CHAPTER 6
Analog Recording
CHAPTER CONTENTS
A Short History of Analog Recording
The Magnetic Recording Process
Head inspection and demagnetization
Mechanical Transport Functions
Successive editions of this book have quite naturally seen the emphasis placed more and more on digital topics and away from analog. Yet even the gramophone record has survived for rather more years than many would have predicted, and the analog open reel tape recorder, particularly in multitrack form in rock recording studios where some engineers and artists value its distortion characteristics, still enjoys a significant amount of use. An analog recording chapter continues to be justified in this edition therefore, albeit in a shortened form.
A SHORT HISTORY OF ANALOG RECORDING
Early recording machines
When Edison and Berliner first developed recording machines in the last years of the nineteenth century they involved little or no electrical apparatus. Certainly the recording and reproduction process itself was completely mechanical or ‘acoustic’, the system making use of a small horn terminated in a stretched, flexible diaphragm attached to a stylus which cut a groove of varying depth into the malleable tin foil on Edison’s ‘phonograph’ cylinder or of varying lateral deviation in the wax on Berliner’s ‘gramophone’ disc (see Figure 6.1). On replay, the undulations of the groove caused the stylus and diaphragm to vibrate, thus causing the air in the horn to move in sympathy, thus reproducing the sound — albeit with a very limited frequency range and very distorted.
FIGURE 6.1 The earliest phonograph used a rotating foil-covered cylinder and a stylus attached to a flexible diaphragm. The recordist spoke or sang into the horn causing the stylus to vibrate, thus inscribing a modulated groove into the surface of the soft foil. On replay the modulated groove would cause the stylus and diaphragm to vibrate, resulting in a sound wave being emitted from the horn.
Cylinders for the phonograph could be recorded by the user, but they were difficult to duplicate for mass production, whereas discs for the gramophone were normally replay only, but they could be duplicated readily for mass production. For this reason discs fairly quickly won the day as the mass-market prerecorded music medium. There was no such thing as magnetic recording tape at the time, so recordings were made directly onto a master disc, lasting for the duration of the side of the disc — a maximum of around 4 minutes — with no possibility for editing. Recordings containing errors were either remade or they were passed with mistakes intact. A long item of music would be recorded in short sections with gaps to change the disc, and possibilities arose for discontinuities between the sections as well as variations in pitch and tempo. Owing to the deficiencies of the acoustic recording process, instruments had to be grouped quite tightly around the pickup horn in order for them to be heard on the recording, and often louder instruments were substituted for quieter ones (the double bass was replaced by the tuba, for example) in order to correct for the poor frequency balance. It is perhaps partly because of this that much of the recorded music of the time consisted of vocal soloists and small ensembles, since these were easier to record than large orchestras.
Electrical recording
During the 1920s, when broadcasting was in its infancy, electrical recording became more widely used, based on the principles of electromagnetic transduction (see Chapter 3). The possibility for a microphone to be connected remotely to a recording machine meant that microphones could be positioned in more suitable places, connected by wires to a complementary transducer at the other end of the wire, which drove the stylus to cut the disc. Even more usefully, the outputs of microphones could be mixed together before being fed to the disc cutter, allowing greater flexibility in the balance. Basic variable resistors could be inserted into the signal chain in order to control the levels from each microphone, and valve amplifiers would be used to increase the electrical level so that it would be suitable to drive the cutting stylus.
The sound quality of electrical recordings shows a marked improvement over acoustic recordings, with a wider frequency range and a greater dynamic range. Experimental work took place both in Europe and the USA on stereo recording and reproduction, but it was not to be until much later that stereo took its place as a common consumer format, nearly all records and broadcasts being in mono at that time.
Later developments
During the 1930s work progressed on the development of magnetic recording equipment, and examples of experimental wire recorders and tape recorders began to appear, based on the principle of using a current flowing through a coil to create a magnetic field which would in turn magnetize a moving metal wire or tape coated with magnetic material. The 1940s, during wartime, saw the introduction of the first AC-biased tape recorders, which brought with them good sound quality and the possibility for editing. Tape itself, though, was first made of paper coated with metal oxide which tended to deteriorate rather quickly, and only later of plastics which proved longer lasting and easier to handle. In the 1950s the microgroove LP record appeared, with markedly lower surface noise and improved frequency response, having a playing time of around 25 minutes per side. This was an ideal medium for distribution of commercial stereo recordings, which began to appear in the late 1950s, although it was not until the 1960s that stereo really took hold. In the early 1960s the first multitrack tape recorders appeared, the Beatles making use of an early four-track recorder for their ‘Sergeant Pepper’s Lonely Hearts Club Band’ album. The machine offered the unprecedented flexibility of allowing sources to be recorded separately, and the results in the stereo mix are panned very crudely to left and right in somewhat ‘gimmicky’ stereo. Mixing equipment in the 1950s and 1960s was often quite basic, compared with today’s sophisticated consoles, and rotary faders were the norm. There simply was not the quantity of tracks involved as exists today.
