TV SynchronisationThis page is best viewed with a screen resolution of 1024 x 768 or higher.
The sync pulses of a tv waveform need to be able to allow the receiver or monitor to reliably produce an accurate picture. The form of the synchronisation waveforms is described here. The description given here applies to the European 625 line CCIR (or PAL) signals but the arguments can be applied to other interlaced tv systems.
A tv picture is built up of a spot scanned rapidly on the faceplate of a crt (cathode ray tube) horizontally and (relatively slowly) vertically to produce a 'raster'. The brightness of the spot is varied to produce the picture. A synchronising signal to control the position of the spot on the screen is combined with the brightness (or luminance) information producing a 'composite' tv signal. The sync and brightness information are kept separate by assigning them unique signal level ranges (see line waveform below). Normally, the total range of a monochrome (black and white) signal is 1 Volt peak-to-peak although the absolute dc levels may vary (usually the signal is ac coupled). The luminance information occupies a range of around 0.7 V and the sync 0.3 V. With respect to the lowest sync level, the brightest parts of the picture are at a level of 1 V with the black level at 0.3 V. Black level is the same as the blanking level in CCIR system (though this isn't necessarily the case for other systems); blanking occurs in non-picture parts of the waveform when it is necessary for the electron gun of the crt to be 'off' so that retrace is not visible on the screen.
The tv waveform carries pulses (at sub-black levels) that allow synchronisation of both the horizontal and vertical deflection circuits. These pulses are separated by the video information using a 'sync separator' - this can be just a comparator with its threshold set around halfway between black level and the sync tips. See 'Notes' below for more information on sync separation.
There are two deflection generators (timebases) - horizontal and vertical. Both must be triggered at the right time to ensure that the picture is correctly reproduced. Therefore both timebases need to be synchronised and there needs to be a method of triggering either one independently using the same single set of sync pulses. The method used to allow the receiver to distinguish between the two types of sync is to use different widths for the horizontal and vertical sync pulses.
Line SyncA single line is shown in the figure below. The sync pulse is separated from the active picture information by the 'porches': the 'back' and 'front' porches. These avoid the picture detail affecting the accuracy of the synchronisation. (A further feature of the back porch is the ability of this otherwise unused period to include a phase reference signal for colour decoding in the PAL and NTSC colour systems).
Interlace was a technique developed in the very early days of television (i.e. mechanical systems) and is used in all analogue broadcasting systems as a bandwidth-conserving measure. The rate of picture presentation needs to avoid jerkiness; this achieved in the cinema by showing 24 pictures per second (although 16 and 18 pictures per second have been used for silent films). So for European tv systems a picture refresh rate of 25 pictures per second was chosen.
However, showing pictures at this rate would produce an intrusive flicker effect. This effect arises because the eye is particularly sensitive to brightness changes at frequencies up to around 40 Hz. With film, at 24 frames/s, flicker is avoided by interrupting the light source at least once during each displayed frame as well as during the pull-down when the film is moved between frames.
In tv, the same effect (of increasing the refresh rate by a factor of 2) is achieved by the use of interlace, that is, splitting each picture into two fields. A field is a single vertical scan. Each field consists of half the total picture lines (312½) which is scanned from top to bottom of the display at twice the picture rate. The second field then completes the picture by scanning its lines between those of the first field. Thus a complete picture is produced in 1/25th of a second but the effective scan rate is twice this (50 Hz). So a picture containing 625 lines is actually made up of two separate vertical scans, each of 312½ lines. This is termed 2:1 interlace (although as this order of interlace is almost universally used, the picture is usually said simply to be 'interlaced').
Interlace is achieved very simply in the master timing generator for a television system by making the total number of lines in the picture odd: the field flyback must then occur halfway through a line on alternate fields, which produces the required effect - see figure below for a 625 line interlaced raster. It is for this reason that all analogue electronic tv systems have used an odd number of lines. It is also necessary for the field timebase and vertical deflection circuitry to operate at a frequency twice the picture repetition rate; i.e. for the European system, which has a picture repetition frequency of 25 pictures/second or 25 Hz, the vertical deflection operates at 50 Hz. Furthermore, in order to get perfect interlace, all vertical deflection cycles and waveforms must be identical.
Interlace will reduce the visibility of large area flicker but small detail that isn't common to both fields will show up as 25 Hz flicker. However, this is not obvious to the viewer at normal distances since the eye is unable to perceive rapid variations in time in areas of very fine detail in a picture. Close examination of a blank scanned raster on a monochrome display will enable the 25 Hz flicker to be seen, though it tends to be rather more visible on picture detail such as sharp edges that are at small angles to the horizontal where there can be very large local differences between the two fields. (Close examination of an interlaced picture may also exhibit the phenomenon of 'line crawl'; this effect gives the appearance of all the lines moving up (or down) the screen. Higher orders of interlace are more prone to this which can be distracting at normal viewing distances; this is partly why 2:1 interlace and not higher orders is used.)
