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Dual Channel Processing Options

Dual channel spectrum analysis provides several very powerful advanced measurements.

You must select the Stereo format before these options are available.  Note that once you have opened or recorded a mono file, you cannot change to stereo; you must first close the file.  If you are in the Real Time mode, you can change between mono or stereo at any time.  It is also possible to combine two mono files into a single stereo file using the <File><Merge> command. 

Independent Scaling and Calibration
This allows each channel to be scaled and calibrated independently.  When selected each channel will be displayed in a separate window instead of in a split screen view.  In addition the Scaling and Calibration dialogs will allow you to choose separate parameters for each channel.  In addition, separate smoothing windows can be selected for each channel.

This feature allows you to analyze two completely different types of signals at once.  For example you can configure the analyzer to show accelerometer data scaled in G's in one channel and 1/3 octave data scaled in SPL from a microphone in the other.

Dual Channel Processing Options:
  • Left Channel Only - processes and displays the auto-spectrum (magnitude) of the left channel only.
  • Right Channel Only - processes and displays the auto-spectrum (magnitude) of the right channel only.
  • Both Left and Right - processes and displays the auto-spectrum (magnitude) of both the left and right channels.  The results are displayed in independent windows.
  • Add Left and Right - add the two channels.  This performs a vector sum - if the channels are 180 out of phase the result is zero.
  • Average Left and Right - Averages the auto-spectrum of the left and right channels.  
  • Real Transfer Function - computes and displays the real transfer function between the two channels.
  • Cross Spectrum - computes and displays the cross-spectrum of both channels.
  • Complex Transfer Function - computes and displays the complex transfer function between the two channels
  • Coherence Function - computes and displays the coherence function between the two channels.

    Notes:
    These are spectral processing options and do not apply to the time series view.

    The Cross-Spectrum, Complex Transfer Function and Coherence Functions operations are very sensitive to phase delays between channels.  Use the Cross Channel Delay feature to delay one channel with respect to the other.

    Definitions
    Auto-spectrum - calculated by multiplying a spectrum by its complex conjugate (opposite phase).  The auto-spectrum is real and is identical to the magnitude response.  The auto-spectrum is what is normally displayed in the single channel spectrum view.

    Cross-spectrum - calculated by multiplying one spectrum by the complex conjugate of a second spectrum.   The cross-spectrum is a complex quantity (has both real and imaginary components).  The magnitude of the cross-spectrum represents the amount of power common to both channels, and the phase represents the phase difference between channels.  For example, If two identical signals are 180 degrees out of phase, the magnitude of the cross spectrum is zero.  If two channels are identical, their cross-spectrum is equal to the auto-spectrum of either channel alone.

    Transfer Function - is the ratio between two channels and can be computed in one of two ways:
    Real - this is the simplest method and is simply the ratio between the auto-spectrum (magnitude) of each channel.  The relative phase of each channel does not contribute to the result.  
    Complex - this is the ratio between the cross-spectrum of the two channels and the auto-spectrum of the reference channel.  The relative phase of each channel contributes to the result.  

    The transfer function is a very powerful feature.  It allows you to quickly and accurately compute the frequency response of a device under test.  See the topic "Transfer Functions" for more information.

    Coherence Function - is the ratio of the squared magnitude of the cross-spectrum and the product of the auto-spectrum of both channels.   It  measures the degree of linearity between the channels and is analogous to the squared correlation coefficient used in statistics.  Two perfectly coherent signals have a coherence value of 1.0 (0 dB).


    See Also:  Cross Channel Delay, Settings, Sampling Format