X and Y Input Channels The X signal represents the input to the
system or device under test (DUT). It is typically referred to as the reference signal. The Y
signal represents the output of the system or device under test (DUT). The X and Y channels can
be selected using the input channel selection menus, which are accessible from the Dual FFT
Analyzer's toolbar.
X Delay
The reference input can be delayed by any amount of time from zero up to the measurement
duration. This allows you to time-align the X and Y inputs for the highest quality measurements.
This can be particularly important for measurements involving significant acoustic delays
(expect roughly an additional millisecond of delay for every foot of sound propagation through
air).
Resolution
Measurement Length
Measurement length corresponds to the amount of time domain data in the final measurement. So,
if you need an impulse response or correlation measurement of a certain length (in time), then
choose a measurement length that is at least that amount of time. Setting this value also sets
the frequency resolution of frequency domain data, as the two are inversely related.
Max Frequency Res
Frequency resolution indicates the spacing between adjacent data points (or frequency bins) in
frequency domain data. It is inversely related to measurement length, above, so setting this
value also sets the measurement length.
Overlap %
It takes time to gather enough data to compute an FFT, so for longer FFT's the spectrum analyzer will update less frequently as it waits for data to be gathered for each FFT. Overlap processing
allows the spectrum analyzer to be updated more frequently, which makes for a smoother response.
The overlap percentage can be manually specified with one of the fixed values (50% overlap means
the analyzer will be updated twice as often and 90% overlap will produce updates 10 times as
often). When Overlap % is set to Max, SignalScope will automatically compute an overlap
percentage that allows the spectrum analyzer to be updated as fast as reasonably possible for
the selected sample rate and frequency resolution. Unless you need a specific amount overlap,
it is generally recommended that Overlap % be set to either Off or Max. High overlap, combined
with certain frequency resolutions, can consume too much available processing time with no
apparent benefit.
Apply Smoothing
This switch enables frequency data to be smoothed into fractional octave bands. Lower
frequencies, below the point at which each fractional octave band includes at least one FFT
frequency bin, are kept, but not smoothed.
Smooth Bandwidth
Use this segmented control to select the fractional octave bandwidth of the spectral smoothing.
Smoothing Type
Use this segmented control to select the type of spectral smoothing that will be applied.
Avg smoothing will simply compute the mean magnitude in each fractional octave frequency
band.
Max smoothing (selected by default) will pick the highest magnitude within each
fractional octave frequency band.
Min smoothing will pick the lowest magnitude within each fractional octave frequency
band.
In general, Max smoothing works better when peaks are present in the spectrum. Avg
smoothing may be more suitable when making basic frequency response measurements using a
broadband excitation signal.
Min Frequency (Fmin)
A minimum frequency can be specified for the spectrum display. Only values that correspond to
frequencies between Fmin and Fmax will be plotted. If Export Full Spectrum is disabled, only
those same values will be included when exporting spectral data to a file. The Cursor Peak Track
function will operate within the bounds of Fmin and Fmax.
Max Frequency (Fmax)
A maximum frequency can be specified for the spectrum display. Only values that correspond to
frequencies between Fmin and Fmax will be plotted. If Export Full Spectrum is disabled, only
those same values will be included when exporting spectral data to a file. The maximum frequency
can be specified independent of the hardware sample rate.
The Cursor Peak Track function will operate within the bounds of Fmin and Fmax.
Auto Adjust Sample Rate
When this option is enabled, the hardware sample rate will be automatically adjusted to an
optimal value for the current Fmax. For example, if Fmax is set to a low frequency (e.g. less
than 4 kHz), the sample rate will automatically be set to 8 kHz, if the hardware supports it (8
kHz is the standard minimum sample rate for iOS devices and many USB audio devices).
Averaging
Averaging
Successive measurements may be averaged according to the selection of this
segmented control. Available average types include: Linear
(Stable) and Exponential. Linear averaging weights
all measurements equally. Exponential averaging applies an exponential
weighting such that more recent measurements contribute to the current
average value more than older measurements. When linear averaging is selected, the
analyzer will automatically stop after the specified number of measurements have been averaged.
Averages
The number of measurements to include in the current average is specified in the Averages text
box. When linear averaging is the selected average type,
the analyzer will automatically stop after the specified number of
measurements have been included in the average. Exponential averaging
allows the analyzer to continue running even after the specified number
of measurements have been processed. The exponential average can be
thought of as including only the most recent N (time-weighted) measurements, where N is the
specified number of averages.
Windowing
X/Y Data Window
SignalScope provides a selection of different data windows that may be applied to the time domain
data, independently for X and Y inputs, before the application of the FFT. See the Data Window Selection page for more information on available
windows and user-defined parameters.
Triggering
Enable
Use this switch to turn input triggering on or off. The Dual FFT Analyzer uses Normal mode
triggering, which means that the measurement is only completed (and averaging updated) when a
trigger event is detected on the reference (X) input.
Slope
A trigger event occurs when the reference (X) signal exceeds some user-defined
threshold value, with a user-defined slope (positive or negative).
You may specify the slope by selecting Positive or Negative in the segment control.
Threshold
A trigger event occurs when the reference (X) signal exceeds some user-defined
threshold value, with a user-defined slope (positive or negative).
You may specify the threshold value by tapping on the text box and entering a desired value.
Pre-trigger Delay
The pre-trigger delay specifies how much of the raw time signal is to be preserved and included
prior to the trigger event.
You may specify a value between 0 and 100 ms. The delay may also be limited by the length of the
measurement.
Manually Accept Data
When this option is enabled, once the triggered data is captured, a preview display will be
presented which allows you to see the raw time data for X and Y inputs and choose whether to
accept or reject it. If it is accepted, the Dual FFT calculation will be completed and the
results updated (including averaging).
This is useful to ensure valid measurements when acquiring data for modal analysis, e.g., using
an impact hammer and accelerometers.
Excitation Signal
Enable Excitation Signal
The Dual FFT Analyzer includes it's own built-in excitation signal generator that keeps
frequency sweeps and pseudorandom noise sequences synchronized with the measurement length.
This allows measurements to be made with high signal-to-noise ratios with little or no
averaging.
Output Channel A/B
The excitation signal will be broadcast through the selected output channels of the current audio
device.
Signal Type
The excitation signal can be a periodic frequency sweep (swept-sine or chirp) or a periodic
pseudorandom noise sequence.
Presends
When measuring an acoustic space, it is often desirable to sufficiently excite that space before
commencing a measurement. The number of pre-sends specifies how many times the selected
excitation sweep or noise sequence should be output before the measurement begins. Each pre-send
is equal in duration to the current measurement duration.
Level
The excitation signal level may be specified in dB, relative to full scale amplitude (0dB
corresponds to maximum amplitude).
Data Export
Current Location
When this option is enabled, the latitude, longitude, and altitude at the time of the
measurement will be included in exported data files. Horizontal and vertical accuracy (in
meters) will also be included.