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A system called quadruplex was developed in 1956. Four record/playback heads are mounted on the circumference of a drum that moves rapidly at a right angle to the direction of the tape. Now the heads can scan the tape at the tape speed equivalent to 3 810 cm per second. The tape itself travels slowly: either 38 cm per second or 19 cm per second. Fixed heads scan the audio and picture control tracks that run linearly near the edge of the tape. The picture control track generates signals that control the speed of the drum the heads are positioned directly over the correct part of the tape. This system came to be generally used from the 1950's in the TV industry.
Helical scan, or slant-track scan, was developed in the 1960's. At first its tape speed was slower than that of the quadruplex system and the band with only 3 MHz. There for its picture resolution was also much weaker than that of the quadruplex system. Only one or two record/playback heads are placed on the circumference of a drum turning rapidly. The tape is wrapped around the drum in a helical manner. VCR's became relatively cheap in the 1980's and were widely available for domestic use. Video brought about new trends in watching television. Eventually at the end of the 1970's new improved versions of helical scan recorders were made using 2.54 cm wide tape and higher tape speeds to equal the quality of quadruplex. Two variants were standardised, "B format" and "C format". C format proved to be more popular and eventually displaced quadruplex machines.
New helical scan recorders that record the
television signal digitally were developed in 1991. The TV signal is first
converted into digital form by an analogue-to-digital converter. After
recording and playback the process can be reversed and an analogue signal
can be created for broadcasting. Otherwise the digital form can be kept
for other purposes such as adding computer-generated effects.