The desirability of stereo reproduction, and what had to be done to realize it, has been known for many years. However, it was only recently that definite techniques were developed and applied to hi-fi equipment. In this Section, these techniques are discussed.
Stereophonic sound equipment utilizes all the circuitry contained in monophonic equipment. In addition, the circuits and devices necessary to add one or more channels and provisions for two, three, or four different outputs must be included in stereophonic equipment.
In the past decade, the reproduction obtained from monophonic and two-channel stereophonic hi-fi equipment has reached a high state of development. The quality of single-source sound and stereo sound, with the best systems, is such that the average human is un able to detect any distortion.
BASIC COMPONENTS OF STEREO SYSTEMS
The basic components of a monophonic system are shown in Fig. 1-2. This arrangement may be adapted to multiple-channel stereo by adding another channel or channels of amplification and additional speaker systems, which will permit stereo reproduction of sound. Then, by adding a stereo phono pickup, tape deck, or multiplex adapter, the input devices can be converted for stereo operation.
Improvements in the form of adapters are available to provide one control for balancing the outputs of the separate channels, and other conveniences are available for patching up the old single-channel system to produce stereo. However, this kind of arrangement is not competitive in convenience of operation, flexibility, simplicity, or other refinements when compared with equipment designed especially for stereo operation.
STEREO RECORDING
To us, as hi-fi listeners, the manner in which recorded or broadcast programs are picked up at the source is only of academic interest.
With discs and tuners, we concentrate on reproducing the music available to us, and the studio techniques over which we have no control do not interest us. However, with the availability of multi track tape recorders for the home, there is growing interest in amateur recording, and the addition of stereo makes it much more interesting. Also, a knowledge of microphone placement problems for pickup helps us in deciding speaker placement for our reproducing system.
The most important general type of stereo pickup technique is called time-intensity pickup. This means that the signals in the two stereo channels will vary in both intensity and time according to the difference in direction and in distance of the sound source from the two microphones.
As explained in Section 2, the stereo effect can be obtained by picking up the sound with two microphones spaced a certain distance apart and located in front of the sound source. But how far should the microphones be separated? If they are too close together, the differences in time and intensity of the two signals are negligible, and the stereo effect is lost. Experiments have been made with micro phones as much as 30 feet apart. At such distances, the reproduced sound seems to the listener to divide into two sources, especially if he is close to the speakers.
Experiments with the relative spacings involved have led to the conclusion that there is a definite optimum relation between the separation of the microphones and their distance from the source.
A similar relation exists between the separation of the listener's speakers and their distance from him. This relation can be illustrated by the triangle formed by the two speakers and the listener in Fig. 3-1A. In practice the angle at the listener should not be less than 30 degrees nor more than 45 degrees. This relation keeps the distance of the listener from the speakers approximately the same as the distance between the speakers, and ensures sufficient time-intensity variation to provide stereo effect.
It is generally agreed that in the ideal two-channel system, the listener's speakers should be the same distance apart as the studio microphones. However, if we consider the size of a full symphony orchestra, and how far the microphones need to be spaced to have a good part of the orchestra between them, we realize that the same spacing at both ends of the circuit is not always practical. In practice, compromises are made. A good rule to follow is to spread the microphones as far apart as you can and still pick up an appreciable amount of sound from the middle portion of the source. Usually the pattern will be such that the microphones can be placed along the sides of a triangle as in Fig. 3-1B.
(A) Speaker placement. PROGRAM SOURCE (ORCHESTRA, CHORUS, STAGE, ETC.) (B) Microphone placement.
Fig. 3-1. Suitable locations for stereo speakers and microphones.
Another method, referred to as the intensity-difference system, is sometimes used in stereo recording. In this system, the microphones are not spaced; however, they have a directional characteristic ( Fig. 3-2). The stereo effect is obtained by proper orientation of the microphones. One method is to mount the microphones at 90 degrees to each other and 45 degrees to the center of the source, as shown in Fig. 3-3A. Another method is to use one microphone aimed directly at the source for the main signal, and a second microphone to pick up side signals. This is illustrated in Fig. 3-3B.
(A) Figure-8. (B) Cardioid.
Fig. 3-2. Typical directional-microphone pickup patterns.
