Adaptive Differential Pulse Code Modulation (ADPCM)

ADPCM is a type of DPCM (Differential Pulse Code Modulation). It is also a differential coding technique that uses differential values like DPCM. ADPCM was developed to overcome the drawbacks of the DPCM (Differential Pulse Code Modulation) and DM (Delta Modulation). However, a method of DM named Continuous Variable Slope Delta Modulation (CVSDM) has more advantages than ADPCM.

Unlike DPCM, the signal variation in ADPCM helps to change the step size of the quantization levels. It reduces the overload error during the modulation process. The step size is directly proportional to the speed rate of the input message. If the variation of the message signal is at a high rate, the step size will be high. Similarly, if the variation of the message signal is at a slower rate, the step size will be small.

DPCM requires fewer levels for the quantization process as compared to the PCM (Pulse Code Modulation) process. ADPCM requires fewer numbers of levels as compared to both the DPCM and PCM. It means that ADPCM provides better bandwidth reduction than DPCM and PCM. The numbers of levels are directly proportional to the bandwidth. The fewer the levels, the lesser the bandwidth will be.

Block Diagram

The block diagram of the Adaptive differential PCM is shown below:

The block diagram of ADPCM contains the quantizer at ends, encoder, predictor (as the feedback loop), and a decoder. The function of the encoder is similar to the modulation process, and the function of the decoder is similar to the demodulation. Various channels of 16k bits, 24, 32, and 40k bits have different applications. For example, 16k and 24k bits channel is used to carry voice with DCME (Digital Circuit Equipment).

Quantizer

A quantizer is present in the circuit of ADPCM at both the transmitting and receiving ends of the circuit. It reduces the number of discrete symbols. It adds some redundant bits and compresses the data to make it suitable for storage and transmission.

Encoder

An encoder is a device that converts the analog signal to digital pulses. It responds to each sample by generating a binary pulse or pattern. It also reduces the transmission bandwidth depending on the step size of between the quantization levels. Different levels quantizer can be used for assigning different number of binary digits.

Quantizer

It works as a reverse quantizer. It generates a quantized opposite signal using these same binary digits.

Adaptive predictor

It completes the feedback loop by generating the estimation of the input signal. The feedback loop detects the variations in the message signal, which helps in changing the step size.

Decoder

The digitally encoded signal arrives at the receiver. It first removes the noise from the signal. The quantization process does not allow the easy separation of the signal and the noise. Hence, it is essential to remove the noise from the signal at the decoding stage. It works similar to the demodulation process and converts the binary pulses to the original form or the analog signal.

Advantages of ADPCM

The advantages of ADPCM are as follows:

• Good bandwidth compression
  ADPCM has better bandwidth compression ability due to the less number of quantization level involved in the quantization process.
• Low power consumption
  ADPCM consumes low power than PCM and DPCM. It is due to its efficient algorithm that uses lower transmission rates.
• No synchronization required
  It does not require a handshake (start and stop bits) to start the transmission process.
• Easy modulation
  The modulation of ADPCM follows the input signal. It helps it to detect any changes in the signal. The variations in the signal are used to change the step size in the quantization process. Hence, the modulation process of ADPCM is easy as compared to DPCM.
• Step size can be changed
  According to the signal variation, the step size in the Adaptive differential PCM process can be changed.

Disadvantages of ADPCM

The disadvantages of ADPCM are as follows:

• High quantization noise than DPCM.
• The continuous variable slope delta modulation, a type of DM (Delta Modulation) has better advantages than ADPCM (Adaptive Differential Pulse Code Modulation).
• The step size variation by the receiver should match the step size variation made by the transmitter.

Applications of ADPCM

ADPCM is used in various applications, such as IP telephony, also known as VoIP (Voice over Internet Protocol). It is a technology that delivers voice transmissions over the IP (Internet Protocol). The ADPCM algorithm is used to increase the line's capacity to twice. This process is named G.726, a speech-coded standard covering the voice transmission at different rates (16k bits, 24, 32, and 40k bits).

ADPCM vs. DPCM

Let's discuss the differences between these two types of PCM.

The differences between ADPCM and DPCM are listed below: