Vestigial-sideband filter, from Croydon Transmitting Station, 1955

Vestigial-sideband filter Type BD 775D made by Marconi's Wireless Telegraph Company Limited, Chelmsford, 1955.

This filter formed part of the Independent Television Authority's [ITA] first transmitter at Croydon. In order to accommodation as many televisions transmissions as possible in the available wavebands, some of the 'sideband' frequencies produced by the modulation process are surpressed at the transmitter; the transmission comprises all sideband frequencies lying to one side of the carrier frequency, but only a narrow 'vestige' of those to the other side.

The unwanted frequencies are reduced in level by appropriate tuning of the transmitter circuits, but to attain the high defree of suppression necessary to prevent interference with other channels, a unti such as that exhibited is employed.

The Independent Television Authority (ITA) was an agency created by the Television Act 1954 to supervise the creation of "Independent Television" (ITV), the first commercial television network in the UK.

The Independent Television Authority (ITA) was formally established in August 1954 to provide “for the period of ten years television broadcasting services, additional to those of the British Broadcasting Corporation . . . for so much of the United Kingdom, the Isle of Man and the Channel Islands as may from time to time be reasonably practicable”. In 1972, the ITA was reconstituted the ITA as the Independent Broadcasting Authority (IBA).

Initially, the BBC agreed to let the ITA share its masts at Crystal Palace in London as well as stations at Sutton Coldfield and Holme Moss but unforeseen technical complications made joint use unfeasible so ITA decided to its own transmitting stations and towers of which Croydon was the first.

The ITA’s first transmitting station was at Croydon and it was from here that the first transmissions of the ITA were made to London and neighbouring counties in September 1955. The transmitting station, of which this component was part, was officially opened by the Mayor of Croydon on 13 September 1955 with full-scale programmes starting on 22 September 1955.

The Croydon transmitting station was the first television station in the UK to operate on Band III (174-216 Mc./s.) carrying a straightforward omnidirectional Band III aerial, with an effective radiated power of 120 kW, uniformly distributed in the horizontal plane. The system used the British standard 405-line system, in common with the existing BBC Band I service. The station was situated on high ground, 375 ft. above sea-level, at Beulah Hill, and the mean height of the aerial above ground-level was 175 feet. The potential audience of the Croydon transmitting station was in excess of eleven million people.

By 1966, the ITA had 30 transmitters covering about 97 per cent of the population; and plans were in hand to build some 20 more relay stations during approximately the next three years.