Digital signal

In a communication system operating at a wavelength of $800 \,nm$, only $1 \%$ of the source frequency is available as signal bandwidth. The number of channels accommodated for transmitting $TV$ signals of bandwidth $6 \,MHz$ is

Audio signals cannot be transmitted directly because $.......$

The $T.V.$ transmission tower in Delhi has a height of $245 \ m$. The distance up to which the broadcast can be received (taking the radius of the earth to be $6.4 \times 10^6 \ m$) is .......... $km$.

For a television network,$5 \times 10^5$ channels are granted. If the central frequency of the microwave link is $25 \text{ GHz}$ and the allotted bandwidth for each channel is $2 \text{ kHz}$,then what percentage of the link is used for the network (in $\%$)?

The ratio of the maximum amplitude to the minimum amplitude of an amplitude-modulated wave is $4$. What is the modulation index of this wave?