The discovery of the photoconductivity of the element selenium by Willoughby Smith in the year 1873 was the major point for establishing a television system. Also importing was the invention of a scanning disk by Paul Nipkow in 1884. All practical television systems use the fundamental idea of scanning an image to produce a time series signal representation. That representation is then transmitted to a device to reverse the scanning process. The final device, the television, relies on the human eye to integrate the result into a coherent image again. While electromechanical techniques were developed prior to World War II, most notably by Charles Francis Jenkins and John Logie Baird, all-electronic television systems relied on the inventions of Philo Taylor Farnsworth, Vladimir Zworykin and others to produce a system suitable for mass distribution of television programming. The first television broadcasts were made in England in 1936. Television did not become commonplace in United States homes until the middle 1950s. While North American over-the-air broadcasting was originally free of direct marginal cost to the consumer and broadcasters were compensated primarily by receipt of advertising revenue, increasingly television consumers obtain their programming by subscription to cable television systems or direct-to-home satellite transmissions. In the United Kingdom, on the other hand, the owner of each television must pay a licence fee annually which is used to support the British Broadcasting Corporation.
Practical television systems include equipment for selecting different image sources, mixing images from several sources at once, insertion of pre-recorded video signals, synchronizing signals from many sources, and direct image generation by computer for such purposes as station identification. Transmission may be over the air from land-based transmitters, over metallic or optical cables, or by radio from synchronous satellites. Digital systems may be inserted anywhere in the chain to provide better image transmission quality, reduction in transmission bandwidth, special effects, or security of transmission from theft by non-subscribers.