Television Standards Relative Merits of the different TV standards

The differences between each of the main TV systems are not quite as clear-cut as one might at first imagine. While NTSC generally has a reputation for poor colour accuracy, this is only really true of broadcast television and as a video format it has some distinct advantages over the other systems. The three systems are a compromise and many efforts have been made over the years to address the shortcomings in each of the systems.

NTSC/525-line Advantages

• Higher Frame Rate - Use of 30 frames per second (really 29.97) reduces visible flicker
• Atomic Colour Edits - With NTSC it is possible to edit at any 4 field boundary point without disturbing the colour signal
• Less inherent picture noise - Almost all pieces of video equipment achieve better signal-to-noise characteristics in their NTSC/525 form than in their PAL/625

NTSC/525-line Disadvantages

• Lower Number of Scan Lines - Reduced clarity on large screen TVs, line structure more visible
• Smaller Luminance Signal Bandwidth - Due to the placing of the colour sub-carrier at 3.58MHz, picture defects such as moiré, cross-colour, and dot interference become more pronounced. This is because of the greater likelihood of interaction with the monochrome picture signal at the lower sub-carrier frequency
• Susceptibility to Hue Fluctuation - Variations in the colour subcarrier phase cause shifts in the displayed colour, requiring that the TV receivers be equipped with a hue adjustment to compensate
• Lower Gamma Ratio - The gamma value for NTSC/525 is set at 2.2 as opposed to the slightly higher 2.8 defined for PAL/625. This means that PAL/625 can produce pictures of greater contrast
• Undesirable Automatic Features - Many NTSC TV receivers feature an Auto-Tint Circuit to make hue fluctuations less visible to uncritical viewers. This circuit changes all colours approximating to flesh tone into a "standard" fleshtone, thus hiding the effects of hue fluctuation. This does mean however that a certain range of colour shades cannot be displayed correctly by these sets. Up-market models often have this (mis)feature switchable, cheaper sets do not.

PAL/625-line Advantages

• Greater Number of Scan Lines - more picture detail
• Wider Luminance Signal Bandwidth - The placing of the colour sub-Carrier at 4.43MHz allows a larger bandwidth of monochrome information to be reproduced than with NTSC/525
• Stable Hues - Due to reversal of sub-carrier phase on alternate lines, any phase error will be corrected by an equal and opposite error on the next line, correcting the original error. In early PAL implementations it was left to the low resolution of the human eye's colour abilities to provide the averaging effect; it is now done with a delay line
• Higher Gamma Ratio - The gamma value for PAL/625 is set at 2.8 as opposed to the lower 2.2 figure for NTSC/525. This permits a higher level of contrast than on NTSC/525 signals. This is particularly noticeable when using multi-standard equipment as the contrast and brightness settings need to be changed to give a similar look to signals of the two formats.

PAL/625 Disadvantages

• More Flicker - Due to the lower frame rate, flicker is more noticeable on PAL/625 transmissions; particularly so for people used to viewing NTSC/525 signals
• Lower Signal-to-Noise Ratio - The higher bandwidth requirements cause PAL/625 equipment to have slightly worse signal-to-noise performance than its equivalent NTSC/525 version
• Loss of Colour Editing Accuracy - Due to the alternation of the phase of the colour signal, the phase and the colour signal only reach a common point once every 8 fields/4 frames. This means that edits can only be performed to an accuracy of +/- 4 frames (8 fields)
• Variable Colour Saturation - Since PAL achieves accurate colour through cancelling out phase differences between the two signals, the act of cancelling out errors can reduce the colour saturation while holding the hue stable. Fortunately, the human eye is far less sensitive to saturation variations than to hue variations, so this is very much the lesser of two evils.

SECAM/625-line Advantages

• Stable Hues and Constant Saturation - SECAM shares with PAL the ability to render images with the correct hue and goes a step further in ensuring consistent saturation of colour as well
• Higher Number of Scan Lines - SECAM shares with PAL/625, the higher number of scan lines than NTSC/525.