MAGNETIC TAPE
Structure
Magnetic tape consists of a length of plastic material which is given a surface coating capable of retaining magnetic flux rather in the manner that, say, an iron rod is capable of being magnetized. The earliest recorders actually used a length of iron wire as the recording medium. In practice all modern tape has a polyester base which was chosen, after various trials with other formulations which proved either too brittle (they snapped easily) or too plastic (they stretched), for its good strength and dimensional stability. It is used throughout the tape industry from the dictation microcassette to the 2 inch (5 cm) multitrack variety. The coating is of a metal oxide, or metal alloy particles.
Open-reel tape
Open-reel quarter-inch tape intended for analog recorders has been available in a variety of thicknesses. Standard Play tape has an overall thickness of 50 microns (micrometers), and a playing time (at 15 inches [38 cm] per second) of 33 minutes is obtained from a 10 inch (25 cm) reel. Long Play tape has an overall thickness of 35 microns giving a corresponding 48 minutes of playing time, which is very useful for live recording work. In the past ‘Double Play’ and even ‘Triple Play’ thicknesses have been available, these being aimed at the domestic open-reel market. These formulations are prone to snapping or stretching, as well as offering slightly poorer sound quality, and should not really be considered for professional use.
Standard Play tape is almost always ‘back coated’. A rough coating is applied to the back of the tape during manufacture which produces neater and more even winding on a tape machine, by providing a certain amount of friction between layers which holds the tape in place. Also, the rough surface helps prevent air being trapped between layers during fast spooling which can contribute to uneven winding. Long Play tape is also available with a back coating, but as often as not it will be absent. It is worth noting that the flanges of a tape spool should only serve to protect the tape from damage. The ‘pancake’ of tape on the spool should not touch these flanges. Metal spools are better than plastic spools because they are more rigid and they do not warp. Professional open-reel tape can be purchased either on spools or in ‘pancake’ form on hubs without flanges. The latter is of course cheaper, but considerable care is needed in its handling so that spillage of the unprotected tape does not occur. Such pancakes are either spooled onto empty reels before use, or they can be placed on top of a special reel with only a lower flange. Professional tape machines are invariably operated with their decks horizontal. Half inch, 1 inch and 2 inch tape intended for multitrack recorders always comes on spools, is always of Standard Play thickness, and is always back coated.
THE MAGNETIC RECORDING PROCESS
Introduction
Since tape is magnetic, the recording process must convert an electrical audio signal into a magnetic form. On replay the recorded magnetic signal must be converted back into electrical form. The process is outlined in Fact File 6.1. Normally a professional tape recorder has three heads, as shown in Figure 6.2, in the order erase-record-replay. This allows for the tape to be first erased, then re-recorded, and then monitored by the third head. The structure of the three heads is similar, but the gap of the replay head is normally smaller than that of the record head. It is possible to use the same head for both purposes, but usually with a compromise in performance. Such a two-head arrangement is often found in cheaper cassette machines which do not allow off-tape monitoring whilst recording. A simplified block diagram of a typical tape recorder is shown in Figure 6.3.
FIGURE 6.2
Order of heads on a professional analog tape recorder.
FIGURE 6.3
Simplified block diagram of a typical analog tape recorder. The bias trap is a filter which prevents the HF bias signal feeding back into an earlier stage.
The magnetization characteristics of tape are by no means linear, and therefore a high-frequency signal known as bias is added to the audio signal at the record head, generally a sine wave of between 100 and 200 kHz, which biases the tape towards a more linear part of its operating range. Without bias the tape retains very little magnetization and distortion is excessive. The bias signal is of too high a frequency to be retained by the tape, so does not appear on the output during replay. Different types of tape require different levels of bias for optimum recording conditions to be achieved, and this will be discussed in bias requirements, below.
FACT FILE 6.1 A MAGNETIC RECORDING HEAD
When an electrical current flows through a coil of wire a magnetic field is created. If the current only flows in one direction (DC) the electromagnet thus formed will have a north pole at one end and a south pole at the other (see diagram). The audio signal to be recorded onto tape is alternating current (AC), and when this is passed through a similar coil the result is an alternating magnetic field whose direction changes according to the amplitude and phase of the audio signal.
