Delta modulation (DM, ΔM, 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 are reduced to a 1-bit data stream representing either up (↗) or down (↘). Its main features are:

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. Differential pulse-code modulation is the superset of DM.

Rather than quantizing the value of the input analog waveform, delta modulation quantizes the difference between the input signal and the integral of all previous quantization steps. This quantized signal effectively represents the derivative of the input signal, so the original signal is recovered by integration, as shown in the block diagram in Fig. 2:

In its simplest form, the quantizer can be realized with a comparator referenced to 0 (a two-level quantizer), whose output is 1 or -1 depending on whether the quantizer's input is positive or negative. The demodulator contains an integrator (just like the one in the feedback loop) whose output rises or falls with each 1 or -1 received. An optional low-pass filter will remove high frequency zigzags (see the blue output signal of Fig. 1), so only frequencies in the band of interest remain, to recover a smooth cleaned version of the original signal.

Because each sample is only 1 bit, the transmission bit rate equals the sampling rate.

The two sources of noise in delta modulation are slope overload, when step size is too small to track the original waveform, and granularity, when step size is too large. But a 1971 study shows that slope overload is less objectionable compared to granularity than one might expect based solely on SNR measures.

In delta modulation, there is no limit to the number of pulses of the same sign that may occur, so it is capable of tracking signals of any amplitude without clipping provided that the signal doesn't change too rapidly. However, if an input signal ${\displaystyle m(t)}$ has a derivative ${\displaystyle {\dot {m}}(t)}$ larger than