In the mid-1920s, records made by mechanical recording method were used to record and restore sound for audio films. For example, the audio feature films don Juan (1926) and jazz singer (1927) made in the United States are recorded mechanically. China's earliest audio feature films "singing girl red peony" and "beauty Yu" were also recorded in this way. Although mechanical recording played a certain role in the early stage of the development of sound films, because records and films are separated, there is often a phenomenon that sound and picture are not synchronized when they are shown in cinemas. Sometimes the picture has already been played, and the sound on the record continues.
Optical recording

As early as the early stage of film development, many people put forward the imagination of using film to record and return sound. It was not until the early 1930s that the optical recording method was successfully applied to film. The earliest feature film produced by optical recording method was the seven heavy days (1937) in the United States. The earliest audio feature films made by optical recording in China include "green after rain" and "spring in the song field". By the 1980s, the vocal cords in general film reproduction were still recorded optically. In film reproduction, it can be seen that there are traces recording sound on one side of the picture, which is called optical sound trace or vocal cord.
Optical recording的进程，是把传声器所拾取的声响转换为相应的模仿电信号，再经过音频放大器把它放大到足以使光－电调幅器起作用的程度，以形成可记载的模仿光信号。光－电调幅器是光学录音的要害部件，它把声响的模仿电信号转换成声响的模仿光信号，并使用胶片对不同曝光量发生不同感光密度的特性，把声响记载下来。对经过录音的胶片进行显影、定影、冲洗加工之后，就能显示出感光密度不同或感光宽度不同的光学声带。和机械录音比较，光学录音的优越性在于它所录制的音域范围较大；它和电影摄影所用的媒介都是感光材料，可以和画面一起印在复制上，这种办法既降低了制造成本、确保了声响质量，又使放映时的声画同步。
Optical recording改进了有声电影。前期的有声电影几乎全部是选用光学录音的。50年代中期曾经，我国的电影出产基本上也选用这种办法。但是，光学录音也有不足之处：①录音后的感光胶片要进行冲洗，才干得到声带底片，制造进程比较复杂。②复制上的声带也象画面相同需要从录音底片上印制出声带正片才干还音，所以光学录音不能立即还音，不能及时查看录音效果。③感光胶片的声带上如有脏点或机械划伤，就会发生噪声，影响声响质量。④感光胶片必须在黑私自操作，工作不方便。⑤因为第②条缺点和剪接的需要，用的胶片许多，增大了制造成本。
Article source: Jiangmen recording production http://www.sjttv.com