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Die Entwicklung der Magnet-Aufnahme aus japanischer Sicht

von Gert Redlich überarbeitet im April 2019 - Durch Zufall habe ich eine sehr ausführliche Zusammenstellung der Geschichte der Entwicklung des Magnetbandtechnik - aus japanischer Sicht - gefunden.

Der Autor Masanori Kimizuka war viele lange Jahre (von 1973 bis 2006) bei SONY, dem zeitweisen Weltmarktführer bei Magnetbandgeräten und natürlich bei der gesamten Unterhaltungselektronik sowie der Profi-Fernsehtechnik.
Nach dem Lesen der 93 Seiten aus dem Jahr 2012 fand ich viel - uns Deutschen - noch nicht bekanntes Wissen, aber auch erstaunliche Lücken in manchen - aus meiner Sicht - wichtigen zeitgeschichtlichen Ereignissen. Es ist für den Vergleich der jeweiligen - teilweise persönlichen - Sichten sehr interessant, wie ein japanischer Diplomingenieur diese technische Entwicklung detailliert zusammengestellt hatte und dazu chronolgisch aufgearbeitet und zusammengefaßt hat.
Herr Kimizuka war in 2012 der "Director of Japan Audio Society", ein vergleichbarer Ton-Ingenieurs-Verein zu "AES" in USA.

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7 Structure and Performance of Compact Cassette-Type Tape Recorders


7.1 Types of Cassette Tape Recorders
According to Head Arrangement

Although Compact Cassette machines started out as so-called memo recorders and were initially limited in terms of function and performance, as the competition for standardisation ended and the Compact Cassette became recognised as the standard format, development rapidly began on increasing its functionality and performance.

The original construction was in "cartridge" (unglücklich, es war eine "Kassette" und hatte 2 Spulen, eine Cartridge hat nur eine Spule) form and presumed a two-head confguration only: an erase head and a recording/ playback head. Various head formations were devised, but in the end the head formation that had been put to use in the freer open-reel era was carried over into the Compact Cassette as well. Fig. 7.1 shows the various Compact Cassette tape recorder formats.

7.2 Basic Structure

Fig. 7.2 shows the basic structure of a tape recorder. The input signal passes through the equaliser after being adequately amplifed; it is then applied to the recording head with the bias signal added to it. The tape moves in front of the head (Fig. 7.3) at a fxed speed and is magnetised in proportion to the strength of the magnetic feld coming from the gap in the recording head, thus recording is achieved.

(a) 1 way playback
(b) 2 head, 1 way
(c) 3 head, 1 way open reel
(d) 3 head, 1 way closed loop dual capstan
(e) rotary head reverse (f) 4 ch head reverse (dedicated playback)

When the recorded portion of the tape is rewound and passed by the playback head at the same speed, the magnetic flux on the tape passes through the gap to the head, inducing a current corresponding to the magnetisation on the tape when the tape was wound. Playback is achieved by this signal being output through the playback amplifer.

7.3 Recording mechanism

The magnetisation curve of tape is represented by what is called a hysteresis curve rather than a straight line. As a current passes through the head coil, the magnetisation of the tape that is in contact with the gap alters in a hysteresis loop pattern in response to variations in the magnetic field H.

As the tape moves, the magnetic field applied to that particular point on the tape returns to 0 once it has gone past the gap. If the applied AC current (recording signal) has a high frequency, the magnetic feld is reversed and returns to 0. Consequently, while residual magnetisation is achieved in a small loop, the recorded magnetisation waveform is greatly distorted because the initial magnetisation curve is non-linear.

Accordingly, a sine wave is passed through the recording head at a higher frequency than the recording current as a means to avoid this distortion. This is called AC bias recording and is an indispensable piece of technology for ensuring good electromagnetic conversion in analogue recording (see Fig. 3.6).

7.4 Loss during Recording

Loss (signal attenuation) at the recording head tends to be greater the higher the frequency, as shown in Fig. 7.4. Self-demagnetisation loss is the mutual negation of adjacent magnetic flux as the recorded wavelength on the tape decreases. Recording demagnetisation loss increases the higher the frequency; like AC bias, at very high frequencies the minor loop converges at 0 and nothing is recorded.

Penetration loss is loss caused by differences in magnetisation between the surface of the tape and the deeper levels of magnetic particles on the tape due to different depths of magnetisation on the tape. Core loss is loss caused by eddy currents in the head core. Recording spacing loss is loss caused by a space forming between the tape and the head gap; however, this has less effect during recording than during playback.

