User:Steve Hosgood/sandbox/System I

System I is an analog broadcast television system. It was first used in the Republic of Ireland starting in 1962 as the 625-line broadcasting standard to be used on VHF Band I and Band III, sharing Band III with 405-line System A signals radiated in the north of the country. The UK started its own 625-line television service in 1964 also using System I, but on UHF only - the UK has never used VHF for 625-line television.

Since then, System I has been adopted for use by Hong Kong, Macau, the Falkland Islands and South Africa. The Republic of Ireland has (slowly) extended its use of System I onto the UHF bands.

As of late 2012, analog television is no longer transmitted in either the UK or the Republic of Ireland. South Africa expects to discontinue System I in 2013, and Hong Kong by 2015.

Specifications
Some of the important specs are listed below. A frame is the total picture. The frame rate is the number of pictures displayed in one second. But each frame is actually scanned twice interleaving odd and even lines. Each scan is known as a field (odd and even fields.) So field rate is twice the frame rate. In each frame there are 625 lines (or 312.5 lines in a field.) So line rate (line frequency) is 625 times the frame frequency or 625•25=15625 Hz.

The total RF bandwidth of System I (as originally designed with its single FM audio subcarrier) was about 7.4 MHz, allowing System I signals to be transmitted in 8.0 MHz wide channels with an ample 600 kHz guard zone between channels.

In specs, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.

Colour TV
System I has only been used with the PAL colour systems, though there's no technical reason why it couldn't have accommodated SECAM or been used with a 625-line variant of the NTSC color(sic) system, but apart from possible technical tests in the 1960s, this has never been done officially.

When used with PAL, the colour subcarrier is 4.43361875 MHz and the sidebands of the PAL signal have to be truncated on the high-frequency side at +1.05 MHz (matching the rolloff of the luminance signal at +5.5 MHz). On the low-frequency side, the full 1.3 MHz sideband is radiated. ( This behaviour would cause some U/V crosstalk in the NTSC system, but delay-line PAL hides such artefacts. )

No colour encoding system has any effect on the bandwidth of system I as a whole.

Improved Audio
Enhancements have been made to the specification of System I's audio capabilities over the years. Starting in the late 1980s and early 1990s it became possible to add a digital signal carrying NICAM sound. This extensions to audio capability has completely eaten the guard band between channels, indeed there would be a small amount of analogue-digital crosstalk between the NICAM signal of a transmitter on channel N and the vestigial sideband of a transmission on channel N+1.

The NICAM system uses an extra 700 kHz, and needs to be placed at least 552 kHz from the audio subcarrier.

Republic of Ireland from 1962
These channels are no longer used for TV broadcasting.

♥ No longer used for TV broadcasting.

United Kingdom from 1964
UHF takeup in Ireland was slower than in the UK. A written answer in the D&aacute;il &Eacute;ireann (Irish parliament) shows that even by mid 1988 Ireland was only transmitting on UHF from four main transmitters and 11 relays.

† Officially these channels "don't exist", being between UHF Band IV and Band V and were supposed to be reserved for radio astronomy. However, from 1997 until the finish of analog TV in the UK in 2012, the UK used channels 35 through 37 for analog broadcasts of Channel 5. § Allocated, but never used in the UK.