Magnetic flux is rather like the magnetic equivalent of electrical current, in that it flows from one pole of the magnet to the other in invisible ‘lines of flux’. For sound recording it is desirable that the tape is magnetized with a pattern of flux representing the sound signal. A recording head is used which is basically an electromagnet with a small gap in it. The tape passes across the gap, as shown in the diagram. The electrical audio signal is applied across the coil and an alternating magnetic field is created across the gap. Since the gap is filled with a nonmagnetic material it appears as a very high ‘resistance’ to magnetic flux, but the tape represents a very low resistance in comparison and thus the flux flows across the gap via the tape, leaving it magnetized.
On replay, the magnetized tape moves across the head gap of a similar or identical head to that used during recording, but this time the magnetic flux on the tape flows through the head and thus induces a current in the coil, providing an electrical output.
Equalization
‘Pre-equalization’ is applied to the audio signal before recording. This equalization is set in such a way that the replayed short-circuit flux in an ideal head follows a standard frequency response curve (see Figure 6.4). A number of standards exist for different tape speeds, whose time constants are the same as those quoted for replay EQ in Table 6.1. Although the replayed flux level must conform to these curves, the electrical pre-EQ may be very different, since this depends on the individual head and tape characteristics. Replay equalization (see Figure 6.5) is used to ensure that a flat response is available at the tape machine’s output. It compensates for losses incurred in the magnetic recording/replay process, the rising output of the replay head with frequency, the recorded flux characteristic, and the fall-off in HF response where the recorded wavelength approaches the head gap width (see Fact File 6.2) Table 6.1 shows the time constants corresponding to the turnover frequencies of replay equalizers at a number of tape speeds. Again a number of standards exist. Time constant (normally quoted in microseconds) is the product of resistance and capacitance (RC) in the equivalent equalizing filter, and the turnover frequency corresponding to a particular time constant can be calculated using:
FIGURE 6.4
Examples of standardized recording characteristics for short-circuit flux. (NB: this is not equivalent to the electrical equalization required in the record chain, but represents the resulting flux level replayed from tape, measured using an ideal head).
Table 6.1 Replay equalization time constants
| Tape speed | Time constants (μS) | ||
| ips (cm/s) | Standard | HF | LF |
| 30 (76) | AES/IEC | 17.5 | - |
| 15 (38) | IEC/CCIR | 35 | - |
| 15 (38) | NAB | 50 | 3180 |
| 7.5(19) | IEC/CCIR | 70 | - |
| 7.5(19) | NAB | 50 | 3180 |
| 3.75 (9.5) | All | 90 | 3180 |
| 1.875(4.75) | DIN (Type I) | 120 | 3180 |
| 1.875(4.75) | DIN (Type II or IV) | 70 | 3180 |
FIGURE 6.5
Examples of replay equalization required to correct for the recording characteristic (see Figure 6.4), re play-head losses, and the rising output of the replay head with frequency.
FACT FILE 6.2 REPLAY HEAD EFFECTS
The output level of the replay head coil is proportional to the rate of change of flux, and thus the output level increases by 6dB per octave as frequency rises (assuming a constant flux recording). Replay equalization is used to correct for this slope.
At high frequencies the recorded wavelength on tape is very short (in other words the distance between magnetic flux reversals is very short). The higher the tape speed, the longer the recorded wavelength. At a certain high frequency the recorded wavelength will equal the replay-head gap width (see diagram) and the net flux in the head will be zero, thus no current will be induced. The result of this is that there is an upper cut-off frequency on replay (the extinction frequency), which is engineered to be as high as possible.
Gap effects are noticeable below the cut-off frequency, resulting in a gradual roll-off in the frequency response as the wavelength approaches the gap length. Clearly, at low tape speeds (in which case the recorded wavelength is short) the cut-off frequency will be lower than at high tape speeds for a given gap width.
At low frequencies, the recorded wavelength approaches the dimensions of the length of tape in contact with the head, and various additive and cancelation effects occur when not all of the flux from the tape passes through the head, or when flux takes a ‘short-circuit’ path through the head. This results in low-frequency ‘head bumps’ or ‘woodles’ in the frequency response. The diagram below summarizes these effects on the output of the replay head.
The LF time constant of 3180μs was introduced in the American NAB standard to reduce hum in early tape recorders, and has remained. HF time constants resulting in low turnover frequencies tend to result in greater replay noise, since HF is boosted over a wider band on replay, thus amplifying tape noise considerably. This is mainly why Type I cassette tapes (120μs EQ) sound noisier than Type II tapes (70μs EQ). Most professional tape recorders have switchable EQ to allow the replay of NAB- and IEC/ CCIR-recorded tapes. EQ switches automatically with tape speed in most machines.
