First a conversion of the speech signal prerecorded on a magnetic tape to amplitude transmittance of photographic film is accomplished with a cathode ray tube ( CRT) using density modulation method. 借助于阴极射线示波器,首先将录制在磁带上的语音信号用密度调制的方法转变为照相底片的振幅透射率的变化,从而实现信号由时域到空域的转变。