DADiSP / Octave
1/N Octave Analysis Module
Octave analysis displays the frequency characteristics of a signal
by passing the signal through a bank of bandpass filters. The
center frequency of each filter is twice the center frequency of
the previous filter. Each filter covers a bandwidth that is twice
as wide as the previous band and half as wide as the next band.
One octave corresponds to a doubling of frequency.
Proportional Bandwidth Octave Filters
Unlike standard FFT spectrum analysis, octave filters exhibit
proportional bandwidth. If the lower and upper cutoff frequencies
of a bandpasss filter are fl
, then the center frequency,
, can be determined with:
This proportional bandwidth property divides frequency information
over a log scale and is very useful in analyzing a variety of
natural systems. For example, the human response to noise and
vibration is very non-linear and many mechanical systems have a
behavior that is best characterized by proportional bandwidth
For greater resolution, the frequency range can be divided into
proportional bandwidths that are a fraction of an octave. For
example, with 1/3 octave analysis, there are 3 bandpass filters per
octave where each center frequency is 21/3
the previous center
In general, for 1/N octave analysis, there are N bandpass filters
per octave such that:
where 1/N is called the fractional bandwidth designator.
Digital Bandpass Filter Method
DADiSP/Octave provides 1/N octave analysis using true IIR digital
filters that conform to ANSI S1.11 and IEC 1260 specifications.
Each band filter is designed in accordance to ANSI S1.11 and IEC
1260 specifications by transforming the original analog transfer
function to the digital domain by means of the bilinear transform.
The filter order can be specified and the frequency ratio can be
calculated using the binary or decimal system.
Standard or Custom Center Frequencies
Center frequencies can be specified as a range of ISO band numbers
for standardized octave analysis. User defined center frequencies as a
range or list of values are supported to facilitate custom applications.
Input and Output Options
A, B, and C weighting filters can be applied to the input data.
Output results include mean power, max power, min power and
un-weighted RMS values. The output can be normalized with a
calibration value. The results can be plotted on log or linear
axes and exact or preferred frequency values are supported.
The output can be displayed in a variety of plot styles including
bar, step and line plots.
DADiSP/Octave requires DADiSP 6.0 B17
for information about updating your current
version of DADiSP.