In motional feedback the mechanical vibrations of the loudspeaker cone are the source of the feedback voltage. Feedback then improves the over-all response characteristic and reduces the total distortion. The theory of this method is presented here in a simplified, though enlightening, way. The treatment is based on an unorthodox theorem on impedance conversion by feedback.
A linear treatment is developed which will predict the conversion efficiency, power output, and power dissipation for sinusoidal Class B push-pull operation of an imperfect device such as a silicon power transistor. Normalized expressions for conversion efficiency, power outputs, and power dissipation as functions of the ratio of load to dc saturation resistance are determined. The results are of particular interest to amplifier designers. Limiting cases are considered in order to indicate the established expressions for the ideal device. Calculated and measured data agree well enough to justify the use of the theory for practical purposes.
A new, inexpensive permanent magnet material is now available which has a higher magnetic energy than any other ceramic magnet material previously known. It is a modified strontium ferrite. One of its most important applications is in moving coil loudspeakers. Sixteen widely used gap configurations, including the nine Electronic Industries Association standards, with gap energies between 0.1 and 20 mega-ergs are described for which strontium-ferrite rings are used. The leakage permeances, the flux densities inside the magnet, and the permeance of the return circuit are estimated. The process of successive approximations is used to calculate the dimensions of the strontium-ferrite rings. It is found that systems with strontium ferrite are smaller, simpler, and lighter in most cases than equivalent systems with Alnico-5 magnets. This is mainly due to the much lower permeability of strontium ferrite as compared to that of Alnico-5. The low permeability makes limb leakage negligibly small in compact strontium-ferrite structures. Strontium-ferrite speaker systems are particularly suitable for low-cost applications and whenever high gap energies and high gap densities are needed.
The simple magnetic recording model which describes the basic phenomena in terms of the growth and decay of a cylindrical magnetization volume from one side of the tape coating is reviewed. The relevance of practical tape hysteresis loops is considered and a model hysteresis loop developed and applied to the description of the recording process. Areas of applicability for the recording model are described including distortion in ac-bias recording.
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.
Since judgement of distance to a sound source is vague, whereas lateral localization is about 2°, stereo tests were conducted with a single row of sound sources. Live sounds were observed and recorded outdoors; reproduction indoors utilized speakers on a 25-foot array. To express the results a criterion of error was computed which ranged from 0.042 for live listening to 0.60 for 2-loudspeaker stereo without toe-in. Three-loudspeaker arrays with toe-in exhibited criterion-of-error values of 0.11 to 0.14 with no significant difference between 2 and 3 electrical channels.
This paper describes a highly-directional electropneumatic loudspeaker system capable of radiating any type of warning signal in the 100-1000 cps range as well as speech. The mechanism of the electropneumatic device is described in detail. Field measurement data on the directional characteristics, maximum range, and the effect of wind, which we gathered on tests performed at Point Montara, Calif., are presented. Aspects of signal duration, threshold, and new requirements for fog signal resulting from new trends in coastal shipping patterns, as for example, the increase in private craft, are discussed.
A tone arm, capable of tracking phonograph records at 2 grams, for use in record changers is described. The low tracking force is accomplished by introducing an additional degree of freedom in the arm. An equivalent circuit analysis of the new design demonstrates its greater stability in comparison with conventional systems. Such matters as alignment of pivots for minimum frequency modulation and reduced damage to records resulting from accidental scraping are discussed.
The results of a study to determine the feasibility of using the PME effect in semiconductors as a pickup means for information stored on magnetic tape are presented, as well as pertinent information on the performance of a completed device. Because the output voltage of the device is proportional to the product of the instantaneous magnetic flux and the light intensity, reproduction of frequencies down to zero cycles per second is theoretically possible, as well as scanning speeds approaching the speed of light. A partial study of the factors affecting the PME voltage in germanium is also presented.
The audio-radar monitor recorder is designed to continuously monitor and record 16 audio channels and one radar display on a single reel of magnetic tape. The tape is 2 in wide and moves at the speed of 15/16 ips. Each reel has a time capacitance in excess of 24 hours. The radar recording is accomplished by means of a quantizing system that separates the video signal into 14 low-frequency channels. The PPI radar display is recorded out to a 30 mile range. Range definition between 0 and 20 miles is 0.2 miles and between 20 and 30 miles is 0.33 miles. The azimuth resolution is 1°. During playback, a PPI display, complete with range marks, is shown on the face of a scope. Stationary magnetic-head and slow-speed tape assure trouble-free operation and long life.
The pickup described will track with a vertical force of 2 grams, on simple but properly designed tone arms. It has a mechanical compliance 6 cm × 10 -6 /dyne and an effective mass of about 1.4 mg. Output is 0.35 volt. A combination of optimum rubber properties and viscous damping provide a response free of resonances. The design is well suited for quantity production for the home phonograph market. Another paper describes successful tracking of this cartridge in commercially available record changers. In a pickup destined for quantity production, several design factors must be met in order to maintain uniform high performance. It is important that the two ceramic elements be correctly positioned during assembly. In the present design this is done by means of octagonal rubber element mounts which are placed into two semioctagonal channels on mating interfaces of two plastic housing parts. In any two-element stereophonic pickup design, a "coupler" is needed to join the two elements to a single stylus arm. Design of the coupler has important bearing on distortion, output level, channel separation, and mechanical impedance. Coupler requirements in the present design have been met by proper configuration and choice of material. Another important design factor is the means provided for pivoting the stylus level arm. For lateral motion, at the stylus, compliance must be high for lateral motion in two degrees of freedom. Rotational compliance, about the lever axis, must be low in order to avoid undesirable rotation of the lever arm. There must be no lateral freedom at the pivot point. Longitudinal compliance of the level arm must be low in the direction of drag from the record groove to avoid frequency modulation effects. The stylus lever mounting arrangement described achieves these design requirements in a simple assembly. A light tube, comprising the lever arm, is flattened at its pivoted end in a manner to produce tapered shoulders. These shoulders are brought to bear against a hole in a small rectangular metal enclosure which is filled with a plastic material having viscoelastic properties. The potting arrangement damps any lever resonances and provides a small restoring force to position the lever for engaging the coupler during change of styli.
Flux sensitive heads using thin semiconductors have been developed for playback of magnetic recordings. The elements have a frequency response inherently flat from dc to tens of megacycles, so that the main limitations are in the associated head structure. Factors entering into optimum design are discussed, including materials and configurations of semiconductor elements and cores. Experimental results for successful designs are given, and a number of applications are suggested.
Crossover distortion in a class B transistor power amplifier can be greatly reduced by driving the output stage from a high-impedance source. Doing this capitalizes upon the fact that the current-to-current gain characteristic of a class B stage is much more linear in the crossover region than the voltage-to-current gain. High-impedance drive can be most easily applied to a complementary output stage, but can also be applied to two transistors of the same polarity if a driver transformer is used. In circuits of this type, no temperature-compensated bias arrangements are necessary, and "thermal runaway" is virtually impossible, as only one of the output transistors can be biased on at a time. Moreover, reverse bias is applied to the "off" transistor, reducing its turn-off time and minimizing the increase in power supply drain at high signal frequencies. Amplifiers designed according to this principle have operated with very low distortion and have exhibited unequalled thermal stability.