Delta Modulation: Definition, Principle, and Applications

Delta modulation is the conversion of analog to digital and vice versa. Modulation is the technique of transforming data into waves by adding information to a carrier signal. A signal is said to have a stable waveform if its amplitude and frequency remain consistent. By altering the frequency, phase, amplitude, and polarization of optical signals, electrical signals, and even quantum signals, one can add information to the carrier.

For improved sampling, a signal's sample rate must be greater than the Nyquist rate. The step size will be very small if this sampling period in Differential PCM is significantly lowered since the sample-to-sample amplitude difference will be as small as 1-bit quantization.

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KeyTerms: Delta modulation, Sampling interval, Quantization, Delta demodulator, Adaptive delta modulation, Granular noise, Slope overload. 

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Delta Modulation 

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Delta modulation is the term given to the form of modulation in which the sampling rate is significantly higher and the step - size following quantization is of a lower value Δ.

Delta Modulation, often known as 1-bit DPCM scheme, is a condensed version of the DPCM technology. The signal correlation will increase when the sampling interval is decreased.

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Features of Delta Modulation

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The features of Delta Modulation are as follows:

 

• To utilize the signal correlation to its fullest potential, an oversampled input is used.
• Quantization's design is simple.
• The input sequence exceeds the Nyquist rate by a wide margin.
• The quality is average.
• The modulator and demodulator have simple designs.
• output waveform approximated by a staircase.
• The step-size, i.e., Δ\ Delta is extremely small.
• The user has control over the bit rate.
• This requires easier implementation.

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Principle of Delta Modulation

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The modulator is created by translating the difference between the input signal and the integral of the earlier steps through a quantizer. The simplest way to implement a quantizer is to use a comparator called 0 (two levels quantizer), whose output depends on whether the input signal is positive or negative and outputs 1 or 0. The demodulator is only an integrator in a feedback loop whose output changes with each 1 or 0 received. In and of itself, the integrator functions as a low-pass filter.

Block diagram of Delta modulator 

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Transfer Properties 

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The two sources of noise in delta modulation are

• "Granularity," which happens when the step size is too high, 
• "Slope overload," which happens when the step size is too small to match the original waveform. 

However, a 1971 study reveals that slope overload is less disturbing than granularity based purely on SNR measures.

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Block Diagram of Delta Modulator 

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According to the above diagram, the delta modulator consists of two summer circuits, a delay circuit, and a 1-bit quantizer. The waveform produced by the delta modulator approximates a staircase. The waveform's step size is indicated by the delta (Δ). The output quality of the waveform is average. To attain a high signal-to-noise ratio, DM needs to alter oversampling methods. In oversampling techniques, the analogue signal is sampled at a rate that is many times faster than the Nyquist rate.

A delta-modulated signal must be transmitted with a certain amount of bandwidth, measured in bits per second. The sampling frequency and this signal's sampling frequency are identical. The bandwidth required to send the modulated signal can be calculated using the formula given below. 

The required bandwidth to transmit the modulated signal = ffs samples/seconds x 1 bit/sample = ffs bits/second

Where, 

ffs is the frequency of sampling signal

Delta Demodulator 

The delay circuit, low pass filter, and summer of the delta demodulator are coupled as shown in the diagram below. The demodulator receives no presumed input because the prediction circuit has been eliminated.

Block diagram of Delta demodulator 

A low pass filter is incorporated into the circuit to improve out-of-band signals and reduce noise. Granular noise, which is also known as the step-size inaccuracy, is eliminated at the transmitter. The modulator output and demodulator input are equal when there is no noise.

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Advantages and Disadvantages of Delta Modulation

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Delta modulation exhibits both advantages and disadvantages. They are: 

Advantages of Delta Modulation

The advantages of delta modulation are as follows: 

• Design is basic and easy.
• A 1-bit quantizer is used.
• Modulator and demodulator design is simple.
• The quantization design for delta modulation is relatively simple.
• The user can create the bit rate.

Disadvantages of Delta Modulation

The disadvantages of delta modulation are as follows: 

• Slope overload distortion, a sort of noise, is visible when the delta value is small.
• Granular noise, a form of noise, appears when the value of the delta is high.

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Adaptive Delta Modulation

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A more advanced kind of delta modulation is adaptive delta modulation. Granular noise and slope overload distortion are the two types of noise that are present in delta modulation, as known. To lessen this kind of noise, an adaptive delta modulation approach was created. The method described here reduces the delta modulation technique's slope inaccuracy. Both slope overload and granular error are completely eliminated by this process. During the demodulation procedure, an LPF is utilized to lessen quantized noise (low pass filter).

Advantages of Adaptive Delta Modulation

Here are a few advantages of adaptive delta modulation. They are: 

• The performance of adaptive delta modulation is very high.
• The use of correction circuits in radio design and error detection is reduced by this method.
• Due to the variable step size's wide range of values, dynamic range is great.
• Granular error and slope overload error are not present.
• Slope error is decreased.

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Applications of Delta Modulator 

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Modern uses for delta modulation include, but are not limited to, reproducing old-school synth waveforms. 

• As FPGAs and game-related ASICs become more widely available, sample rates may now be readily controlled to prevent slope overload and granularity issues. 
• The sampling rate of the C64DTV, for instance, was 32 MHz, which offered adequate dynamic range to mimic the SID output at tolerable levels.

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Things to remember 

• Modulation is the technique of transforming data into waves by adding information to a carrier signal.
• Delta modulation is the conversion of analog to digital and vice versa
• Granular noise and slope overload distortion are the two types of noise that are present in delta modulation. 
• The modulator output and demodulator input are equal when there is no noise.
• An adaptive delta modulation approach was created to overcome noise in delta modulation.
•  The signal correlation will increase when the sampling interval is decreased.