Helical scan - AbsoluteAstronomy.com

Helical scan is a method of recording high bandwidth signals onto magnetic tape

Magnetic tape

Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...

. It is used in reel-to-reel video tape recorder

Video tape recorder

A video tape recorder is a tape recorder that can record video material, usually on a magnetic tape. VTRs originated as individual tape reels, serving as a replacement for motion picture film stock and making recording for television applications cheaper and quicker. An improved form included the...

s, video cassette recorders, digital audio tape

Digital Audio Tape

Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As...

recorders, and some computer tape drive

Tape drive

A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...

Comparison to linear tape recording

In a fixed tape head

Tape head

A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa.-Principles of operation:...

system, video tape is drawn past the head at a constant speed. The head creates a fluctuating magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

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 channel capacity

Channel capacity

In electrical engineering, computer science and information theory, channel capacity is the tightest upper bound on the amount of information that can be reliably transmitted over a communications channel...

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.

Digital video

Digital video

Digital video is a type of digital recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...

and digital audio

Digital audio

Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...

need considerably more bandwidth than analog audio, so much so that tape would have to be drawn past the heads at very high speed in order to capture this signal. This is impractical, since tapes of immense length would be required. 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. This is known as a helical

Helix

A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

scan because the tape wraps around the circular drum at an angle, travelling up like a helix.

Practical problems

There was 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 smoothly, and the head made of a hard wearing material. In addition, most systems operate with an air bearing

Fluid bearing

Fluid bearings are bearings which support the bearing's loads solely on a thin layer of liquid or gas.They can be broadly classified as fluid dynamic bearings or hydrostatic bearings. Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and...

separating the heads from the surface of the tape. Supplying signals to a rotating head is also problematic: this is usually accomplished by coupling the signal(s) inductively

Inductor

An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

through a rotary transformer

Rotary transformer

A rotary transformer is a specialized transformer used to couple electrical signals between two parts which rotate in relation to each other....

. The transport mechanism is also much more complex than a fixed head system, since, during loading, the 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.

Transport systems

Two transport systems evolved in the early video machines, known as the alpha wrap and the omega wrap. In the alpha

Alpha

Alpha is the first letter of the Greek alphabet. Alpha or ALPHA may also refer to:-Science:*Alpha , the highest ranking individuals in a community of social animals...

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

Omega

Omega is the 24th and last letter of the Greek alphabet. In the Greek numeric system, it has a value of 800. The word literally means "great O" , as opposed to omicron, which means "little O"...

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.

Slant 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) must not interfere with information on 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. All the early reel-to-reel machines and the first cassette formats, the Philips VCR and the Sony U-matic

U-matic

U-matic is an analog recording videocassette format first shown by Sony in prototype in October 1969, and introduced to the market in September 1971. It was among the first video formats to contain the videotape inside a cassette, as opposed to the various Reel-to-Reel or open-reel formats of the...

used this system.

Later helical scanning recorders instead usually use a method called slant azimuth recording, also called Symmetric Phase Recording

Symmetric Phase Recording

Tape recording technology developed by Quantum Corporation packs data across a tape's recording surface by writing adjacent tracks in a herringbone pattern: track 0 = \\\\\, track 1 = /////, track 2 = \\\\\, track 3 = /////, etc....

. The head drum usually contains two heads with the magnetic gap of one head slanted slightly leftwards and the magnetic gap of the other head slanted slightly rightwards. (The slant of a magnetic head is referred to as its azimuth adjustment). Because of the alternating slants, 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 slant on one television field and right slant on the next television field. (In practice, it's not uncommon for the recorded stripes to overlap somewhat). Later machines including the JVC VHS

VHS

The Video Home System is a consumer-level analog recording videocassette standard developed by Victor Company of Japan ....

and the Sony Betamax

Betamax

Betamax was a consumer-level analog videocassette magnetic tape recording format developed by Sony, released on May 10, 1975. The cassettes contain -wide videotape in a design similar to the earlier, professional wide, U-matic format...

used slant azimuth recording as well as all later machines and their digital derivatives.

Using slant azimuth recording, the need for guard bands is completely eliminated, allowing more recording to be placed on a given length of tape.

Contrast with quadruplex recording

Helical scanning was a logical progression beyond an earlier system (pioneered by Ampex

Ampex

Ampex is an American electronics company founded in 1944 by Alexander M. Poniatoff. The name AMPEX is an acronym, created by its founder, which stands for Alexander M. Poniatoff Excellence...

) known as quadruplex recording, also referred to as transverse recording. In this scheme, the rotating head drum ran essentially perpendicular to a 2 inches (50.8 mm) tape, and the slices recorded across the tape were nearly perpendicular to the tape's motion. U.S.

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

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

Field (video)

In video, a field is one of the many still images which are displayed sequentially to create the impression of motion on the screen. Two fields comprise one video frame...

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.

Gallery

Ref. and ext. links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.