Delta modulation

Delta modulation (DM or Δ-modulation) is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. DM is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples is encoded into n-bit data streams. In delta modulation, the transmitted data is reduced to a 1-bit data stream.

Its main features are:

• the analog signal is approximated with a series of segments
• each segment of the approximated signal is compared to the original analog wave to determine the increase or decrease in relative amplitude
• the decision process for establishing the state of successive bits is determined by this comparison
• only the change of information is sent, that is, only an increase or decrease of the signal amplitude from the previous sample is sent whereas a no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous sample.

To achieve high signal-to-noise ratio, delta modulation must use oversampling techniques, that is, the analog signal is sampled at a rate several times higher than the Nyquist rate.

Derived forms of delta modulation are continuously variable slope delta modulation, delta-sigma modulation, and differential modulation. The Differential Pulse Code Modulation is the super set of DM.

Principle

Fig. 1 - Block diagram of a Δ-modulator/demodulator

Delta modulation is based on quantizing, not the absolute value of the input analog waveform, but only the difference between the current and the previous step. Let's examine a system block diagram.

The modulator is made by a quantizer which converts the difference between the input signal and the average of the previous steps. In its simplest form, the quantizer can be realized with a comparator referenced to 0 (two levels quantizer), whose output is 1 or 0 if the input signal is positive or negative. The demodulator is simply an integrator (like the one in the feedback loop) whose output rises or falls with each 1 or 0 received. The integrator itself constitutes a low-pass filter.

See also

• Analog-to-digital conversion - ADC
• Codec
• Pulse-code modulation - PCM
• Pulse-density modulation - PDM
• Direct-Stream Digital - DSD

Bibliography

• Steele, R. (1975). Delta Modulation Systems. London: Pentech Press. ISBN 0-4708-2104-3.

External links

Delta Modulator