Tape head
This article's factual accuracy is disputed. (June 2017) |
A tape head is a type of
Principles of operation
The
Reversibility
While a head is reversible in principle, and very often in practice, there are desirable characteristics that differ between the playback and recording phases. One of these is the impedance of the coil - playback preferring a high impedance, and recording a low one. In the very best tape recorders, separate heads are used to avoid compromising these desirable characteristics. Having separate heads for recording and playback has other advantages, such as off-tape monitoring during recording, etc.
Head gap width
The width of the head gap is also critical - the narrower the gap, the better the head will be - a narrow gap gives much better transcription in the magnetic domain (which equals to more output with high frequency signals in the case with playback heads). The desirability for a narrow gap means that most practical heads are made by forming a narrow V-shaped groove in the back face of the core, and grinding away the front face until the V-groove is just breached. In this way, gaps of the order of micrometres are achievable.
A record head, on the other hand, has a gap typically six times larger than that of the replay head, this gives a larger flux to magnetise the tape. The ideal gap size in a cassette deck are; wide record head gap and narrow playback head. The larger gap does not affect frequency response because the 'image' is largely made by the trailing edge of the gap. A combined record/replay head has a compromise size gap typically three times that of a replay only head.
There are also negative aspects of narrow head gaps, particularly for
Types
The physical design of a head depends on whether it is fixed or rotating. In either case, the face of the head where the gap is must be made hard wearing and highly smooth to avoid excessive head wear. It can also be seen that due to the construction method of the head gap, head wear will tend to widen the gap, reducing the head's performance over time. The vertical alignment of the heads (the
Rotating heads
Rotating play heads, as used in video recorders,
Erase heads
An erase head is constructed in a similar manner to a record or replay head, but has a much larger gap, or more frequently, two large gaps. The erase head is powered during recording from a high frequency source (usually the same oscillator that provides the AC bias). In some inexpensive cassette recorder designs, the erase head is a permanent magnet that is mechanically moved into contact with the moving tape only during recording. Permanent magnet erase heads are also sometimes used in machines that are equipped with DC bias.
Cross-field heads
Instead of feeding both the
Head materials
Record and replay heads are traditionally made of soft iron (the softness is an essential requisite for good record and replay characteristics). This material features extremely good electro-acoustical properties, but wears away fairly rapidly with a consequent deterioration of performance. Some higher end recorders featured heads made from ferrite, which features excellent electro-acoustical properties while being a very hard material which resists wear. Its two main disadvantages are that it is brittle and easily damaged, and that it has a much higher noise output due to the Barkhausen effect. In more recent years, more exotic materials have appeared, some involving ceramics, which offer the best of both of the traditional materials.
Cleaning
With use the head will become dirty with loose tape shedding.
Gallery
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D6 HDTV VTR Scanner and video head, removed
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Inside a D6 HDTV VTR Tape Deck, VTR Scanner and video head in place.
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Type B videotape video Scanner Head
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Type B VTR, BCN 20 Tape Desk and video Scanner
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Quadruplex videotape Ampex AVR-2 Video Head
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Sony U-Matic Video head
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Ampex audio recorder without covers
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VHS heads
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Nagra audio heads: erase, record and play
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SONYUVW-1400P_1400AP Betacam SP video heads
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Sony UVW-1800P Betacam SP flying erase head
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HP-67(1976) with transport mechanism
See also
- Recording head
- Magnetic tape sound recording