Fig. 3-3. Placement of two directional microphones for stereo pickup. (A)
Two microphones mounted at 45° from center of source. (B) One microphone
for main signal, an other for side signals.
A directional microphone especially designed for stereo pickup is pictured in Fig. 3-4A. This microphone contains two directional elements, as shown in the cutaway side view of Fig. 3-4B. Each element has a cardioid pickup pattern, and the two elements are aimed 90 degrees apart. Thus, each element is aimed 45 degrees from the center of the sound source, producing the response given in Fig. 3-4C.
Fig. 3-4. A directional microphone designed for stereo pickup. (A) Photo.
(B) Construction. (C) Response pattern. Courtesy North American Philips
Corp.
DISC RECORDING
It was not until practical methods of stereo disc recording were developed that standardized stereo could reach our homes. Although other good methods have been considered, the Westrex method of stereo recording is the one adopted by the recording industry. We shall, therefore, confine our discussion here to that method.
Requirements
Let us review briefly the general requirements of a stereo disc re cording method. They are as follows:
1. Two completely independent left and right signals must be re corded separately in such a way that there is a minimum of interference or mixing between them, and so that, in playback, they can be separately recovered in the same form as that in which they were recorded.
2. The system must be compatible with monophonic recording and playback systems. A stereo playback system should play monophonic records without loss of fidelity; likewise, a mono-phonic system should play stereo records and produce as good a monophonic output as when it is playing monophonic records.
When Edison first developed phonograph recording, he used vertical motions of the recording stylus to record the sound vibrations.
This came to be known as the "hill-and-dale" method of recording.
However, in such a system, the "hills" in the record groove had to lift the playback arm and cartridge. Since early arms and cartridges were relatively heavy, this motion caused excessive wear. Because of this, the record industry adopted the lateral recording method, in which the needle goes from side to side in accordance with the sound vibrations. All commercial monophonic records for home use are laterally recorded, and monophonic pickups are designed accordingly.
The Westrex System
Fig. 3-5. Basic principle of Westrex system of stereo recording.
Fig. 3-6. Principle of operation of stereo phonograph cartridge. (A) Rest
position. (B) Movement to left. (C) Movement to right.
In the Westrex stereo recording system, both lateral and vertical motions are employed. Modem pickup and arm design is such that vertical motions are usable under the proper conditions. The basic principle of the Westrex system is illustrated in Fig. 3-5. The sound signal for each stereo channel is recorded along a direction at 45 degrees to the horizontal. In other words, each channel is recorded on one side of the record groove in such a way that its undulations will move a playback stylus back and forth along one of the 45 degree directions. The two directions are at right angles ( 90 degrees) to each other, and motions along either one alone do not affect motion along the other. Thus, the motions of one channel do not interfere with the motions of the other, and two separate output signals, each corresponding to the motions for its respective channel, are obtained from the stereo cartridge.
A symbolic representation of how the axial motions in the two 45 degree directions can generate two separate signals is given in Fig. 3-6. Imagine the assembly of the stylus and the two bar magnets free to move in any direction but not to rotate. Then, if the stylus is pushed to the left, it goes into a position like that of Fig. 3-6B. The magnet at the right merely moves across the diameter of the coil perpendicular to its axis. There is no axial motion in or out of the right coil, so no current is generated in this coil.
Now suppose that the stylus-magnet assembly is pushed to the right, as shown in Fig. 3-6C. Conditions are now the reverse of those in Fig. 3-6B. A current is generated in the right coil, but none is generated in the left coil.
Thus, vibrations recorded in the groove so that they push the stylus at 45 degrees toward the left produce pickup output current only in the left coil. Likewise, vibrations recorded in the groove so that they push the stylus at 45 degrees toward the right produce pickup output current only in the right coil.
