Dual FFT Analyzer Options

Input Options

  • 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.
  • 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.
  • 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 averages has been reached.
  • 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.

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).

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.

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.

Loaded Measurements

  • Trace Opacity
    Use this slider control to adjust the opacity of static data traces (loaded from data files) in the plot.
  • Trace Thickness
    This segmented control can be used to select the thickness of the static frequency data traces in the plot.
  • Load From Data File
    Up to 7 static frequency data traces may be displayed in the plot by loading them from data files that were previously exported from the Dual FFT Analyzer tool.
  • Add New Average Curve
    Static DFFT spectral data traces may also be created by loading data from 1 or more DFFT data files and averaging the data to create a new spectrum. The loaded files must contain data with the same measurement units. When source data with differing frequency resolutions is loaded, the resolution of the new average curve will be defined by the largest (coarsest) resolution of all spectra included in the average. Time data, such as impulse response, is not averaged.