Types and types of magnetic tapes. Tape
Is it true that magnetic tape with a working layer of chromium dioxide wears out magnetic heads with a permalloy core faster?
Indeed, the chrome dioxide working layer has a higher hardness than gamma iron oxide and has an increased abrasive effect on the head. On the one hand, its greater hardness makes it possible to achieve ideal polishing with higher smoothness than that of gamma iron oxide. In addition, it is necessary to take the so-called running-in period, during which the abrasiveness of the belt is most pronounced, after which the abrasiveness decreases sharply (the working surface of the belt is, as it were, polished) and further wear of the head core occurs very slowly.
Tests of various tapes have shown that if for tapes with a working layer of gamma-iron oxide, the running-in period lasts 5-7 passes of a 525 m long tape, then for chrome dioxide tape it usually stops after the second pass. Therefore, a magnetic tape with a working layer of chromium dioxide, which has a high degree of initial polishing, wears out the head core at a speed of 4.76 cm/s no less than a tape with a working layer of gamma iron oxide.
To reduce the abrasiveness of the tape, you can artificially break it in. To do this, you need to take a strip of steel grade 20 - 40 with a width of 3.5 mm, anneal it well, bend it to the body of the universal head, stick a piece of yarn inside and, putting the strip on the head, pass several passes of the tape in both directions. After this, the abrasiveness of the tape is noticeably reduced.
Can a tape with a working layer of chromium dioxide be used in tape recorders designed to work with a tape whose working layer is made of gamma iron oxide?
Chrome dioxide tape requires higher bias and erase currents, as well as an increased recording current and modified frequency response correction in the high-frequency part of the operating range compared to a tape with a working layer of gamma iron oxide. In order for the tape recorder to work with tapes whose working layers are made of different magnetic powders, a switch is introduced into the circuit that changes the recording, bias and erase currents when moving from one tape to another, and also changes the frequency response correction. In some simple tape recorders, such a switch only changes the bias and erase current, which does not allow using all the positive properties of chrome dioxide tape. In tape recorders that do not have such a switch, it is not advisable to use chrome dioxide tape.
Are there any other higher quality magnetic tapes available?
The trend towards improving the quality indicators of cassette tape recorders required the creation of tapes capable of providing high parameters of the devices at low speeds. One of the first such tapes was a tape with a working layer of gamma iron oxide powder of a finer-grained structure, which had an improved polishing of the working surface. 3a due to the better fit of the tape to the head and the finer structure of the working layer powder, the dynamic range of the phonogram on such a tape is 2 - 4 dB better than on a regular one. Higher sound frequencies are recorded and reproduced better on it, which further improves the quality of the phonogram. (Foreign cassettes with such a tape were equipped with the inscription “Low noise” - small). Let us also add that its use is advisable only in cassette tape recorders at low speeds, and the hardness of the surface of the working layer makes it possible to achieve almost perfect polishing and, therefore, a better fit to the head and greater output at high frequencies.
Relatively recently, a tape with a working layer of gamma-iron oxide with a cobalt additive, which is called cobaltized, has become widespread. The main advantage of such a tape is a higher level of recording. When using it, it becomes possible to increase the tape magnetization from 250 to 320 nWb/m in reel-to-reel tape recorders and from 160 to 250 nWb/m in cassette tape recorders. Such tapes also include domestic tapes of types A4309-6B, A4409-6B and A4205-ZB.
One of the varieties of tapes with a working layer of gamma iron oxide is a tape that can provide an increased dynamic range of the phonogram and a slightly higher level of high-frequency recording. Improvement in the parameters of the tape was achieved by reducing the size of the ferroparticles of the working layer (0.4 microns instead of 1 microns in a conventional tape), high density and their uniform distribution in the working layer. Abroad, such a tape was called “Super Dynamic” (SD).
The latest innovation is the so-called “metal” tape, the working layer of one of the variants of which is made on the basis of powdered pure iron. “Metal” tape has a higher coercive force than chrome dioxide and requires even higher bias and erase currents. So, for example, for such a tape, the bias should be approximately 6 dB more than for chrome dioxide, and 9 dB more than for a tape with a working layer of gamma iron oxide. For a “metal” tape at a speed of 4.76 cm/s, the magnetization level at a frequency of 12 kHz is almost 12 dB higher than for a conventional tape. The domestic industry does not yet produce such a tape.