Additional adjustable HF and LF EQ is provided on many tape machines, so that the recorder’s frequency response may be optimized for a variety of operational conditions, bias levels and tape types.
THE TAPE RECORDER
Studio recorder
Professional open-reel recorders fall into two categories: console mounted and portable. The stereo console recorder, intended for permanent or semipermanent installation in a recording studio, outside broadcast truck or whatever generally sports rather few facilities, but has balanced inputs and outputs at line level (no microphone inputs), transport controls, editing modes, possibly a headphone socket, a tape counter (often in real time rather than in arbitrary numbers or revs), tape speed selector, reel size selector, and probably (though not always) a pair of level meters. It is deliberately simple because its job is to accept a signal, store it as faithfully as possible, and then reproduce it on call. It is also robustly built, stays aligned for long periods without the need for frequent adjustment, and will be expected to perform reliably for long periods. A typical example is pictured in Figure 6.6.
FIGURE 6.6 A typical professional open-reel two-track analog tape recorder: the Studer A807-TC. (Courtesy of FWO Bauch Ltd.)
The inputs of such a machine will be capable of accepting high electrical levels — up to at least +20dBu or around 8 volts — so that there is virtually no possibility of electrical input overload. The input impedance will be at least 10 kΩ. The outputs will be capable of driving impedances down to 600 ohms, and will have a source impedance of below 100 ohms. A facility will be provided for connecting a remote control unit so that the transport can be controlled from the mixing console, for instance.
Its semi-professional counterpart will be capable at its best of a performance that is a little inferior, and in addition to being smaller and lighter will sport rather more facilities such as microphone inputs and various alternative input and output options. Headphone outlets will be provided along with record-level meters, source/ tape monitor switching, variable output level, and perhaps ‘sound on sound’-type facilities for simple overdub work. The semi-professional machine will not usually be as robustly constructed, this being of particular concern for machines which are to be transported since rough treatment can easily send a chassis askew, causing misalignment of the tape transport system which will be virtually impossible to correct. Some chassis are constructed of pressed steel which is not very rigid. A casting is much better.
The professional portable tape machine, unlike its console equivalent, needs to offer a wide range of facilities since it will be required to provide such things as balanced outputs and inputs, both at line and microphone level, phantom and A–B mic powering, metering, battery operation which allows usefully long recording times, the facility to record timecode and pilot tone for use in TV and film work, illumination of the important controls and meters, and possibly even basic mixing facilities. It must be robust to stand up to professional field use, and small enough to be carried easily. Nevertheless, it should also be capable of accepting professional 10 inch (25 cm) reels, and adaptors are usually available to facilitate this. A lot has to be provided in a small package, and the miniaturization necessary does not come cheap. The audio performance of such machines is at least as good as that of a studio recorder. A typical commercial example is pictured in Figure 6.7.
FIGURE 6.7 A typical professional portable two-track recorder: the Nagra IV-S. (Courtesy of Nagra Kudelski (GB) Ltd.)
The multitrack machine
Multitrack machines come in a variety of track configurations and quality levels. The professional multitrack machine tends to be quite massively engineered and is designed to give consistent, reliable performance on par with the stereo mastering machine. The transport needs to be particularly fine so that consistent performance across the tracks is achieved. A full reel of 2 inch tape is quite heavy, and powerful spooling motors and brakes are required to keep it under control. Apart from the increased number of tracks, multitrack machines are basically the same as their stereo counterparts and manufacturers tend to offer a range of track configurations within a given model type. Alignment of course takes a lot longer, and computer control of this is most welcome when one considers that 24 tracks implies 168 separate adjustments!
A useful feature to have on a multitrack recorder is an automatic repeat function or autolocate. The real-time counter can be programmed so that the machine will repeat a section of the tape over and over again within the specified start and end points to facilitate mixdown rehearsals. Multitrack recorders will be equipped with a number of unique features which are vital during recording sessions. For example, sync replay (see Fact File 6.3), gapless, noiseless punch-in (allowing any track to be dropped into record at any point without introducing a gap or a click) and spot erasure (allowing a track to be erased manually over a very small portion of tape).
FACT FILE 6.3 SYNC REPLY
The overdubbing process used widely in multitrack recording requires musicians to listen to existing tracks on the tape whilst recording others. If replay was to come from the replay head and the new recording was to be made onto the record head, a recorded delay would arise between old and new material due to the distance between the heads. Sync replay allows the record head to be used as a replay head on the tracks which are not currently recording, thus maintaining synchronization. The sound quality coming off the record head (called the sync head in this mode) is not always as good as that coming off the replay head, because the gap is larger, but it is adequate for a cue feed. Often separate EQ is provided for sync replay to optimize this. Mixdown should always be performed from the replay head.
