Tape head
Encyclopedia
A tape head is a type of transducer
used in tape recorder
s to convert electrical signals to magnetic
fluctuations and vice versa.
arrangement of a tape head is generally similar for all types, though the physical design varies considerably depending on the application - for example videocassette recorder
s (VCR) 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. This gap is filled with a paramagnetic material, such as gold. This forces the magnetic flux out of the gap into the magnetic tape
medium more than air would (it also prevents the gap filling with rubbed off material from the tape which would have the reverse effect). 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.
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.
There are also negative aspects of narrow head gaps, particularly for magnetic recording. The narrower the head gap, the more bias signal must be used to maintain linearity of the signal on tape. This reduces high frequency head room, particularly at lower tape speeds. Manufacturers must find a compromise between intended tape speeds and head gaps for this reason.
. 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.
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 helical scan
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.
signal and the audio
signal into the same recording head, a few brands of audio tape recorder
, notably Tandberg
, 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 cross-field
.
. In more recent years, more exotic materials have appeared , some involving ceramics, which offer the best of both of the traditional materials.
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
used in tape recorder
Tape recorder
An audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage...
s to convert electrical signals to magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
fluctuations and vice versa.
Principles of operation
The electromagneticElectromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
arrangement of a tape head is generally similar for all types, though the physical design varies considerably depending on the application - for example videocassette recorder
Videocassette recorder
The videocassette recorder , is a type of electro-mechanical device that uses removable videocassettes that contain magnetic tape for recording analog audio and analog video from broadcast television so that the images and sound can be played back at a more convenient time...
s (VCR) use rotating heads which implement a helical scan
Helical scan
Helical scan is a method of recording high bandwidth signals onto magnetic tape. It is used in reel-to-reel video tape recorders, video cassette recorders, digital audio tape recorders, and some computer tape drives....
, whereas most audio recorders have fixed heads. A head consists of a core of magnetic material arranged into a doughnut shape or toroid
Toroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....
, into which a very narrow gap has been let. This gap is filled with a paramagnetic material, such as gold. This forces the magnetic flux out of the gap into the 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...
medium more than air would (it also prevents the gap filling with rubbed off material from the tape which would have the reverse effect). 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
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
in the case of playback. The basic head design is fully reversible - a variable magnetic field at the gap will induce
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
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 impedanceElectrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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 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.There are also negative aspects of narrow head gaps, particularly for magnetic recording. The narrower the head gap, the more bias signal must be used to maintain linearity of the signal on tape. This reduces high frequency head room, particularly at lower tape speeds. Manufacturers must find a compromise between intended tape speeds and head gaps for this reason.
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 responseFrequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
. 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.
Play heads
Play heads as used in video recorders, digital audio tapeDigital 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...
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 helical scan
Helical scan
Helical scan is a method of recording high bandwidth signals onto magnetic tape. It is used in reel-to-reel video tape recorders, video cassette recorders, digital audio tape recorders, and some computer tape drives....
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
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
- there is no direct connection to the head coils.
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 really cheap 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 AC bias.osbourne heads
Instead of feeding both the biasTape bias
Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal to the audio...
signal and the audio
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
signal into the same recording head, a few brands of audio tape recorder
Tape recorder
An audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage...
, notably Tandberg
Tandberg
This article is about the video-conferencing vendor, a Cisco company. Other companies with the same name such as Tandberg Television or Tandberg Data for data storage: see here and here...
, Akai
Akai
Akai is a consumer electronics brand, founded by Saburo Akai as , a Japanese manufacturer in 1929. It is now headquartered in Singapore as a subsidiary of Grande Holdings, a Hong Kong-based conglomerate, which also owns the formerly Japanese brands Nakamichi and Sansui. The Akai brand is now used...
and its US cousin Roberts, used a separate bias
Tape bias
Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal to the audio...
head on the opposite side of the tape
Magnetic tape sound recording
The use of magnetic tape for sound recording originated around 1930. Magnetizable tape revolutionized both the radio broadcast and music recording industries. It did this by giving artists and producers the power to record and re-record audio with minimal loss in quality as well as edit and...
from the recording head
Recording head
A recording head is the physical interface between a recording apparatus and a moving recording medium. Recording heads are generally classified according to the physical principle that allows them to impress their data upon their medium...
; this system was termed cross-field
Cross-field head
A cross-field head, sometimes referred to as X-field, is an additional recording head in a tape recorder that improves the ability to record high-frequency sounds. The concept was first introduced by Tandberg in their TB-6X 1960s, and more widely used by Akai and their US brand, Roberts.-Bias:A...
.
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 unfortunately 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 effectBarkhausen effect
The Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed...
. In more recent years, more exotic materials have appeared , some involving ceramics, which offer the best of both of the traditional materials.