SECAM/625 Disadvantages

• Greater Flicker - (See PAL/625)
• Mixing of two synchronous SECAM colour signals is not possible - Most TV studios in SECAM countries originate in PAL and transcode prior to broadcasting. More advanced home systems such as SuperVHS, Hi-8, and LaserDisc work internally in PAL and transcode on replay in SECAM market models.
• Patterning Effects - The FM subcarrier causes patterning effects even on non-coloured objects
• Lower monochrome Bandwidth - Due to one of the two colour sub-carriers being at 4.25MHz (in the French version), a lower bandwidth of monochrome signal can be carried
• Incompatibility between different versions of SECAM - SECAM being at least partially politically inspired, has a wide range of variants, many of which are incompatible with each other. For example between French SECAM with uses FM Subcarrier, and MESECAM which uses an AM Subcarrier.

Additional Features:

Stereo Sound

• MTS - Used in conjunction with NTSC/525. Consists of two independent carriers each carrying a discrete channel. One channel provides stereo sound by providing left/right channel difference signals relative to transmitted mono audio track. The second carrier carries the Secondary Audio Program (SAP) which is used for a second language or a descriptive commentary for the blind. Uses a technique based on the DBX noise reduction to improve the frequency response of the audio channel
• FM-FM - dual carrier FM coded discrete stereo transmissions, analogue. Can be used for bi-lingual operation under user selection, but no auto-selection is available. Audio characteristics better than standard mono soundtrack
• NICAM - (full name: NICAM 728) Digital two-channel audio transmissions with sub-code selection of bi-lingual operation. Stereo digital signals with specifications approaching those of Compact Disc are possible. NICAM stands for Near Instantaneously Compounded Audio Multiplex and uses a 14-bit sample at a 32KHz sampling rate which produces a data stream of 728KBits/sec.

Subtitling

• CC - (full name: Closed Captioning) Transmitted on line 21 of NTSC/525 transmissions, contains subtitling information only. CC has no support for block graphics or multiple pages but it can support 8-colours and the use of an italic typeface. Frequently found on pre-recorded VHS cassettes and LDs, also used on broadcast. Also found on PAL/625 pre-recorded VHS cassettes in a modified version.
• Teletext - An information service of 200-700 "pages" covering a wide range of topics including TV Schedules, News, Financial Market prices, Comment, Reviews, Concert & Theatre information. Subtitles are typically transmitted on page 888 in the UK, on pages 199/299/399 in Belgium and Holland, on page 150 in Germany and on page 777 in Italy. There are a number of variant character sets used, but the encoding is identical and all English alphabet characters plus numbers and most punctuation can be handled by any decoder. Includes support for 8 colours, limited block graphics and selective revealing of underlying TV picture. Transmitted on a variable number of lines (specified in header which contains basic information such as time, date and channel), starting on line 12 and continuing for 7-8 lines typically. Found on broadcasts and some LaserDiscs; recording of Teletext signals is marginal on S-VHS, almost impossible on VHS hence the PAL/625 version of CC.

The BBC-designed Teletext system is known in some quarters as World Standard Teletext. There is an enhanced version known as FastText which defines four links to additional pages that can be followed with one of four coloured buttons on the Teletext receivers remote control. The US "CC" is known widely in technical circles as LITO = LIne Twenty-One! where it lives.

Why different standards?

Not all the worlds TV receivers work in the same way. Different countries use different types of broadcast TV system, most of which are to varying extents incompatible with each other.

Unfortunately, video recordings retain many of the characteristics of the original signal of which they are a recording. In general, recordings are more likely to be compatible than receiving equipment, but only with their own "family".

Why did this occur?

In order to work, TV receivers require a source of field timing reference signals. These are signals that tell the TV receiver to be ready to receive the next picture in the stream of images. Early set designers decided to use the mains power supply frequency as this source for two good reasons. The first was that with the older types of power supply, you would get rolling hum bars on the TV picture if the mains supply and power source were not at exactly the same frequency. The second was that the TV studios would have had enormous problems with flicker on their cameras when making programmes.

There are two mains power frequencies widely used around the world: 50Hz and 60Hz. This immediately divided the worlds TV systems into two distinct camps, the 25 frames per second camp (50Hz) and the 30 frames per second camp (60Hz).

Later, the 60Hz camp made a small adjustment and changed the field rate to 59.94Hz when they added colour to the signals. The issue of field frequency remained sufficiently deep-rooted in both TV standards that the vested interest remained long after the original technical justification had gone.

