It would become increasingly difficult for TV tuners to accurately tune into a 7 MHz channel. Therefore, for frequencies above 300 MHz, the PAL-G frequency plan is used. Further the exact frequency should also not drift or change over a period of time. If the carrier frequency of 1 channel drifts, it could over lap with or cause disturbance to the adjacent channel. To ensure stable carrier frequencies, most modulators utilise a crystal oscillator.
Crystal oscillators are low cost and provide excellent stability.
Infact, we all know the accuracy of a digital watch or clock, which is driven by a crystal oscillator. The signal generated by a modulator has certain unwanted components. Simple Modulators also referred to as VSB or Vestigial Side Band Modulators include an output for about 1.25 MHz before the video carrier. This implies that a VSB Modulator is an alternate channel modulator. If a VSB Modulator is used for channel 4, Channel 3 will have to be kept unutilised on the system.