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Difference between Delta Modulation (DM) and Differential Pulse Code Modulation (DPCM)
In digital communication, Delta Modulation and Differential Pulse Code Modulation are the techniques with fixed step size and it has updated ones with varying step size as Adaptive DM and DPCM. The Correlation between the current sample and a predicted value is quantized for both approaches to function. The main distinction between the two is that while DPCM sends multiple bits per sample, DM only sends one bit per sample. DM is a less complex approach than DPCM, but it also has a worse signal−to−noise ratio.
What is Delta Modulation (DM)?
It follows a linear type and the circuits of both transmitter and receiver are very simple to construct. There are primarily two levels and the step size is not variable. The input is approximated to the step signal ${(\delta)}$. There are two cases in which the step signal can be positive or negative. When the correlation between the input x(t) and the predicted data x^(t) is positive, it is increased by one step ('1'). Similarly, when the difference between them is negative, it is decreased by one step ('0').
Delta Modulation Transmitter
Delta Modulation involves the transmission of input signals like voice signals, and images and is then passed through the quantizer. A quantizer is a device that carries out quantization. Because the encoder's bit value determines the level of quantization, the quantization level is dependent on the encoder. One bit of delay is used in the transmitter to get the same sample signal. Accumulator adds the quantizer output with the preceding sample signal.
Delta Modulation Receiver
At the receiver, Low Pass Filter (LPF) is used to get the original signal along with it a delay circuit is used. The cutoff frequency of LPF is the highest signal frequency. To reduce quantization error and slope overload distortion. , step signal should be small. The sampling duration is also small and the hardware cost is reduced. The Signaling rate(r) increases in delta modulation.
Advantages of Delta Modulation
The Signaling rate is very small.
Simple and no analog to Digital Converter involved.
Disadvantages of Delta Modulation
Slope Overload distortion: The Error arises between the staircase approximated signal and the input signal. When the input signal rises high and so the step signal becomes too small.
Granular Noise: It occurs when the step size is too large.
What is Differential Pulse Code Modulation?
It works on the principle of prediction and converts the analog signal to a digital signal then upon finding the difference it quantizes them. The current value is identified from the previous value and it can either be the true value or not. It involves analog to digital signal conversion. The Signal to noise ratio in DPCM is improved by the prediction gain rather than Pulse code modulation. These methods are used for compressing the images and then transmitting them.
Transmitte
The difference error signal is obtained between the input signal and the reconstructed signal. The transmitter consists of components like Quantizer, an error signal, and a predictor.
Receiver
The output signal from the receiver is more accurate as the predictor considers the complete history of the SNR ratio. The Receiver consists of only the predictor which predicts based on the previous value.
Difference between Delta Modulation and Differential Pulse Code Modulation
Basic Feature |
Delta Modulation (DM) |
Differential Pulse Code Modulation (DPCM) |
|---|---|---|
Bits per sample |
DM transmits only one bit per sample. |
DPCM transmits multiple bits per sample. |
Principle |
DM works on the principle of slope overload distortion. |
DPCM works on the principle of prediction. |
Complexity |
DM is a simpler technique compared to DPCM. |
DPCM is more complex compared to DM. |
Advantages |
It provides a very simple transmitter and receiver circuit. The channel bandwidth and signaling rate are very low. |
DPCM can achieve greater compression ratios compared to PCM by reducing short−term redundancy in the signal. |
Disadvantages |
The main drawbacks are slope overload distortion and granular noise. |
If there is a loss of synchronization between the transmitter and receiver, DPCM may experience error propagation. |
Application |
It is used in a speech transmission system that reproduces sound that is very close to the original sound in terms of accuracy and quality. |
It also produces high fidelity that it reproduces sound as image and video compression. |
Speed |
It has a lower bit rate than pulse code modulation. |
It has a higher bit rate compared to DM |
Conclusion
Although Delta modulation is a less complex method than DPCM, it has a worse signal−to−noise (SNR) ratio. On the other hand, DPCM can achieve better compression ratios compared to PCM by reducing short−term redundancy in the signal and can be employed in applications where high fidelity is required, such as picture and video compression.
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