Helical Scan - Transport Systems

Transport Systems

Two transport systems evolved in the early video machines, known as the alpha wrap and the omega wrap. In the alpha wrap machines the tape was wrapped around the head drum for a full 360 degrees (the tape looking like the lowercase Greek letter alpha). There was only one head which wrote a complete stripe for every revolution of the head. This system had problems when the head transited from one piece of tape to the next giving a large signal gap between fields. The machine had to fill this gap with the frame synchronizing pulses. Such machines were constrained to using guard band recording (see below).

In the omega wrap machines, the tape was only wrapped around the head for 180 degrees. Two video heads were required, each writing alternate fields. This system had a much smaller signal gap between fields, but the frame synchronizing pulses were able to be recorded on the tape. Cassette based systems could only utilize the omega wrap technique, since it was geometrically impossible for an automatic loading system to introduce a loop into the tape. Early omega wrap systems utilized guard band recording, but the presence of two heads permitted the development of the slant azimuth technique. Later developments used increasing numbers of heads to record video using smaller drums and for recording HiFi sound as well.

A variation of the omega wrap, such as that used by Echo Science Corporation of Mountain View, California in its instrumentation and high resolution video recorders in the late 1970s and 1980s, wrapped the 1-inch tape about 190 degrees around the two-headed drum, so there was signal overlap between the two heads. Head switching in video recorders occurred instantaneously in the video models, during an horizontal sync interval. With a standard NTSC video signal a head could cover one-sixth of a field each time it passed across the tape. Switching in instrumentation models was gradual, so the signals from both heads overlapped briefly, producing a transient-free output signal where the original signal did not contain convenient dead intervals during which a switching transient could be hidden.