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Die Entwicklung der Magnet-Aufnahme aus japanischer Sicht

überarbeitet im April 2019 - Durch Zufall habe ich eine 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.
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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 und dazu chronolgisch aufarbeitet und zusammengefaßt hat.
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Herr Kimizuka war in 2012 der "Director of Japan Audio Society", ein vergleichbarer Ton-Ingenieurs-Verein zu "AES" in USA.

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9.3 Advances in Materials for Magnetic Heads

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9.3.1 Ferrite Heads

The Compact Cassette started out using ferric oxide tape. As these tapes became more widely used, it became a major challenge to solve the quality performance issue resulting from narrow track width and slow tape speed.

In the late 1960s, stereo records, which were already firmly established as a music medium, improved greatly in sound quality due to improvements in recording and cutting technology, creating an even greater demand to improve the sound quality of consumer tape recorders.

While development continued on high-performance open-reel machines such as studio recorders, this was increasingly heavy-duty equipment for analogue recording and therefore highly priced.

Naturally, this resulted in a greater demand in the home audio and car audio markets for a small-scale, affordable machine that could record and play music on Compact Cassettes.

As discussed in Section 8.2, researchers worked on improving the magnetic substance on tape, creating chrome tape in 1970. This tape was superior in a wide range of applications and succeeded in expanding the existing recording and playback bandwidth of 10kHz to around 15kHz, which could comfortably be called Hi-Fi.

In terms of compatibility, a new “Chrome Position” (later Type-II) was defned, with a high-frequency playback time constant changed from 120µs to 70µs to match the rise in high-frequency sensitivity.

Das Band war auf einmal härter als der Kopfspiegel

While the greater recording and playback performance of chrome tape played a major part in Compact Cassette machines being accepted as audio equipment, the surface of the tape was harder than ferric oxide tape and caused problems with wear on the heads.

To solve this problem, developers brought out an abrasion-resistant ferrite head (Fig. 9.5). Since ferrite is made of compression-moulded powder (in Formen gepresster Puder), it has relatively low manufacturing costs. It forms a hard core material following sintering, but this is easy to grind or cut and therefore easy to achieve dimensional accuracy.

As well as being very hard and resistant to wear, ferrite heads also offer a very dimensionally stable gap, with glass layers fused between layers of ground core material. They maintain their initial properties for a very long time and are extremely stable even with changes in temperature and humidity (Fig. 9.6).

Sony had used ferrite heads in open-reel machines from a very early stage. It used ferrite not only for the core material, but also the entire contact surface of the head, including the dummy segment. This rugged and highly abrasion-resistant head was called the “F&F Head” (Fig. 9.7) and also became widely used for Compact Cassettes.

9.3.2 Sendust Heads for Metal Tape

Metal tape appeared in 1978 at the onset (Ausbruch/Beginn) of the digital audio era. This tape was the music tape trump card for the Compact Cassette, able to record and play back sounds that required a high resolution and a large dynamic range, like computer music.

While it was understood that metal tape had ground-breakingly superior magnetic properties because it used a magnetic substance of metal powder rather than oxide, this contravened the Compact Cassette policy of “maintaining compatibility”.

In short, metal tape used a strongly magnetic substance. Compared in terms of coercive force (saturation flux density) Hc and maximum residual magnetisation Br, the values for the existing chrome (cobalt) music tape were in the vicinity of Hc=600-700Oe, Br=1500G, while the values for metal tape were almost double at Hc=1000Oe, Br=3000G.

Hc represents the high frequency recording level (the quality of the high-frequency response) as well as the ease of erasing or the strength of the magnetic feld required for magnetisation.

The ferric oxide tape first used in Compact Cassettes had an Hc of 350-400 Oe (Oerstaedt). While this value was higher for chrome tape, the existing heads were still adequate for erasing and recording.

Das Problem ist der extrem hohe Löschstrom

However, to record at full capacity on Hc=1000 Oe metal tape required a stronger magnetic force and a greater bias current in the head.

Since the existing head materials (permalloy or ferrite) had a low saturation flux density, any increase in current flowing through them would simply convert to heat rather than provide any effective increase in magnetic flux.

Likewise, there was a fatal compatibility faw (Hinkefuß) for erasing, as the existing erase heads would not erase the tape.