It is difficult to imagine any case in which both left ( L) and right ( R) signals would not be present. Because of this, you may wonder how the playback needle can move in both 45-degree directions at once. Naturally, it cannot do this. Instead it moves in a direction and with an amplitude dictated by the vector resultant of the two forces along the 45-degree paths. This means the needle can move as a whole in any resultant direction, including side to side and straight up and down. Five combinations of relative L and R signals are analyzed in Fig. 3-7. Only one signal is present in Figs. 3-7 A and 3-7B. Accordingly, in each of these cases the needle moves along the direction of the single channel present. Thus if the motion is along one of the 45-degree sides of the groove, the pickup "senses" that it is receiving signal from only one channel. Fig. 3-7C illustrates the special case of equal signals from L and R. In this case, the resultant motion is always vertical, and the record "looks" to the needle like a "hill-and-dale" recording. The vertical motion indicates that both signals are equal in amplitude, and the amount of motion indicates their amplitude. Fig. 3-7D indicates the motions when the L signal is much stronger than the R signal. The larger the L signal with respect to the R signal, the farther the resultant is to the left of vertical. The angle of resultant motion "tells" the pickup the ratio between the two amplitudes, and the amplitude of the resultant indicates their amplitudes. The situation depicted in Fig. 3-7E is the same except that the R signal is stronger than the L signal. The examples in Fig. 3-7 show that for any combination of resultant needle angle and amplitude of motion, there is a distinct amplitude for both the L signal and the R signal.
(A) L signal but no R signal. (B) R signal but no L signal. (C) Equal L and R signals. (D) Strong L signal, weak R signal. (E) Strong R signal, weak L signal
Fig. 3-7, Resultant motion for various combinations of L and R components.
Since the amplitudes of the L and R signals are usually approximately equal at any instant, the conclusion to be drawn from Fig. 3-7 and the preceding discussion is that the groove modulation in stereo records is predominantly vertical. However, vertical groove modulation tends to be more distorted and produce more record wear than lateral modulation. To change the Westrex recording system to one in which groove modulation is predominantly lateral, the phase of one of the signals driving the stereo record cutter is reversed. This changes the resultant needle force from a predominantly vertical motion to one which is predominantly lateral. This method is illustrated in Fig. 3-8. Note that Figs. 3-8A, 3-8B, and 3-8C are the same as Figs. 3-7C, 3-7D, and 3-7E, respectively, except that the R vector is reversed. The Record Industry Association of America ( RIAA), which accepted the Westrex system, has stipulated that "equal in phase signals in the two channels shall result in lateral modulation of the groove." This requirement is satisfied by the phase reversal of one of the signals to the cutter as shown in Fig. 3-8; but a compensating reversal must take place at the pickup or later in the system for playback. However, this is a simple matter, because it involves only reversing the connections of one of the pairs of leads from the pickup.
Fig. 3-8. Vector addition of L and R components with one channel electrically
inverted. (A) L and R signals of equal strength. (B) Strong L signal,
weak R signal. (C) Strong R signal, weak L signal.
Fig. 3-9. Cause of the pinch effect.
Special Factors in Stereo Disc Systems
Although the phase reversal of one of the signals to the recording head makes the modulation of a stereo record predominantly lateral, the fact still remains that vertical motions play a vital part in re cording and playback. This fact brings with it some special problems in stereo pickup design.
The stereo pickup must have high vertical compliance; that is, it must allow for easy motion of the needle in the vertical direction.
If it does not, the needle exerts undue pressure on the record, causing rapid wear. A good stereo pickup is designed to have the necessary vertical compliance, but monophonic pickups are not. Thus, al though as far as motions required of the pickup needle are con concerned there is compatibility, use of a monophonic pickup on a stereo record will ruin the record after relatively few playings.
Therefore, never play stereo records with a monophonic pickup.
Another important factor is the pinch effect illustrated in Fig. 3-9.
It arises from the fact that the cutting stylus has a flat front edge.
At A in Fig. 3-9, there is no signal and the stylus is laterally motion less; the flat front of the stylus is at right angles to the direction of the groove, which at this point has its greatest width (W). However, at B modulation has been applied; the stylus still has the motion it had at A, but now it also has the lateral motion imparted by the signal applied to the cutting head. The Hat front of the stylus, with its cutting edges, now plows sideways through the record material, so the width of the groove here is W', which is considerably less than W, the width at A. Unless the tip of the playback needle is extremely fine, the narrowing at B tends to lift it out of the V-shaped groove.
This lifting motion is the same as might be experienced with vertical groove modulation. It does not affect a monophonic pickup ( except to make the record wear faster), but with the vertical-sensitive stereo pickup it introduces an unwanted signal and distortion.