The biggest compatibility problems between TV standards remain related to the field rate; these are also the hardest problems to solve.

Compounding the Problem

Beyond the initial divide between 50 and 60Hz based systems, further sub-divisions have appeared within both camps since the inception of Colour broadcasting. The majority of 60Hz based countries use a technique known as NTSC originally developed in the United States by a committee called the National Television Standards Committee. NTSC (often scurrilously referred to as Never Twice the Same Colour) works perfectly in a video or closed-circuit environment but can exhibit problems of varying hue when used in a broadcast environment.

This hue change problem is caused by shifts in the colour sub-carrier phase of the signal. A modified version of NTSC soon appeared which differed mainly in that the sub-carrier phase was reversed on each second line; this is known as PAL, standing for Phase Alternate Lines (it has a wide range of facetious acronyms including Pictures At Last, Pay for Added Luxury (re: cost of delay line) and People Are Lavender). PAL has been adopted by a few 60Hz countries, most notably Brazil.

Among the countries based on 50Hz systems, PAL has been the most widely adopted. PAL is not the only colour system in widespread use with 50Hz; the French designed a system of their own - primarily for political reasons to protect their domestic manufacturing companies - which is known as SECAM, standing for SEquential Couleur Avec Memoire. The most common facetious acronym is System Essentially Contrary to American Method, SECAM was widely adopted in Eastern Block countries to encourage incompatibility with Western transmissions - again a political motive.

In general, since the field and scan rates are identical, you can expect to get a monochrome picture from a PAL video recording replayed on SECAM equipment and vice versa. Transmission frequencies and encoding differences make equipment incompatible from a broadcast viewpoint. Transcoders between PAL and SECAM, while often difficult to find, are reasonably cheap.

In Europe, a few Direct Satellite Broadcasting services use a system called D-MAC. Its use is not very widespread and is transcoded to PAL or SECAM to permit video recording of its signals. It includes features for 16:9 (widescreen) aspect ratio transmissions and an eventual migration path to Europe's proposed HDTV standard. There are other MAC-based standards in use around the world including B-MAC in Australia and B-MAC60 on some private networks in the USA. There is also a second European variant called D2-MAC which supports additional audio channels making transmitted signals incompatible, but not baseband signals.

In addition to the incompatibilities of 50 and 60Hz systems and the different Colour systems, there is a further barrier to compatibility. Fortunately, video recordings themselves are not affected by this; only TV signal reception equipment. For various reasons of number of stations and terrain, TV pictures can be transmitted in any of three main frequency ranges, VHF, UHF and Microwave (Satellite Direct Broadcasting). Equipment designed to receive signals in only one of these bands cannot receive transmissions in any of the other bands.

Further, there are differences between the encoding of the sound between countries using the same frequency bands. Within 50Hz PAL UHF transmissions, audio signals can be at 5.5Mhz offset (system G), or at 6MHz offset (system I). Similar differences exist between the Middle Eastern versions of SECAM (MESECAM) and the Eastern Bloc (OIRT) version.

Further complications

In addition to standard combinations of Scan Rate, Colour System and transmission frequencies, there are further complications when it comes to additional features like stereo sound, sub-titling and information services. Fortunately, such differences do not effect the basic operation of equipment conforming to the same broadcast standard, but they can restrict the use of various features.

In the cases of both stereo sound and additional textural information carried in the top few lines of the picture, there are three competing systems of varying technical merit. The oldest still operational of the stereo sound systems is the American MTS system based on NTSC transmissions, only slightly more recent is the twin channel FM-FM system used in Germany, Austria, Australia, the Netherlands and Switzerland. The most recent system, NICAM 728, was designed by the BBC in the late 1980s using digital audio technology.

The oldest of the subtitling systems is almost definitely the BBC- and IBA-designed Teletext system which has been is use in the UK since the mid-1970s. It is also the most widespread and the most flexible of the systems in widespread use. The US Closed Captioning system came about through political pressure from organisations for the deaf in the USA and has not been developed beyond the simple job of producing subtitles. The French developed a subtitling and information system called Antiope which has not found favour elsewhere, largely due to the existing widespread use of the BBC-developed Teletext system. A few US stations have now adopted BBC-style Teletext and a few manufacturers, most notably Zenith, fit the decoders to their sets.

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