The Appearance of a Raster on a CRT (with zero flyback time)
Field SynchronisationBroad pulses or serrations are used for triggering the field scan generator. In order to keep the line sync generator running continuously, falling edges appear every 64 microseconds throughout the entire field blanking interval for this purpose. Because there is an effective half line offset between the two sets of field sync pulses it is necessary to double the frequency of the line syncs during the field sync pulses. This allows identical pulse trains to be used for both field syncs.
Equalisation PulsesThe field sync pulses can be distinguished from the line syncs using simple analogue techniques. This was the method universally used until recently when pseudo digital techniques became easier and took over. One method is, having ac coupled the sync pulses (after stripping the picture information), to integrate them; this will result in a drift of level downwards during the field pulses of the integrated waveform, this negative signal can be used to trigger the vertical sync generator. Now, for proper interlace to be achieved it is necessary for the field sync generator to be triggered after identical delays after the last picture line of both odd and even fields. If the field sync pulses occurred immediately following the end of the picture, then there would be a potential difference in triggering times since, at the end of the odd field, there would be only half a line duration following the line sync pulse before the start of the field sync pulses whereas at the end of the even field there would be a full line delay.
The equalisation pulses, which are inserted before the field syncs, allow the integrator to settle and be minimally influence by the presence of the line syncs. It is worth noting that the equalisation pulse width is half that of the line sync pulses - this compensates for the doubling in frequency, so that the dc level at the integrator output is unaffected. Further equalisation pulses are inserted after the field syncs to ensure that the trailing edge of the integrated waveform is the same for both fields for those circuits where this may be critical (e.g. where the area under the waveform can influence the triggering point).
Field Sync Waveforms (CCIR 625 line system)
It is easy to display the two sets of sync waveforms one above the other on an oscilloscope (as shown in the diagram above) - apply the tv signal to both channels of a dual beam 'scope, switch triggering to TV FRAME SYNC or TV FIELD SYNC, the trigger source to ALT (rather than CH1 or CH2) and the display mode to ALT (rather than CHOP). Which field appears on which Y-channel is a matter of luck, so use the shift controls to correctly position the waveforms. Simple analogue 'scopes (like mine) won't show the first group of equalisation pulses and probably won't show all the field sync ones either.
The equalisation pulses and the field sync are identical for both fields and are shown in detail below:
Designating Lines and FieldsEach line of a complete picture is individually numbered. Lines are always numbered consecutively in time and not as displayed on the crt screen. However, since it is necessary to distinguish between the two fields which go to make up each picture the terms 'odd' and 'even' are used. The 'odd' field is defined as the one which ends in a half-line of picture information; whilst that which ends in a full-line of picture information is termed the 'even' field Note 3. See diagram above.
Choice of Field FrequencyIt is easy to see why 25 pictures per second rather than 24 was chosen. The effective vertical scan frequency is 50 Hz and in the early days of tv, mains (at 50 Hz) could break through from imperfectly filtered power supplies. This would manifest itself on the picture by modulating the brightness of the picture or the position of the raster. If there were a difference of 1 Hz (i.e. 25 - 24 Hz) this modulation would show up very noticeably by rolling up the screen once every second. Using a 50 Hz scan causes the modulation to be stationary on the screen which is far less noticeable. In the US and elsewhere where the mains frequency is 60 Hz the field rate is also 60 Hz.
However, choosing a picture rate related to the local mains frequency means that televising cinema films (filmed at 24 frames per second - fps) isn't straightforward. In Europe cinema films are transmitted at 25 fps, this means that the action is marginally increased in speed and the sound raised in frequency by roughly a semitone. This is not normally considered to be a problem. In the US, on the other hand, it would be impractical to show films at 30 fps. But by adding a degree of complexity to the scanning process, they are shown at a film rate of 24 fps and the higher tv rate is accommodated by taking pairs of film frames and spreading them between 5 tv fields. The first film frame takes 2 tv fields and the second film frame, 3 tv fields (i.e. 1½ pictures). The sequence ends after 10 complete tv fields (i.e. 5 pictures) which takes the same time as 4 film frames.
Filmed material, destined to be shown on tv only, is very often shot at 25 or 30 fps depending on the intended broadcast system - this does away with speed errors or complications of scanning.
Variations on the themeThe synchronisation waveforms look complicated and are difficult to produce (although, up until recently, single ICs could be bought to generate all the signals). But usually a designer can get away with very much simpler waveforms which most tvs (and vcrs) are happy to lock to. Commonly, with games and home computers, the equalisation pulses are done away with and the serrated field sync pulse replaced by a single wide pulse. Other 'tricks' use an even number of lines (with slightly different picture rates as a consequence if the line duration is unchanged) producing a non-interlaced picture with 312 or 313 lines and a repetition rate of about 50 Hz.