A discussion commenced on whether or not to change the high frequency time constant for chrome tape from 70µs to 50 or 35µs to make a marked improvement in high-frequency response during playback.

Die Standards verbessern/ergänzen oder aufheben

One opinion said that the latest technology should be actively incorporated, because fussing (aufregen/hinwegsetzen) over standards and compatibility prevented technological progress, but this meant that the compatibility that was such an important factor in the Compact Cassette would be likely to come undone.

However, the latest improvements in performance were sought after in the music scene and could not be ignored. The EIAJ (now JEITA) formulated an international standard in a short space of time with the help of international organisations such as IEC and metal tape Compact Cassettes were introduced.

The playback time constant was set at 70µs, the same as for chrome tape. Although the compatibility policy had been closely adhered to for playback, such as maintaining playback on existing machines, a new head (with more efficient materials and design to supress heat and prevent saturation even with a high bias current) was absolutely necessary for recording.

While ferrite was used in large quantities in high-performance heads due to its superior abrasion-resistance and the ease and low cost of manufacture and production, it was unsuitable for metal tape due to its low saturation flux density of around 5000G.

"Sendust" von 1935 kommt wieder ins Gespräch

Sendust gained much attention at this stage (Table 9.1). Sendust is an iron-aluminium-silicon alloy invented in 1935 by Dr. Hakaru Masumoto and others at the Tohoku University Institute for Materials Research and used as a magnetic powder core before ferrite was discovered.

Although it has superior magnetic properties and has cost benefts due to the abundance of raw materials for it, it has limited uses as it is extremely hard and brittle as a metal (alloy) and difficult to roll out like permalloy.

Nevertheless, it gained immediate attention with the advent of metal tape. The worst characteristic of Sendust was its difficulty in processing.

While vacuum-melted Sendust ingots (Block/Barren) were cheap, it was expensive to grind and polish them to make heads. Usually, heads had a laminated structure, made up of ground and polished cores 0.2-0.3mm thick.

This thickness was a compromise to keep processing costs down; even thinner layers would have been better for high-frequency response.

Various attempts were trialled to make a thinner product, such as the method of melting at high temperatures and then rapidly cooling the alloy in ribbons (ribbon Sendust) or an attempt at rolling the alloy, but it was not suitable for mass head production due to the difficulty in handling the brittle ribbon.

One possible method to curb high-frequency loss was the composite S&F Head (Figs. 9.8, 9.9), with the tip of the head (nur die Spitze) made out of a small block of Sendust, thus making the gap area highly magnetic, while the rest, which did not need to be so strongly magnetic, was made out of ferrite, which has hardly any high-frequency loss.

The gap area had more or less the same structure as that of a ferrite head (Fig. 9.10), with the abrasion-resistance of Sendust rivalling that of ferrite. The product was welcomed as a highly reliable, long-life device. Sony, Matsushita and other companies made this type of head; these were widely used in high-performance cassette tape recorders from the metal-tape era onwards.

9.3.3 Adoption of Amorphous Alloy Head Material

Although a lot of Sendust was used in heads for metal tape, its saturation flux density was slightly reduced, as other elements such chrome and molybdenum were being added to it to increase its durability as an audio head.

While successive studies were made on how to improve it by modifying the type or amount of additives, it was impossible to achieve a saturation flux density any higher than 10,000G.

Amorphous alloys emerged as a more effective head material that could transcend this limitation. Non-crystalline amorphous alloys differed in various ways from existing metals with a crystalline lattice structure. When metals melt at high temperature, they have a completely random arrangement of atoms; if they are rapidly cooled, they retain this property even at normal temperatures, thus forming an amorphous alloy.

The cooling rate to achieve this is around 10,000-1,000,000ºC/s. In practice, amorphous alloys are made by pouring molten metal from a fine nozzle over a cooling roller rotating at high speed, thus creating a ribbon (Fig. 9.12).

At around 30-50µm, this ribbon is of a suitable thickness for use as a head material; it is easy to make into a laminate core by layering it (Fig. 9.13).

Soft magnetic amorphous alloys are made from a combination of ferromagnetic metals such as iron, cobalt and nickel, and metalloids necessary for amorphisation, such as phosphorus, carbon, boron and silicon.