7.5 Loss during Playback

Ideally, a playback head should increase in output in proportion to frequency. In reality, however, various losses occur when a tape is played back (Fig. 7.5). Gap loss is loss caused by gap width: if the recorded wavelength is equal to the gap width, there is no output.

Playback spacing loss is loss due to a space forming between the tape and the gap in the playback head. This can be caused by the surface properties of the tape, the tape driving performance, the tape tension or other factors. Azimuth loss is caused by a misalignment between the playback head gap and the azimuth angle of the recorded signal (determined by the relative position of the recording head gap and the tape).

Spacing loss and azimuth loss are largely due to tolerances of the core mechanisms of the tape recorder, including the tape drive system. Particular attention needs to be paid to this when designing mechanisms. Since there are limited options for head arrangement on Compact Cassette machines in particular, it is very diffcult to improve azimuth precision on reverse machines and three-head machines.

Head arrangements with no pads inside the cassette (such as heads that are inserted through a small window) cannot ensure proper contact between the tape and the head and are thus susceptible to spacing loss. Thickness loss is loss related to the effective depth of magnetisation during playback.

7.6 Playback Equaliser

A characteristic of magnetic recording is that although the output of the playback head is relative to the speed of variation in magnetic flux, certain losses are evident at high frequencies.

Consequently, a balanced frequency response can be achieved in a tape recorder through playback compensation (equalising) that separates the mid and low frequencies, where loss is negligible, from the high frequencies, where loss is not negligible.

This had to be standardised relatively early on, because there would be little compatibility between machines if every different device were to make this compensation in a different way. Compact Cassette machines had different playback equalisers for each
tape type. Equalising low frequencies meant increased noise with any rise in gain, so the low frequency time constant was fattened (Fig. 7.6).

8 Magnetic Tape in Compact Cassettes


8.1 Development of Magnetic Substances

Development of ferric oxide magnetic tape continued with the basic aim of trying to improve its recording performance. Unlike the open-reel system, the Compact Cassette tape recorder system meant very little freedom in terms of tape width, tape speed and head formation.

Since compatibility had to be strictly adhered to, there was a strong demand to improve the performance of the magnetic tape itself. The early Compact Cassette tapes were lacking in performance in terms of music recording.

Development of magnetic substances quickly got under way in order to make this system into an all-purpose tape recorder for recording everything from conversations to music.

Hier sollte die Pyramide der bandqualitäten stehen

Fig. 8.1 is a chart proposed in the 1970s showing the estimated development of magnetic substances on tape. First, attempts were made to physically improve the tape and magnetic substance by developing and improving the magnetic ferric oxide powder, increasing the packing ratio by making the particles smaller and improving the tape’s adherence to the head by making the tape surface smoother.

At the same time, improvements were being made to the process of applying the magnetic powder to the tape, as well as to the adhesives and additives, the so-called binders, that went into the magnetic powder.

Work was also being done to improve the mechanical precision of the tape by improving the tape cutting machines (slitters) and other elements in the manufacturing process, enabling the creation of a more delicate and higher-precision tape than open-reel tape.

Wichtiger aber, das Bandgerät mußte kleiner werden

However, the most important tasks were to downsize the product and improve its ease of use. Developers had very constrained conditions under which to achieve proper recording performance, so they had to focus on developing and improving the magnetic substance itself to improve tape performance.

The Compact Cassette philosophy of strictly maintaining compatibility not only spurred on product development, providing users with small, lightweight tape recorders that were easy to use, but also forced developers to improve the performance of the technology under very restrictive conditions. This was the driving force (der Bandzug) that made the development of the Compact Cassette so defnitive.

8.2 Development of Chrome Tape

In 1970, the German BASF released a “chrome tape” that used chromium dioxide as the magnetic substance. This tape was a major improvement in terms of a high-frequency response that could not be achieved with ferric oxide tape.

Up until this time, all Compact Cassette players had a playback equaliser time constant of 120µs; the arrival of the chrome tape meant a time constant of 70µs could be used. This became the standard value for high frequency compensation for high-performance tape.

Once BASF developed its chrome tape, Japanese manufacturers also came up with their own chromium dioxide tape.

