In 1922, according to Nahin, John Renshaw Carson defined the instantaneous frequency of a signal "as the time derivative of the signal's phase angle." In frequency modulation, instantaneous frequency describes the frequency varying above and below the carrier frequency, at the audio tone frequency.
Instantaneous phase and frequency are important concepts in signal processing that occur in the context of the representation and analysis of time-varying functions. The instantaneous phase (also known as local phase or simply phase) of a complex-valued function s(t), is the real-valued function:
where arg is the complex argument function. The instantaneous frequency is the temporal rate of change of the instantaneous phase.
And for a real-valued function s(t), it is determined from the function's analytic representation, s<sub>a</sub>(t):
where represents the Hilbert transform of s(t).
When ÃÂ(t) is constrained to its principal value, either the interval or , it is called wrapped phase. Otherwise it is called unwrapped phase, which is a continuous function of argument t, assuming s<sub>a</sub>(t) is a continuous function of t. Unless otherwise indicated, the continuous form should be inferred.
where ÃÂ > 0.
In this simple sinusoidal example, the constant ø is also commonly referred to as phase or phase offset. ÃÂ(t) is a function of time; ø is not. In the next example, we also see that the phase offset of a real-valued sinusoid is ambiguous unless a reference (sin or cos) is specified. ÃÂ(t) is unambiguously defined.
where ÃÂ > 0.
In both examples the local maxima of s(t) correspond to ÃÂ(t) = 2N for integer values of N. This has applications in the field of computer vision.
Instantaneous angular frequency is defined as:
and instantaneous (ordinary) frequency is defined as:
where ÃÂ(t) must be the unwrapped phase; otherwise, if ÃÂ(t) is wrapped, discontinuities in ÃÂ(t) will result in Dirac delta impulses in f(t).
The inverse operation, which always unwraps phase, is:
This instantaneous frequency, ÃÂ(t), can be derived directly from the real and imaginary parts of s<sub>a</sub>(t), instead of the complex arg without concern of phase unwrapping.
2m<sub>1</sub> and m<sub>2</sub> are the integer multiples of necessary to add to unwrap the phase. At values of time, t, where there is no change to integer m<sub>2</sub>, the derivative of ÃÂ(t) is
For discrete-time functions, this can be written as a recursion:
Discontinuities can then be removed by adding 2 whenever ÃÂÃÂ[n] ⤠âÂÂ, and subtracting 2 whenever ÃÂÃÂ[n] > . That allows ÃÂ[n] to accumulate without limit and produces an unwrapped instantaneous phase. An equivalent formulation that replaces the modulo 2 operation with a complex multiplication is:
where the asterisk denotes complex conjugate. The discrete-time instantaneous frequency (in units of radians per sample) is simply the advancement of phase for that sample
In some applications, such as averaging the values of phase at several moments of time, it may be useful to convert each value to a complex number, or vector representation:
This representation is similar to the wrapped phase representation in that it does not distinguish between multiples of 2 in the phase, but similar to the unwrapped phase representation since it is continuous. A vector-average phase can be obtained as the arg of the sum of the complex numbers without concern about wrap-around.