TAPE consists of a magnetic coating bonded to a base of some plastic. The coating contains a tremendous number of minute "domains," each of which behaves like a bar magnet.
Recording and playback processes
When the tape is in an unmagnetized condition, the domains point in random directions. Their magnetic fields cancel and there is very little net magnetic flux on the tape. When subjected to the magnetic flux emanating from the record head, the domains align themselves in accordance with the polarity of the head flux.
In short, the tape becomes magnetized. The amount of audio current through the record head, and thus the strength of the head's magnetic field, determines the number of domains that become aligned in a given direction at any instant (or point on the tape). If the current is great enough, all the domains are aligned. The tape is saturated in the sense that further increases in audio current do not produce proportionate increases in the tape's magnetization. Obviously, saturation results in distortion inasmuch as the signal recorded on the tape cannot correspond to the signal entering the record head.
In playback, the magnetic flux produced by the aligned domains enters the gap of the playback head and passes through its core, thereby inducing a voltage in the winding. This happens provided the magnetic flux is a changing one and the playback gap is small compared with the length of the flux field (wavelength) on the tape.
The use of an alternating "bias" current is of fundamental importance to the recording process. The bias current is fed into the record head simultaneously with the audio current to accomplish two things: (1) reduce to an acceptably low level the distortion that occurs; (2) raise the level of the recorded signal; other wise a relatively tremendous output of magnetic flux from the record head would be required to put a useful amount of signal on the tape.
Demagnetization
A phenomenon of vital importance that occurs during recording-varying somewhat among brands and kinds of tape-is de magnetization. This is one of the principal factors responsible for the high-frequency losses that take place in the recording process.
A sine wave recorded on tape is equivalent to a series of bar magnets. Each magnet equals half of a wave; one pole corresponds to the upper peak of the wave and the other pole to the lower peak; from peak to peak is half a cycle. As the audio frequency increases, the number of bar magnets formed on a given length of tape increases. For example, at a frequency of 750 hz per second and a tape speed of 7.5 ips, the wavelength of 1 cycle in inches is 7.5/750, or 0.01 inch. In other words, there are 100 wavelengths per inch, which means 200 magnets, each 0.005 inch long. But at a higher frequency of 7,500 hz and a tape speed of 7.5 ips, the wavelength of 1 cycle in inches is 7.5/7,500, or 0.001. In other words, there are 1,000 wavelengths per inch, which means 2,000 bar magnets, each 0.0005 inch long. As a bar magnet becomes shorter, magnetic strength decreases because the opposite 'poles tend to cancel; the closer the poles are to each other, the greater the cancellation. This is the demagnetization effect.
Therefore, in magnetic recording, as the frequency increases, the amount of magnetic flux recorded on the tape decreases al though the strength of the magnetic flux applied to the tape by the record head remains the same.
On the other hand, in playback, the core of the playback head acts as a sort of magnetic "keeper" at the shorter wavelengths, serving to neutralize opposite poles and thereby tending to eliminate the cancelling effect. This restores, somewhat, high-frequency output. However, such restoration is far short of the loss caused by self-demagnetization and may be essentially ignored.
Magnetic recording properties of tape
Tape has two fundamental magnetic properties, retentivity (Br) and coercivity (He). This is illustrated in Fig. 309 in Section 3. The drawing shows how the flux density (B) induced in the tape by the record head varies with the magnetizing force (H). Strictly speaking, the term B represents "surface induction," which refers to the density of the flux at right angles to the tape, this being the amount of flux that enters the playback head. Retentivity is the flux that remains in the tape after it is saturated and the magnetizing force is returned to zero. Thus (as shown in Fig. 309) after the tape's flux is brought to point c (saturation), when the magnetizing force H returns to zero, B does not also go to zero but remains, instead, at the level denoted by Br.
To reduce the remaining flux to zero, a magnetizing force of opposite polarity to the original force is required. When a magnetizing force of opposite polarity attains the magnitude He, B is reduced to zero. He therefore is a measure of the force necessary to remove Br.
The values of Br and H,, vary with the nature of the magnetic coating found on each brand and kind of tape. Large values of retentivity are desirable for they increase the amount of flux re corded on the tape and therefore increase the signal output of the tape, particularly at low and mid-range frequencies. At high frequencies, however, greater values of Br accomplish relatively little in the way of increasing signal output. Instead, the important factor at high frequencies tends to be coercivity-the ability of the tape to withstand record losses. Inasmuch as these losses are most pronounced in the treble range, coercivity plays a role in determining treble response.
