LM1894 principle and application (a)

A typical representative of the dynamic noise reducer is the LM 1894 (see Table 1-3), which was first successfully developed and integrated by the National Semiconductor Company of the United States. Its principle is shown in Figure 1-107. The upper part of the figure is the same as the left and right. There are two main signal channels,
the left and right signals are input from pin 132 respectively and output from pins 11 and 4 after noise reduction processing. A2 and A2 between the input and output
form a variable bandpass low-pass filter with a gain of OdB and controlled by the channel.
    The lower part of the figure is the secondary signal control channel. The left and right share a control channel here, which is helpful to simplify the circuit.
    The left and right input signals are first mixed together in the adder A3, then amplified by the secondary signal amplifier and
detected by the peak detector composed of As and VD, and then a DC control voltage is obtained on pin 10, and then the built-in voltage/ current (V/,)
converter turns into a DC control current and adds it to Ai to change the main channel bandwidth . According to the design requirements of the noise reducer, the richer the high-frequency components in the input signal
, the greater the DC control voltage and the smaller the converted DC control current, and
the wider the bandwidth of the corresponding main channel. Specifically, every time the control current decreases by 1, the bandwidth increases by 27Hz. On the contrary, when
there are fewer high-frequency components in the input signal , the bandwidth of the main channel becomes narrower, generally 800Hz ~ 1,1Ⅺ7o.