In 1973, Sony released its “DUAD tape”, double-coated with ferric oxide and chrome. The ferric oxide layer on this tape compensated for the lack of energy (or lack of sensitivity) in the mid and low frequency range that was a shortcoming in chrome tape.

In turn, the high frequency response of chrome made this a high performance tape. Although this type of tape became standardised as the Type-III, it was not highly popular and went out of use in the 1980s.

Although double-coated tape had a reputation for good sound quality and had also been used in open-reel systems, it faced pressure from the high performance and price competitiveness of Type-II tapes such as cobalt tape.

When metal tapes appeared, it lost much of its purpose and exclusive Type-III products faded (ausgeblendet) from the market.

While chrome tape was ground-breaking in terms of high frequency response and played a huge part in improving the performance of Compact Cassettes, it gradually disappeared off the market in Japan due to patent licencing issues and pollution issues with hexavalent chromium effluent from coating factories.

  • Anmerkung : Die Lizenzfrage war bei den japanischen Herstellern eminent wichtig. Sie suchten immer sofort nach einer Alternative, bei der sie ein (das eigene) Patent anmelden konnten.

Instead, cobalt-deposited ferric oxide (CoFe2O4, crystal growth of cobalt ferrite on the surface of ferric oxide) tape with similar properties became the main high-performance music tape in use. Eventually, most of the Type-II tape was this so-called “cobalt tape”.

8.3 Appearance of Metal Tape

With stereo records becoming popular and FM broadcasts reaching full swing in the late 1960s, the audio market flourished. The Japanese audio industry entered a phase of furious development in exports.

While Compact Cassette machines were a mainstay of this trend, as the audio market expanded, there was increasing demand for further performance improvements in the Compact Cassette.

The magnetic properties of the tape had to be improved, but the standards had to be strictly maintained. The race was on to get a new, high-performance magnetic substance out of the laboratory and onto shop shelves.

Developers first came up with chromium oxide; the next development was cobalt. Finally, they came up with metal tape (metal powder tape) and set about putting it to practical use.

Nagai, Iwasaki, et al. had already reported some research results at Tohoku University regarding tape for short wave recording around 1963. Although this was recognised to have better qualities in theory, there was no obvious need for the product and no incentive to solve the various difficulties faced in manufacturing it.

But high-performance Compact Cassettes presented such a need, and metal tape, originally considered to be for video use, was put to practical use.

Es war gar keine neue Erfindung

Despite being called metal tape, it was actually a type of coated tape very similar in composition to the existing ferric oxide tape rather than a completely new composition, such as the evaporated tape discussed in a later section.

The difference was that the magnetic substance used was metal particles (iron). This meant a superior magnetic substance compared to ferric oxide because the lack of oxygen atoms increased the density of the metal, thereby adding to the magnetism of the substance; however, it was also very susceptible to oxidation.

It was important to develop anti-oxidation or rust-proofng measures to be incorporated into the tape manufacturing process, as well as a method for producing metal particles.

A number of companies tried to solve this issue through various means, such as coming up with different binder materials or putting anti-rust properties into the magnetic particles themselves.

A major argument was the lack of compatibility: the tape was highly retentive, and could not be recorded onto or erased using existing machines. However, such concerns were far outweighed by the desire for better sound quality, and a new standard, the Type-IV, was established.

The final stage in the evolution of the Compact Cassette had taken place. The development of metal tape not only proved useful for audio, but also made a major contribution to the improvement, downsizing and digitalisation of small-scale video tape recorders and the development of audio and video equipment.

8.4 Creation of Practical Evaporated Tape

During the same period of time, Matsushita achieved another dream: thin metal film tape. Although thin metal film tape was considered ideal, as it does not require a binder like coated tape does, it required a new manufacturing technique to achieve such a product in tape form.

Matsushita developed the evaporation coating method, in which an evaporated coating could be continuously formed on a base product (Fig. 8.2).

It was originally developed as a means of evaporation coating for the production of film capacitor electrodes (Elektroden in Dünnfilmkondensatoren).

While evaporated tape offered superior magnetic properties, it also had several flaws when used in tape recorders. It adhered to the head very well, as the surface of the tape was far smoother than coated tape; however, it could cause too much friction against the head and the tape guide, so an appropriate lubricant was required.

The magnetic layer was very thin, so care had to be taken to ensure it did not deteriorate, even when highly durable materials such as nickel or cobalt were used for the magnetic film.