Some manufacturers have optimized their head technology such that record and replay heads are exactly the same, and thus there is no difference between true replay and sync replay.
FIGURE 6.8 The available dynamic range on an analogue tape lies between the noise floor and the MOL. Precise figures depend on tape and tape machine.
MAGNETIC RECORDING LEVELS
It has already been said that the equivalent of electrical current in magnetic terms is magnetic flux, and it is necessary to understand the relationship between electrical levels and magnetic recording levels on tape. The performance of an analog tape recorder depends very much on the magnetic level recorded on the tape, since at high levels one encounters distortion and saturation, whilst at low levels there is noise (see Figure 6.8). A window exists, between the noise and the distortion, in which the audio signal must be recorded, and the recording level must be controlled to lie optimally within this region. For this reason the relationship between the electrical input level to the tape machine and the flux level on tape must be established so that the engineer knows what meter indication on a mixer corresponds to what magnetic flux level. Once a relationship has been set up it is possible largely to forget about magnetic flux levels and concentrate on the meters. Fact File 6.4 discusses magnetic flux reference levels.
FACT FILE 6.4 MAGNETIC REFERENCE LEVELS
Magnetic flux density on tape is measured in nanowebers per meter (nWbm−1), the weber being the unit of magnetic flux. Modern tapes have a number of important specifications, probably the most significant being maximum output level (MOL), HF saturation point and noise level. (These parameters are also discussed in Chapter 18.) The MOL is the flux level at which third-harmonic distortion reaches 3% of the fundamental’s level, measured at 1 kHz (or 5% and 315 Hz for cassettes), and can be considered as a sensible peak recording level unless excessive distortion is required for some reason. The MOL for a modern high-quality tape lies at a magnetic level of around 1000nWbm−1, or even slightly higher in some cases, and thus it is wise to align a tape machine such that this magnetic level corresponds fairly closely to the peak level indication on a mixer’s meters.
A common reference level in electrical terms is 0dBu, which often lines up with PPM 4 or −4VU on a mixer’s meter. This must be aligned to correspond to a recognized magnetic reference level on the tape, such as 320nWbm−1. Peak recording level, in this case, would normally be around 8dBu if the maximum allowed PPM indication was to be 6, as is conventional. This would in turn correspond to a magnetic recording level of 804nWbm−1, which is close to the MOL of the tape and would probably result in around 2% distortion.
There are a number of accepted magnetic reference levels in use worldwide, the principal ones being 200, 250 and 320nWbm−1. There is 4dB between 200 and 320nWbm−1, and thus a 320nWbm−1 test tape should replay 4dB higher in level on a meter than a 200nWbm−1 test tape. American test tapes often use 200nWbm−1 (so-called NAB level), whilst German tapes often use 250nWbm−1 (sometimes called DIN level). Other European tapes tend to use 320nWbm−1 (sometimes called IEC level). Test tapes are discussed further in the main text.
There is currently a likelihood in recording studios that analog tapes are being under-recorded, since the performance characteristics of modern tapes are now good enough to allow higher peak recording levels than before. A studio which aligned PPM 4 to equal 0dBu, in turn to correspond to only 200nWbm−1 on tape, would possibly be leaving 4–6dB of headroom unused on the tape, sacrificing valuable signal-to-noise ratio.
WHAT ARE TEST TAPES FOR?
A test tape is a reference standard recording containing pre-recorded tones at a guaranteed magnetic flux level. A test tape is the only starting point for aligning a tape machine, since otherwise there is no way of knowing what magnetic level will end up on the tape during recording. During alignment, the test tape is replayed, and a 1 kHz tone at the specified magnetic flux level (say 320nWbm−1) produces a certain electrical level at the machine’s output. The output level would then be adjusted for the desired electrical level, according to the studio’s standard (say 0dBu), to read at a standard meter indication (say PPM 4). It is then absolutely clear that if the output level of the tape machine is 0dBu then the magnetic level on tape is 320nWbm−1. After this relationship has been set up it is then possible to record a signal on tape at a known magnetic level — for example, a 1 kHz tone at 0dBu could be fed to the input of the tape machine, and the input level adjusted until the output read 0dBu also. The 1 kHz tone would then be recording at a flux level 320nWbm−1.
