Delta Modulation

The Nyquist rate, often known as frequency, is the smallest rate at which a finite bandwidth signal must be sampled in order to retain all of its information. The signal’s sampling rate is kept higher than the Nyquist rate in the differential pulse code modulation procedure to acquire a comparatively better sampling rate.

When the sampling interval is reduced in the Differential Pulse Code Modulation (DPCM) process, the sample-to-sample amplitude difference becomes minimal, similar to a 1-bit quantization difference. As a result, the step size will be quite small.

Delta modulation

A delta modulation (DM or ∆-modulation) is an analog-to-digital and digital-to-analog signal conversion technique used for speech transmission when quality isn’t a priority. The difference between successive samples is encoded into n-bit data streams in DM, the simplest form of differential pulse-code modulation (DPCM). The transmitted data is reduced to a 1-bit data stream in delta modulation. Its main characteristics are as follows:

• A succession of segments are used to approximate the analogue signal.

• The preceding bits are compared to each segment of the estimated signal, and the subsequent bits are determined by this comparison.

• Only the change of information is conveyed, i.e. only an increase or decrease in the signal amplitude from the previous sample, whereas the no-change condition keeps the modulated signal in the same 0 or 1 state as the previous sample.

Delta modulation requires oversampling techniques to produce a high signal-to-noise ratio, which means the analogue signal is sampled at a rate several times greater than the Nyquist rate.

Principle

Delta modulation quantizes the difference between the current and previous steps, rather than the value of the input analogue waveform, as seen in the block diagram in Fig.

Fig: Block diagram of a Delta modulator

A quantizer translates the difference between the input signal and the integral of the preceding stages to create the modulator. The quantizer can be realised in its simplest version using a comparator referred to 0 (two levels quantizer), whose output is 1 or 0 depending on whether the input signal is positive or negative. The demodulator is just a feedback loop integrator whose output increases or decreases with each 1 or 0 received. The integrator is a low-pass filter in and of itself.

Transfer characteristics

“Slope overload,” which occurs when the step size is too tiny to match the original waveform, and “granularity,” which occurs when the step size is too high, are the two sources of noise in delta modulation. However, based only on SNR measurements, a 1971 study suggests that slope overload is less disagreeable than granularity.

Block diagram of delta modulator

“Slope overload,” which occurs when the step size is too tiny to match the original waveform, and “granularity,” which occurs when the step size is too high, are the two sources of noise in delta modulation. However, based only on SNR measurements, a 1971 study suggests that slope overload is less disagreeable than granularity.

The delta modulator consists of a 1-bit quantizer, a delay circuit, and two summer circuits, as shown in the diagram above. The delta modulator’s output is a stair-case approximated waveform. The delta () is the waveform’s step size. The waveform’s output quality is average. DM must modify oversampling techniques in order to achieve a high signal-to-noise ratio. The analogue signal is sampled many times faster than the Nyquist rate in oversampling techniques.

For the transmission of a delta-modulated signal, the bandwidth in bits/second is required. This signal has the same sampling frequency as the sampling frequency. Using the formula below, we can determine the bandwidth required to transport the modulated signal.

ffs samples/second X 1 bit/sample = needed bandwidth to transmit the modulated signal

ffs bits per second

Where,

The sampling frequency of the signal is given by fs.

Demodulator Delta

The delta demodulator is depicted in the diagram below. As shown in the diagram, the delta demodulator consists of a delay circuit, a low pass filter, and a summer. There is no presumed input sent to the demodulator because the prediction circuit has been eliminated.

The circuit includes a low pass filter for noise reduction and improved out-of-band signals. Granular noise is eliminated at the transmitter, and the step-size inaccuracy is referred to as granular noise. When there is no noise, the modulator output is equal to the demodulator input.

Adaptive delta modulator

Adaptive delta modulation is a more advanced kind of delta modulation. The types of noise encountered in delta modulation are granular noise and slope overload distortion, as we all know. An adaptive delta modulation technique was developed to reduce this type of noise. In this method, the slope inaccuracy exhibited in the delta modulation technique is decreased. This procedure entirely eliminates slope overload and granular mistake. An LPF is used to reduce quantized noise during the demodulation process (low pass filter).

Advantages of adaptive delta modulation

• Adaptive delta modulation is a high-performance technique.

• In radio design and error detection, this strategy reduces the need for corrective circuits.

• Because the changeable step size covers a wide range of values, the dynamic range is great.

• There is no evidence of slope overload error or granular error.

• The amount of slope error is reduced.

Advantages of delta modulator

• The technique of adaptive delta modulation is a high-performance one.

• This method eliminates the requirement for correction circuits in radio design and mistake detection.

• The dynamic range is large because the variable step size covers such a wide range of values.

• There is no evidence of granular or slope overload mistake.

• Slope inaccuracy is decreased to a lesser extent.

Disadvantages of delta modulator

• Slope overload distortion, which is a sort of noise, is visible when the delta is tiny.

• Granular noise, a form of noise, is visible when the value of delta is significant.

Applications of delta modulator

Delta Modulation’s modern applications include, but are not limited to, replicating vintage synthesiser waveforms. Sample rates may now be easily regulated to avoid slope overload and granularity difficulties, thanks to the growing availability of FPGAs and game-related ASICs. The C64DTV, for example, had a sampling rate of 32 MHz, which provided enough dynamic range to replicate the SID output to acceptable levels.

Conclusion

An analogue signal’s Delta Modulation and Demodulation techniques. Delta Modulation transfers one bit every sample, although it has downsides (slope overload distortion and granular noise), as we all know. To solve these issues, we can employ Adaptive Delta Modulation Signal. In practice, we receive the output of delta modulation, although there is some noise, which has been avoided by applying appropriate parameters with the correct values.

Utilise the proper parameters with the appropriate values.

As a result, we can observe that it is working and that it is being used in real-world situations.

Limitation: Different components of our circuit operated admirably, and we were able to obtain the correct waveform and results for each one. However, to obtain a perfect output waveform of the entire circuit, the input-output independence of separate sections must be synced.