Modulation is the process whereby the digital baseband signal or waveform is superimposed onto a carrier. The carrier may be manipulated in amplitude, frequency and phase.

The most frequently used modulation techniques are 2FSK using two frequencies, MFSK with four or more tones, and differential phase modulation methods such as 2DPSK, 4DPSK, 8DPSK or 16DPSK. On satellite links phase modulation methods such as BPSK or QPSK are used. Other modulation types include AM, FM or phase sub-carrier modulation of an FM, AM or phase shift main carrier.

Modulation methods which directly manipulate the carrier may be decoded using the receiver IF output, or for smaller shifts in the case of FSK, also the AF output. In addition many VHF/UHF receivers also have a direct discriminator output which may be used.

In contrast, modulation methods using sub-carrier modulation must be decoded using the receiver AF output. Decoding indirect modes can only take place using the receiver AF output. The receiver serves as a demodulator for the FM or AM carrier, while the decoder demodulator processes the sub-carrier modulation.