To attempt to overcome the effects of pinching, the tip radius of stereo pickup needles has been reduced from the over 1-mil ( 0.001 inch) monophonic microgroove value to 0.5-0.7 mil. However, this reduction in tip diameter introduces another disadvantage-the tip pressure on the record is increased. Pressure is force per unit area, so even if the force of the needle on the record has not increased, a decrease in its area causes a corresponding increase of pressure.
Pressure is a main factor in wear on records, so the smaller needle tip results in greater wear, if the other factors remain equal. To keep wear down, stereo pickups are operated at very low needle force.
Four grams is usually considered a reasonable maximum.
The same types of pickup devices are used for stereo as for monophonic reproduction. They include crystal cartridges, ceramic cartridges, and moving-coil and moving-magnet magnetic cartridges.
The suspensions used for the needle and its associated mechanical vibrating assembly are very critical, and many clever arrangements have been worked out by the engineers.
The principle of how moving magnets are used to generate the two stereo outputs was illustrated in Fig. 3-6. Although that diagram does not correspond to any manufactured cartridge, the manufactured ones do have a small magnet ( or two) moving near a core having a pair of coils. The motion of the magnet( s) generates volt ages in the fixed coils. The pole pieces and moving magnets are oriented so that motion of the needle in one 45-degree direction causes magnetic lines of force to be cut by one coil but moves the lines of force parallel to the other coil so that no voltage is generated in it. For movement in the other 45-degree direction, the situation is reversed and voltage is generated in the other coil.
As implied by the name, in the moving-coil magnetic-type ( dynamic) pickup, the coils move and the magnet stands still. A special linkage mechanism with jeweled bearings imparts rotational motion to either or both of the coils as the needle is moved by the record undulations. Each coil responds to needle motion in one of the 45 degree directions. The coils operate in a magnetic field, so an audio voltage is generated in them when they turn.
(A) No signal. (B) Signal moves needle up and to left. (C) Signal moves needle up and to right.
Fig. 3-10. Stereo playback with ceramic elements.
Ceramic elements can also be used in a stereo pickup, as illustrated in Fig. 3-10. As a ceramic crystal is bent, a voltage is generated. As the needle moves to the left, one ceramic element is bent (Fig. 3-lOB); similarly, movement to the right bends the other element (Fig. 3-lOC). Thus, two isolated stereo outputs are provided.
Four-Channel Record Discs
The Westrex recorded disc contains two separate signals on the 45°-45° sound groove. A new system has been introduced to record and play back four discrete signals ( four channels) on a similar groove. This system, however, requires considerably expanded capability of components such as the record cutter, disc, pickup cartridge, and needle, all of which must be able to operate together with a flat response curve up to 50 kHz.
This system utilizes direct recording of frequencies up to 20 kHz for the left and right channels (similar to the standard stereo process), and a modulated 20- to 45-kHz subcarrier to carry encoded difference information for the other two channels.
The bearings (positions) of the channel inputs (microphones) and outputs ( speakers ) are as follows:
Channel One: Channel Two: Channel Three: Channel Four: Left Front Left Rear Right Front Right Rear
The sum of channels one and two is recorded on one side of the record groove, and the sum of channels three and four is recorded on the other side of the groove. Thus, when this four-channel stereo disc is played on standard two-channel stereo equipment, full compatibility of two-channel reproduction from a four-channel disc is obtained.
To reproduce four-channel sound from this same record, a special decoding device is required. The four independent channels are extracted through the use of dematrixing techniques ( addition and subtraction of signals), and then the four decoded signals are fed to a four-channel stereo system for reproduction.
Tracking Problems
Trackability distortion is produced when a phono stylus-cartridge transducer does not track, or "trace," the grooves of a record in the same manner as originally cut by the cutting stylus. This action causes all kinds of distortions to be produced, and parts of the original sound are not reproduced. As techniques of record and tape making improve, wider frequency ranges and greater transients are accurately stored in the recording mediums. These improvements create a requirement all the way through the hi-fi system for improved pickup, amplification, and reproducing techniques, and for the equipment to keep pace. However, the essence of a quality sys tem is the ability of its pickup device to provide precise dynamic coupling to the medium and accurately transduce the program material as it was recorded.
