Question

why digital modulation is not enough for radio transmission

Answer 1

ans=    Digital modulation in many cases also involves coding. A simple kind of digital coding and modulation is called Pulse Code Modulation or PCM. Once you have the digital modulated signal, which is the digital bits of the original signal, then you can use these bits to modulate an analog signal for transmission in the RF frequency. A very very simple example is to us OOK or On-Off-Keying. If the digital bits, which is in binary form, is a "1", you turn on the RF carrier and a "0", you turn it off. Other ways to transmit the RF signal is to modulate the digital coded modulation in other forms, i.e. use Amplitude (PAM or Pulse Amplitude Modulation) or PSK (Phase Shift Keying) etc. These modulated signals are actually in analog form and will need to be up-converted to RF frequency so that it can be transmitted.

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