Why are all radio receptions except FM band affected when cable TV is switched on?

When electrical equipment is switched on, there are electrical currents in different parts of the circuit. And an electrical current is associated with a magnetic field in its neighbourhood; its strength reduces as the distance increases. If the current is alternating with time as in the case of AC, the equipment acts as a source of electromagnetic waves, often called the EM noise for other equipment. Wireless equipment are fitted with suitable tuned antennas designed to pick up the EM waves, which are further detected and processed by the receiver circuit.

A TV set handles time varying high frequency electric currents at many places including the signal feeding cable itself. And these currents generate EM waves at such frequencies that can be detected by a radio receiver tuned to an amplitude-modulated band. These bands are marked as AM and SW bands but all of them operate as amplitude modulated (AM) signal detectors. These signals are essentially radio frequency carrier waves whose amplitude is modified as to represent an audio signal. Therefore, in the neighbourhood of a noise producing equipment like a TV, the signal received by the antenna of the radio receiver is altered by the picked up EM waves; and it would be taken by the receiver as the total signal to process. The noise has a predominantly higher frequency than the audio range. The detection and amplification process in an AM band is done using a `mixer' circuit, which mixes the received signal with a sinusoidal signal generated by a local oscillator operating at 470 kHz. The noise picked up by the antenna passes for a genuine signal and does not get eliminated in the signal processing in the receiver. Actually, it appears in the output mixed with the genuine audio signal. Hence the noise component appears to affect the reception of the AM bands.

However in the case of a radio receiver tuned to FM band instead of the amplitude of the carrier wave, the frequency is modulated to the time variation of the loudness of the audio signal and the amplitude remains unaffected. Further, during FM transmission, the high frequency region, where the noise band falls, is deliberately emphasized by a large factor. Thus the picked up noise makes only a small contribution to the total signal, which goes through a stage of de-emphasis in the receiver. This treatment of the signal further reduces the effect of picked up noise in the radio tuned to the FM band. Note that this is not done in the AM transmission reception process because the detection method adopted in the receiver would eliminate the pre-emphasis in the first stage itself.

Source: thehindu.com