Harder than ferrite at Hv=800+ and with the same or slightly higher abrasion-resistance than Sendust, they are perfectly suitable material for audio heads. The higher performance amorphous heads took over from Sendust heads, first used in high-end machines such as three-head decks, then in other devices competing for high sound quality (Fig. 9.14).

10 Development of Three-Head Compact Cassettes

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10.1 Advantages of the Three-Head Format

The two-head type is generally thought of as the standard Compact Cassette. The idea is that the cassette is vertically symmetrical and both sides can be used by turning the cassette over.

This basic concept is very important, as it rapidly boosted usability, providing tape recorders with a necessary and sufficient function.

The basic specifications, such as tape speed, width and magnetic properties, were also thought to provide suffcient functions for the expected users and usage environment.

Compact Cassette machines, which had started out as simple “memo tape recorders”, had successfully become the de facto standard.

Once they came to be regarded as mainstream tape recorders, rapid technological developments took place to improve their sound quality and performance.

Improving the performance of open-reel tape recorders

When improving the performance of open-reel tape recorders, developers had been quite free to select any tape speed as well as the shape and number of heads.

Since it was straightforward to use heads optimally designed for recording, playback and erasing, it was inevitable that a three-head system was adopted.

Of course, even for open-reel machines, two-head systems were more typical among popular-model machines and simple head arrangements, such as DC erasing, were used in small, battery-operated machines.

However, one major advantage in all of these machines, from popular-model machines to high-performance machines and even business machines, was that there was free choice in terms of heads.

In practice, most stereo tape decks used the three-head format. While the main aim of the three-head format was to improve the sound quality when recording, a major advantage was that running the playback head during recording made it possible to have a “recording monitor”.

When recording on ordinary two-head tape recorders, the sound from the microphone or line in was adjusted as necessary using a preamplifier then sent to the recording amplifier.

The sound monitored during recording was the output from the preamplifer, or the “sound being recorded”. Of course, some machines had level meters for adjusting the volume and other functions that made it easier to produce a better recording, such as automatic levelling functions, but ultimately, the recording could only be checked by rewinding and playing back the tape after recording.

While this presented no real problems in general use, in instances where the recording was vital, such as live recording, being able to monitor the recording, or the “recorded sound”, during recording provided a definite way to check the recording levels and to check for any faws in the basic components, such as the tape or heads, thus providing a very effective way to prevent mishaps.

Monitoring during recording can only be achieved by having a three-head system with dedicated recording and playback heads. This is the “three-head” format. For open-reel systems, the head arrangement could be freely designed to suit the required performance and functions.

10.2 Three-Head Format for Compact Cassette

The Compact Cassette running system serves as a tape recorder by inserting heads and a pinch roller into the cassette. Fig. 10.1 shows the main components of a typical Compact Cassette tape recorder running system.

There are three large apertures (generally called “windows”) on the face of the cassette for inserting the heads and the pinch roller.

The central window has a felt-like component called a pad behind the tape, held in place by a leaf spring. When the head is inserted, this pad pushes the tape against the head to ensure contact is maintained between the tape and the head.

Fig. 10.2 shows the face of a cassette. The left and right windows, for the erase head and the pinch roller, are symmetrical, so that when the cassette is turned over, they each insert into the opposite window on the so-called B side.

These pinch roller windows are the large windows; there are also smaller apertures between these large windows and the central head window. These are the small windows; while they are designed to have various uses, such as using tape tension to detect the end of the tape and detecting the transparent leader tape (the start and end of the reel of tape, mostly with no magnetic substance on it), they are hardly ever used.

Ein 3-Kopf System in die CC-Kassette "implementieren"

Given these constraints /Zwänge), it is diffcult to implement a three-head format on a Compact Cassette and work out which head to put where. While various companies investigated various proposals (Fig. 10.3), the first format implemented used a system of standalone heads as shown in Fig. 10.3 (1).

Although this format meant that heads very similar to the existing recording/playback heads could be used as playback heads, it was difficult to get any performance out of the recording head, as it had to be made smaller in order to fit into one of the small windows.

Since the small windows had no tape pads, it was also diffcult to maintain contact between the head and the tape. As a result, a high-end running system called a closed loop dual capstan had to be implemented to maintain tape tension within the loop (the area between the two capstans) to ensure recording performance.

The erase head was placed on the outside of the upstream capstan; this head also had to have quite a special shape to prevent it from interfering with the pinch roller.

Der Nakamichi 1000 war der erste 3-Kopf CC-Recorder

This three standalone head system was frst incorporated into the Nakamichi 1000 (Fig. 10.4) by Nakamichi Corporation. This was an ultra-high-end tape deck with a price tag of over ¥200,000 when it frst went on sale in 1973.

Major tape recorder companies such as Sony and Matsushita were also working on similar developments around the same time; Sony released the TC-6150SD (Figs. 10.7, 10.8) in 1973, while Matsushita released the RS-690 (Figs. 10.5, 10.6) in 1975, both decks with three standalone heads.

This proved that the Compact Cassette could have ("könnte" haben !!) the same level of performance as open-reel machines. Users gained confidence that it could cover all areas from taking memos to high performance, again boosting the popularity of the Compact Cassette.
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  • (1) Three standalone heads format
    - Optimal head performance
    - Requires specially-shaped heads
  • (2) Combination head format
    - Easy to access each head
    - Manufacturing of a combination head is key
  • (3) Monitor head format
    - Recording/playback performance identical to the two-head format
    - Monitoring function during recording has been added

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3 Köpfe - getrimmt auf Performance

The three standalone heads format was optimised for performance. While it answered the call for the Compact Cassette to function as a music player, cost was a major limitation due to the special head shape; it was difficult to develop a low-cost model.

As the recording head was inserted through the small window, it was sandwiched in the relatively narrow space between the guide pins on the cassette side, making it diffcult to maintain precision in tape operation and contact between the head and the tape.

As a result, the cassette was susceptible to azimuth loss during recording and playback and spacing loss during recording. The difficulty in maintaining mechanical precision and the adjustment processes required during manufacture only added to the cost.

If a three-head system could be achieved using heads similar in size to the standard recording/playback head, then the same mechanism could be used, meaning a major reduction in cost.

2 Köpfe in einem Gehäuse

A “recording/playback combination head” was developed, integrating the recording and playback heads in one casing; this later became the main three-head system in use.

While contact between the tape and the heads relied on a central pad, as it had previously, the pad was smaller than before, with a width of 4-5mm. It was therefore necessary to reduce the gap between the heads to around 2-3mm, which required advanced head processing technology.

In the 1970s, the design and manufacturing technology for such electronic devices was quite advanced in Japan, meaning it was possible to produce a superior combination head (Fig. 10.9).

The combination head had to accurately integrate the respective track heights (positions) for recording and playback, while at the same time eliminating any difference in head protrusion (front position) and also preventing azimuth loss by minimising the relative angle deviation between the two gaps.

Accuracy had to be maintained on all these fronts during manufacturing and assembly. Another type of head – the “independent suspension type” – required such precise processing methods as well as azimuth adjustment following assembly (Fig. 10.10).

11 Noise Reduction Systems

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11.1 Introduction of Noise Reduction Systems

Compared to the popular open-reel tape recorders, the early Compact Cassette machines performed too poorly at recording music to be placed among the ranks of serious audio equipment.

Chapters 8 to 10 discuss the active development undertaken to improve the heads and the magnetic substance on the tape in order to boost the recording performance.

However, one major factor in Compact Cassette machines gaining acceptance as proper audio equipment was the success of the noise reduction systems that began to be used in household tape recorders as a means of increasing the dynamic range by expanding the signal to noise ratio (S/N).

When listening to sound recorded on tape, there was sometimes a bothersome hissing noise. This was often particularly noticeable when the music or audio was very quiet (low level); this is called a hiss noise and is quite audible because it largely comprises high-frequency components.

The simplest way to reduce this noise is to cut out the high frequencies with a flter; however, this also cuts out the high-frequency component of the signal (music), resulting in a muffed sound with a narrower frequency response.

One characteristic of the human ear is that background noise is very audible when listening to low-level sounds, although it is hardly noticeable when listening to loud sounds.

It is possible to utilise this effect when recording quiet sounds by increasing the volume during recording and then decreasing the volume during playback, effectively playing back the sound in its original state and suppressing only the noise.

In other words, this is a process of compressing/expanding the sound during the recording and playback processes. The US company Dolby Laboratories, Inc. applied this principle and perfected a system that could be put to use in a business tape recorder.

In 1966, British company Decca introduced the system for their in-house master tapes. This system, called the Dolby A, split the 20Hz-20kHz frequency range into four bandwidths, gaining a 10-15dB improvement in the S/N ratio through compression and expansion in each bandwidth.

11.2 Dolby B

The Dolby A system worked by compressing/expanding the entire sound bandwidth. While this method was easy to implement, it meant that the whole bandwidth would be manipulated even if only one frequency band had a high-level signal. This meant that the background noise would audibly fuctuate, resulting in a sound that was unnatural to the ear.

This side effect was a relatively minor problem if there were favourable noise conditions to begin with, but the strict Compact Cassette standards meant that they were very noise-prone. This made the sound distorted and muddy, which was diffcult to correct by a masking effect alone.

The Dolby A system was also complex in its circuitry, with the frequency range divided up and different operations for each bandwidth. Another issue was that this also cost a lot.

The sensitivity of the human ear to different frequencies is well understood through the so-called “Fletcher-Munson curves”; there is actually a limited bandwidth that the ear hears well.

Fig. 11.1 shows that humans are sensitive to sounds around 1-6kHz and less sensitive to higher or lower sounds. The Dolby B system achieved effective noise reduction by targeting and compressing/expanding sounds in this range of sensitivity as a countermeasure for the tape recorder hiss noise.

Erklärungen zu den Grafiken

Fig. 11.2 shows the playback frequencies when the whole-bandwidth level compression method is used. The signal is expanded irrespective of frequency. In such cases, if there is a high-level signal at particular frequency, the entire bandwidth is subject to compression/expansion, resulting in less effective noise reduction.

By contrast, Fig. 11.3 shows the playback frequencies when the “sliding-band” method adopted by the Dolby B system is used. The high-frequency range is expanded a fixed amount according to signal level. Even if there is a loud signal in the low- and mid-frequency range, a fixed amount of attenuation is ensured in the high-frequency range, which also suppresses other side effects, such as breathing*.

* When a signal-compressing/expanding noise reduction system such as the Dolby encounters a signal that varies greatly at a certain frequency, the noise reduction effect fluctuates as compression/expansion is carried out in response to this signal. This results in a fluctuation in background noise, which is unnatural to the ear. This phenomenon is called breathing.

By keeping compression/expansion to a minimum, the Dolby B system prevented other side effects and was also relatively cost effective. As a consequence, it became widely used as a noise reduction system for the Compact Cassette. However, since it manipulated the signal in a relatively moderate manner, the resulting noise reduction effect was not all that large, usually around a 10dB improvement in the S/N ratio.

As the digital audio era approached, further competition ensued to develop more effective noise reduction systems. Fig. 11.4 shows a comparison between the effect produced by the Dolby B and the more effective Dolby C, which was introduced later.

11.3 Growing Popularity of Dolby Noise Reduction (NR)

The Dolby B noise reduction (NR) system was frst released as a standalone unit, but it soon became incorporated into cassette decks. The first cassette deck with an inbuilt Dolby system was put out by TEAC in 1971 (Fig. 11.5 - TEAC A350). The Dolby B system suddenly grew in popularity as a standard cassette deck feature, with Sony releasing its first cassette deck with an inbuilt Dolby system, the TC-2250SD (Fig. 11.6), in 1972.

Following the development of chrome tape, the Dolby NR system played a huge part in improving recording and playback performance. It later became monumental in tape recorder development history as having elevated the Compact Cassette from a memo recorder to a serious piece of audio equipment.

While compressing/expanding noise reduction systems such as the Dolby are expected to have a greater effect and ensure a better sound quality than simply cutting noise with a flter, tapes with compressed recordings on them lose their compatibility, as they must be expanded and played in a predetermined manner.

When the Dolby B came to the fore, it had no obvious rivals; it had the good fortune to take hold as the standard system in a relatively smooth manner. While Dolby Laboratories, Inc. started out in technology development, the company also became well-known for its successful licencing business model, having employed some clever technology agreement strategies when the Dolby NR system became popular.

Although various other systems were later developed for noise reduction and some market competition ensued, the Dolby B continued to be used throughout as the standard NR system. The fact that its successor, the Dolby C, kept a number of rivals at bay and maintained its position as the standard system was probably due to a sense of security associated with the name “Dolby”.
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