Left channel simplified circuit

The characteristic of Dolby Pro Logic decoding is to highlight the directionality of the sound source through logical operations and improve the separation between channels.
It uses a counter-divergent differential amplifier to detect and compare the left and right channel signals , the center channel (L+R) signal and the surround sound (L-R)
signal respectively, to determine the dominant directions of the left, right and front and back, and Automatically change
the working time constant of the logic circuit with the complex changes of the signal. When the measured sound pressure difference is greater than 6dB, the time constant is set to 30ms, and the polarity separation
circuit responds quickly in 30ms to control
the gain of the voltage-controlled amplifier (VCA) of the channel with the lower sound pressure. , so that the gain is reduced, and the sound pressure of the channel with higher sound pressure appears larger, thereby highlighting the directional
effect of the sound with large changes. When the measured sound pressure difference is below 6dB, the time constant is converted to 1s, that is It is said that it does not respond to small sound pressure differences with a duration of Is or less
. In this way, when small sound pressure differences occur frequently between channels, the logic circuit will not act too
frequently to avoid affecting the stability of the sound field. This decoding method is also called active decoding.
    The encoded left and right channel composite signals Lt.Re also enter the voltage-controlled amplifier, where they are weighted and changed in level,
and then added and subtracted from the original left and right channel signals. After
directional processing, the eight groups of voltage-controlled amplifiers finally output LR, s, and
c four-channel signals . This processing method can offset
the crosstalk between channels to the greatest extent without degrading the sound quality. The following
describes the voltage-controlled amplifier output circuit of the left channel. The simplified circuit of the left channel is as shown in the figure