Television Standards

Frames

Before we dive deep into the various TV Standards we shall take a look at a few basics of TV transmission. A television transmission consists of a set of rapidly changing pictures to provide an illusion of continuous moving picture to the viewer. The pictures need to come at a rate of 20 pictures per second to create this illusion. Each of these "rapidly changing" pictures is a frame. A typical TV transmission is at 25-30 frames per second (fps).

Lines

Each frame consists of several closely spaced lines. The lines are scanned from left to right and from top to left. A typical TV picture consists of 525 to 625 lines. Considering this large number of lines, if all were to be written one after another the picture would begin to fade at the top by the time the last line is written. To avoid this, the first frame carries the odd numbered lines and the next frame carries the even numbered lines. This provides uniformity in the picture and this is called interlacing.

Timing

TV receivers require a source to time the rapid succession of frames on the screen. Designers decided to use the Mains power supply frequency as this source for two good reasons. The first was that with the older type 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 studio lights or for that matter all fluorescent, non incandescent lights flicker at the mains frequency. Since this flicker is much higher than 16 times per second the eye does not detect it. However this flicker could evolve into an extremely pronounced low frequency flicker on TV screens due to a "beat" frequency generated between the light flicker and the mains frequency. This would have made programmes un-viewable particularly in the early days of development of TV receivers.

The two mains power frequencies worldwide are 50Hz and 60Hz. This meant that there was an immediate division in the TV standards - the one with 25 frames per second (50 Hz) and 30 frames per second (60 Hz). Most of the compatibility problems between TV standards across the world stem from this basic difference in frequencies.

NTSC (National Television Standards Committee)

The majority of 60Hz based countries use a technique known as NTSC originally developed in the United States by a focus committee called the National Television Standards Committee. NTSC (often funnily referred to as Never Twice the Same Colour) works perfectly in a video or closed circuit environment but can exhibit problems of varying colour when used in a broadcast environment.

PAL (Phase Alternate Lines)

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 funny acronyms including Pictures At Last, Pay for Added Luxury etc). PAL has been adopted by a few 60Hz countries, most notably Brazil.

SECAM

Amongst 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 ON PAL

Some Satellite TV transmissions (usually Russian) that are available over India, are in SECAM Since the field (25 frames /sec) and scan rates are identical, a SECAM signal will replay in B&W on a PAL TV and vice versa. However, transmission frequencies and encoding differences make equipment incompatible from a broadcast viewpoint. For the same reason, system converters 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 wide-spread at present and it is trans-coded 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.[1]

Quick Facts:

• NTSC and PAL are video standards that are recorded on the cassette. These videos send and electronic signal to the television, then only it can be viewed.
• In, India, PAL video format is supported.
• NTSC is the video standard commonly used in North America and most of South America.
• PAL is the video standard which is popular in most of the European and Asian countries.
• The difference between NTSC and PAL is the transmission of number of frames per second. In NTSC, 30 frames are transmitted per second. Each frame is constituted up of 525 scan lines.
• In PAL, 25 frames are transmitted per second. Each frame consists of 625 scan lines.
• Second, the power frequency used in NTSC is 60 Hz. While in PAL, the power frequency is 50 HZ.

Suggested Readings

• NTSC
• PAL
• SECAM

[1].From World TV Standards (http://www.scatmag.com/technical/worldtv.pdf)