Does the speed of magnetic tape affect the quality of recording (playback)?
Affects. To explain this, we must remember that the records TO is directly proportional to the forward speed V of the tape recording medium and inversely proportional to the recording frequency f (see p. 4). It should also be recalled that e. d.s. playback head depends on the length of the recorded oscillations and decreases as the recording wavelength approaches the effective width of the working gap of the head, and when the recording wavelength becomes equal to the width of the working gap - e. d.s. playhead will be zero. This is called "gap loss" and is described by the so-called "gap function".
It has been practically established that the minimum wavelength of effectively reproduced oscillations should be twice the effective width of the working gap of the GV. Let's illustrate this with an example. Let's say we have a magneto with a belt speed of 9.53 cm/s, in which a GW is installed with a geometric working gap width of 3 microns. Since the effective width of the working gap l is usually 20 - 25% greater than the geometric width, then l = 3-1.25 = 3.75 microns. Replacing the recording wavelength with twice the effective width of the working gap, we determine the upper frequency of the operating range f= =V/2l=95,300/7.5=12,707 Hz. This approximately upper operating frequency range (12500 Hz) is established by regulatory documents. Under the same conditions, at a speed of 19.05 cm/s, recording and playback of frequencies up to 25400 Hz is possible, and at a speed of 4.76 cm/s - up to 6347 Hz. It is also necessary to take into account the fact that as the quality indicators of tapes and magnetic heads improve, the working range of recorded and reproduced frequencies continuously expands.
It is known that the working gap of a magnetic head is characterized by its width, depth and length. What is the effect of the depth and length of the working gap on sound recording and playback?
The influence of the depth and length of the working gap (the influence of the width is described in the previous answer) of the magnetic head (Fig. 3) is not so obvious and is often not taken into account, since radio amateurs use ready-made heads with known parameters.
The length of the working gap, which is the same as the width of the head core, is determined by the width of the recording track. The use of four-track recording in modern tape recorders has led to a reduction in the core width to 1 and 0.66 mm with a magnetic tape width of 6.25 and 3.81 mm, respectively, and this, in turn, affected the residual magnetic flux of the phonogram, lowering it compared to two-track recording. Under these conditions: reducing the width of the working gap leads to a deterioration in the signal-to-noise ratio and a decrease in dynamic range phonograms. One of the ways to combat this is to increase the efficiency of the main zone and the return of the hot water by reducing the depth of the working gap.
Rice. 3. Working gap of the magnetic head and its parameters
The effectiveness of the GB is determined by the cross-section of the core in the zone of the working gap of the firebrand. The smaller the core cross-section, the higher the GB efficiency, which determines the write current required to create the required GB working gap magnetic field records. With an increase in the efficiency of the GB, the recording current can be reduced, which is important for tape recorders powered by autonomous current sources and especially cassette tape recorders.
GW recoil is e.g. . s., induced in the winding when playing a phonogram. The electromotive GW is proportional to the rate of change of the magnetic flux in the GW core and depends on the residual magnetic field of the phonogram and the parameters of the GW magnetic circuit. For effective closure of the magnetic flux of the phonogram through the GV core, and not through the working gap, it is necessary that the magnetic resistance of the GV working gap be significantly greater than the resistance of the core. For a given width of the working gap, this is achieved by reducing its depth. In modern HV and GU reel-to-reel tape recorders, the depth reaches 0.15 - 0.25 mm, and in cassette recorders - about 0.1 mm.
Reducing the gap depth entails a decrease in the durability of the head due to the abrasion of the working surface of the head by the working layer of the magnetic tape. However, modern tapes with a base made of polyethylene terephthalate and a high degree of polishing of the working surface make it possible to build tape drive mechanisms with a pressing force of the tape to the head of about 4 - 6 N (400 - 600 g) in reel-to-reel tape recorders and about 2 N (200 g) - in cassette and receiving heads up to 1000 hours or more.
