In signal processing, group delay is the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component. Phase delay, in contrast, is the time delay of the phase as opposed to the time delay of the amplitude envelope.
All frequency components of a signal are delayed when passed through a device such as an amplifier, a loudspeaker, or propagating through space or a medium, such as air. This signal delay will be different for the various frequencies unless the device has the property of being linear phase. The delay variation means that signals consisting of multiple frequency components will suffer distortion because these components are not delayed by the same amount of time at the output of the device. This changes the shape of the signal in addition to any constant delay or scale change. A sufficiently large delay variation can cause problems such as poor fidelity in audio or intersymbol interference (ISI) in the demodulation of digital information from an analog carrier signal. High speed modems use adaptive equalizers to compensate for non-constant group delay.
For a device such as an amplifier or telecommunications system, group delay and phase delay are device performance properties that help to characterize time delay, which is the amount of time for the various frequency components of a signal to pass through the device from input to output. If this timing does not sufficiently meet certain requirements, the device will contribute to signal distortion. For example, sufficient amounts of distortion equates to poor fidelity in video or audio, or to a high bit-error rate in a digital bit stream.
Phase delay directly measures the device or system time delay of individual frequency components. For a modulated signal, which must be demodulated to recover the original signal intelligence, group delay must be used with the modulated signal to determine the time delay of the demodulated signal.
Discussed in this article is background theory on a device’s phase response property, from which the device’s phase delay and group delay properties can be calculated exactly. The article then goes on to illustrate the theory and use cases of these related device properties.