A number of factors must be just right to ensure proper tracking, that is, to ensure that the needle stays in the groove and does not cause excessive wear. Good tracking is achieved when the needle follows both sides of the groove with equal pressure on each side at all times. Tracking is much more important in stereo playback than in monophonic playback because the needle will wear one side of the groove more than the other if it is forced against the side of the groove. In stereo, wear on one side of the groove reduces the amplitude of one stereo channel with respect to the other stereo channel; therefore, if the wear is appreciable, a stereo record would be come useless much sooner than a monophonic record under the same conditions.
This wear on the sides of the grooves is another reason why the needle force of a stereo pickup must be less than that of a mono phonic pickup. But, reduction of needle force means more difficulty in maintaining tracking because the needle will sometimes have a tendency to "skate" across the record. Fortunately, the reduction of the needle-tip radius to 0.5 to 0.7 mil to overcome pinch effect ( as explained before) also improves tracking.
It can be seen from the preceding discussion that the pickup arm for stereo cannot be too carefully adjusted. It must have an absolute minimum of resistance to lateral motion so that it tracks smoothly when balanced for the recommended needle force.
Adjustment of the pickup arm and needle is easier with a "single play" turntable than with a record changer because of the absence of the complex change mechanism. With a changer, the needle must rest heavily enough in the grooves to allow the pickup arm to trigger the change mechanism. This takes a great deal more force than is necessary to move the arm across the record. In addition, because the records are stacked on the turntable during operation of a changer, the needle has a different angle for each record. Thus distortion is introduced in the reproduction, and record wear is much greater than with single-play mechanisms. This does not mean that changers cannot give top-quality reproduction, but, everything else being equal, the single-play setup is simpler to adjust and operate.
Turntables
At first thought, it would seem that the requirements for turntables and turntable drives would be the same for stereo as for monophonic systems. However, this is not so. Stereo turntable and drive requirements are more exacting because of the greater inherent sensitivity of stereo pickups to vertical vibration and the susceptibility ( in the case of magnetic pickups) to hum pickup from the motor.
Rumble--The effect produced in the sound output by low frequency signals generated by vibrations in the motor and drive systems is called rumble. These signal components usually have frequencies of from 30 to 60 Hz. Therefore, if the system as a whole does not have extended low-frequency response, rumble is not such a great problem. Thus, if your speaker system cuts off at about 100 Hz, you can stop worrying about the fine points of rumble production. However, any high-fidelity system worthy of the name reproduces signal components down to 50 Hz or below, and turntable rumble is an important factor.
A stereo pickup is more sensitive to rumble than a monophonic pickup because it is sensitive to vertical vibrations, and the vertical vibrations are usually two or three times as strong as the horizontal vibrations in a turntable drive assembly. Therefore, special measures must be taken for stereo-system turntables to minimize vibration and its effects. Otherwise, severe rumble is present in the out put signal of the pickup. Although there is no standard, rumble is usually measured with respect to a fairly strong 1000-Hz signal obtained from a standard test record. Low-level passages of music may be as much as 40 dB below the test-record output, so rumble should be at least 45 dB down. At -60 dB, rumble usually is completely inaudible, so this is a desirable objective.
In record players designed for stereo, rumble should be minimized by damping in the drive system and by the use of motors which deliver power as smoothly as possible.
Hum Pickup--The reason for the greater susceptibility of some stereo pickups to hum is the fact that there are two channels instead of one. The hum currents in the two coils of a magnetic-type stereo pickup combine in the output during operation. When a mono phonic record is being played, the hum currents can be made to cancel by connecting the coils in parallel.
The source of most hum pickup is the turntable drive motor. Induction and synchronous motors have coils carrying alternating cur rent from the power line, and thus they radiate hum. In general, the higher the motor power, the more hum is radiated. Because of this, it might seem that the motor power should be made as low as possible. However, the lower the motor power, the more difficult it is to get good speed regulation and the more likely it is that wow will be introduced. It is general practice to make the turntable relatively heavy to provide the inertia for good speed regulation. But more motor power is required to drive the heavier turntable; hence more hum is produced. Record changers require more drive power than single-play systems, so changers tend to be more subject to hum.
TAPE RECORDING
Stereo recording and playback from tape may soon be as common as stereo records, because of technical improvements, price reduction, and greater convenience in use. Many audiophiles believe that tape offers the greatest opportunity for the ultimate in high-fidelity stereo reproduction.