What caused the increase in the nominal value of the short-circuit magnetic flux to 320 nWb/m in reel-to-reel tape recorders and to 250 nWb/m in cassette tape recorders?
The short-circuit flux of the phonogram characterizes the quantitatively but useful effect of recording and is represented through a GW core with zero magnetic resistance. The normalized value of the recording level is called nominal. It is easy to show that the recording level under these conditions largely depends on the quality of the magnetic tape. With the advent magnetic tapes with improved properties and especially high-coercivity recording tapes can be increased. The introduction of new magnetic tapes of types A4409-6B and A4205-ZB made it possible to increase the nominal value of the short-circuit flux to 320 nWb/m for a speed of 19.05 cm/s in reel-to-reel tape recorders and to 250 nWb/m for speed 4. 76 cm/s in cassette. This allows tape recorder developers to expand the -mic recording range, reduce nonlinear distortion and improve a number of other parameters of the tape recorder.
What other requirements apply to magnetic tapes?
In modern tape recorders, when the width of the recording track has become less than 1 mm, and the geometric width of the working gap of the head is approaching 1 micron, to achieve high-quality performance, a magnetic one must be used, which allows to ensure the best between the working layer of the tape and the head.
To ensure this, high elasticity of the base material of the tape is required. All newly developed tapes, especially for cassette tape recorders, are therefore made with a base of polyethylene terephthalate (trade name ""). New tapes of types A4309-6B, A4409-6B, A4205-ZB, etc. have this basis.
Another feature of the tapes is high degree polishing the working layer. With a well-polished surface of the working layer, the contact between the tape and the head is noticeably improved, the wear of the heads is reduced, the recording and playback of high frequencies is improved due to the reduction of contact losses, and the signal-to-noise ratio also increases.
Another specific quality is the absence of defects in the working layer. It is known that the tape’s own noise is determined by the composition, uniformity and homogeneity of the magnetic material of the working layer. The entry of foreign inclusions into the working layer or the appearance of microbubbles in it leads to signal loss and, likewise, to loss of information. This is especially noticeable in music recordings.
What should the signal level indicator show?
In household magnetic sound recording equipment, a built-in indicator is used to constantly monitor the level of the signal sent for recording. Since most tape recorders have a universal amplifier, the signal level indicator is turned on at its output. With separate recording and playback amplifiers and separate heads, built-in indicators allow you to monitor both the signal supplied for recording and the already recorded signal, thereby monitoring the end-to-end signal. Under these conditions, the indicator must show the values of the monitored signals, and the maximum permissible signal must correspond to the nominal recording level.
Conventional solid magnets are used to solve a huge number of applied problems, but in some cases the possibilities of their use are limited. In particular, in the production souvenir products or outdoor advertising, there is a need for flexible and flat materials. In search of the best option for such situations, users will learn what magnetic tape is. This material is one of the varieties of magnetic plastics - magnetic vinyl, cut into strips of a certain width. The tape is characterized by the following features:
Application of magnetic tape
So, we figured out what magnetic tape is. Now let's find out how it can benefit you. This flexible material is used in the following areas:
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"LP Disk"!
Connoisseurs of top-class (Hi-Fi, High-End) stereo sound still prefer vinyl records. A significant portion of amateurs and specialists in the field of sound reproduction agree that gramophone record(vinyl record, LP, vinyl) has excellent sound fullness and greater naturalism compared to CD (compact disc).
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Unfortunately, in our country production vinyl records ceased in the mid-1990s.
On this site, the sale of vinyl records is carried out strictly FROM STOCK!!! Overwhelmingly gramophone records have a diameter of 300 mm (12"" inches) and a rotation speed of 33 rpm, unless otherwise specified in the description.
Your wishes about what vinyl records If you would like to buy (order) in the future, please send it to the address indicated in the "Contacts" section. Indicate the name of the album, artist, and the subject of the letter, for example, “Desired purchase.”
To find vinyl records on the site, use the "Search" box. Gramophone records will be found even by incomplete artist name and album, provided that they are available. For example, you don't have to enter "Black Sabbath" in its entirety. Just enter the short "sabb" after which vinyl records and their prices will be presented in the form of a list. Please note that gramophone records of Soviet and Russian production may have names in both Russian and English languages. For example, the records "Pink Floyd" and "Pink Floyd" are two different names one rock band.