Test tapes also contain tones at other frequencies for such purposes as azimuth alignment of heads and for frequency response calibration of replay EQ (see below). A test tape with the required magnetic reference level should be used, and it should also conform to the correct EQ standard (NAB or CCIR, see ‘Equalization’, p. 179). Tapes are available at all speeds, standards and widths, with most being recorded across the full width of the tape.
TAPE MACHINE ALIGNMENT
Head inspection and demagnetization
Heads and tape guides must be periodically inspected for wear. Flats on guides and head surfaces should be looked for; sometimes it is possible to rotate a guide so that a fresh portion contacts the tape. Badly worn guides and heads cause sharp angles to contact the tape which can damage the oxide layer. Heads have been made of several materials. Mu-metal heads have good electromagnetic properties, but are not particularly hard wearing. Ferrite heads wear extremely slowly and their gaps can be machined to tight tolerances. The gap edges can, however, be rather brittle and require careful handling. Permalloy heads last a long time and give a good overall performance, and are often chosen. Head wear is revealed by the presence of a flat area on the surface which contacts the tape. Slight wear does not necessarily indicate that head replacement is required, and if performance is found to be satisfactory during alignment with a test tape then no action need be taken.
Replay-head wear is often signified by exceptionally good high-frequency response, requiring replay EQ to be reduced to the lower limit of its range. This seems odd but is because the replay gap on many designs gets slightly narrower as the head wears down, and is at its narrowest just before it collapses!
Heads should be cleaned regularly using isopropyl alcohol and a cotton bud. They should also be demagnetized fairly regularly, since heads can gradually become slightly permanently magnetized, especially on older machines, resulting in increased noise and a type of ‘bubbling’ modulation noise in the background on recordings. A demagnetizer is a strong AC electromagnet which should be switched on well away from the tape machine, keeping it clear of anything else magnetic or metal. This device will erase a tape if placed near one! Once turned on the demagger should be drawn smoothly and slowly along the tape path (without a tape present), across the guides and heads, and drawn away gently on the far side. Only then should it be turned off.
Replay alignment
Replay alignment should be carried out before record alignment, as explained above. The method for setting replay and record levels has already been covered in the previous section. HF tones for azimuth adjustment normally follow (see Fact File 6.5). The test tape will contain a sequence of tones for replay frequency response alignment, often at 10 or 20 dB below reference level so that tape saturation is avoided at frequency extremes, starting with a 1kHz reference followed by, say, 31.5Hz, 63Hz, 125Hz, 250Hz, 500Hz, 2kHz, 4kHz, 8kHz and 16kHz. Spoken identification of each section is provided. As the tape runs, the replay equalization is adjusted so as to achieve the flattest frequency response. Often both LF and HF replay adjustment is provided, sometimes just HF, but normally one should only adjust HF response on replay, since LF can suffer from the head bumps described in Fact File 6.2 and a peak or dip of response may coincide with a frequency on the test tape, leading to potential misalignment. Also full-track test tapes can cause ‘fringing’ at LF, whereby flux from the guard band leaks on to adjacent tracks. (Although it seems strange, replay LF EQ is normally adjusted during recording, to obtain the flattest record-replay response.)
FACT FILE 6.5 BIAS ADJUSTMENT
Bias level affects the performance of the recording process and the correct level of bias is a compromise between output level, distortion, noise level and other factors. The graph below shows a typical tape’s performance with increasing bias, and it can be seen that output level increases up to a point, after which it falls off. Distortion and noise go down as bias increases, but unfortunately the point of minimum noise and distortion is not quite the same as the point of maximum output level. Typically the optimum compromise between all the factors, offering the best dynamic range, is where the bias level is set just slightly higher than the point giving peak output. In order to set bias, a 10kHz tone is recorded at, say, 10dB below reference level, whilst bias is gradually increased from the minimum. The output level from the tape machine gradually rises to a peak and then begins to drop off as bias continues to increase. Optimum bias is set for a number of decibels of fall-off in level after this peak — the so-called ‘overbias’ amount.
The optimum bias point depends on tape speed and formulation, but is typically around 3dB of overbias at a speed of 15ips (38cms−1). At 7.5ips the overbias increases to 6dB and at 30ips it is only around 1.5dB. If bias is adjusted at 1kHz there is much less change of output level with variation in bias, and thus only between 0.5 and 0.75dB of overbias is required at 15ips. This is difficult to read on most meters.
Fact File 6.6 Azimuth Alignment
Azimuth
Azimuth describes the orientation of the head gap with respect to the tape. The gap should be exactly perpendicular to the edge of the tape otherwise two consequences follow. First, high frequencies are not efficiently recorded or replayed because the head gap becomes effectively wider as far as the tape is concerned, as shown in the diagram (B is wider than A). Second, the relative phase between tracks is changed.
