In order to improve the sound reproduction quality of conventional AC bias magnetic recorders, a method in which the input sound signal is coded by PCM (Pulse Code Modulation) and recorded on magnetic tape digitally has been developed. In this case, the frequency band width of the PCM signal must be several tens of that of the conventional tape recorder. Therefore, VTR has mainly been used. However, to simplify the mechanism, a fixed head system has also been proposed, in which coded digital signals are distributed onto multi-tracks. In this paper, the signal distributing method, the detection of error codes and the characteristics of an experimental PCM tape recorder are reported. Frame distribution, to minimize the influence of signal dropouts in the tape-head system and its basic design are described. In this distributing method, as shown in Figs. 1 (b), 2 and 3, the input signal is divided into frames and recorded on the magnetic tape at a reduced pulse transfer rate. The distinctive features of this method are as follows: 1) The number of error samples caused by the dropouts is less than that of the bit distribution method. 2) it is unnecessary that the number of tracks be equal to that of the coding bits. The number of tracks is determined only by the tape-head recording density limit. 3) The number of shift registers is less than that of the bit distribution method. With respect to the errors in the reproduced codes, it is observed experimentally that burst errors are dominant (Table 2). From this result, a method of group checking and processing of each frame when the errors are detected is proposed is proposed. In the group check method, "1's" included in high-rank bits in a sample are counted in binary and the lower 2 bits of this counted value are used as error check bits. However, this method is not practical when the number of error codes is a multiple of 2^2. Furthermore, modified FM is used to record the digital signal, so there is the possibility of generating meaningless random pulses in the demodulation process during the dropout interval. These pulses cause misdetection, hence the method described below was tested. Even in the case where a one-bit error is detected, the frame including the error bit is considered to be an error frame. For this reason, all bits in the frame are reset to "0", the correct sample value just before the error frame held during this interval. The result is shown in Table 3. Using this method, the probability of misdetection is reduced significantly. A system block diagram and specifications of the PCM tape recorder are shown in Fig. 5 and Table 4, respectively. By adopting independent synchronization, the influence of wow and flutter caused by tape transportation is eliminated.
In designing the miniaturization of a condenser microphone, the most important condition is the adequate miniaturization of the diaphragm which determines the external dimensions. In order to satisfy this condition, it is desirable first to determine the minimum area of the diaphragm satisfying the design target and then to determine the various constants of the microphone elements. This paper describes the design method and conditions for the DC biased directional condenser microphone studied with the above design policy. For the miniaturization of the diaphragm, determination of the adequate electrode spacing is an important condition. Furthermore, it is necessary to restrict the lower frequency limit of the front characteristics in the mechanical system because of the necessity for high equivalent stiffness of the diaphragm. First, the relations between electrode spacing and minimum diaphragm area, sensitivity, bias voltage, and constants of the mechanical system which satisfy the design target, like stability, frequency range, inherent noise, dynamic range and directivity, are analyzed. On the basis of these results, the design conditions for the electrode spacing in order to design a directional condenser microphone with minimum diaphragm is considered. Furthermore, two techniques to correct the decreasing of the front characteristics in the mechanical and electrical system, which occurs in the design of high equivalent stiffness diaphragms, are discussed, and the design conditions are obtained.
A novel electromechanical pressure transducer is proposed. Essentially, as shown in Fig. 1, it operates using the acoustical resonance due to the stiffness of a fluid in cavities and the mass of a vibrating body. This transducer is suited for application to telemetering systems because: (1) The absolute pressure, not the pressure difference, is transformed into an electrical signal. No parts of the transducer support any difference of static pressure. Therefore, error due to creep is avoidable. (2) The output signal is the frequency, not the amplitude. Pressure is transformed directly into a high-power ac signal by a simple electrical circuit. This paper includes the operation analysis of this transducer, an experimental check of its operation and the trial construction of a rather practical model. The input-output relation of this transducer, shown in Fig. 2, is given by Eq. (8) where k_V_0 is the stiffness of the cavity at pressure P=P_0. The symbols k_e and k_<ee> are the elastic support stiffness related to two respective deformation modes shown in Fig. 3. When the condition given by Eq. (10) or (12) occurs, Eq. (8) is reduced to the approximate Eq. (11) or (13). A novel electromechanical transducer having no biasing magnet is utilized in the first experimental model shown in Fig. 6. There are no large attractive forces and no fragile voice coils. Fig. 8 shows the oscillatory circuit using the first model. The input-output relation of this circuit is shown in Fig. 10, which satisfies Eq. (11). A rather practical small model was constructed using two elastic support thicknesses, 15μm and 6μm. The input-output relation of this model is shown in Fig. 12, which fits Eq. (13), because the smaller support size causes greater stiffnesses k_e and k_<ee>. The dependence of the output period of this model on temperature is shown in Fig. 13. This effect can be compensated for, just as with conventional strain-gauge type pressure transducers.
