A modulation is a process by which the properties of a particular radio signal, including phase, frequency or amplitude, are changed to transmit data. Some of the types of modulations include amplitude-shift keying, frequency-shift keying, and phase-shift keying. These types of modulations are part of a category called quantized radio modulation modes, meaning the modes are generally utilized and defined as quantized points. Amplitude-shift keying is a design of the key of an old telegraph, and is used in digital modulation where two of a certain carrier signal’s amplitudes represent two values that are binary. A signal’s amplitude changes while phase and frequency remain constant, creating signal elements from the carrier signal’s amplitude variations.
Modulated signals in amplitude-shift keying are represented as either on or off. Voice signals vary in voltage, and this gets applied to carrier signals by amplitude modulation, or amplitude-shift keying. In communication of digital signals, two or more distinct amplitude levels are represented in the process of amplitude modulation. Usually, the modulated waveforms are binary, consisting of two on or off levels that are represented as 1 or 0, respectively. The modulated waveforms of the carrier signal are sinusoid bursts while silent voice signals are modulated as flat lines.
At transition points of the modulated signal, there are jagged discontinuities that result in bandwidth being unnecessarily wide. In digital communication, it is desirable that modulated signals have no distortions, including overlaps and temporal spreading of single phases. These distortions, which are known as intersymbol interferences, degrade a communication system’s error performance. This makes it difficult to reliably differentiate between state changes of received signals. Achieving the conditions where no intersymbol interference occurs is called Nyquist ISI criterion.
Nyquist ISI criterion conditions can be met by two different methods. One of these methods involves equalization, or filtering a received signal. This cancels out intersymbol interference that is introduced in a response of channel impulse.
Another method to achieve Nyquist ISI criterion is designing pulses of band limited transmission to minimize and reduce ISI effects. In band limiting, the jagged discontinuities are rounded off; this can be applied to a modulated signal or transmitted digital message before the transmission. Linearity is a factor that is important and amplitude-shift keying can make signal processing more difficult because of this. Amplitude-shift keying has no constant envelope, whereas frequency-shift keying and phase-shift keying do have one.