The high-frequency tone on a test tape (8, 10, or 16kHz) can be used with the outputs of both channels combined, adjusting replay azimuth so as to give maximum output level which indicates that both channels are in phase. Alternatively, the two channels can be displayed separately on a double-beam oscilloscope, one wave being positioned above the other on the screen, where it can easily be seen if phase errors are present. Azimuth is adjusted until the two sine waves are in step. It is advisable to begin with a lower-frequency tone than 8kHz if a large azimuth error is suspected, since there is a danger of ending up with tracks a multiple of 360° out of phase otherwise.
In multitrack machines a process of trial and error is required to find a pair of tracks which most closely represents the best phase alignment between all the tracks. Head manufacturing tolerances result in gaps which are not perfectly aligned on all tracks. Cheap multitrack machines display rather wider phase errors between various tracks than do expensive ones.
Azimuth of the replay head is normally adjusted regularly, especially when replaying tapes made on other machines which may have been recorded with a different azimuth. Record-head azimuth is not modified unless there is reason to believe that it may have changed.
Height
Absolute height of the head should be such that the center of the face of the head corresponds with the center of the tape. Height can be adjusted using a test tape that is not recorded across the full width of the tape but with two discrete tracks. The correct height gives both equal output level from both channels and minimum crosstalk between them. It is also possible to buy tapes which are only recorded in the guard band, allowing the user to adjust height for minimum breakthrough onto the audio tracks. It can also sometimes be adjusted visually.
Zenith
Zenith is the vertical orientation of the head with respect to the surface of the tape. The head should neither lean forwards towards the tape, nor lean backwards, otherwise uneven wrap of the tape across the surface of the head results causing inconsistent tape-to-head contact and uneven head wear. Zenith is not normally adjusted unless the head has been changed or there is reason to believe that the zenith has changed.
Wrap
Wrap is the centrality of the head gap in the area of tape in contact with the head. The gap should be exactly in the center of that portion, so that the degree of approach and recede contact of the tape with respect to the gap is exactly equal. Uneven frequency response can be caused if this is not the case. Wrap can be adjusted by painting the head surface with a removable dye and running the tape across it. The tape will remove the dye over the contact area, and adjustments can be made accordingly.
Record alignment
The frequency response of the machine during recording is considerably affected by bias adjustment, and therefore bias is aligned first before record equalization. The effects and alignment of bias are described in Fact File 6.5. It is wise to set a roughly correct input level before adjusting bias, by sending a 1 kHz tone at reference level to the tape machine and adjusting the input gain until it replays at the same level.
After bias levels have been set, record azimuth can be adjusted if necessary (see Fact File 6.5) by recording an HF tone and monitoring the now correctly aligned replay output. It may also be necessary to go back and check the 1 kHz record level if large changes have been made to bias.
Record equalization can now be aligned. Normally only HF EQ is available on record. A 1kHz tone is recorded at between 10 and 20 dB below reference level and the meter gain adjusted so that this can be seen easily on replay. Spot frequencies are then recorded to check the machine’s frequency response, normally only at the extremes of the range. A 5 kHz tone, followed by tones at 10 kHz and 15 kHz can be recorded and monitored off tape. The HF EQ is adjusted for the flattest possible response. The LF replay EQ (see above) can similarly be adjusted, sweeping the oscillator over a range of frequencies from, say, 40Hz to 150Hz, and adjusting for the best compromise between the upper and lower limits of the ‘head bumps’.
Some machines have a built-in computer which will automatically align it to any tape. The tape is loaded and the command given, and the machine itself runs the tape adjusting bias, level and EQ as it goes. This takes literally seconds. Several settings can be stored in its memory so that a change of tape type can be accompanied simply by telling the machine which type is to be used, and it will automatically set its bias and EQ to the previously stored values. This is of particular value when aligning multitrack machines!
Once the tape machine has been correctly aligned for record and replay, a series of tones should be recorded at the beginning of every tape made on the machine. This allows the replay response of any machine which might subsequently be used for replaying the tape to be adjusted so as to replay the tape with a flat frequency response. The minimum requirement should be a tone at 1 kHz at reference level, followed by tones at HF and LF (say 10kHz and 63Hz) at either reference level (if the tape can cope) or at −10 dB. The levels and frequencies of these tones must be marked on the tape box (e.g. ‘Tones @ 1kHz, 320nWbm−1 (=0dB); 10kHz and 63Hz @ −10 dB). Designations on the tape box such as ‘1kHz @ 0VU’ mean almost nothing, since 0VU is not a magnetic level. What the engineer means in this case is that he/she sent a tone from his/her desk to the tape machine, measuring 0VU on the meters, but this gives no indication of the magnetic level that resulted on the tape. Noted on the box should also be an indication of where peak recording level lies in relation to the 1 kHz reference level (e.g. ‘peak recording level @ 8dB above 320nWbm−1), in order that the replay chain can be set up to accommodate the likely signal peaks. In broadcasting, for example, it is most important to know where the peak signal level will be, since this must be set to peak at PPM 6 on a program meter, corresponding to maximum transmitter modulation.