Coercivity principally affects high-frequency response while retentivity helps determine low-frequency response. Therefore, the ratio of coercivity to retentivity, measured in the appropriate units, is an index of the relationship between high- and low-frequency response. The ability of tape recorders to achieve ex tended treble response at relatively low tape speeds is in substantial part due to the fact that manufacturers of tape have been able to increase the ratio of coercivity to retentivity, at the same time maintaining a high value of retentivity to insure high output.
The thickness of the magnetic coating is another factor in determining how well high-frequency response compares with low frequencies. The recorded flux on the tape penetrates the coating to a greater depth at low frequencies than at high ones. There fore a thick coating tends to augment output at low frequencies.
Conversely, a thin coating tends to be detrimental to low frequencies, which means a relative improvement in high-frequency response.
There are variations in B, among brands and kinds of tape and, as a result, for a given amount of signal input to the record head, the resulting output from the tape will vary. Within a reel of good-quality tape, the output variation in the low- and mid-frequency range is ordinarily ±1/2 db, which is insignificant for audio purposes. From one reel to another of the same kind and same manufacturer, the variation may be on the order of-1:1 db, which is still quite satisfactory. However, between brands there may be differences as great as 3 or 4 db. Consequently, if two brands of tape are spliced into the same reel, there may be a sudden and noticeable change in playback level!
Effect of recording level
Distortion on the tape is primarily determined by the strength of the magnetic field produced by the record head rather than by the amount of signal which gets recorded. Consequently, it is not feasible to compensate losses in the amount of signal re corded at high frequencies simply by increasing treble boost in the record amplifier. Before such treble losses can be completely compensated, the strength of the magnetic field applied to the tape is sufficient to cause excessive distortion.
The permissible record current and, therefore, the amount of magnetic flux which can be applied by the record head to the tape for a given level of distortion varies somewhat with recorded wavelength. Translating wavelength into frequency at a speed of 15 ips, f = S/A, where f is the frequency in hz per second, S is tape speed in ips, and A is wavelength in inches per cycle.
The permissible record current tends to be constant over part of the low range and throughout the mid-frequencies. In the vicinity of 2,000 hz, the permissible record current starts to increase until it is something like 4 or 5 db higher at 15,000 hz. At a speed of 7.5 ips, this means that the rise begins at 1,000 hz and is some 4 or 5 db higher at 7,500 hz. At the low end, below 100 hz or so, the permissible record current tends to drop a few db. The relationship between permissible record current and wavelength (which may be translated into frequency) varies somewhat among brands and kinds of tapes.
When all the magnetic domains on the tape are aligned in accordance with the magnetic flux produced by the record head, there can be no further increase in the amount of signal recorded on the tape. This is the saturation point. However, distortion becomes appreciable at a signal level several db below that which produces saturation. It is important to distinguish between harmonic and intermodulation distortion, because IM distortion can become extremely severe at input signal levels which permit harmonic distortion to remain at a fairly acceptable point.
Fig. 401. Measurements of input level are based on the peak value of
the test signals. The bias current is approximately optimum for the machine
and tape used in making these measurements. 0-db input level corresponds
approximately to a 0 reading on the tape recorder's VU meter. (Source:
Audio, October, 1956.)
Fig. 401 indicates how harmonic and IM distortion vary with input signal and, consequently, with the magnetic flux recorded on the tape. It can be seen that IM distortion begins to rise considerably earlier than harmonic distortion and at a much faster rate. Thus, under the particular recording conditions present when the data in Fig. 401 were measured (tape used, bias setting, frequency equalization, etc.), harmonic distortion was only about 2%-not an unacceptable figure-while IM distortion was at a very pronounced level of 35%. Thus, when a recordist attempts to gain a few db of signal-to-noise ratio by increasing signal input to the record head, he runs the very serious risk of gaining it at the cost of a vast increase in IM distortion.
Tapes differ in their distortion characteristics. A test made by the authors, using five brands of tape, showed the following minimum amounts of IM distortion obtainable by varying the bias current until minimums were reached. A relatively high signal input was employed so that pronounced indications of IM distortion would be obtained.
