why the picture signal is amplitude modulated and sound signal frequency modulated?

Answer 1

First of all, amplitude modulation (AM) is easier to do than frequency modulation (FM).
But, as probably can imagine, FM produces better quality at the end as there is no clipping with FM.

When dealing with audible sound, the frequency range is only from 20Hz to 20 KHz.

A picture signal however, deals with millions of frequencies (each color has its own frequency). In addition, you must also be able to transmit other information (such as dimensions).

While it is at least theoretically possible to FM a picture signal, it may not be practical or even necessary with cable. But I don't know that much about cable to know it this last part is true.

Rob

Answer 2

bandwidth, pure and simple.

fm modulation occupies a bandwith that is several times the highest frequency to be demodulated, and video has such high frequencies, this is way too much BW for terrestial use. Satellite video is FM modulated, and the transponder BW is on the order of 30 MHz.

The main element to prevent interference to the video is the FM carrier level is only 5 to 10 percent of the video carrier, even less in Cable systems where 1 to 2 percent is common. The FM uses filters to keep the video from being demodulated as sound. However when highly saturated yellow color is overmodulated, it encrouches on the audio and is heard as a raspy buzz. The Video luminence is a 4.2 MHz signal, that is why the audio subcarrier is at 4.5 MHz. If the carrier were to be AM modulated, that would occupy a bandwidth of 8.4 MHz. With TV channels allocated on 6 MHz spacing, this would not fit. So the lower sideband is mostly eliminated leaving just 1.5 MHz as a vestige. This gives an occupied BW of 5.7 MHz for the video which does fit. As long as there's a carrier, diode detectors don't need both sidebands. When color was added, it's subcarrier at 3.57 MHz was done in such a way as to interleave with the visual, so it does not add to the overall BW.

FM was the choice for audio mostly because its greater RF BW makes it more immune to noise on demodulation. Random noise in the picture was not considered to be as serious a problem as the eye could 'average out' the sparkles.

Answer 3

The audio carrier is about 4.5 MHz above the video carrier frequency and they don't share any spectrum. Since the bandwidth of the TV signal is so large, FM was chosen for the audio for the highest quality. Using AM (vestigial single sideband) modulation for the video resulted in the smallest bandwidth for the signal.

Answer 4

Sound is already measured by its frequency, so it makes sense to modulate sound onto a carrier frequency.

That leaves amplitude modulation left over for video.

Answer 5

The first guy had it right -- interference. You can absolutely limit the bandwidth of an FM signal with good filters, while keeping the fidelity of the modulating audio signal. Limiting the FM bandwidth with sharp filters eliminates the majority of interference with the AM portion of the TV channel bandwidth.

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Answer 6

Only because you have two information sources being transported over 1 carrier wave, in the vhf/uhf band, its cost-effective, no other reason!
Preferably a carrier wave is channelized frequency modulated in the microwave band for performance but this method requires more equipment and technology.

Answer 7

The sound can cause problems with video, by being able to suppress the audio amplitude from channel to channel the audio interference can be eliminated.