Helical scan

From Academic Kids

Helical Scan, or striping is a method of recording higher bandwidth signals onto magnetic tape than would otherwise be possible at the same tape speed with fixed heads. It is used in video cassette recorders, digital audio tape recorders, and numerous computer secondary storage and backup systems.

In a fixed head system, tape is drawn past the head at a linear speed. The head creates a fluctuating magnetic field in response to the signal to be recorded, and the magnetic particles on the tape are forced to line up with the field at the head. As the tape moves away, the magnetic particles carry an imprint of the signal in their magnetic orientation. If the tape moves too slowly, a high frequency signal will not be imprinted - the particles' polarity will simply oscillate in the vicinity of the head, to be left in a random position. Thus the bandwidth capacity of the recorded signal can be seen to be related to tape speed - the faster the speed, the higher the frequency that can be recorded.

Video and digital audio need considerably more bandwidth than analogue audio, so much so that tape would have to be drawn past the heads at very high speed in order to capture this signal. Clearly this is impractical, since tapes of immense length would be required. (However, see VERA for details of a partially-successful linear videotape system.) The generally adopted solution is to rotate the head against the tape at high speed, so that the relative velocity is high, but the tape itself moves at a slow speed. To accomplish this, the head must be tilted so that at each rotation of the head, a new area of tape is brought into play, each segment of the signal is recorded as a diagonal stripe across the tape. Because the tape is moving, the diagonal is actually curved, which is why it is known as a helical scan.

There are a number of practical problems to be overcome with this system. The high tape/head speed could lead to rapid wear of both the tape and the head, so both need to be polished extremely smooth, and the head made of a hard wearing material. In addition, supplying signals to a rotating head is problematic - this is usually accomplished by coupling the signal(s) inductively through a rotary transformer. The transport mechanism is also much more complex than a fixed head system, since tape must be pulled around a rotating drum containing the head(s) so that a complete stripe can be recorded on each revolution. In a VCR for example, the tape must be pulled right out of the cassette case and threaded around the drum, and between the capstan and pinch roller. This leads to complex and potentially unreliable mechanics.

Azimuth recording

Every videotape system attempts to pack as much video as possible onto a given-sized tape, but information from one recording stripe (pass of the video head) can't be allowed to contaminate information on the adjacent stripes. One method to provide isolation between the stripes is the use of guard bands (unrecorded areas between the stripes), but this wastes valuable tape space.

Helical scanning recorders instead usually use a method called azimuth recording. The head drum usually contains two heads with the magnetic gap of one head tilted slightly leftwards and the magnetic gap of the other head tilted slightly rightwards. (The tilt of a magnetic head is referred to as its azimuth adjustment.) Because of the alternating tilts, each head will not strongly read the signal recorded by the other head and the stripes can be recorded immediately next to each other, alternating between left tilt on one television field and right tilt on the next television field. (In practice, it's not uncommon for the recorded stripes to overlap somewhat.)

Using azimuth recording, the need for guard bands is completely eliminated.

Contrast with quadrature recording

Helical scanning was a logical progression beyond an earlier system (pioneered by Ampex) known as quadrature recording. In this scheme, the rotating head drum ran essentially perpendicular to a 2 inch wide tape and the slices recorded across the tape were nearly perpendicular to the tape's motion. U.S. quadrature 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.ja:ヘリカルスキャン方式


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