soundgen (version 3.0.0)

getPeakFreq: Get peak frequency

Description

Performs STFT and finds the dominant frequency within a target range for each frame, together with its spectral purity (proportion of normalized spectral magnitude at the peak) and, when a meaningful DC component is present, the modulation depth derived from the AC/DC ratio. Used by "getAM_env" for measuring amplitude modulation and by analyze for measuring frequency modulation.

Usage

getPeakFreq(
  x,
  samplingRate,
  freqRange = NULL,
  overlap = 75,
  parab = TRUE,
  plot = FALSE
)

Value

A dataframe with one row per STFT frame:

time

time stamp, ms

freq

peak frequency, Hz (NA for frames with no usable energy in the target frequency range)

purity

peak magnitude as a proportion of the frame's total spectral magnitude. Normally approximately 0 to 1, but not clamped; values outside this range are preserved as diagnostic information.

dep

approximate modulation depth, on a 0 to 100 scale for an ideal sinusoidal envelope. Not clamped; values outside 0-100 can occur for non-sinusoidal or otherwise uncalibrated inputs. NA if the DC component is zero or non-finite.

Arguments

x

numeric vector (amplitude envelope, pitch contour, etc.)

samplingRate

sampling rate of x, Hz

freqRange

a vector of length 2: the frequency range (Hz) in which to search for the peak. The DC component (0 Hz) is always extracted from the full spectrum, regardless of freqRange.

overlap

overlap between consecutive STFT frames, % (default 75, i.e. step = window length / 4)

parab

if TRUE, refines the peak location by parabolic interpolation on a log10 scale. Purity is based on the raw peak bin, not on the interpolated amplitude.

plot

if TRUE, produces a simple plot

Details

For a sinusoidal modulator with depth m = amDep / 100, the one-sided (un-doubled) envelope spectrum satisfies |X(f_am)| / |X(0)| = m / (4 - 2m), giving amDep = 400 * purity / (dc + 2 * purity). The sum-to-one normalization cancels in this ratio, so the formula is unaffected by spectral leakage.

Examples

{
# White noise with sinusoidal AM
amFreq = 10; amDep = 60
am = .5 + .5 * cospi(pi * amFreq * (1:1000) / 4000)
env = rnorm(4000) * (1 - am * amDep / 100)
plot(env, type = 'l')

soundgen:::getPeakFreq(env, samplingRate = 4000, freqRange = c(5, 50))
}