Field 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

The electromagnetic arrangement of a tape head is generally similar for all types, though the physical design varies considerably depending on the application - for example video recorders use rotating heads which implement a helical scan, whereas most audio recorders have fixed heads. A head consists of a core of magnetic material arranged into a doughnut shape or toroid, into which a very narrow gap has been let (an air gap or other insulating material). This gap causes the magnetic flux to spill out of the material at that point, which is where the tape is made to run. The flux thus magnetises the tape at that point. A coil of wire wrapped around the core opposite the gap interfaces to the electrical side of the apparatus; thus either supplying a signal in the case of recording, or being fed to an amplifier in the case of playback. The basic head design is fully reversible - a variable magnetic field at the gap will induce an electric current in the coil, and an electric current in the coil will induce a magnetic field in the core and hence in the tape drawn across the gap.

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 (the magnetic flux density is higher, and hence will give stronger recording for a given signal, or greater signal pickup for a given tape). A narrow gap also permits higher frequency signals to be recorded for a given tape speed. 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.

Types of tape heads

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 tape or head wear. It can also be seen that due to the construction method of the head gap, tape wear will tend to widen the gap, reducing the head's performance over time. The vertical alignment of the heads (the azimuth) must also match between recording and playback for good fidelity, and the gap should be as close to exactly vertical as possible for highest frequency response. Most tape transport mechanisms will allow fine mechanical adjustment of the azimuth of the heads. Sometimes this can be achieved by automatic circuitry - the actual mechanical azimuth adjustment being carried out by taking advantage of the piezo effect of certain types of crystal material.

Rotating heads

Rotating heads as used in video recorders, digital audio tape and other applications are used to achieve a high relative head/tape speed while maintaining a low overall tape transport speed. The wear characteristics of such heads are even more critical, and highly polished heads and tapes are required. The electrical signals of rotating heads are coupled either inductively or capacitively - there is no direct connection to the head coils.

Erase heads

Erase heads have a similar design to the signal heads, but are generally far less critical in design. In very cheap audio cassette players (particularly very early designs), a permanent magnet has occasionally been used as an erase head, which simply wipes the tape as it is drawn across the head. The disadvantage of this approach is that the head must be held away from the tape when it is not required, and the tape ends up with a lot of random noise on it. Most recorders use a high frequency alternating current signal to erase a tape - this can be switched on only when required so the head can remain in contact with the tape at all times, making it mechanically simpler, and the way the tape is erased leads to much lower noise. The erase signal can be conveniently derived from the same oscillator used to generate the bias signal.

Bias heads

Instead of feeding both the bias signal and the audio signal into the same recording head, a few brands of audio tape recorder, notably Akai and its US cousin Roberts, used a separate bias head on the opposite side of the tape from the recording head; this system was termed a cross-field head.

See also

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