User:Misternuvistor/Westinghouse VIDAC

Westinghouse VIDAC

VIDAC (VIDeo/Audio Compressed) was an early method developed by Westinghouse Laboratories in 1971 to compress the time needed for the transmission of televised still-video and audio content over a television channel or circuit in a non-realtime fashion. It allowed for far more material to be sent over the air in a compressed form in the same time than with traditional television broadcasting, where upon receiving the compressed programming it is then expanded into real-time form through special receiving equipment for presentation to an audience. Due to its design (and limitations of the technology at the time), it is suited only for television material with still visuals and narrative audio (much like a slide show or filmstrip presentation).

How it works

VIDAC allows for a 480:1 compression of a still-visual and audio television program to be transmitted without modification of the NTSC video signal. The compression is performed by sending the video stills as single frames of video consecutively, and by encoding the audio portion digitally and storing the audio data on a line-by-line basis in following frames of video. One video frame containing this audio data can yield up to 16 seconds of audio. The digital audio frames also carry auxiliary data, such as program metadata, text material relating to the program, and other ancillary data. The compressed VIDAC programming can be recorded to a videotape ready for transmission, which can carry over 900 15-minute programs on one hour of tape.

The VIDAC receiving terminal or terminal buffer installed at the receiving premises then receives and decompresses the compressed programming into real-time human-watchable form. The terminal buffer employs a 16-inch-diameter magnetic disk array with each track of the disc able to carry one frame of analog video, where the buffer then records to this disk array each frame of the compressed VIDAC transmission. The VIDAC system's program converter then reads the tracks of video frames, and determines which frames are still video frames, and which are video frames containing digital audio data. It then plays out both the video & audio frames by playing the same video track on the disk over and over for the length according to the audio content decoded from the audio frames also read from the disk array, which is now played back with the video. This results in a viewable real-time television program of stills and sound that can be recorded to videotape for later viewing.

Use in the field

In 1974, Westinghouse (along with the involvement of the Veterans Administration (VA), Georgia Department of Education, the US Department of Health, Education, & Welfare (now DHHS), and NASA) developed a study of distributing educational television programming to schools and hospitals using VIDAC technology to determine the practicality and improved efficiency of transmitting educational television programming using VIDAC. Westinghouse's VIDAC study also was part of a larger experiment jointly developed by the DHEW and NASA in 1971 called the Health, Education, Telecommunications experiment, or HET, to see how effectively the experimental ATS-6 satellite launched by NASA in 1969 could perform for teleeducation and telecommunications duties. The educational programming used for Westinghouse's VIDAC study was relayed through the ATS-6 satellite for reception by Veteran's Administration hospitals for medical training, and for other groups in the experiment for general educational programming to schools, as well as other VIDAC-compressed educational material being transmitted traditionally using terrestrial TV means during station break times between programs on Georgia Public Broadcasting's schedule (outside of the HET project) during the VIDAC study.

Outside of its use for Westinghouse's feasiblity study for VIDAC (including the ATS-6 HET experiment), the VIDAC system was not commercially introduced by Westinghouse afterward, possibly due to production costs.

External links

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