電話機における非直線歪の通話品質評価とその応用
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概要
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According to the recent progress of integrated circuit technology, it has become easy to use active elements for a telephone speech circuit. Instead of the carbon transmitter used up to this time, the combination of a stable, small, light weight, reciprocal type transmitter and a sending amplifier will be used still more. The telephone speech circuit is fed electric power from the telephone office. So it is difficult to suply sufficient bias voltage to the amplifier. Then its circuit design has to be done carefully based on the knowledge about the relation of input-output nonlinearity of the circuit to the transmission performance. It is a disadvantage of the carbon transmitter that the response and speech resistance are rather unstable. And the carbon transmitter is not suitable for miniaturization. However, the nonlinearity of its response improves receiving AEN. But the said reciprocal type transmitter has not such characteristic. It must be studied to realize the nonlinear characteristic at the part of the sending amplifier circuit. Problems mentioned above are investigated in this paper. The detection level measurements of saturated distortion on telephone transmission are made. The distortion characteristics are shown in Fig. 3 and the results of measurements are shown in Fig. 4. The curve of both input and output band limitation in Fig. 4 shows the detection level in the telephone transmission performance. Using this result, the proper design parameter level of the amplifier can be decided, when the saturation characteristics of the amplifier and the output peak to peak voltage which corresponds to the prescribed sending transmission level are given. The effects of center clipping distortion on the detectability of sound are measured by the Thurston's psychometric scale and opinion test. Fig. 10 shows the relation between the psychometric scale and the clipping level which is defined by Eq. (1). From these results the center clipping distortion is allowed up to the clipping level of 0. 3 dB in the telephone transmission performance. To reduce the room noise through a telephone side tone path, it is investigated to use a sending amplifier with slight center clipping characteristics. The relation of the clipping level to the degree of receiving AEN improvement and sending AEN detrioration is measured, Fig. 16 shows a case of the total AEN improvement value. This value is 3 to 4 dB within the permitted clipping level, when the AEN is rather bad (27 to 28 dB) and the line loss is 6 dB.
- 社団法人日本音響学会の論文
- 1974-08-01
著者
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