These various hurdles were cleared and the frst evaporated tape – “Ångrom” tape – was created for the microcassette in 1978. In 1984, an "Ångrom Compact Cassette" went on sale as a Type-II tape.

This evaporated tape was later developed for use in video, just as the metal tape was, and full use was made of its high density recording properties. It became a very important tape in the era of audio/video digitisation.

9 Advances in Magnetic Heads for Compact Cassettes


9.1 Recording/Playback Head Structure

Magnetic heads used in tape recorders comprise a magnetic circuit of a coil wrapped around an iron core with a part cut away. Magnetic recording occurs when a magnetic field is applied to the tape through this cut-away section, or gap (der Spalt).

For playback, the magnetic flux on the tape passes through the gap, inducing a current in the coil, which is then amplified (see Fig. 9.1).

The core material must be sensitive to microscopic changes in magnetism. Since high magnetic permeability is needed to increase the effectiveness of the magnetic circuit during recording, many of the early heads were made of permalloy, an alloy of nickel and iron with high magnetic permeability. The magnetic head is a wound coil with impedance that increases in proportion to frequency.

As a result, the higher the frequency, the greater the “core loss”, loss caused by eddy currents. To reduce this core loss, a laminate structure is used, made up of several layered sheets of permalloy (Fig. 9.2).

The head is also designed so that the magnetic circuit is narrower near the gap, so as to increase the effectiveness of the exchange of magnetic flux.

The front part of the head that comes into contact with the tape is called the tape contact surface (der Kopfspiegel). Since it is in constant contact with the tape, it is very important for it to be abrasion-resistant (Widerstand gegen Abrieb); it must have very smooth frictional properties so as not to adversely affect the running of the tape.

In actual heads, the contact surface comprises a shield (ein Schild doer Isolator) between tracks (if there are multiple track heads, such as for stereo), a dummy segment without tracks and a resin material to hold these in place.

If these different materials do not wear evenly, this affects the contact with the tape and may result in spacing loss.

Accordingly, every head-manufacturing company has come up with its own ideas on which materials to select and how to polish the contact surface.

9.2 Types of Heads and their Characteristics


(1) Recording Heads

The aim of these heads is to generate a large and effective magnetic field with very little current and magnetise the tape precisely and accurately. They are configured to have a slightly larger gap of around 3-5µm to avoid magnetic saturation at the gap and increase the effectiveness of the recording penetrated into the tape. Impedance is set to 10 Ohm at 1kHz for an electrical circuit providing a bias current.

(2) Playback Heads

The priority for these heads is to reduce gap loss in order to play back at as high a frequency as possible. Accordingly, they have a far narrower gap than recording heads, at around 1.0µm.

Since they have an increased number of winds in the coil to make the playback output as high as possible (increased sensitivity), they automatically have a high impedance of 1kOhm at 1kHz. As well as being highly sensitive, playback heads are connected to a playback amplifier, so require strong magnetic shielding to prevent external magnetic induction and noise generation.

(3) Recording/Playback Combination Heads

The basic two-head type Compact Cassette tape recorders use the same head for recording and playback in order to make the device simpler and keep the price down.

Many of these recording/playback heads have a gap of 1.3-1.6µm in consideration of magnetic saturation during recording as well as gap loss during playback. Since the parameters of these heads are not optimum for either recording or playback, they are inferior in performance to the three-head type, which uses individual recording and playback heads. However, for as long as Compact Cassettes used ferric oxide, they achieved adequate high-end performance.

(4) Erase Heads

Many AC erase heads use ferrite materials, which generate strong magnetic fields with little heat generated from eddy currents (Wirbelströme).

But with the appearance of metal tape, erase heads needed to generate even stronger magnetic felds. Although Sendust erase heads were developed that could achieve this, they were extremely cost-prohibitive, so other alternatives were devised, such as improving the ferrite material and increasing the number of gaps (Fig. 9.3).

Low-cost, popular-model machines were not really capable of playing and recording on high-performance tape such as metal tape. Many of them used a DC erase method with erase heads made from permanent magnets. In such cases, rather than simply using a magnet, the head surface was magnetised in an N-S-N-S arrangement to give the effect of AC erasure as the tape passed by. Combination products such as radio cassette players often used permanent magnet erase heads to avoid causing radio interference with a strong magnetic feld (Fig. 9.4).

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