EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: AutogolazzoJr on May 27, 2020, 06:10:56 am
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Here is a spectrum graph of the signal on a VHS tape:
https://imgur.com/a/OH5AxWV
The text says that there's interference between the luma and chroma signals, which is shown by the trapezoid things. My question is - what are these trapezoid things on the graph? If the luma signal is 3.4mhz at it's lowest and the chroma signal is 629khz, how is there interference? Thanks in advance.
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There is no way you can have perfectly square and sharp signal, so those edges represent real signal, whose spectrum is a bit wider.
3.4 MHz is the lowest for the carrier, not for the whole spectrum.
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So because the signal is imperfect, the luma carrier can go from 3.4mhz to 629 khz and interfere with the chroma signal? That sounds like quite the jump.
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3.4 MHz is a carrier frequency. The carrier can't jump this much. But the signal has some bandwidth, and the edge of that band may move into 629 kHz range.
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I'm still confused. The spectrum shows that the signal has 1mhz between it's minimum frequency and it's maximum. What is the point of the trapezoidal shape around it? Is there some sort of article that I can read to explain this? Thanks.
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Nevermind, it's because of sidebands.
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They are not side bands, they are part of the useful signal. Amplitude variation is part of the signal, and it results in a wider spectrum than a regular carrier would. If they were just side bands, you would be able to filter them out, you can't do that without also damaging the useful signal.