Vinyl records (LPs) today
Vinyl records are coming back into our lives. They are becoming popular again! Their sound is difficult to confuse with digital media. You can argue for a long time about “which is better?”, but it is enough to give one very powerful argument in favor of gramophone records: over the entire existence of the music industry, vinyl records have been the most released, especially by rock bands. Moreover, many of them have never been republished digitally. And some publications are very interesting and unique. Since their appearance on the world market, digital discs have brought with them somewhat different music - commercial.
Vinyl records are not subject to the same fate as digital discs: they are technically difficult to counterfeit and pass off as licensed ones. Their production requires expensive equipment that cannot be placed in a basement, garage or apartment. It is enough to provide statistical data at the beginning of 2009 regarding pirated CDs and DVDs released in Russia: their share reached 75 - 80% of the market. Globally, record sales are increasing little by little every year.
The best vinyl records produced in Japan. By adding special components to the plastic mass - vinylite, the Japanese managed to reduce the noise from the needle sliding along the sound grooves, which is noticeably audible during pauses between songs. These components also made it possible to minimize the appearance of electrostatic charges and increase the service life of the record. All this, naturally, affects the cost: Japanese vinyl records are the most expensive in the world.
Vinyl discs collected not only by ordinary citizens, but also by very famous people. Some music lovers have a collection of vinyl records that numbers several thousand. All this “wealth” is carefully stored on shelves, occupying space from floor to ceiling. And especially “advanced” ones measure vinyl records not by pieces, but by linear meters.
To vinyl records To produce your unique sound, you need the appropriate equipment. It is important to take into account every element of the path along which sound passes: from the needle to the acoustic systems. The final sound picture that vinyl records produce is influenced by: the pickup head (characteristics and geometric shape of the stylus), the tonearm of the player (design, availability of settings), the vinyl record player (design, type of drive, body weight), the vinyl records themselves (wear state , absence of dust and dirt), electric wires(cables), phono stage (is it there or not), stereo amplifier (tube or transistor), speaker cables, Acustic systems(design, shape, characteristics, power). All this adds up to the sound quality.
Room acoustics also affect how a recording will sound. vinyl records. Here it is necessary to take into account the volume of the room, the ratio of length, width, and height, clutter with furniture, the presence of carpets, rugs and a closing door. Not a large number of furniture and good soundproofing of the room will affect the sound quality and make listening to music more enjoyable.
Vinyl record (LP) - CD - MP3
Digital recording on disk appeared as a result of technological progress in laser technology. The new optical media had a number of advantages over the vinyl record: lighter weight, compact size, unlimited number of plays, cheaper production. All this was reflected in its name - “Compact Disc”. In the 90s of the last century, when vinyl record factories were closing in our country, a boom in CDs began. Of the gushing stream, a small part of them were licensed. The main one is fake “pirate”. At first, discs were imported from other countries, for example, Bulgaria. A little later they began to produce them clandestinely within the country.
It seemed that time vinyl records has come to an end. They began to be thrown out in large quantities... The turning point came around 2000-2003. When CD saturation occurred, people, sorting through old things, took out a stack of old vinyl records and a record player from the mezzanine. The nostalgia of how they listened to audio recordings before made them remember part of their life or experience for themselves - how it was 10-15 years ago. Those who had an ear or were involved in music at one time immediately felt how “live” and “real” the sound of vinyl records was.
The CD euphoria waned, especially with the advent of the MP3 format. Now, due to information compression, the same disc could fit 10-15 times more music than on a CD. Compression is impossible without loss of quality. Therefore, the MP3 format can be called “introductory” due to its prevalence and low cost. After all, before buying vinyl records, it is wise to first listen to the musical material you are interested in in MP3 format.
Currently, there are a large number of resources on the Internet that offer free big choice music in mp3 format: "Yandex Music", "VKONTAKTE audio recordings" and others.
The records sold by the vinyl record store "LP Disk" are mostly used. For designations, see table. 1. section "Evaluation".
