Adaptive Differential Pulse Code Modulation – ADPCM

We know that pulse code modulation is classified into two types namely DPCM-differential pulse code modulation & ADPCM-adaptive differential pulse code modulation. So, previously we have discussed PCM and DPCM. This article discusses an overview of ADPCM which is known as Adaptive differential pulse code modulation. The modulation like ADPCM was developed in the year 1970 by Bell Labs. It is an extremely efficient digital coding of signals for voice coding purposes.

In the year 1990, this kind of modulation was used by IMA which is known as Interactive Multimedia Association for the legacy audio codec development. So, this modulation is also called ADPCM IMA ADPCM, DVI, or DVI4. Some of the methods of ADPCM are used in VoIP communications.

What is an ADPCM (Adaptive Differential Pulse Code Modulation)?

ADPCM (Adaptive Differential Pulse Code Modulation) is also known as Delta Pulse Code Modulation. For a signal, it is a compression coding using differential values like DPCM or Differential Pulse Code Modulation, where the quantization stages scaling is adapted additionally based on the signal curve
This modulation is used in the structure of a variety of ITU-T standards like G.726 in audio signals.

In these applications, the data rate of the output can be adjusted dynamically among 16 kbit/s & 64 kbit/s. Another example of this is DECT (Digital Enhanced Cordless Telecommunications) which is used in cordless telephones. The ADPCM uses the signal performance in the past and predicts it in the future and the output signal will signify the prediction error, which has no importance.

So, the signal should be decoded to restore a strong and original signal. The main function of ADPCM is to transmit sound signals with the help of fiber-optic long-distance lines. This modulation is very helpful particularly for associations that arrange digital lines among remote sites for transmitting both voices as well as data. Before they transmitted, the voice signals are digitized

The ADPCM method is mainly used in the telecommunication field for compression of speech because this technique will reduce the flow of bit without reducing its quality. This kind of modulation can be used in all waveforms, images, and audio with high-quality & data in other moderns.

Operating Principle

ADPCM operating principle is, it is one kind of pulse code modulation (PCM) using a prediction function. Once the signal is processed, an effort can be made to forecast the additional path of the signal in the next segment. For the signal quantization within the next time step only the disparity among predicted as well as the real signal can be used. Because of this different configuration, a smaller amount of bits can be utilized to explain the signal.

By using this technique, both the quantization level & prediction function are adapted one more with every work step. As compared to DPCM, this control loop gives an enhanced signal prediction. This kind of modulation is used for imitating computers as well as arcade games.

Adaptive Differential Pulse Code Modulation Block Diagram

The following characteristics are optional for the adaptation of a 64 Kbps A-law otherwise μ-law PCM channels from a 16-Kbps channel or 24-, 32-, 40-. The adaptation can be applied to the pulse code modulation bitstream through an ADPCM transcoding method.

The main relationship among the voice frequency signals as well as the encoding/decoding laws of PCM is completely specific in proposal G.711. The main application of 24- & 16-Kbps channels is to carry voice within DCME (digital circuit equipment).

The 40-Kbps channel’s main application is to hold the signals of the data modem in DCME, particularly for the modems which operate above 4800 Kbps. The Adaptive Differential Pulse Code Modulation Block Diagram is simplified like encoder and decoder are shown below.

Encoder

Subsequent to the exchange of both the laws like μ-law & A-law, the input signal of PCM to consistent PCM, a disparity signal can be acquired through subtracting an approximation of the i/p signal from the i/p signal itself. An adaptive 7-, 15-, 31- otherwise 4 level quantizer can be for assigning 5,4,3, or 2 binary digits correspondingly to the difference signal value for broadcast toward the decoder.

An inverse quantizer generates a quantized dissimilarity signal using these binary digits correspondingly. The estimation of the signal can be added toward this quantized difference signal for generating the reconstructed edition of the i/p signal. Both the signals like quantized difference as well as reconstructed are functioned ahead through an adaptive predictor that generates the estimation of the i/p signal, thus finishing the feedback loop.

Decoder

This decoder uses an identical structure toward the feedback part of the encoder, as one through a consistent PCM toward the conversion of A-law otherwise μ-law & an adjustment of synchronous coding.

The adjustment of synchronous coding stops increasing distortion happening on synchronous tandem coding like PCM, ADPCM, ADPCM, etc, under specific conditions. The adjustment of synchronous coding can be achieved through adjusting the output codes of pulse code modulation in a way that efforts to remove quantizing distortion within the subsequent ADPCM encoding phase

Difference between DPCM and ADPCM

The difference between DPCM and ADPCM includes the following.

The term DPCM stands for Delta Pulse Code Modulation. In this modulation, a multi-bit disparity value can be stored. A bipolar digital to analog converter can be used for playback to change the consecutive disparity values toward an analog waveform.

The term ADPCM stands for Adaptive Delta Pulse Code Modulation. In this modulation, a disparity value can be regulated mathematically based on the input waveform’s slope is stored. So, a bipolar digital to analog converter can be used to change the digital code which is stored to analog intended for playback.

Please refer to this link to know more about Pulse Code Modulation

Please refer to this link to know more about Differential Pulse Code Modulation

Advantages and Disadvantages of ADPCM

The advantages and disadvantages of adaptive differential pulse code modulation are discussed below.

The advantages of ADPCM are

• The modulator doesn’t follow the input signal for a long time in delta modulation once a change within the signal is little as well as the modulator overworks once the signal’s slope is extremely high. So, the DPCM problems are eliminated through the ADPCM method. According to the signal variation, the step size will be changed.
• The compression of bandwidth is one more benefit of ADPCM because simply a single wire is utilized for transmission in place of several like in a usual digital communication system. Furthermore, it uses low power for the transmission of bits.
• The elimination of synchronization trouble can also be done because there no requirement for a start as well as stop bits for handshaking.
• It assists in attaining financial & proficient digital coding for speech through bit rate within the range from 24 Kbps to 32 Kbps.
• This kind of modulation is very easy to implement.

The disadvantages of ADPCM are

• In this modulation, the receiver should be capable to adjust step sizes within accurately the same method as the transmitter.
• Quantization noise is there however no other errors.
• Continuous Variable Slope Delta Modulation (CVSDM) gives development over ADPCM.

Thus, this is all about an overview of Adaptive Differential Pulse Code Modulation which is known as ADPCM. The coder and decoder-like codec of APDCM is used in the PHS (personal handy phone systems) & PAS (personal access systems). Here is a question for you, what are the different kinds of modulation techniques available?