When this tape comes to be replayed, the engineer will adjust the replay level and EQ controls of the relevant machine, along with replay azimuth, to ensure that the recorded magnetic reference level replays at his or her studio’s electrical reference level, and to ensure a flat response. This is the only way of ensuring that a tape made on one machine replays correctly on another day or on another machine.
MECHANICAL TRANSPORT FUNCTIONS
Properly, mechanical alignment of the tape transport should be looked at before electrical alignment, because the electromagnetic performance is affected by it, but the converse is not the case. Mechanical alignment should be required far less frequently than electrical adjustments, and sometimes it also requires rather specialized tools. Because most mechanical alignments are fairly specialized, and because they differ with each tape machine, detailed techniques will not be covered further here. The manual for a machine normally details the necessary procedures. Looking at the diagram in Figure 6.9, it can be seen that the tape unwinds from the reel on the left, passes through various guides on its way to the head block, and then through various further guides and onto the take-up reel on the right. Some tape guides may be loaded with floppy springs which give on the instant of start-up, then slowly swing back in order to control the tension of the tape as the machine starts. The capstan is the shaft of a motor which pokes up through the deck of the machine by a couple of centimeters or so (more of course for multitrack machines with their increased tape widths) and lies fairly close to the tape when the tape is at rest, on the right-hand side of the head block. A large rubber wheel will be located close to the capstan but on the opposite side of the tape. This is called the pinch roller or pinch wheel. The capstan motor rotates at a constant and carefully controlled speed, and its speed of rotation defines the speed at which the tape runs. When record or play is selected the pinch roller rapidly moves towards the capstan, firmly sandwiching the tape in between the two. The rotation of the capstan now controls the speed of tape travel across the heads.
FIGURE 6.9 Typical layout of mechanical components on the deckplate of an analog open-reel recorder.
The take-up reel is controlled by a motor which applies a low anticlockwise torque so that the tape is wound on to it. The supply reel on the left is also controlled by a motor, which now applies a low clockwise torque, attempting to drag the tape back in the opposite direction, and this ‘back tension’ keeps the tape in firm contact with the heads. Different reel sizes require different degrees of back tension for optimum spooling, and a reel size switch will usually be provided although this is sometimes automatic. One or two transports have been designed without pinch rollers, an enlarged diameter capstan on its own providing speed control. The reel motors need to be rather more finely controlled during record and replay so as to avoid tape slippage across the capstan. Even capstan-less transports have appeared, the tape speed being governed entirely by the reel motors.
When fast wind or rewind is selected the tape is lifted away from the heads by tape lifters, whilst spooling motors apply an appropriately high torque to the reel which is to take up the tape and a low reverse torque to the supply reel to control back tension. The tape is kept away from the heads so that its rapid movement does not cause excessive heating and wear of the tape heads. Also, very high-level, high-frequency energy is induced into the playback head if the tape is in contact with it which can easily damage speakers, particularly tweeters and HF horns. Nevertheless, a facility for moving the tape into contact with the heads during fast spooling is provided so that a particular point in the tape can be listened for.
Motion sensing and logic control is an important feature of a modern open-reel machine. Because the transport controls are electronically governed on modern machines, one can go straight from, say, rewind to play, leaving the machine itself to store the command and bring the tape safely to a halt before allowing the pinch wheel to approach the capstan. Motion sensing can be implemented by a number of means, often either by sensing the speed of the reel motors using tachometers, or by counting pulses from a roller guide.
The tape counter is usually driven by a rotating roller between the head block and a reel. Slight slippage can be expected, this being cumulative over a complete reel of tape, but remarkably accurate real-time counters are nevertheless to be found.
RECOMMENDED FURTHER READING
Jorgensen, F., 1995. The Complete Handbook of Magnetic Recording, fourth edition. McGraw-Hill.
See also ‘General further reading’ at the end of this book
USEFUL WEBSITES
www.taperecorder.co.uk: A UK source for spare parts and other information.
www.servicesound.com: A US source for spare parts and information.
www.reelprosoundguys.com: A US source for spare parts and information.