Spectrum display with memory function (b)

The structure and working principle of this circuit are basically the same as the previous circuit, except for the components used and some functions.
First, the display frequency band of this circuit is divided into seven frequency bands, and its center frequency is selected according to 2.5 octaves, respectively. For
60Hz, 150Hz, 400Hz, 1kHz, 2-5" seven, 6kHz: and 15kHz; secondly, the voltage comparator of this circuit uses
an LB1404 to replace the two operational amplifiers, and the comparison is increased from 8 levels to 9 levels By comparison, the display matrix is ​​changed from 8×8
to 7×9; third, an input amplifier and automatic level control circuit are added before the band-pass filter to overcome
the phenomenon of full screen for large signals and dark screen for small signals. At the same time, a memory selection circuit is added, which is controlled by a switch.
When the switch is turned on, the spectrum being displayed is fixed, so that the frequency components and signal strength of the signal can be carefully analyzed.
    The whole circuit is composed of input signal mixing amplification, automatic level It is composed of control, electronic switch, scan pulse distribution, bandpass filter
and memory selection circuit. The circuit is shown in Figure 4-64 o
    input mixing circuit. After the left and right channel input signals are input from the input terminal, they pass through the mixing resistor Rl, After the mixing of R2,
cl is coupled to ICI-4 for isolation amplification. The values ​​of Rl and R2 are larger to reduce the impact on the separation. The signal is
amplified by ICl.4 and divided into two channels. One channel is coupled by BA and After VDi, rDz voltage doubling rectification and C3 filtering,
a DC voltage reflecting the signal strength is generated, which is added to the base of VTi through the motor. After being amplified by VTi, the on-resistance of VTzC-E is controlled, and the input is controlled
accordingly to the signal strength of ICI-4, thereby stabilizing the output level; the other channel is added to
the 7-level band-pass filter composed of Icl-l~lCl.3, IC2-1~IC2-4 and surrounding components for frequency division Processing. The signal voltage output
    by the band-pass filter
needs to be rectified by the diode, D4~VDio to obtain a DC voltage that reflects the signal strength of each frequency band. Since the conduction voltage of the silicon diode (also called the dead zone) is high, In order to overcome the influence of the dead zone voltage
, this circuit is equipped with a dead zone circuit, which is composed of Rg and VI. VIb is
the voltage drop between eb poles of a germanium low-frequency transistor, forming a stable voltage of 0.3V with good performance. To reduce the impact of silicon diode dead zone voltage.