Broadcast television systems

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, 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 2000s.

With the introduction of digital terrestrial television (DTT), they were replaced by four main systems in use around the world: ATSC, DVB, ISDB and DTMB.

Analog television systems

thumb|500px|Analog television system by nation

Every analog television system bar one began as a black-and-white system. Each country, faced with local political, technical, and economic issues, later adopted a color television standard which was grafted onto an existing monochrome system such as CCIR System M, using gaps in the video spectrum (explained below) to allow color transmission information to fit in the existing channels allotted. The grafting of the color transmission standards onto existing monochrome systems permitted existing monochrome television receivers predating the changeover to color television to continue to be operated as monochrome television. Because of this compatibility requirement, color standards added a second signal to the basic monochrome signal, which carries the color information. The color information is called chroma with the symbol C, while the black and white information is called the 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 systems had been developed before the WW2. The first ones were mechanical, had low resolution (180 lines in Germany, 240 lines in the UK), sometimes with no sound.

Later TV systems were electronic, and usually mentioned by their line number: 240-line (used in the US), 343-line (used in the US, USSR), 375-line (used in Germany, Italy, US), 405-line (used in the UK), 441-line (used in Germany, France, Italy, US, adopted but not widely used in the USSR) or 567-line (used in the Netherlands). These systems were mostly experimental and national, with no defined international standards, and did not resume broadcasting after the war except for the UK 405-line system, which resumed broadcasts and was the first to be standardized by ITU as 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). 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.

; 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.

; C: Early VHF system; used only in Belgium, Italy, the Netherlands, and Luxembourg, as a compromise between Systems B and L. Discontinued in 1977.

; F: Early VHF system used only in Belgium and Luxembourg; allowed French 819-line television programming to be broadcast on the 7 MHz VHF channels used in those countries, at a substantial cost in horizontal resolution. Discontinued in 1968 (Belgium) and 1971 (Luxembourg).]]

ATSC

The terrestrial ATSC system (unofficially ATSC-T) uses a proprietary Zenith-developed modulation called 8-VSB; as the name implies, it is a vestigial sideband technique. Essentially, analog VSB is to regular amplitude modulation as 8VSB is to eight-way quadrature amplitude modulation. This system was chosen specifically to provide for maximum spectral compatibility between existing analog TV and new digital stations in the United States' already-crowded television allocations system, although it is inferior to the other digital systems in dealing with 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.

On cable, ATSC usually uses 256QAM, although some use 16VSB. Both of these double the 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 Mainland China, Hong Kong and Macau. This is a fusion system, which is a compromise of different competing proposing standards from different Chinese Universities, which incorporates elements from DMB-T, ADTB-T and TiMi 3.

DVB

DVB-T uses coded orthogonal frequency division multiplexing (COFDM), which uses as many as 8000 independent carriers, each transmitting data at a comparatively low rate. This system was designed to provide superior immunity from multipath interference, and has a choice of system variants which allow data rates from 4 MBit/s up to 24 MBit/s. One US broadcaster, Sinclair Broadcasting, petitioned the Federal Communications Commission to permit the use of COFDM instead of 8-VSB, on the theory that this would improve prospects for digital TV reception by households without outside antennas (a majority in the US), but this request was denied. (However, one US digital station, WNYE-DT in New York, was temporarily converted to COFDM modulation on an emergency basis for datacasting information to emergency services personnel in lower Manhattan in the aftermath of the September 11 terrorist attacks).

DVB-S is the original Digital Video Broadcasting forward error coding and modulation standard for satellite television and dates back to 1995. It is used via satellites serving every continent of the world, including North America. DVB-S is used in both MCPC and SCPC modes for broadcast network feeds, as well as for direct broadcast satellite services like Sky and Freesat in the British Isles, Sky Deutschland and HD+ in Germany and Austria, TNT Sat/Fransat and 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 digital television over cable. This system transmits an MPEG-2 family digital audio/video stream, using a QAM modulation with 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

{| class="wikitable collapsible collapsed sortable" style="text-align:center;"

|+ World television systems

|- style="background:rgb (170, 160, 150);"

! System

! Year ratified

! Digital Modulation

! Resolution (Lines)

! Frame rate

! Data rate

! Hierarchical Mod.

! Ch. B/W (MHz)

! Video B/W

! Audio offset

! Video Coding

! Audio Coding

! Interactive TV

! Digital subchannels

! Single-Frequency Network

! Predecessor format(s)

! Mobile

| ATSC 1.0

| 8VSB, A-VSB and E-VSB in the works

| 1080

| Up to 60p

| 19.39 Mbit/s

| No

| 6

| 4.25 digital carrier at 1.31 MHz

| ?

| H.262

| Dolby Digital, AC3, MPEG-1 Layer II

| DSM-CC, MHEG-5, PSIP

| Yes

| Partial

| NTSC

| Not yet, ATSC-M/H in the works

| ATSC 3.0

| 2016

| COFDM (QPSK, 4096QAM)

| 2160p/4K

| Up to 120p

| 57 Mbit/s

| Yes

| 6

| 4.5

| ?

| H.265/Scalable HEVC

| Dolby AC-4, MPEG-H

| Yes

| NTSC, ATSC 1.0

| Yes

|| DVB-T

| 1997

| COFDM (QPSK, 16QAM/64QAM)

| 1080 (typical, not defined)

| Up to 50p

| Up to 31.668 Mbit/s

| Yes

| 5, 6, 7, or 8

| ?

