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**Digital Modulation Schemes**

This is possible to transmit the analog signal i.e., speech, video etc, in digital format. Some digital modulation schemes are given below

**Digital Carrier Modulation:**Commonly used digital modulation schemes are Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK).**Amplitude Shift Keying (ASK)**: The amplitude of a high-frequency carrier is varied in accordance with digital data (0 or 1).

S(t) = A_{c} cos 2πf_{c}t; 0 ≤ t ≤ T_{b}

= 0; otherwise

Bandwidth = 2 × 1/T_{b}

= 2 × bit rate

- For digital input 1 amplitude level is high and for digital input 0 amplitude level is low.
- Signalling used is on-off signalling.

**Demodulation of ASK:**

For binary digit 1, A_{c} cos 2π f_{c}t × A_{c} cos 2πf_{c}t = (A^{2}_{c} /2)[1 + cos 4πf_{c}t]

Output of LPF = (A^{2}_{c }/2)

For binary digit 0 output of LPF = 0

In ASK, probability of error (P_{e}) is high.

In ASK, SNR is less.

**Phase Shift Keying (PSK):**

In phase shift, keying phase of high-frequency carrier is varied in accordance with digital data 1 or 0.

- NRZ signalling is used.

S(t) = A_{c} cos 2πf_{c}t for bit 1

= – A_{c} cos 2pf_{c}t for bit 0

A frequency of the carrier must be a multiple of a bit rate.

T_{b} = n/f_{c}

F_{c} = nr_{b}

- In case of PSK, a probability of error is less.
- In case of PSK, SNR is high.
- Mainly used a technique in wireless transmission.

**Frequency Shift Keying (FSK):**

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 f_{2}.

- NRZ signalling is used here
- VCO The schematic diagram of VCO is given below

Bandwidth = 2Δf + 2f_{m}

Bandwidth = f_{1} + (1/T_{b}) – f_{2} + (1/T_{b})

= f_{1} – f_{2} + (2/T_{b}); f_{1} – f_{2} = 2Δf

**Key Points**

- In case of FSK, Pe is less but SNR is high.
- Multiplexing is difficult in FSK.

**Differential Phase Shift Keying (DPSK): **

In PSK it needs a complicated synchronising circuit at the receiver, this disadvantage of PSK is removed in DPSK.

A cos ω_{0}t = ± A cos ω_{0}t

Note: Advantage of DPSK over PSK is, DPSK does not require a coherent carrier for demodulation.

**Comparison of Digital Modulation Schemes**

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.

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