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Magnetic tapes are a composition of a supporting base made of plastic material and a working layer in the form of a mixture of ferromagnetic powder with a binder. Currently, polyethylene terephthalate (lavsan), which has high strength, elasticity, moisture resistance and manufacturability, is usually used as a base. In addition to lavsan, there are tapes on acetate and other bases.
The magnetic materials used are y-iron oxide (y-Fe 2 O 3), chromium oxide (CrO 2), pure iron, cobalt compounds (Co) and some other substances. The most widely used tapes are those based on the compound y-Fe 2 O 3 , with tapes based on CrO 2 in second place in popularity. There are also varieties of tapes with iron oxide modified with cobalt, with two working layers (inner - ferrooxide, outer - chrome dioxide), etc.
After magnetization of the magnetic tape material and removal of the external magnetic field, it continues to retain residual induction. In Fig. Figure 4.25 shows magnetization curves for various materials, that is, the dependence of magnetic induction B, measured in teslas (T), on the external magnetic field strength H, measured in units of amperes per meter (A/m). The curves have a hysteretic character. As the magnetic field strength increases in the positive direction, the magnetic induction increases at first quite sharply, then the magnetization curve becomes flat and finally reaches the magnetic saturation value V n. With a subsequent decrease in the magnetic field strength H, the induction B also decreases. When the H value drops to zero, the material remains magnetized (Bremain > 0).
Rice. 4.25. Dependence of magnetic induction B on the external magnetic field strength H in various materials
Residual induction B ost is the most important characteristic magnetic tape material. The higher it is, the greater the maximum residual magnetic flux and, therefore, better characteristics This tape will provide playback recordings. The value of Hc, equal to the magnetic field strength required to change the induction from B rest to zero, is called the coercive force by induction. In addition, ferromagnetic materials are characterized by magnetic permeability μ, which shows how many times the magnetic induction in a ferromagnet is greater than in air.
To reduce nonlinear distortions and increase the residual magnetization of the tape, tape recorders use the recording of signals with high-frequency bias. Then the recorded low-frequency (sound) vibration S zp. (Figure 4.26) is summed up with the bias fluctuation S P (Figure 4.26). the frequency Pn of which is much higher than the upper sound frequency and amounts to tens of kilohertz. As a result, a signal S ZP arises (Fig. 4.26), with the help of which the range of change of the recorded audio signal is shifted to the linear section of the magnetization curve. In this case, the high-frequency oscillation itself is not recorded on the magnetic tape. The optimal value of the high-frequency bias current depends on the magnetic properties of the tape used.
Magnetic tape can be used for recording and playback repeatedly. If you do not demagnetize it before recording a new fragment of a phonogram, the recordings will overlap each other. To remove previous information, it is erased by exposing the active layer of the tape to a strong external magnetic field, as a result of which the working layer is first magnetized to saturation and then demagnetized. This field can be either variable or constant. In the first case, oscillations of an erasing and bias current generator (GSC) are used, which generates a harmonic signal, in accordance with which the magnetic field of a special erasing head changes. In the second case, the erasing head is a permanent magnet.
Very high level standardization has been achieved in the production of magnetic tapes. According to the international classification Electrotechnical Commission(IEC-IEC) magnetic tapes for audio cassettes are divided into 4 groups depending on the required values of the optimal high-frequency bias current and the parameters for correcting the amplitude-frequency characteristics of the tape paths:
- IEC 1 (IEC 1) - tape with a ferrooxide working layer (Fe 2,O 3), “regular” or “normal”;
- IEC II (IEC II) - tape with a working layer of chrome dioxide (CrO 2) or substitutes;
- IEC III (IEC III) - tape with two working layers (inner - ferrooxide, outer - chrome dioxide);
- IEC IV (IEC IV) - tape with a working layer of metal iron powder (Metal).
Rice. 4.26. Formation of a recording signal with high-frequency bias
Comparing the first two, most common, types of magnetic tapes, we can identify a number of advantages of magnetic tapes based on chromium dioxide. When used to record audio signals, the achieved signal-to-noise ratio is 12-16 dB better than when using ferrooxide-based tapes. Nonlinear distortions and self-demagnetization at high frequencies will also be less.
