Broadcast television systems
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.
Each country, faced with local political, technical, and economic issues, 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.
These systems were mostly experimental and national, with no defined international standards, and didn't resume broadcasting after the war.
The French 819 line system E was a post-war effort to advance France's standing in television technology.
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.
Typically, for 25 frame/s formats (European among other countries with 50 Hz mains supply), the content is PAL speedup, while a technique known as "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.
The image on a CRT is painted by a moving beam of electrons which hits a phosphor coating on the front of the tube.
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).
Broadcasters later developed mechanisms to transmit digital information on the phantom lines, used mostly for teletext and closed captioning: 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.
For digitally recorded material it becomes necessary to rearrange the field order when conversion takes place from one standard to another.
Another parameter of analog television systems, minor by comparison, is the choice of whether vision modulation is positive or negative.
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.
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.
Given all of these parameters, the result is a mostly-continuous analog signal which can be modulated onto a radio-frequency carrier and transmitted through an antenna.
Most digital television systems are based on the MPEG transport stream standard, and use the H.262/MPEG-2 Part 2 video codec.
Japan uses a third system, closely related to DVB-T, called ISDB-T, which is compatible with Brazil's SBTVD.
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.
DTMB is the digital television broadcasting standard of the Mainland China, Hong Kong and Macau.
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.
DVB-S is the original Digital Video Broadcasting forward error coding and modulation standard for satellite television and dates back to 1995.
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.
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.
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.
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).
