This page on PCM vs DPCM vs ADPCM describes difference between PCM, DPCM and ADPCM modulation techniques. All these are pulse digital modulation techniques.

## PCM

The short form of the Pulse Code Modulation is PCM. In PCM, the analog speech waveform is sampled and converted directly into a multibit digital code by an Analog to Digital converter. The digital code is stored in the memory and which is later re-called for the playback.

In this type of modulation, analog data is sampled and quantized before being represented to digital binary form. Hence using PCM, continuous amplitude and continuous time signal waveform is converted into discrete amplitude and discrete time waveform.

If there is a n bit quantizer and sampling rate is Fs then bit rate will be
Rb(bits/sec) = n * Fs
Sampling rate must be at a rate greater than or equal to nyquist rate to avoid aliasing. Higher the sampling rate easier is the reconstruction at the receiver. Bandwidth requirement is minimum Rb/2 and maximum Rb.

Signal to Quantization Noise Ratio for PCM for sinusoidal input is
SQNR (dB) = 6*n + 1.76, where n is bit of uniform quantizer.

## DPCM

The short form of Delta Pulse Code Modulation is DPCM. In DPCM, a multi-bit difference value is stored. A bipolar D/A converter is used for playback to convert the successive difference values to an analog waveform.

Figures 1 and 2 depicts DPCM encoder and DPCM decoder block diagram.

This modulation scheme encodes difference in current as well as predicted values.

Simple block diagram of linear predictor used in DPCM is depicted in the fig3.

When the samples of a signal are highly correlated then we go for DPCM in order to save bandwidth or using the same bandwidth at higher data rate.

## Delta Modulation

It is a special case of differential PCM. It is called 1 bit DPCM as it transmits only one bit per sample.

In delta modulation problem of slope overload occurs if input is changing very fast that is :
| Δ/Ts | < | dm(t)/dt |

To overcome slope overload error we choose optimum size of Δ such that :
Δopt/Ts = | dm(t)/dt |max

For m(t) = Am*cos(2*π*fm*t)
Δopt/Ts = 2*π*fm*Am

The second problem occurs is hunting, which occurs when message is almost constant.

In adaptive delta modulation, step size is chosen in accordance with message signal sampled value to overcome slope overload error and hunting.
If message is varying at a high rate then step size is high and if message is varying slowly then step size is small.

The short form of Adaptive Delta Pulse Code Modulation is ADPCM. In ADPCM, a difference value that has been mathematically adjusted according to the slope of the input waveform is stored. Bipolar D/A converter is used to convert the stored digital code to analog for playback.

Figures 4 and 5 depicts ADPCM encoder and ADPCM decoder block diagram.

## Modulation types

BPSK -This page describes BPSK modulation technique with equation and constellation diagram.
QPSK -This page describes QPSK modulation technique with equation and constellation diagram.