation, a form of amplitude modulation
in which one sideband is partially removed. This reduces the bandwidth of the transmitted signal, enabling narrower channels to be used.

Audio

In analog television, the

MTS), which multiplexes additional audio channels into the FM audio carrier. All three systems are compatible with monaural FM audio, but only NICAM
may be used with the French AM audio systems.

Digital television systems

The situation with worldwide digital television is much simpler by comparison. Most digital television systems are based on the MPEG transport stream standard, and use the H.262/MPEG-2 Part 2 video codec. They differ significantly in the details of how the transport stream is converted into a broadcast signal, in the video format prior to encoding (or alternatively, after decoding), and in the audio format. This has not prevented the creation of an international standard that includes both major systems, even though they are incompatible in almost every respect.

The two principal digital broadcasting systems are

DMB-T/H
.

DTT broadcasting systems.[8]

ATSC

The terrestrial ATSC system (unofficially ATSC-T) uses a proprietary

multipath interference; however, it is better at dealing with impulse noise which is especially present on the VHF bands that other countries have discontinued from TV use, but are still used in the U.S. There is also no hierarchical modulation. After demodulation and error-correction, the 8-VSB modulation supports a digital data stream of about 19.39 Mbit/s, enough for one high-definition video stream or several standard-definition services. See Digital subchannel: Technical considerations
for more information.

On November 17, 2017, the FCC voted 3-2 in favor of authorizing voluntary deployments of ATSC 3.0, which was designed as the successor to the original ATSC "1.0", and issued a Report and Order to that effect. Full-power stations will be required to maintain a simulcast of their channels on an ATSC 1.0-compatible signal if they decide to deploy an ATSC 3.0 service.[9]

On cable, ATSC usually uses

throughput to 38.78 Mbit/s within the same 6 MHz bandwidth
. ATSC is also used over satellite. While these are logically called ATSC-C and ATSC-S, these terms were never officially defined.

DTMB

DTMB is the digital television broadcasting standard of the

.

DVB

DVB-T uses

September 11 terrorist attacks
).

DVB-S is the original

CanalSat in France, Dish Network in the US, and Bell Satellite TV in Canada. The MPEG transport stream
delivered by DVB-S is mandated as MPEG-2.

DVB-C stands for Digital Video Broadcasting - Cable and it is the DVB European consortium standard for the broadcast transmission of

channel coding
.

ISDB

ISDB is very similar to DVB, however it is broken into 13 subchannels. Twelve are used for TV, while the last serves either as a guard band, or for the 1seg (ISDB-H) service. Like the other DTV systems, the ISDB types differ mainly in the modulations used, due to the requirements of different frequency bands. The 12 GHz band ISDB-S uses PSK modulation, 2.6 GHz band digital sound broadcasting uses CDM and ISDB-T (in VHF and/or UHF band) uses COFDM with PSK/QAM. It was developed in Japan with MPEG-2, and is now used in Brazil with MPEG-4. Unlike other digital broadcast systems, ISDB includes digital rights management to restrict recording of programming.

Comparison of digital terrestrial television systems

Line count

As interlaced systems require accurate positioning of scanning lines, it is important to make sure that the horizontal and vertical timebase are in a precise ratio. This is accomplished by passing the one through a series of electronic divider circuits to produce the other. Each division is by a prime number.

Therefore, there has to be a straightforward mathematical relationship between the line and field frequencies, the latter being derived by dividing down from the former. Technology constraints of the 1930s meant that this division process could only be done using small integers, preferably no greater than 7, for good stability. The number of lines was odd because of 2:1 interlace. The 405 line system used a vertical frequency of 50 Hz (Standard AC mains supply frequency in Britain) and a horizontal one of 10,125 Hz (50 × 405 ÷ 2)

Notes
  1. The division of the 240-line system is academic as the scan ratio was determined entirely by the construction of the mechanical scanning system used with the cameras used with this transmission system.
  2. The division ratio though relevant to are not constrained to having the scanning in precise ratios. The 1080p high definition system requires 1125 lines in a CRT display.
  3. The System I version of the 625-line standard originally used 582 active lines before later changing to 576 in line with other 625-line systems.

Conversion from one system to another system

Converting between different numbers of lines and different frequencies of fields/frames in video pictures is not an easy task. Perhaps the most technically challenging conversion to make is from any of the 625-line, 25-frame/s systems to system M, which has 525-lines at 29.97 frames per second. Historically this required a frame store to hold those parts of the picture not actually being output (since the scanning of any point was not time coincident). In more recent times, conversion of standards is a relatively easy task for a computer.

Aside from the line count being different, it's easy to see that generating 59.94 fields every second from a format that has only 50 fields might pose some interesting problems. Every second, an additional 10 fields must be generated seemingly from nothing. The conversion has to create new frames (from the existing input) in real time.

There are several methods used to do this, depending on the desired cost and conversion quality. The simplest possible converters simply drop every 5th line from every frame (when converting from 625 to 525) or duplicate every 4th line (when converting from 525 to 625), and then duplicate or drop some of those frames to make up the difference in frame rate. More complex systems include inter-field interpolation, adaptive interpolation, and phase correlation.

See also

Transmission technology standards

Defunct analog systems

Analog television systems

Analog television system audio

Digital television systems

History

References

  1. ^ a b Final acts of the European Broadcasting Conference in the VHF and UHF bands. Stockholm, 1961.
  2. ^ a b c d "C.C.I.R - DOCUMENTS OF THE Xlth PLENARY ASSEMBLY OSLO, 1966" (PDF).
  3. ^ "C.C.I.R. Report 624-4 Characteristics of television systems, 1990" (PDF).
  4. ^ "The UK 405-Line Television Network". 2012-02-12. Archived from the original on 12 February 2012. Retrieved 2021-12-31.
  5. ^ a b c "World Analogue Television Standards and Waveforms". 2012-03-06. Archived from the original on 6 March 2012. Retrieved 2021-12-31.
  6. ^ "625-Line Television Broadcast Standards - UK Vintage Radio Repair and Restoration Discussion Forum". www.vintage-radio.net. Retrieved 2021-12-31.
  7. ^ "World Analogue Television Standards and Waveforms". 2012-08-30. Archived from the original on 30 August 2012. Retrieved 2021-12-31.
  8. ^ DVB.org Archived 2011-03-20 at the Wayback Machine, Official information taken from the DVB website
  9. ^ "FCC Authorizes Next Gen TV Broadcast Standard". Federal Communications Commission. 16 November 2017. Retrieved 2017-11-18.
  10. ^ On the beginning of broadcast in 625-lines 60 year s ago, 625 magazine (in Russian). Archived 2016-03-04 at the Wayback Machine
  11. ^ https://web.archive.org/web/20041230091501/http://www.ebu.ch/en/technical/trev/trev_255-portrait.pdf. Archived from the original (PDF) on 30 December 2004. {{cite web}}: Missing or empty |title= (help)
  12. ^ https://web.archive.org/web/20070221210300/http://cra.ir/FTD/Static/RRC/RRCFile10.pdf. Archived from the original (PDF) on 21 February 2007. {{cite web}}: Missing or empty |title= (help)
  13. ^ Observer, Reflective (2021-12-23). "Where did 625-line TV come from?". Medium. Retrieved 2021-12-31.
  14. ^ "625-Line Television System Origins - UK Vintage Radio Repair and Restoration Discussion Forum". www.vintage-radio.net. Retrieved 2021-12-31.

Further reading

External links