Although stereo tapes were available long before stereo records came into general use, certain basic problems have slowed them from enjoying a wide distribution. The first of these problems is price. At present, stereo tapes cost more than comparable discs, and they must be played on a machine of relatively high quality, which also costs more than record players providing comparable reproduction. The second basic problem, now overcome, was the inconvenience of handling. A disc can easily be put on a turntable, the pickup placed on it, and music obtained with little delay. When a tape is to be played, ordinary rolls must be carefully keyed into position, and the tape threaded through the guides and past the heads of the machine to the pickup reel. Magazine-type tapes are now available which eliminate such time-consuming operations, but they are still expensive.
In spite of the price disadvantage, tape does have many advantages. It is practically immune to wear and deterioration of quality with playing. A tape can be played thousands of times without noticeable degradation, providing reasonable care is used in its storage.
The transfer from storage on the tape to an electrical signal in the amplifier is accomplished without the necessity of mechanical parts that vibrate at sound frequencies as in phonograph pickups. Thus wear and resonance effects are minimized.
The difference between a stereo tape system and a monophonic tape system is that the stereo system uses two, four, or eight tracks recorded on the tape. The recording and playback heads each have two or four units, one for each channel. The tracks and the gaps in the head are separated by a guard band so that there is no interaction between the two signals.
The arrangement in Fig. 3-11, used with open-reel systems, allows two tracks to be recorded or played back simultaneously for two channel stereo. Four-track tape permits additional playing time for the same length of tape. The tape can be played in one direction first and then turned over and played in the other direction. The four tracks are recorded on the tape as illustrated in Fig. 3-12A. The guard bands do not need to be as large as for the two-track tape in Fig. 3-11, because in Fig. 3-12A only alternate tracks are used during tape travel in a given direction. Fig. 3-12B shows the open-reel tape format when the tape is intended for application to four-channel stereo. The tape is played in one direction only, and all four tracks are used simultaneously.
Fig. 3-11. Tracks on two-track stereo tape.
The open-reel tape system has inconveniences in handling, loading, threading, playing, and storage, and the tape is susceptible to breaking and tangling. To overcome these problems, the cartridge and the cassette have been developed. These are permanent containers in which internal spools hold the tape in a protected position, ready to play at any time.
The mechanical arrangement of the cartridge is shown in Fig. 3-13. The cartridge is primarily intended for playback of mass produced prerecorded programs, although recorders for cartridge tapes are available. Fig. 3-14 shows the tape formats for two and four-channel cartridges. The cartridge tape operates at 3¾ inches per second, in one direction only, and has eight tracks. For two channel stereo operation, the tape carries two channels of information in each of four pairs of tracks ( Fig. 3-l 4A) . The first pass plays tracks 1 and 5. Then the tape head indexes to pick up tracks 2 and 6, and this process is repeated through tracks 3 and 7 and, finally, 4 and 8. The heads then return to replay tracks 1 and 5; the machine plays continuously until shut off.
(A) Two-channel stereo. (B) Four-channel stereo.
Fig. 3-12. Track arrangements on four-track open-reel tape.
Fig. 3-13. Construction of a tape cartridge.
(A) Two-channel stereo. (B) Four-channel stereo.
Fig. 3-14. Track arrangements for cartridge tape.
The four-channel cartridge format is shown in Fig. 3-14B. In this arrangement, alternate tracks-for example, tracks 1, 3, 5, and 7 are used in one pass. Then the tape head indexes, and tracks 2, 4, 6, and 8 are used in the second pass. Only two passes are required to play the entire tape, and the available playing time for a given length of tape is only half as great as for the two-channel system.
The spacing between tracks, however, is sufficient to provide adequate separation between channels.
The cassette is a miniature reel-to-reel tape device, as shown in Fig. 3-15. Tapes are threaded permanently, and each end is coiled and attached to a hub. The capstan, pinch roller, and heads of the cassette deck can pass through holes in the plastic tape housing to operate the tape to record and play back.
The cassette is suitable for use as a home or business recording and playback medium, or it may be used for prerecorded programs.
