Definition: Delta modulation is a modulation technique introduced to overcome the disadvantages of pulse code modulation. Delta modulation transmit only one bit per sample. Hence helps in reducing the bandwidth and signaling rate too.
Drawbacks of Pulse Code Modulation:
- PCM used encoding, decoding and quantizing circuits which makes PCM complex.
- PCM required large bandwidth and signalling rate is high too.
To overcome the drawbacks of PCM delta modulation is introduced.
Working Principle of Delta Modulation:
Delta modulation transmit only one bit per samples. Thus, present value of sample is compared with previous sample value and outcome whether the amplitude is increased or decreased is transmitted. Input signal is approximated to step signal by the delta modulator. The step size is kept fixed. The difference between input signal and staircase approximated signal is confined to two levels +Δ and -Δ. If the difference is positive then approximated signal is increase by one step that is Δ. If the difference is negative then the approximated signal is decrease by Δ.
When step is reduced ’0′ is transmitted and is step is increased then ’1′ is transmitted. Hence for each sample one bit is transmitted. Hence delta modulation is achieved.
Advantages of delta modulation
- Since one bit is transmitted per sample therefore signalling rate and bandwidth required in delta modulation is less as compared to PCM.
- There is no use of analog to digital convertor or digital to analog convertor in delta modulation hence circuit is simple.
Drawbacks of delta modulation
- Slope overload distortion
- Granular noise
Slope overload distortion occurs because of large dynamic range of input signal. The rate of rise of input signal is so high that the staircase signal was not able to approximate it. The step size Δ becomes too small to follow the increased input signal. Hence this large difference between input signal and approximated signal is referred to as slope overload distortion. To reduce this error step size Δ must be increased.
Granular noise occurs when step size is too large as compared to small variation in input signal. Hence for a small change in input signal their will be large change in approximated signal. This error can be reduced by decreasing the step size Δ.