ADPCM: Advantages and Disadvantages

This page covers the advantages and disadvantages of ADPCM (Adaptive Differential Pulse Code Modulation). It mentions ADPCM benefits and drawbacks.

What is ADPCM?

Introduction:

In adaptive delta modulation, the step size is chosen according to the message signal’s sampled value to overcome slope overload error and hunting. If the message is varying at a high rate, then the step size is high, and if the message is varying slowly, then the step size is small. ADPCM stands for Adaptive Delta Pulse Code Modulation.

In ADPCM, a difference value that has been mathematically adjusted according to the slope of the input waveform is stored. A bipolar D/A converter is used to convert the stored digital code to analog for playback. ADPCM has been developed to overcome disadvantages seen in the Delta modulation system.

When signal changes are small, we would like to reduce the step size, whereas to avoid slope overload, we would like to increase the step size. This is taken care of by the ADPCM technique.

ADPCM adjusts its step size automatically according to the error between m(t) and m^(t).

The figures above depict the ADPCM encoder and ADPCM decoder block diagrams.

Besides digital telephone applications, the utilization of ADPCM coding is seen in the computer storage of digital speech. It is also used in mobile radio telephony, deep space communication, and digital encryption.

Benefits or Advantages of ADPCM

Following are the benefits or advantages of ADPCM:

• In delta modulation, the modulator no longer follows the input signal when changes in the signal are small, and the modulator overloads when the slope of the signal is too high. These problems of DPCM are eliminated or overcome by the ADPCM technique. Step size varies according to signal variation.
• Bandwidth compression is an important advantage of ADPCM, as only one wire is used for transmission instead of many, as in a normal digital communication system. Moreover, it requires less power to transmit bits.
• The synchronization problem is also eliminated as there is no need for start and stop bits for handshaking.
• It helps in achieving economic and efficient digital coding of speech with a bit rate in the range from 24 Kbps to 32 Kbps.
• It is simple to implement.

Drawbacks or Disadvantages of ADPCM

Following are the drawbacks or disadvantages of ADPCM:

• In ADPCM, the receiver must be able to adapt step sizes in exactly the same manner as the transmitter.
• Quantization noise is present, but no other errors.
• CVSDM (Continuously Variable Slope Delta Modulation) offers improvement over ADPCM.