Group delay
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In physics, and in particular in optics, the study of waves and digital signal processing, the term group delay has the following meanings:
1. The rate of change of the total phase shift with respect to angular frequency, d θ/d ω, through a device or transmission medium, where θ is the total phase shift, and ω is the angular frequency in radians per second, equal to 2πf, where f is the frequency in hertz.
2. In an optical fiber, the transit time required for optical power, traveling at a given mode's group velocity, to travel a given distance.
Note: For optical fiber dispersion measurement purposes, the quantity of interest is group delay per unit length, which is the reciprocal of the group velocity of a particular mode. The measured group delay of a signal through an optical fiber exhibits a wavelength dependence due to the various dispersion mechanisms present in the fiber.
Source: from Federal Standard 1037C
It is often desirable that group delay be constant across all frequencies; otherwise there is temporal smearing of the signal. Because group delay is -d θ/d ω, as defined in (1), it therefore follows that a constant group delay can be achieved if the transfer function of the device or medium has a linear phase response (i.e., θ = θ0 + Kω where K is a constant). The deviation of the group delay from a constant indicates the degree of nonlinearity of the phase.
Group delay in the audio field
Group delay has some importance also in the audio field and especially in the sound reproduction field. Many components of an audio reproduction chain, notably loudspeakers and multiway loudspeakers crossover networks, introduce group delay in the audio signal. It is therefore important to know the threshold of audibility of group delay with respect to frequency, especially if the audio chain is supposed to provide a high fidelity reproduction. At the time of writing no extensive data is available and the best thresholds of audibility table has been provided by Blauert and Laws:
| Frequency | Threshold |
|---|---|
| 500 Hz | 3.2 ms |
| 1 kHz | 2 ms |
| 2 kHz | 1 ms |
| 4 kHz | 1.5 ms |
| 8 kHz | 2 ms |
The table above has been published into the following article:
Blauert, J. and Laws, P "Group Delay Distortions in Electroacoustical Systems", Journal of the Acoustical Society of America, Volume 63, Number 5, pp. 1478–1483 (May 1978)