This is possible to transmit the analog signal i.e., speech, video etc, in digital format. Some digital modulation schemes are given below
S(t) = Ac cos 2πfct; 0 ≤ t ≤ Tb
= 0; otherwise
Bandwidth = 2 × 1/Tb
= 2 × bit rate
For binary digit 1, Ac cos 2π fct × Ac cos 2πfct = (A2c /2)[1 + cos 4πfct]
Output of LPF = (A2c /2)
For binary digit 0 output of LPF = 0
In ASK, probability of error (Pe) is high.
In ASK, SNR is less.
In phase shift, keying phase of high-frequency carrier is varied in accordance with digital data 1 or 0.
S(t) = Ac cos 2πfct for bit 1
= – Ac cos 2pfct for bit 0
A frequency of the carrier must be a multiple of a bit rate.
Tb = n/fc
Fc = nrb
In frequency shift keying, a frequency of the carrier is varied in accordance with digital data (1 or 0).
For digital data 1 we use frequency f1 and for digital data 0 we use frequency f2.
Bandwidth = 2Δf + 2fm
Bandwidth = f1 + (1/Tb) – f2 + (1/Tb)
= f1 – f2 + (2/Tb); f1 – f2 = 2Δf
In PSK it needs a complicated synchronising circuit at the receiver, this disadvantage of PSK is removed in DPSK.
A cos ω0t = ± A cos ω0t
Note: Advantage of DPSK over PSK is, DPSK does not require a coherent carrier for demodulation.
In QAM, digital information is content in a both amplitude and phase of the signal. It is used in both digital modulation scheme and analog modulation scheme. Digital cable television and in cable modem applications QAM is used.