固定ヘッドPCM磁気録音機の試作

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.