Minimum IM
Tape Distortion (%)
A 7.6
B 9.0
C 11.0
D 10.0
E 3.5
While tape E seems preferable from the point of view of distortion characteristics, the final decision on which tape to use must include additional factors such as the tape's high-frequency response relative to the low and mid-ranges, whether the point at which high-frequency response begins to fall is very critical or fairly broad, and the tape's noise characteristics.
Tape noise and imperfections
In a tape recorder with heads well shielded to prevent hum pickup and with a well-designed and constructed amplifier, the principal component of noise will be that produced by the tape.
Such noise takes two forms: tape hiss and "modulation noise." When a tape is in the erased condition, its magnetic domains point in random directions. Although random orientation causes the magnetic fields of the domains to cancel, this cancellation, be cause it is random, is not complete. The small remaining amount of magnetization corresponds to an infinite number of frequencies more or less equally distributed through the sonic and ultrasonic spectrums. This is a form of noise known as tape hiss.
Inasmuch as the upper part of the audio spectrum contains more frequencies per octave than does the lower part, more noise energy is present in the upper octaves than the lower ones. Consequently the noise takes on a high-frequency characteristic, although it contains low-frequency components as well.
Modulation noise arises partly from the fact that the tape's coating is not perfectly homogeneous from a magnetic point of view; that is, the magnetic particles are not evenly distributed throughout the resinous binder but tend to cluster somewhat.
Also, it is partly due to the fact that the coating is not perfectly uniform in thickness, either because of defects in the coating or irregularities in the base material. Both the magnetic and physical irregularities are translated into irregularities in recorded flux when the tape is subjected to a magnetic field in recording. These flux irregularities constitute noise.
The modulation noise, so to speak, is "developed" in the presence of a magnetic field, which may be either ac or dc. Fortunately, modulation noise is ordinarily greatest when the record signal is greatest and least when the signal is smallest, so this noise does not become more obtrusive during quiet passages.
Since both the dc and ac magnetic fields can produce modulation noise, it is important to avoid any dc components in addition to the audio signal. Such dc components can arise in various ways from a magnetized head or an asymmetrical bias or erase wave form, which, in effect, contains a dc component.
A common tape imperfection, but one that manufacturers have been overcoming with increasing success, is that of "dropouts." This refers to small areas on the tape where the coating is too thin or contains an insufficient amount of magnetic material, resulting in sudden and brief decreases in output level.
Tape erasure
As seems reasonable to expect, the loudest signals recorded on the tape are the hardest to eradicate. Also, the mid-range and low frequencies, for a given recorded level, are the most difficult for the erase head to remove. Therefore, particularly if the tape has been recorded at an excessively high level, it is often necessary to employ a bulk eraser to obtain complete erasure. The bulk eraser-a relatively large ac electromagnet directly powered by the 117-volt 60-cycle line-can erase an entire reel in a few seconds.
Erasure is more difficult for recorded tapes that have been stored a long time or at warm temperatures. A "memory" effect has also been noted; that is, after a tape has been erased and stored, a slight amount of the erased signal reappears. This memory effect may be reduced by placing the tape in a warm place immediately after erasure and erasing again at a later date. How ever, for most audio recording purposes memory effect is not great enough to be noticeable.
Signal transfer
Signal transfer, sometimes referred to as print-through, post echo or pre-echo, refers to the fact that the signal on one layer of tape may appear on one or more adjoining layers above or be low it on the reel. That is, the magnetic flux on the tape may be strong enough to magnetize adjoining sections of the tape through which this flux passes.
Since print-through is typically some 50 db below the original signal, signal transfer is unlikely to be noticeable at a moderate record level. But as the record level goes up, signal transfer increases at an even faster rate. Therefore, a high recording level (often found on commercial prerecorded tapes) or peaks on a normally recorded tape are likely to result in signal transfer. The pre-echo or post-echo that one sometimes hears on a phonograph record might be due to the original master tape, although the phenomenon is also common in the disc recording process.
Elevated temperatures can produce a substantial amount of signal transfer. So can the presence of magnetic fields although, of course, a sufficiently strong field will cause erasure. In fact, it is possible to copy, more or less successfully, one tape from an other by running them together (magnetic coating against magnetic coating) through a properly adjusted magnetic field. Signal transfer increases slightly with storage, but not by an important amount.