Digital Signal Processing Reference
In-Depth Information
Sonogram
Frequency - time- landscape
Narrow
GAUSSIAN window in the time domain
means
poor
resolution in the frequency domain
and
high
resolution in the time domain
Δ
f
t
Δ
Time domain of the entire signal
Sonogram
Frequency - time- landscape
Wide
GAUSSIAN window in the time domain
means
high
resolution in the frequency domain
and
poor
resolution in the time domain
Δ
f
Δ
t
Illustration 61:
Sonogram: "Satellite picture" of the frequency-time landscape
The sonogram shown here corresponds to the frequency-time landscape from Illustration 60. Here different
colours are used to highlight the “third dimension” - the level of the amplitudes. The horizontal axis is the
time axis, the vertical axis is the frequency axis.
Sonograms are used in technology and science as “acoustic finger prints”. Whether the problem is to iden-
tify bird songs or the voices of criminals the sonogram provides an un mistakable complex picture full of
information in the frequency time landscape.
In the present case the relatively good time and the relatively poor frequency resolution with a narrow
short time window can be seen in the top picture and in the lower picture with a wider window. What is
particularly striking in the lower picture is the time overlapping - that is the time uncertainty! - of the
individual signal segments. The “noise band” on the left is much wider and protrudes into the field of the
100Hz signal segment.
