Dynamic noise reduction circuit for left and right channel separation

The sound sources of today's audio equipment mainly come from three categories: original tapes, electromagnetic record players, and ∞ record players.
The content included in the sound sources can also be summarized into three categories: light music, pop music, and symphony. No matter what kind of music it is, the position of the band and musicians
in the entire sound field has certain standards. In the production process of traditional two-channel stereo sound sources,
it often happens that the high range of a certain section of the music is on the left. The distribution of the right channel and the right channel is uneven. Only through the artistic processing of the recording engineer can
the listener have a strong sense of presence and accurate sound and image positioning. For LM1894, since its left and right main channels are controlled
by the same sub-channel , when the high-pitched ranges of the left and right channels are uneven, that is, when a strong high-frequency signal appears in a certain channel, it This will trigger two channels of widening at the same time. In this way, the channel with high audio signal input will be unobstructed, and the "noise flow" of the other channel without high audio signal will also pass smoothly, resulting in a serious "dereliction" of the noise reduction function.     From the above analysis, it can be seen that two problems must be solved. The best way for channels to be triggered simultaneously is to separate the control channels. According to the characteristics of LM1894 that does not have strict requirements on the input signal level, it is also considered that if the left and right channels are used in series, the noise reduction amount of the noise reduction circuit can also be improved. Therefore, an LM1894 is used for the left and right channels respectively, and the two internal main signal channels are connected in series to form two noise reduction circuits controlled by their respective control channels . This not only solves the mutual influence of the two channels, but also increases the amount of noise reduction , killing two birds with one stone. The circuit is as shown in the figure