Search

A "compatible" stereophonic record is one which will reproduce full stereophonic sound when played on a stereophonic phonograph and will be indistinguishable from an LP record when played on a monaural phonograph. Such a record may be obtained by recording the sum signal S=L+R as lateral modulation and the difference signal D=L-R as vertical modulation and suitably modifying D so that the tracking possibilities of monaural pickups are not exceeded.

A literature study and listening tests have been conducted to contribute to an understanding of the stereophonic effect as a function of frequency. The literature study failed to reveal tests showing loss of stereophonic direction for any part of the audio spectrum and pointed to arrival time difference of the transient portion of sound waves as the significant contributor to stereophonic perception. Tests employing actual program material with a specially developed Stereo Spectrum Selector showed the extreme lower frequencies to have equal or superior directional content to the higher frequencies. The perception loss at any frequency may be of a quantitative nature rather than strictly related to certain wavelengths. Test results and consideration for future developments suggest adoption of full frequency stereophonic systems.

A considerable number of observations as well as formal experiments in High Fidelity Stereo sound are reported. Three-channel stereo with corner flanking speakers has been shown to achieve accuracy in both tonality and geometry. A large center speaker with small outriggers is observed to lack both tonality and geometry. A center speaker in the corner with wall outriggers is observed to produce a deeply curved stereo geometry. Limiting speakers to a bass range down to 300 cycles may preserve geometry, but not tonality. Overly large bass speakers lead to a spatial and delay separation of bass and treble events. There is an optimum bass speaker size. Wide spacing of speakers offers improved accuracy of stereo geometry. The array cannot normally exceed the speaker spacing, but because of the focusing effect of the center channel, it may be much narrower than the speaker spacing. Numerous minor observations of radiation angle and polar response point up the desirability of corner speaker placement. Large speakers may be placed at low or high level, but the dynamic range of small speakers is limited. Corner speaker placement affects apparent room size and may be advantageously employed to improve the reverberation effects. Wide Stage Stereo demonstrations using speaker spacing up to 50 feet or more are compared to home applications with 15-foot spacing. High Fidelity Stereo is both high fidelity and stereo, and entails meeting the requirements of both.

The high-speed duplication of previously recorded magnetic tapes has made the tape radio network practical. For example, each of the two major radio networks in Mexico duplicates over 19,000 seven-inch reels of taped programs per month for their several hundred affiliated stations. Since the tapes are erasable and wire program circuits are seldom used, these networks operate very economically, and the quality of their recorded programs is unimpaired by losses in transmission. Likewise, the adoption of Automatic Programming Systems for radio broadcasting will create a demand for syndicated magnetic tape programs and tape libraries. High-speed magnetic tape duplicators are also utilized for the mass production of tape recordings for entertainment, education, churches, libraries for the blind, reviews of professional publications, and sales information for large field sales organizations. The Ampex Model S-3200 tape duplicator with ten slaves will produce up to ten copies at one time of 15, 7\frac{1}{2} , or 3\frac{3}{4} ips master recordings at 60 ips. The copies can be recorded to be reproduced either at the speed of the master or at half speed; i.e., 3\frac{3}{4} ips copies from a 7\frac{1}{2} ips master. The duplicates may be either half-track, double-track, full-track, or two-track stereophonic. Comparisons are made of the degradation of the signal-to-noise ratio, frequency response, and distortion with respect to the number of generations of the duplicates. With optimum adjustment of the equipment, it is very difficult to identify the fourth generation copy from the master recording.

Subjective listening tests have been completed which indicate that stereo in any of the forms compared is preferred over monaural, but normal stereo from two full range speakers well physically separated is, in all probability, the most preferred.

A stereophonic test record has been designed for measurement of pickup response on a curve recorder. The principles of stereophonic disk recording which have governed the design of this record, are reviewed. The new test record contains two sweep-frequency bands for the left and right channels, the signal frequency varying logarithmically with time at a rate of 1 decade each 24 seconds covering a range of 40-20,000 cps. Left and right spot-frequency tones with voice announcements have also been provided covering a range of 20-20,000 cps. Automatic recording of pickup response has been estimated to save \frac{1}{2} to \frac{1}{3} of manpower used in pickup and phonograph development. For automatic measurement of preamplifier characteristics, an RC RIAA generating network has been designed. A circuit for automatic starting of recorder by keying tones on the record is described.

