Quote:
Originally Posted by DVtyro
Um, sort of. I think I understood that the whole spectrum would move, but I did not understand how a TV set would know that the frequencies that moved to a different location represent the same image.
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It is not the TV that interprets the FM signal. The VCR has an FM decoder that reconstitutes the baseband video signal you see in the top row of drawings.
The time domain plot of the FM signal during the pattern shows the carrier frquency during the pattern varying directly according to the instantaneous baseband voltage in the top row diagram.
So, just picking some numbers out of the air thatcould apply to the HI-8 system:
If the baseband video pattern average is mid gray voltage, meaning the sine wave varies equally above and below mid gray, the instantaneous FM time signal might vary in frequency from 6.8 MHz to 7.2 MHz. The instantaneous FM frequency corresponds exactly to the instantaneous video baseband voltage. The FM demodulator converts this instantaneous frequency back to the exactly corresponding video baseband voltage, forming the signal that is sent to the TV. The FM spectrum is only telling you that you can make this varying frequency FM time signal by adding togehter a certain collection of sine waves that each have their own constant unvarying frequency and amplitude.
This is the genius of Fourier, that he showed this is possible - to decompose a varying waveform into a sum of unvarying waveforms. In this case, exact replication of the FM signal requires a center frequency and two sidebands. (As mentioned before, the tape system loses the upper sideband, so the recovered M signal then has amplitude modulation in addition to frequency modulation - but the FM decoder ignores the amplitude modulation.)
Now suppose you have in the baseband video signal the same pattern with the same peak-to-peak amplitude, but its positive and negative peak voltages are both shifted lower by the same amount. All instantaneous FM time signal frequencies are now shifted lower by the same amount compared to the mid-gray case (for example, 6.5 to 6.9 MHz), because the video baseband voltages that control the frequency have been shifted by a particular amount. The FM demodulator sees these shifted frequencies and produces a baseband video with shifted voltages that match the original. The spectrum plot just shows that all the frequencies of the first case are replaced with shifted frequencies in the second case, and the amount of shift is the same for all frequencies.