Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

DELTA MODULATION

• In PCM the signaling rate and transmission channel bandwidth are quite large since it transmits all the bits which are used to code a sample. To overcome this problem, Delta modulation is used.
• Delta modulation transmits only one bit per sample. Here, the present sample value is compared with the previous sample value and this result whether the amplitude is increased or decreased is transmitted.

Transmitter

• Fig. 1 shows the transmitter . It is also known as Delta modulator.

Fig.1 Delta Modulation Transmitter

• It consists of a 1-bit quantizer and a delay circuit along with two summer circuits.
• The summer in the accumulator adds quantizer output (±Δ) with the previous sample approximation . This gives present sample approximation. i.e.,

• The previous sample approximation u[(n-1)Ts ] is restored by delaying one sample period Ts .
• The samples input signal x(nTs ) and staircase approximated signal xˆ(nTs ) are subtracted to get error signal e(nTs ).
• Thus, depending on the sign of e(nTs ), one bit quantizer generates an output of +Δ or -Δ .
• If the step size is +Δ, then binary ‘1’ is transmitted and if it is -Δ, then binary ‘0’ is transmitted .

Receiver

• At the receiver end also known as delta demodulator, as shown in Fig.2, it comprises of a low pass filter(LPF), a summer, and a delay circuit. The predictor circuit is eliminated here and hence no assumed input is given to the demodulator.

Fig.2 Delta Modulation Receiver

• The accumulator generates the staircase approximated signal output and is delayed by one sampling period Ts.
• It is then added to the input signal.
• If the input is binary ‘1’ then it adds +Δ step to the previous output (which is delayed).
• If the input is binary ‘0’ then one step ‘Δ’ is subtracted from the delayed signal.

• Also, the low pass filter smoothens the staircase signal to reconstruct the original message signal x(t) .

• The delta modulation has two major drawbacks as under :

  (a)Slope Overload Distortion (b) Granular Noise

Slope Overload Distortion

• This distortion arises because of large dynamic range of the input signal.

Fig.3: Quantization Errors in Delta Modulation

• We can observe from Fig.3 , the rate of rise of input signal x(t) is so high that the staircase signal can not approximate it, the step size ‘Δ’ becomes too small for staircase signal u(t) to follow the step segment of x(t).
• Hence, there is a large error between the staircase approximated signal and the original input signal x(t).
• This error or noise is known as slope overload distortion .
• To reduce this error, the step size must be increased when slope of signal x(t) is high.