Cassette track formats are shown in Fig. 3-16. Fig. 3-16A shows the standard licensed format for two-channel stereo; it is compatible with mono use. Figs. 3-16B and 3-16C show proposed formats for use in four-channel applications.
RECORD DEFEAT TABS TAPE VIEW WINDOW FEED HUB TAKE-UP HUB TAPE GUIDE OPENING FOR RECORD/PLAY HEAD PRESSURE PAD OPENING FOR PINCH ROLLER
Fig. 3-15. Construction of a tape cassette.
The cassette is recorded from one end of the tape to the other in one direction. Then the cassette is turned over for recording in the opposite direction. ( Some recorders reverse the head arrangement so that it is not necessary to remove the cassette, and continuous play is provided. ) In order to get two sets of four-channel stereo tracks ( four discrete channels) on the narrower tape, smaller track widths are used ( Fig. 3-16C). The location of the tracks on the cassette tape is different from the track positions for open-reel or cartridge tape. Head gaps are spaced to give the least possible cross talk between the two adjacent tracks while maintaining adequate stereo separation. Since the tracks of a stereo pair are side by side, a single, double-width mono phonic head can pick up and blend the two stereo channels. Cassette decks can incorporate rewind and fast-forward modes. Because a very low tape speed ( 1 ¼ ips) and thin tapes are used, the cassette can hold up to two hours of information (one hour in each direction). Four basic cassette recording times are available for program material: 30, 60, 90, and 120 minutes.
To prevent accidental erasure of a cassette, you may knock out the record-defeat tab ( Fig. 3-15) for either or both sides. This pre-vents most recorders from switching into the record mode. Pre recorded cassettes are delivered with the tabs removed.
Recent improvements in techniques for reducing tape noise and in chemical formulas for the manufacture of tape have opened the way for cartridges and cassettes to provide sound quality equal to that of discs. There are several new mediums such as Crolyn, titanium, Coboloy, and other formulations to replace ferric oxide.
(A) Two-channel stereo. (B) Four-channel, four-track. (C) Four-channel, eight-track.
Fig. 3-16. Track arrangement for cassette tape.
Also, several manufacturers are improving the results obtainable with present formulations by increasing their density in the application process. These materials and techniques have demonstrated an ability to hold more "bits" of information on a given area of tape than do the former ferric-oxide applications. This ability is related to the signal frequency response, the level of output, and the signal to-noise ratio. However, use of some of these new formula tapes requires a change in the level of bias necessary to be applied in the recorder. Therefore, a recorder must have a bias control or switch to be compatible with all kinds of tapes currently available. These improvements apply to all tapes, including those used in open-reel, cartridge, and cassette systems.
All stereo tape machines now use "in-line" heads, in which the gaps for adjacent channels are exactly centered along the same vertical line. However, there were some machines made with staggered heads; in these the tape passed over first one head, then the other.
Tape recorded for this arrangement is obviously not playable on the in-line-head machine. It was at first thought that staggering was necessary to prevent cross talk between the channels, but the de sired isolation is now obtained by proper spacing between the in-line gaps.
One of the advantages of tape as a stereo medium is the fact that isolation between channels is inherently much better than for phono graph pickups. Even with the small spacing between the tracks of a two-track system, 40 dB of separation is normally obtained. The desired channel separation is obtained much more easily in the four track system because in this system the head gaps have more physical separation.
STEREO BROADCASTING AND RECEPTION
We have already considered two sources of stereo high-fidelity music for the home listener: disc recordings and tape recordings. A third source is the signal of a radio broadcast station. Instead of a record player or tape machine, a tuner is used. Tuners for stereo are shown in Section 4. They are similar to single-channel tuners except for the technique of multiplexing, which will be described in this Section.
Two-Station Stereo Broadcasts
The first method used for stereo broadcasting and reception is illustrated in Fig. 3-17. Two complete transmitters were used, one sending out the left ( L) stereo signal, the other the right ( R) stereo signal of the same program. All combinations of a-m broadcast, fm broadcast, and television sound-channel transmitters have been tried. Most popular were the "fm/ am" ( one fm station and one a-m station) and the "fm/fm" (two fm stations) methods.