Playback localization plots, similar to the technique employed by Steinberg and Snow in the Bell Laboratory experiments of 1933, afford perhaps the only means of evaluating quantitatively the performance of representative stereophonic systems. In the present tests, which deal with two-channel systems comprising a center bridged loudspeaker in addition to the two flanking loudspeakers, it is seen that the performance of wide-angle loudspeaker arrays can be optimized to give accurate localization over a large listening area. As an aid to the evaluation of test data, brief discussions of loudness and time-delay effects-the essential factors providing localization-are included.

An improved understanding of some stereophonic phenomena may be obtained by use of acoustical pressure phasors to portray sound pressure at the ears of the observer. With the help of phasors, it is possible to expand and modify certain conclusions of previous observers and to validate some previously unpublished observations: a stereophonic "law of sines" is derived. The existence and location of the "out-of-bounds" stereophonic image is analyzed and verified. The "allowed maximum out-of-phase ratio" is derived, together with the observation that this maximum is exceeded by certain microphone arrays. The motion and elevation of the center image in stereophonic reproduction is observed and explained.

Magnetic recording is recognized as a superior medium for stereophonic entertainment, but its popularity has been handicapped by inconvenience of threading and high cost. A new approach is a tape cartridge of very low cost, which is compact, and fully protects the record. The cartridge is completely automatic on a machine designed for its use, and yet will operate manually on present tape recorders.

Designed to drive two transistor-power amplifiers previously described, this preamplifier contains all the controls necessary for stereophonic and monophonic programs. The first two of the three stages form a complementary circuit readily adaptable for the various input sources, which provides the necessary gain and equalization for low-level phonograph and tape inputs, guard voltages to reduce the capacitive effects of the input cables, and impedance transformation for high-level inputs from tuners and other tube circuitry. Loudness control is achieved by the combined effect of variable negative feedback and a novel tone-control arrangement. The frequency range, including the power amplifiers, extends from below 6 cycles to above 50 kilocycles per second.

Derivation of a third playback channel from two stereo sound tracks may be accomplished by several means. The center channel may be derived by recombination prior to power amplification; acoustically, after amplification, by using two center speakers; and in a variety of phase relationships, including the limiting case of equal signals (monophonic) in which either sum or difference combination may be chosen by polarity selection.

The primary aim of this work has been to develop a stereophonic system for recorded music, using a small, inexpensive and practical cartridge with magnetic tape as the information carrier. In order to make the cost of the recorded cartridge comparable with a disk containing an equivalent amount of music, it was realized that basic developments in magnetic recording such as efficiency of recording and reproducing techniques were necessary to meet the packing density requirements. This paper describes developments which have led to a recorded cartridge one fifth of the volume of a disk and capable of playing more than one hour of stereophonic sound uninterrupted. In order to obtain the desired signal-to-noise ratio and frequency response to 15 kc, radical improvements have been made to tape, recording system and playback head. In conjunction with this work, a fully automated tape machine has been developed. The machine is equipped with a changer-type mechanism and accommodates a number of cartridges which are played and rewound completely automatically, one after another, furnishing music for several hours. The machine requires no manual threading and has a rewind cycle of less than twenty seconds for an hour-long tape.

Following the teaching of the early Bell Telephone Laboratories' experiments in auditory perspective, wide speaker spacing is needed to realize accuracy of geometry. This results in substantially random phase, so that polarity is relatively unimportant. This is supported by Lissajous figures of two-channel stereo signals. It is still good practice, however, to observe polarities, if for no other reason than to permit monophonic reproduction over a stereophonic array. Where the stereo signals contain a strong monophonic component, correct system polarity is better than random polarity. In some stereo situations, bass is improved by correct polarities.

A system is described for home stereophonic reproduction from a single cabinet by reflection of sound from the room boundaries. The effect is emphasized by using loudspeakers which maintain a uniform cardioid directional pattern over the useful frequency range. The directional properties are obtained with acoustic phase-shift networks.