Shown in Fig. 4.27 magnetization curves of tapes of types I, II and IV indicate that tape of type IV (Metal) is capable of providing a significant gain in the level of the recorded signal compared to chromium dioxide and ferrooxide tapes. In addition, metal powder tapes are characterized by minimal distortion and a wide frequency range. Another advantage is their absolutely smooth surface, which significantly reduces abrasive wear of the magnetic heads. However, the cost of such tapes is significantly higher, they require a significantly higher bias current: not all household tape recorders are able to record on them due to the lack of the necessary correction circuits. In playback mode, this drawback can be ignored: cassettes with type IV (Metal) tape can be listened to without loss of quality when the tape switch is in the “CrO 2” (type II) position.
Fig. 4.27. Dependence of the third harmonic coefficient and the emf of the outflow bias of the reproducing head
Type III magnetic tapes are not widely used. As already noted, the characteristics of magnetic tape largely determine the quality of recording and playback of phonograms. The most important parameters are:
- relative sensitivity;
- the magnitude of nonlinear distortions;
- signal-to-noise ratio.
The sensitivity of a tape is characterized by the degree of its magnetization, which is defined as the ratio of the residual magnetic flux to the low-frequency field of the head created by the recording current. Simply put, the higher the tape sensitivity, the lower the gain the recording amplifier can have.
The relative sensitivity of a tape is defined as the ratio of the signal level on a given magnetic tape to a similar signal level on standard or reference tapes of the same type produced by manufacturing companies. This parameter is measured at frequencies of 315 Hz and 10 kHz and characterizes the level at which the signal is actually recorded on tape when the recording indicator is zero (it means the signal level in decibels).
Having the results of sensitivity measurements at frequencies of 315 Hz and 10 kHz, it is possible to estimate the amplitude-frequency response (AFC) of the magnetic tape. An accurate frequency response is obtained by measurements at several frequencies. The resulting curve should be straight and parallel to the x-axis in the audio frequency range, and the value at 315 Hz should be as close to 0 dB as possible. Typically, the frequency response of a magnetic tape is indicated on the insert of the tape cassette.
Changes in sensitivity are mainly determined by the uneven thickness of the working layer of the tape and the concentration of ferromagnetic powder in it. An increase in unevenness can be caused by dust, as well as wear products of the tape and magnetic heads on the surface of the working layer.
The uniformity of the frequency response of magnetic tapes is significantly affected by the magnitude of the high-frequency bias current. With optimal bias current, the highest recording level is ensured. Exceeding it beyond the optimal level causes a sharp weakening of the recording level of high sound frequencies and a slight increase in it when recording low sound frequencies. As the bias current decreases, the picture reverses. The optimal high-frequency bias current is set according to the maximum output (sensitivity) of the magnetic tape at frequencies of 400 Hz or 1000 Hz.
The unevenness of the frequency response determines the linear distortion of the signals. In addition, the magnitude of nonlinear distortions, which are the main part of the total nonlinear distortions of the magnetic recording channel, depends on the magnetic properties of the working layer and the high-frequency bias current. The greater the residual magnetization of the material, the smaller they are. To evaluate them, a parameter called the harmonic coefficient is used. , and, most often, the third harmonic coefficient K 3. Modern tapes have a K 3 value in the range of 0.4-2.2%. Approximate view The dependence of K 3 and the emf of the reproducing head E at different frequencies on the ratio of the magnitude of the bias current I p to its optimal value I p opt is shown in Fig. 4.27. At optimal choice This parameter provides some compromise between the uniformity of the amplitude-frequency response and the amount of nonlinear distortion.
Also, the amount of nonlinear distortion is affected by the correct choice of the level of the recorded signal, because increasing the recording level above the permissible level leads to overmodulation of the tape and the appearance of increased nonlinear distortion, and its decrease reduces the signal-to-noise ratio. Therefore, the recording level should be maintained at a value that achieves a compromise between the maximum possible recordable tape magnetization level.
The maximum recording level, selected in accordance with these criteria, allows us to judge the overload capacity of the tape and determines the upper limit of the dynamic range of the recording channel. The wider this range, the higher the quality of recording and playback of phonograms. Its lower limit is determined by the amount of magnetic tape noise, which depends on the magnetic state of the tape. There are several types of noise signals obtained during playback:
- pause noise;
- noise of demagnetized tape;
- magnetized tape noise;
- modulation noise.