The two-transmitter method of transmitting stereo was convenient because it utilized existing transmitting and receiving equipment with no circuit changes. However, it did have the following disadvantages:
1. It wasted frequency spectrum space. Two station channels had to be used for each program.
2. Differences in propagation characteristics of the waves radiated by the two transmitters led to variation in signal amplitude and quality within the separate channels. This was particularly true of fm/am combinations, where the carrier frequencies are so widely separated.
3. In some cases, especially with fm/fm, the duplication of complex receiving equipment represented an excessive expense.
Ninety percent of the programs were non-stereo, requiring only one receiver; but to receive stereo, two receivers were needed.
Also, a number of am/fm tuners could not receive both bands simultaneously, and so they had to be supplemented with additional receiving equipment.
FM TRANSMITTER
Fig. 3-17. Two-transmitter method for radio transmission of stereo signals.
Stereo Multiplex
The disadvantages of the two-station method made it imperative that a method of transmitting both stereo signals on one carrier be devised. The result is the method called stereo multiplex. "Multiplex" means "a method or arrangement for sending two or more messages simultaneously on one carrier or circuit." This is exactly what stereo multiplex does-it transmits both channels on the same fm carrier.
The standard fm broadcast system is based on an audio frequency response of approximately 50 Hz to 15 kHz. There is nothing about the basic modulating system to prevent extension of this audio frequency range to 75 kHz ( further extension would require more bandwidth than is currently allotted). Therefore, if at some point above the limit of human hearing (but still within the 75-kHz band width) another signal is added, it will be amplified and detected by the regular receiver circuits. However, being above the range of hearing (supersonic), it will not interfere with the regular audio signal. Thus, the carrier is modulated by the regular audio signal plus the supersonic signal.
This supersonic modulation component is known as the subcarrier. It, in turn, is modulated by another audio signal. As long as the bandwidth of the subcarrier is not allowed to extend downward into the range of the regular carrier, no interference will occur. This complete modulated subcarrier signal is part of the composite signal which modulates the main carrier. One of the stereo signals is trans mitted as modulation of the main carrier, and the other signal as modulation of the subcarrier.
In receiving, two demodulating circuits are needed. First the main carrier is demodulated, to get the first stereo signal and the modulated subcarrier. Then the modulated subcarrier is separated and demodulated, to get the second stereo signal.
There will always be some listeners who don't care about being equipped for stereo reception. Also, for portable use, and where cost is to be minimized, a conventional single-channel receiver is called for. For these reasons, it is important that the stereo system be compatible. By "compatible" we mean that while stereo multiplex transmission is in progress a listener with a conventional receiver (one not equipped for stereo) should be able to receive the transmission as a full monophonic signal.
If one channel of the multiplex system is used for the L signal and the other for the R signal, compatibility is not provided. The owner of the conventional receiver hears only the modulation of the main carrier, which in this case would be just the L or R signal; the sub carrier frequency is above audibility, so neither the subcarrier nor its modulation is used. For a truly compatible system, the non-stereo listener should be able to hear the combination of the signals of both the L and R channels.
To meet this compatibility requirement, the main carrier can be modulated with a full monophonic signal ( L plus R), and the sub carrier channel can be modulated by the difference between the two signals ( L-R). The L + R modulation signal provides the non stereo listener with his full monophonic reception; his receiver does not respond to and thus ignores the subcarrier.
For stereo reception, the Land R signals are recovered by demodulating the subcarrier signal and adding or subtracting the difference signal to or from the main carrier ( L + R) signals. Electrical addition of the signals can be accomplished simply by applying both signals across a common impedance. Subtraction can be accomplished by first inverting one of the signals ( changing its phase by 180 degrees), then adding it to the other signal.
STEREO VERSUS HIGH FIDELITY?
One point should be emphasized: Stereo reproduction is part of high fidelity, not an "additional feature." The impression is some times wrongly given that "hi-fi'' and stereo are two separate stages of development of audio reproduction. This is a fallacy, unfortunately sometimes encouraged by a statement that some equipment features "high fidelity and stereo." Stereo is a "fine point" in high fidelity; that is, if you do not have all the good qualities of high-fidelity monophonic systems to start with, other forms of distortion will prevent appreciation of the benefits of the stereo effect. If the reproduction of your system is clean and clear of harmonic, intermodulation, and transient distortion, it is likely that improvements in spatial sound effects through stereo will be appreciated.