| H.262, H.264

| Dolby Digital, MPEG-1 Layer II, HE-AAC

| DSM-CC, MHEG-5, DVB-SI

| Yes

| PAL, SECAM

| Yes (DVB-H)

| DVB-T2

| 2008

| COFDM (QPSK, 16QAM, 64QAM, 256QAM)

| 1080 (typical, not defined)

| Up to 50p

| Up to 50.34 Mbit/s

| Yes

| 1.7, 5, 6, 7, 8, or 10

| ?

| H.262, H.264, H.265

| Dolby Digital, MPEG-1 Layer II, HE-AAC

| DSM-CC, MHEG-5, DVB-SI

| Yes

| DVB-T

| DVB-NGH

| DTMB

| 2006

| TDS-OFDM

| 1080

| Up to 50p

| ?

| 6, 7, or 8

| ?

| MPEG-2, H.264/MPEG-4 AVC, AVS

| MPEG-1 Audio Layer II, AC3, DRA

| Yes

| ?

| Yes

| PAL

| Yes

| ISDB-T

| 1999

| 16/64QAM-OFDM (QPSK-OFDM/DQPSK-OFDM)

| 1080

| Up to 60p

| 23 Mbit/s

| Yes

| 6 (5.572 + 428 kHz guard band)

| ?

| H.262 H.264 (1seg)

| AAC

| No

| Yes

| NTSC

| Yes, ISDB-Tmm/1seg

| ISDB-Tb (SBTVD)

| BST-OFDM

| 1080

| ?

| Yes

| 6

| ?

| H.264

| HE-AAC

| Yes, Ginga

| Yes

| PAL-M, PAL-N, NTSC

| Yes, 1seg

| MediaFLO

| OFDM (QPSK/16QAM)

| ?

| 5.55

| ?

| Yes

| ?

| NTSC (Channel 55)

| Yes

| T-DMB

| OFDM-DQPSK

| ?

| H.262/H.264

| HE-AAC

| ?

| NTSC

| Yes

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)

2 × 3 × 3 × 5 gives 90 lines (non interlaced)
2 × 2 × 2 × 2 × 2 × 3 gives 96 lines (non interlaced)
2 × 2 × 3 × 3 × 5 gives 180 lines (non interlaced) (used in Germany in mid-1930s before switch to 441-line system)
2 × 2 × 2 × 2 × 3 × 5 gives 240 lines (used for the experimental Baird transmissions in Britain [See Note 1])
3 × 3 × 3 × 3 × 3 gives 243 lines
7 × 7 × 7 gives 343 lines (early North American system also used in Poland and in Soviet Union before WW2)
3 × 5 × 5 × 5 gives 375 lines
3 × 3 × 3 × 3 × 5 gives 405 lines System A (used in Britain, Ireland and Hong Kong before 1985)
2 × 2 × 2 × 5 × 11 gives 440 lines (non interlaced)
3 × 3 × 7 × 7 gives 441 lines (used by RCA in North America before the 525-lines NTSC standard was adopted and widely used before WW2 in Continental Europe with different frame rates)
2 × 3 × 3 × 5 × 5 gives 450 lines (non interlaced)
5 × 7 × 13 gives 455 lines (used in France before WW2)
3 × 5 × 5 × 7 gives 525 lines System M (480i) (a compromise between the RCA and Philco systems. Still used today in most of the Americas and parts of Asia)
3 × 3 × 3 × 3 × 7 gives 567 lines (Developed by Philips used for a while in the late 1940s in the Netherlands)
5 × 11 × 11 gives 605 lines (proposed by Philco in North America before the 525 standard was adopted)
5 × 5 × 5 × 5 gives 625 lines (576i) (designed by Soviet engineers during the mid-late 1940s, introduced to Western Europe by German engineers.)
2 × 3 × 5 × 5 × 5 gives 750 lines at 50 frames (used for 720p50)
2 × 3 × 5 × 5 × 5 gives 750 lines at 60 frames (used for 720p60)
3 × 3 × 7 × 13 gives 819 lines (737i) (used in France in the 1950s)
3 × 7 × 7 × 7 gives 1,029 lines (proposed but never adopted around 1948 in France)
3 × 3 × 5 × 5 x 5 gives 1,125 lines at 25 frames (used for 1080i50 but not 1080p25)
3 × 3 × 5 × 5 x 5 gives 1,125 lines at 30 frames (used for 1080i60 but not 1080p30)

;Notes

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.
The division ratio though relevant to CRT-based systems is largely academic today because modern LCD and plasma displays are not constrained to having the scanning in precise ratios. The 1080p high definition system requires 1125 lines in a CRT display.
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.

References

External links

FARWAY IRFC, TV and Radio Transmission, Radio Data System Encoders, Broadcasting Technologies
World Analog Television Standards and Waveforms by Alan Pemberton
Analog TV Broadcast Systems by Paul Schlyter
European Television Stations in 1932 a scan from a 1932 French magazine

Broadcast television systems

Analog television systems

Pre–World War II systems

ITU standards

ATSC

DTMB

DVB

ISDB

Comparison of digital terrestrial television systems

Line count

Conversion from one system to another system

See also

References

Further reading

External links