In addition, according to the sources of origin, noise is divided into contact and structural. The former arise due to the inconstancy of the tightness of the magnetic tape to the heads, and the latter - due to the magnetic inhomogeneity of the working layer.
Rest noise is the noise of a tape that has been demagnetized by the erase head and then exposed to the high-frequency bias field of the write head. The relative noise level of a pause during playback is defined as the ratio of the tape noise voltage to the voltage corresponding to the nominal recording level.
The relative noise level of the magnetized tape is used to estimate interference, which manifests itself in the form of so-called modulation noise, which is superimposed on the recorded signal and grows with increasing amplitude. Modulation noise is determined by the uneven structure of the working layer of the tape and fluctuations in the speed of its movement. When played back, it can be heard as rustling noises. Despite the relatively low level, such noise is clearly noticeable by ear, since it is practically not affected by existing noise reduction systems.
The manifestation of the so-called copy effect depends on the magnetic properties of the tape, the thickness of the working layer, and its overall thickness. It is as follows: when storing magnetic tape in a roll (cassette, reel), highly magnetized areas can magnetize other areas of the tape adjacent to them and located on adjacent turns of the tape. During listening, this property manifests itself in the form of an echo. The influence of the copy effect is most pronounced when a copy is applied to an area with a pause. Note that there is a certain dependence of its manifestation on temperature (at elevated temperatures he is stronger). This should be taken into account when storing magnetic tapes and operating the tape recorder in specific conditions, for example in a car in the summer.
As stated above, in order to re-record a magnetic tape, the previous one must be erased. The erasability of a tape depends on its magnetic properties, but in addition, the parameters of the erasing and bias current generator, the erasing head, the previous recording mode, as well as storage conditions also influence. It is believed that when reusing magnetic tape, the old recording should be attenuated by at least 70 dB.
In addition to the magnetic properties of tapes, the quality of recording and playback of audio signals is also significantly affected by their physical and mechanical properties. These include:
- elongation (under load and residual);
- sabre;
- warping;
- roughness;
- adhesive strength;
- heat and moisture resistance;
- elasticity;
- wear resistance;
- abrasiveness.
During operation of the tape transport mechanism (TDM) and in contact with other parts of the tape recorder, for example magnetic heads, the tape is subjected to mechanical stress and itself affects the parts of the path. Particularly sensitive to increased loads thin tapes with a thickness of 9 microns (C-120), so their use on cheap tape recorders with low quality CVL performance is not recommended. The particles of ferromagnetic material that make up the working layer of the tapes have high mechanical hardness, therefore, when the surface of the tape comes into contact with the magnetic heads, both the tape itself and the heads are abraded, their working gap expands and the quality of recording/reproduction of high frequencies deteriorates.
Cassette tape recorders use magnetic tape with a width of 3.81 mm and a thickness of 18, 12 and 9 microns. In this case, naturally, a standard cassette can accommodate different amounts of tape, which, in turn, determines full time sound. The cassette labeling indicates its size: S-60, S-90, S-120 or MK-60, MK-90. Cassettes are also produced with non-standard playing times: S-30, S-45, etc. Until recently, reel-to-reel tape recorders were used in everyday life, where the tape width was 6.25 mm and the total thickness, depending on the base material, was 55 microns or 37 microns with a working layer thickness of 15 microns and 11 microns, respectively.
On a cassette recorder, during the recording process, the magnetic tape is divided into two halves (Fig. 4.28), on each of which the recording is made in one direction, and with stereo recording, information is recorded channel by channel on two tracks (right and left channels), and with monophonic recording in each direction one combined track is used, equal in width to the sum of the two tracks used in stereo mode and the space between them. This ensures compatibility of magnetic tapes recorded in Stereo and Mono modes. The body of the tape cassette must meet certain requirements to ensure the stability of the movement of the magnetic tape under external mechanical and thermal influences. For this purpose, the casings and mechanical elements of the cassettes are made of heat-resistant durum varieties plastics or ceramics. They contain:
- high-precision rigid guides;
- special stiffeners;
- additional elements of tape laying;
- special spring gaskets;
- pressing brushes made of special anti-friction and antistatic materials.
