Allocation of frequency bands for television signal Transmission
The following section gives an overview about the allocation of frequency bands for television signal Transmission.
Allocation of frequency bands for television signal transmission:
- For effective amplitude modulation and better selectivity at the RF and IF tuned amplifiers in the receiver, it is essential that the carrier frequency be chosen about ten times that of the highest modulating frequency.
- Since the highest modulating frequency for picture signal transmission is 5 MHz, the minimum carrier frequency that can be employed, cannot be much less than 40 MHz.
- As an illustration consider a carrier frequency fc = 10 MHz. With the highest video modulating frequency = 5 MHz, a deviation of 50 per cent from the centre frequency would be necessary in any tuned circuit to accommodate the lower and upper sideband frequencies.
- However, if the carrier frequency is fixed at, say 50 MHz, the percentage deviation required to pass the upper and lower sideband frequencies for the same modulating frequency would be only 10 per cent.
- It is obvious from these observations that selectivity is bound to be poor at the receiver tuned amplifiers with a carrier frequency of 10 MHz.
- The 3 db down points with a carrier frequency of 50 MHz are within 5 per cent deviation from the carrier frequency and thus the selectivity is bound to be much better.
- Further, each television channel occupies about 7 MHz. In order to accommodate several TV channels, the carrier frequencies have to be in the region of the spectrum above about 40 MHz.
- This explains why television transmission has to be carried out at very high frequencies in the VHF and UHF bands.
- In radio broadcast where the highest modulating frequency is only 5 kHz, lower carrier frequencies can be used, and accordingly transmission is carried out in the medium wave band (550 kHz to 1600 kHz) and short wave bands extending up to about 30 MHz.