Phase modulation
Angle modulation encoding message signal as carrier phase variations.
Phase modulation (PM) is a technique used to prepare communication signals for transmission by altering a carrier wave. It works by encoding the message signal into changes in the carrier wave's instantaneous phase. Along with frequency modulation, it is one of the two main types of angle modulation. In this process, the baseband signal’s instantaneous amplitude changes the phase of the carrier, while the carrier’s amplitude and frequency stay the same. This method is a key component of many digital transmission coding systems used in technologies such as Wi-Fi, GSM, and satellite television. However, it is rarely used for sending analog audio signals over radio, because the receiver required is relatively complex and offers no advantage for audio. Phase modulation also appears in digital synthesizers—like the Yamaha DX7—to produce FM synthesis. A related technique, phase distortion, is used in Casio CZ synthesizers. In general, an analog modulation process for a sinusoidal carrier can be written as: `m(t) = A(t) · cos(ωt + φ(t))`. Here, `A(t)` is the carrier’s time-varying amplitude, `ω` is its angular frequency, and `φ(t)` is the instantaneous phase deviation. This equation directly defines two major modulation groups: amplitude modulation, where the angle term stays constant, and angle modulation, where `A(t)` is constant and the second term depends on the modulating signal. The cosine term’s argument—`ωt + φ(t)`—separates angle modulation into frequency modulation (FM) and phase modulation (PM). In FM, the message signal changes the carrier frequency, controlled by both the modulating wave’s frequency and amplitude. In PM, the modulating waveform controls the instantaneous phase deviation `φ(t)`, while the main frequency remains constant. In principle, the modulating signal for either FM or PM can be analog or digital. The mathematics of the spectrum shows two regions of particular interest. The modulation index measures how much the modulated variable deviates from its unmodulated level. For phase modulation, it relates to the carrier’s phase variations: `h = Δθ`, where `Δθ` is the peak phase deviation. Compare this to the modulation index for frequency modulation.
- Modulation index formula
- h = Δθ, where Δθ is the peak phase deviation
- Carrier parameters constant
- amplitude and frequency
- Principal forms of angle modulation
- phase modulation and frequency modulation
Lore & Background
Phase modulation is an integral part of many digital transmission coding schemes that underlie a wide range of technologies like Wi-Fi, GSM and satellite television. However, it is not widely used for transmitting analog audio signals via radio waves, because of the relative complexity needed in the receiver, for no added benefit with audio signals. It is also used for signal and waveform generation in digital synthesizers, such as the Yamaha DX7, to implement FM synthesis. A related type of sound synthesis called phase distortion is used in the Casio CZ synthesizers.
In general form, an analog modulation process of a sinusoidal carrier wave may be described by the equation m(t)=A(t)·cos(ωt+φ(t)). A(t) represents the time-varying amplitude of the sinusoidal carrier wave and the cosine-term is the carrier at its angular frequency ω, and the instantaneous phase deviation φ(t). This description directly provides the two major groups of modulation, amplitude modulation and angle modulation. In amplitude modulation, the angle term is held constant, while in angle modulation the term A(t) is constant and the second term of the equation has a functional relationship to the modulating message signal.
The functional form of the cosine term, which contains the expression of the instantaneous phase ωt+φ(t) as its argument, provides the distinction of the two types of angle modulation, frequency modulation (FM) and phase modulation (PM). In PM, the instantaneous phase deviation φ(t) (phase angle) of the carrier is controlled by the modulating waveform, such that the principal frequency remains constant. In principle, the modulating signal in frequency modulation or phase modulation may either be analog in nature, or it may be digital.
Reader's Guide
Phase modulation is notable as one of the two principal forms of angle modulation, alongside frequency modulation. Its significance lies in its integral role in many digital transmission coding schemes that underlie technologies such as Wi-Fi, GSM, and satellite television. Despite this digital importance, phase modulation is not widely used for transmitting analog audio signals via radio waves, due to the relative complexity needed in the receiver with no added benefit for audio signals. The modulation index for phase modulation is defined as h = Δθ, where Δθ is the peak phase deviation. In the broader context of modulation theory, phase modulation is distinguished from frequency modulation by the fact that in PM the instantaneous phase deviation of the carrier is controlled by the modulating waveform while the principal frequency remains constant, whereas in FM the message signal causes a functional variation of the carrier frequency. Phase modulation also finds application in digital synthesizers, such as the Yamaha DX7, to implement FM synthesis, and a related synthesis method called phase distortion is used in the Casio CZ synthesizers.
Did You Know?
- Phase modulation is one of the two principal forms of angle modulation, together with frequency modulation.
- In phase modulation, the peak amplitude and frequency of the carrier signal are maintained constant.
- Phase modulation is an integral part of digital transmission coding schemes underlying Wi-Fi, GSM, and satellite television.
- The Yamaha DX7 digital synthesizer uses phase modulation to implement FM synthesis.
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