Contrast With Quadruplex Recording
Helical scanning was a logical progression beyond an earlier system (pioneered by Ampex) known as quadruplex recording, also referred to as transverse recording. In this scheme, the rotating head drum ran essentially perpendicular to a 2-inch-wide (51 mm) tape, and the slices recorded across the tape were nearly perpendicular to the tape's motion. U.S. quadruplex systems revolved the head drum at 14,400 revolutions per minute (240 revolutions per second) with four heads on the drum so that each television field was broken into sixteen stripes on the tape (which required appropriately complex head-switching logic). By comparison, the longer stripe recorded by a helical scan recorder usually contains an entire TV field and the two-headed head drum spins at the frame rate (half the field rate) of the TV system in use.
Recording an entire field in a single pass allowed these machines to play back a viewable still frame when the tape was stopped, and display a viewable image sequence while shuttling forwards or backwards. This greatly facilitated the editing process. The quadruplex systems were unable to display video from tape except while playing at normal speed.
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