Modulation technology is a technology that converts baseband signals into transmission signals. After sampling and quantizing the analog signal, the optical carrier is modulated with binary digital signal "1" or "0" for pulse coding (PCM). The advantage of digital modulation is that it has strong anti-interference ability, and the influence of noise and dispersion will not accumulate during relay, so it can realize long-distance transmission. Its disadvantage is that it needs a wider frequency band and the equipment is complicated.

Baseband signal is the original electrical signal, which generally refers to the basic signal waveform and the corresponding electrical pulse in digital communication. In wireless telemetry and remote control system and radio technology, modulation is to control the parameters (amplitude, frequency and phase) of high-frequency carrier with baseband signal, so that these parameters change with baseband signal. Baseband signals used to control the parameters of high-frequency carriers are called modulation signals. Unmodulated high-frequency electric oscillation is called carrier wave (it can be sine wave or non-sine wave, such as square wave, pulse sequence, etc. ).

When 1 is sent, the carrier phase of the last symbol changes by 180 relative to the previous symbol, while the carrier phase between the previous symbols remains unchanged when 0 is sent. Therefore, demodulation only depends on the relative change of carrier phase. Don't look at its absolute phase. As long as the phase jumps 180, it means transmitting 1. If the phase is unchanged, 0 is transmitted. Differential phase shift keying (DPSK) has strong anti-interference ability, no need to transmit reference phase, and its implementation is simple.