Usually, acoustical characteristics of cylindrical cavities are calculated using the onedimensional model in which are present only plane-waves travelling in the axial direction. However, the results of calculations are not always in good agreement with the results of measurements even in the range below the transverse resonant frequency. In this paper, the four-terminal constants of the co-axial cylindrical cavity are deduced by means of the velocity-pontential in the cavity in which the entrance and the exit are replaced by pistons. The velocity-potential Φ in a cavity of radius a and length l, illustrated in Fig. 1, is given by the summation of Φ_1 and Φ_2, where Φ_1 is the velocity-pontential when the exit piston is fixed and Φ_2 is the one when the entrance piston is fixed, these are given by equations (2) and (4). When the cavity is not extremely flat, and when ka is smaller than λ_1 and is not too near λ_1, where ka=λ_1 is the first transverse resonance, the sound pressure P_i(r, z_0) on the end-plate of the vibrating piston and P_i(r, z_1) on the end-plate of the fixed piston, the average sound pressure P^^^-_<ii> on the vibrating piston and P^^^-_<ii> on the fixed piston are given by equations (13), (14) and (15), respectively, where i=1 or 2 corresponding to Φ_1 or Φ_2 and i′=1 or 2 (i′≠i). An example of P_i(r, z_0) and P^^^-_<ii> is shown in Fig. 2, where P′_i(r, z_0) is the sound pressure of the one-dimensional model [eq. (16)]. Fig. 3 shows some results of calculations of G_i using eq. (12), and when ka is smaller than about 1. 5, G_i is found to have the form given by eq. (17). Fig. 4 shows g_i of eq. (17), and when a_i/a is smaller than 0. 4, g_i is found to be given by eq. (18). Since total sound pressures are given by eq. (19) and the relation of the volumevelocities is given by eq. (21) when the exit impedance is equal to zero, the four-terminal constants lead to eq. (23), and when ka<λ_1 these constants are given by eq. (25). If G_i=0, eq. (25) gives the constants of a one-dimensional model. The results of calculations are shown in Figs. 5 and 6. Fig. 7 shows the constant A obtained by various methods. Though the boundary conditions are somewhat different from each other, the results seem to be nearly equal. Total four-terminal constants of the network illustrated in Fig. 8 are given by eq. (26), where A′, B′, C′ and D′ are the four-terminal constants of the one-dimensional model and X_1 or X_2 is the reactance. By comparing eq. (25) with eq. (26), in the range below the transverse resonance, it is assumed that the co-axial cylindrical cavity is equivalent to a one-dimensional model of same dimensions which is connected by reactances X_i[eq. (27)] at each end. When ka<1. 5, X_i is equal to the reactance of a pipe of length ΔZ_i=a_ig_i connected to the entrance or to the exit. As a result, the four-terminal constants or other characteristics of co-axial cylindrical cavities cannot be calculated using only a one-dimensional model, but require a model with end reactances. In the appendix, it is shown that the exact expression of the "transmission loss" TL for cylindrical cavities is given by eq. (33), and the "noise reduction" NR is given by eq. (34) which is nearly equal to the 20 log |A|. However, the author emphasizes TL and NR cannot show the effectiveness of mufflers.
In the holographic recording of the vibrating mode of the cone of a loudspeaker which has a large diameter and a high compliance edge, reconstruction of the vibrating mode is generally impossible by reason of the random vibration of the cone driven by the external disturbance which consists of air motion and acoustic noise. For the purpose of improving this defect, a motion compensation method is already well-known. The optical arrangement of this method is indicated in Fig. 4. However, the amplitude of the cone recorded by this method cannot be computed from the mode. In the case of partial vibration, a different mode is sometimes obtained for the same vibration in accordance with a mirror position on the cone (shown in Fig. 6). In order to determine the exact vibrating mode recorded with the external disturbance, we developed three holographic recording methods. First, we attempted a time-averaged method using a soundproof box. In this method, the loudspeaker cone is freed from acoustic noise by using the soundproof box and, considering the volume of this box, air motion can be neglected. The appearance of this soundproof box is shown in Fig. 7. According to many experimental results, in high-frequency and in small-amplitude operation, such as the measurement of interference fringes, no modification of the vibrating mode could be found using this soundproof box (shown in Fig. 8). Generally, the cone vibration due to external disturbance is piston motion and its amplitude is maximum near the fundamental resonant frequency f_0. Accordingly, it can be considered that two loudspeakers with identical mechanisms are influenced identically in the same air disturbance. Fig. 10 shows the optical arrangement of the holographic recording method using a completely equal loudspeaker (the non-driven reference loudspeaker) as the object loudspeaker. In this method, since the reference beam is reflected by a mirror fixed on the cone of reference loudspeaker, its phase is modulated by the external disturbance. Therefore, when it interferes with the object beam, the effect of the external disturbance is cancelled. In Fig. 10, if the condition of Eq. (12) is satisfied, the same vibrating mode as recorded by the time-averaged method can be obtained (Fig. 11). In this case, it is difficult to prepare a reference loudspeaker having exactly the same characteristics as the object loudspeaker, but this can be approximately satisfied using a reference loudspeaker with an identical resonant frequency f_0. This method, a reference beam modulated method using a pair of loudspeakers, has the defect that reconstruction of the stationary part (the speaker holder) of the object cannot be achieved. In order to improve this defect, we developed a combined method using both a non-modulated reference beam and a phase-modulated reference beam with the reference loudspeaker. As indicated in Fig. 12, the reference beam is divided into two paths by a beam splitter. One beam is reflected by the stationary mirror and the other beam is reflected by the mirror fixed on the cone of the reference loudspeaker, and each beam interferes with the object beam on the hologram plate. Fig. 13 shows the effect of this combined recording method. This method is effective in recording the vibrating mode of a multiple loudspeaker system. Small diameter loudspeakers used as tweeters and large diameter loudspeakers used as woofers are generally influenced differently in the same external disturbance. Therefore, the complete vibrating mode of the multiple system could not be recorded on a single hologram. However, by using the combined method, the complete vibrating mode of the system could be recorded on one hologram plate and reproduced at the same time (Fig. 14).