Magnetic tapes of audio cassettes are designed for operation at temperatures from -10 o C to +45 ° C.
Fig. 4.28 Placement of recording tracks on a cassette tape recorder: a – monophonic,
b - stereophonic
The tapes are characterized by three groups of indicators: physical and mechanical, magnetic and working.
Main physical and mechanical properties tapes are: load corresponding to the fluidity of the base material; residual relative elongation after removing the load, relative elongation when exposed to impact load; adhesive strength; sabreability and warping (saberability is determined by the degree of deviation of a piece of tape 1 m long, loosely laid on a flat surface, from a straight line, and warpage is determined by the degree of deformation of the surface of the tape); heat and moisture resistance.
The strength characteristics of magnetic tape are almost entirely determined by its base. The lavsan base, as a rule, provides the strength characteristics required for the tape.
Saber and warping are types of deformation of magnetic tapes that occur due to improper cutting, drying or winding them during the production process, as well as violations of storage conditions. The consequence of these deformations is a poor fit of the tape to the magnetic head, which leads to defects during recording and playback of the phonogram.
Below are the main physical and mechanical characteristics for a magnetic tape with a width of 3.81 mm on a lavsan base with a thickness of 12 microns:
Magnetic properties of tapes characterized by coercive force (ranges from 20 to 80 kA/m for various types of tapes); residual saturation magnetic flux (5-10 nWb); saturation magnetization (90 - 120 kA/m); residual saturation magnetization (70 - 100 kA/m); relative initial magnetic permeability (1.7 -2.2).
The basic magnetic properties of the tape can be determined from the magnetization curves of the working layer of the tape, which have the form of hysteresis loops. Figure 4.2 shows magnetization curves related to three different compositions of the working layer of the tape based on Fe 2 O 3, CrO 3 and metal powder. Residual induction is the most important characteristic of the magnetic tape material. The higher this indicator, the greater will be the maximum residual magnetic flux of the tape and, therefore, the greater, all other things being equal, the maximum achievable signal-to-noise ratio.
The magnetization characteristic shows that “metal” tape is capable of providing approximately a twofold gain in the level of the recorded signal compared to chromium dioxide and ferrooxide. “Metal” tapes have minimal distortion and a wide frequency range, but to realize these characteristics, special heads are required that ensure the creation of a significantly higher field strength both when recording a signal and when erasing it.
To the main performance characteristics include: relative sensitivity of the tape and its maximum level; signal-to-noise ratio; signal/echo ratio; frequency range; erasability.
Rice. 4.2. Magnetization curves of tapes with different compositions of the working layer: 1 - Fe 2 O 3 ; 2 - СrO 2; 3 - Me
Relative tape sensitivity - the ratio of the sensitivity of the test tape to the sensitivity of the primary standard tape. The sensitivity of the tape is characterized by the degree of its magnetization, which is defined as the ratio of the residual magnetic flux to low frequency field heads created by the recording field. The higher the sensitivity, the lower the gain the recording amplifier can have.
Primary standard tapes are batches of magnetic tapes with the most optimal properties, produced by leading manufacturers. They are like a standard with which the parameters of the tested tapes are compared when evaluating them. Typical tapes and their characteristics are established by the IEC - the International Electrotechnical Commission.
Uneven sensitivity is characterized by fluctuations in sensitivity along the length of the tape and depends mainly on the uneven thickness of the working layer and the concentration of magnetic powder in it, the deposition of wear products of the tape and dust on the working layer. Within one roll of magnetic tape, sensitivity unevenness should not exceed ± 0.6 dB.
Signal-to-noise ratio is determined by the ratio of the voltage of the maximum reproduced signal to the noise voltage of a tape magnetized by a constant field. Modern tapes have a signal-to-noise ratio of 57 - 62 dB.
Third harmonic coefficient - the ratio of the third harmonic voltage of the reproduced signal with a frequency of 400 Hz to the signal voltage at the output of the reproduction amplifier. The value of this parameter is usually 0.5 -3%.