Stereophonic playback systems are studied to determine the accuracy with which they can reproduce the geometry of the original sound. Sounds were generated in a geometric pattern, recorded, and reproduced over a loudspeaker array. The methods used were similar to those used by Steinberg and Snow in 1933. Using two sound tracks, a derived center playback channel, and corner placement of flanking speakers, geometry plots were made with almost as good accuracy as when observers listened to an actual person speaking at the indicated stations in the geometric array. Since wide speaker spacing was used, corner placement becomes natural. An evaluation of corner speaker placement from the tonal standpoint (as contrasted with the geometric) shows that there is a large increase in quality available by taking advantage of the reflections of the floor and walls. It is shown that corner placement of flanking speakers and use of a derived-channel center speaker affords the best reproduction of geometry as well as tonality.

Stereophonic sound has become of vital importance to industry. The subject has been studied for many years, but the published material is scattered. This paper summarizes the fundamental theory underlying stereophonic sound so far as it has been published, and gives examples of how the theory is employed in representative practical situations. Fundamental differences between ordinary binaural listening and stereophony are pointed out, as well as similarities. It is shown that much qualitative but little quantitative information has been reported. Factors which aid some stereophonic effects are shown to be detrimental to others, and methods of minimizing the undesirable conditions are suggested. Applications to recording are discussed.

A central feature of the new design of a stereo amplifier is an output transformer with original features that makes possible reduced cost and improved performance at the same time. This paper discusses a varied possibility of design objectives for a stereo system, and explains the way in which the new output transformer functions. By variation in its method of use, or in choice of parameters, a whole range of amplifiers can apply advantages in different proportions or degrees. The basic design of an output transformer, which is essentially inexpensive to make, provides for separation between "left" and "right" as well as crossover, and combining networks for mixed lows, if desired, without additional external circuits. It makes possible a new type of tone control, achieving high performance economically, using feedback, and/or improved matching between amplifier and loudspeakers over the entire frequency range as well as better separation and efficiency than the single-ended and push-pull transformer matrix can give. One particular amplifier is discussed in detail, while a more general discussion shows possible application to more diverse design objectives.

The paper reviews briefly the history of stereophonic reproduction. The principal basic systems with their underlying ideas are described and compared. Some account is given of the supposed mechanism of natural binaural listening from the viewpoint of direction localization. The principles and practice are discussed of a particular system for domestic use, derived from the early work of Blumlein, and characterized by the use of spaced loudspeakers driven in phase, to which the name "stereosonic" has been given. The aims of this system are defined, and the mathematical theory involved in its use is developed. Limitations and sources of error in the results achieved are described. Equipment used in the making of master recordings and some of the problems of studio technique involved are described. Consideration is given to the form which a domestic stereophonic record should take, and the standards to which such a record should conform, together with the requirements which these impose on the reproducing equipment.

Good audio sound quality in stereophonic reproduction may be summarized in the "eight cardinal points of loudspeakers," which are: 1) minimal distortion, 2) optimum size, 3) avoidance of rattles, 4) avoidance of shadows, 5) avoidance of cavities, 6) wide spacing, 7) proper number, and 8) toe-in.

The stereophonic composite signal may be decoded in adaptor circuitry by time multiplex or sum-and-difference approaches. To obtain suppression of SCA (background music) signals while maintaining adequate separation of the two stereophonic audio signals double, single, or residual sideband demodulation of the stereo subcarrier can be employed. Satisfactory reception of FM stereo signals requires tuner circuitry of higher performance than for equal monophonic performance. Calculated and measured stereophonic separation, distortion, and signal-to-noise ratio are shown with major causes of these performance aspects.

The need for conserving technical manpower in broadcasting operations has spurred interest in automatic level controlling systems to achieve optimum modulation. A set of operating standards can be established so that any system of level control, manual or automatic, can be appraised. These standards temper technical and aesthetic considerations. An examination of the prior art in the field discloses the shortcomings of systems using the conventional approach and the large degree of compromise that must be accepted by their use. A completely new system, the Audimax, employing simple switching logic has been devised which satisfies the requirements of the operating standards with negligible compromise. This system is described in detail. In order to satisfy the level control problems for stereophonic broadcasting, an adapter is provided for use with two Audimax units which makes the system responsive to the sum of the left and right signals. In this way, the stereophonic perspective is maintained and maximum modulation ensured. Operational experience with the Audimax is described briefly.