RC low-frequency noise removal circuit

Since the built-in microphone, BM is installed on the casing of the recorder, and the casing is also equipped with a speaker when the speaker emits sound, the casing will vibrate, and this vibration will cause the built-in microphone BM to vibrate, resulting in the BM output. A vibration noise with a very low frequency causes the low-frequency noise of "boom" in the work of the internal microphone. For this reason, an RC series circuit should be added to the input circuit of the internal microphone to remove the low-frequency noise to eliminate this low-frequency noise.

It can be seen from the impedance characteristic curve of the RC series circuit that the total impedance increases with the decrease of the input signal frequency, which presents a high impedance characteristic to the low-frequency noise in the machine, hinders the passage of the low-frequency noise current, and achieves the removal of low-frequency noise. the purpose of noise. Although this can eliminate low-frequency noise, it also has a certain impact on low-frequency useful signals, which has become the main contradiction in eliminating low-frequency noise.

RC de-emphasis circuit

De-emphasis circuits appear in FM radio circuits and television sound channel circuits.

FM radios do not have the same noise at different frequencies as AM radios do, but increase as the frequency increases, which shows that the high-frequency noise of FM is serious. In order to improve the signal-to-noise ratio of the high-frequency band, the FM transmitter should pre-emphasize the high-frequency signal in the audio signal before transmitting the FM signal, that is, to increase the signal of the high-frequency band first, and set a de-emphasis circuit in the FM radio circuit. to restore the original characteristics of the audio signal. In the de-emphasis circuit, high-frequency noise removal and de-emphasis circuits are also divided into two types: mono and dual-channel. The mono-channel de-emphasis circuit is connected behind the frequency discriminator, while the dual-channel de-emphasis circuit requires After the discriminator is connected to the stereo decoding circuit, the emphasis circuit is then connected. The de-emphasis circuit is actually a voltage divider circuit composed of a resistor and a capacitor. Since the capacitive reactance of the capacitor to the high-frequency signal is relatively low, the impedance of the high-frequency signal pre-emphasized to the transmitter is small, and the capacitor will absorb the high-frequency signal. The pre-emphasis signal achieves de-emphasis.

Field integrator circuit

The field integrator circuit in the scanning circuit of black and white TV and color TV is composed of two integrator circuits. The horizontal and vertical sync signals have the same amplitude, but different widths. The horizontal sync pulse is narrow and the vertical sync pulse is wide. The field integrator circuit is to take the field synchronization signal from the horizontal and vertical synchronization composite signal. The pulse width of the vertical synchronization signal is relatively long, and the output signal voltage is large, while the horizontal synchronization signal is the opposite. The output signal voltage is small. After passing through the field integration circuit (that is, the two integration circuits), the voltage of the horizontal synchronization signal is reduced twice to achieve attenuation, so that the field synchronization signal is selected.

After reading the information above, you will have a deep insight into the RC circuit.