In the early 1970s, the BBC considered a proposal to replace the field synchronisation waveform with a simpler format. The intention was not to make life easier for SPG designers but to free up more time between the end of one field picture information and the start of the next field picture information so that there was more time available during the field blanking period to insert test signals or data (such as text) - see note 7 below. The proposal was to reduce the number of broad pulses per field from five to three and to have only one equalisation pulse per picture. Production tv receivers were tested to see if they would be able to adequately lock to tv signals using the proposed format. It turned out that although all the receivers could adequately lock to the new waveform, some were unable to produce pictures with an acceptable degree of interlace. For this and other reasons the proposal was abandoned. What this illustrates is not an inherent superiority of the existing waveform shape but only that a receiver designed with one waveform in mind should not be expected to work with a different one however desirable the consequences of that expectation might be. See the BBC report at: An investigation into a proposal for a simplified television field synchronisation signal.
Simple systems (in the days of vidicon tubes) used 'random interlace'; what this means is that the field and line oscillators were entirely independent (so the timings would drift with respect to each other) consequently the pattern traced out on the tube face changes from field to field in a non-predictable way. With modern CCD cameras this can't happen.
- SYNC SEPARATORS: Chips that will perform this function (as well as having some other useful features) are the National Semiconductor LM1881 or the (more sophisticated) Elantec (now Intersil) EL4581.
- SYNC PULSE GENERATORS: The idea of an SPG is that a fixed frequency clock goes in and the correct waveforms come out. Locking to a timebase supplied externally is usually an option (Genlocking). In practice timings (pulse durations) can be a bit off and there may be other quirks.
- The first single chip SPG was the Ferranti (later GEC Plessey) ZN134 (16 pin DIL); I mention this as (I believe) it was the first (produced some time in the late seventies). It was able to produce either 625 or 525 line waveforms correctly. Long obsolete.
- The RCA (now Intersil) CD22402 is basically a 525 line SPG, it can produce a correct 525 line waveform (which has 6 equalisation and field sync pulse) but produces 6 of each in the 625 mode rather than the correct 5. In practice, this defect is unlikely to be a real problem but it looks like a silly oversight by the designers. Obsolete, but some may still be around.
- My favourite sync pulse generator is the Philips SAA1101. This is programmable to provide 625 or 525 line standards (correctly) and signals for PAL, NTSC or SECAM encoding. Interlace can be switched off as well. This device is now a discontinued product but the datasheet is well worth studying for insight into the synchronising waveforms (but note that some of the waveforms in the datasheet haven't been drawn particularly accurately).
- The only single chip SPG currently in production that I know of is the Fairchild 74ACT715. This is a wholly programmable device which can produce almost any set of video timing waveforms. It is designed to give NTSC 525 line waveforms by default but while other line system waveforms can be generated it will require some ingenuity to achieve this - registers need to be loaded with the correct numbers every time it is switched on. Fairchild haven't given any application notes, which, had they done, would make the chip possibly more attractive to use in a design. Has anyone any experience of this chip? Let me know.
- The definition of Odd and Even Fields follows the nomenclature used in Report No. 124 adopted by the International Radio Consultative Committee at its Plenary Assembly in Los Angeles in 1959. Return to text (or press 'Back' button). People who should know better manage to get things quite wrong; see what they say.
- The timing datum for all the pulses (line sync, field sync and equalisation) is the falling edge. Falling edges are separated by either 32 or 64 microseconds).
- Equalisation pulses are a desirable feature of a tv waveform. However, many tv systems (as well as home computers and games) can produce adequately interlaced pictures without them. Two such systems were the British 405 and the French 819 line systems. The absence of equalisation pulses may result in a defect called line pairing - its appearance is as described - the lines of the raster are not evenly spaced but appear on the screen with gaps alternating between wide and not-so-wide. Provided that the lines are distinct, however, this impairment is not serious - its main effect is to make the line structure more visible at normal viewing distances.
- In the UK, some broadcasters, at least sometimes, choose to extend the field blanking by half a line at the end of the odd and the beginning of the even fields thereby extending the field blanking by one line in total. This means that there are no active half picture lines but it also means that the blanking signal is different for the two fields (and therefore harder to produce).
Additionally, now that we're all going digital, many programmes in the UK are transmitted with fewer active lines than is specified. This is equivalent to increasing the number of lines blanked.
- Extra signals are inserted during the field blanking interval and before the 'official' start of the picture (lines 23 or 336). These signals are for in-service network testing using the Insertion Test Signal (ITS) or for teletext transmission. Lines 7 - 22 and 320 - 335 are available for text data although, in practice, not all are usually used. The ITS usually is broadcast on two lines per field: 19, 20, 332 and 333 are used.
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