Modulation changes radio signal properties to transmit data. Types include amplitude, frequency, and phase shift coding. Amplitude-shift keying is used in digital modulation. Modulated signals have jagged discontinuities causing intersymbol interference, which can be cancelled out by equalizing or filtering received signals or designing bandwidth-limited transmit pulses. Amplitude-shift coding does not have a constant envelope.
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 coding, frequency shift coding, and phase shift coding. These types of modulations are in a category called quantized radio modulation modes, which means that the modes are typically used and defined as quantized dots. Amplitude-shift keying is an older telegraph key design and is used in digital modulation where two of a certain carrier signal represent two binary values. The amplitude of a signal changes while phase and frequency remain constant, creating signal elements from changes in the amplitude of the carrier signal.
Signals modulated in amplitude-shift coding are represented as on or off. Speech signals vary in voltage and this is applied to carrier signals by amplitude modulation or amplitude keying shift. In digital signal communication, two or more distinct amplitude levels are represented in the amplitude modulation process. Typically, modulated waveforms are binary, consisting of two levels of on or off represented as 1 or 0, respectively. Carrier signal modulated waveforms are sine bursts while silent speech signals are modulated as flat lines.
At the transition points of the modulated signal, there are jagged discontinuities which result in an unnecessarily large bandwidth. In digital communication, it is desirable that the modulated signals have no distortions, including overlaps and time spreading of the individual phases. These distortions, known as intersymbol interference, degrade the error performance of a communication system. This makes it difficult to reliably distinguish between state changes of received signals. The achievement of the conditions in which no intersymbol interference occurs is called the Nyquist ISI criterion.
The conditions of the Nyquist ISI criterion can be satisfied by two different methods. One such method involves equalizing or filtering a received signal. This cancels out the intersymbol interference that is introduced into a channel impulse response.
Another method of achieving the Nyquist ISI criterion is the design of bandwidth-limited transmit pulses to minimize and reduce ISI effects. In band limiting, jagged discontinuities are rounded; this can be applied to a modulated signal or broadcast digital message prior to transmission. Linearity is an important factor and for this reason manipulating the amplitude shift can make signal processing more difficult. Amplitude-shift coding does not have a constant envelope, while frequency-shift coding and phase-shift coding have one.
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