Data simply means unprocessed facts. Data can be analog or digital.
The term analog data refers to information that is continuous; digital data refers to information that has discrete states.
Analog data, take on continuous values.
Digital data take on discrete values.
Digital-to-digital conversion
Digital-to-digital conversion involves three techniques: line coding, block coding, and scrambling.
Line coding is the process of converting digital data to a digital signal.
The most common technique to change an analog signal to digital data (digitization) is called pulse code modulation (PCM).
Digital-to-Analog conversion
Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in the digital data.
Digital-to-analog conversion can be accomplished in several ways: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). Quadrature amplitude modulation (QAM) combines ASK and PSK.
In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes.
In frequency shift keying, the frequency of the carrier signal is varied to represent data. The frequency of the modulated signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes. Both peak amplitude and phase remain constant for all signal elements
In phase shift keying, the phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude and frequency remain constant as the phase changes.