ST_CM_EnvelopeSpectrum_InitPars

Function block-specific structure with initialization parameters, which are analyzed when the function block is initialized.

TYPE ST_CM_EnvelopeSpectrum_InitPars EXTENDS ST_CM_InitPars :
STRUCT
    nFFT_Length          : UDINT := 512;                            (* Length of FFT. *)
    nWindowLength        : UDINT := 400;                            (* Length of analysis window. *)
    bTransformToDecibel  : BOOL  := TRUE;                           (* Transform to decibel. *)
    fDecibelThreshold    : LREAL := cCM_MinArgLog10;                (* Minimum argument of decadic logarithm for 64-bit IEEE 754 arithmetic. *)
    eWindowType          : E_CM_WindowType := eCM_HannWindow;       (* Window type. *)
    aWindowParameters    : T_CM_WindowParameters := [2.5,1,1,1,1];  (* Window parameters for specific windows, e.g. FlatTop. *)
    nOverlap             : UDINT := -1;                             (* Size of overlap in samples. *)
    eScalingType         : E_CM_ScalingType := eCM_DiracScaling;    (* Scaling type. *)
    nChannels            : UDINT := 1;                              (* Number of channels. *)
END_STRUCT
END_TYPE

• nFFT_Length is the length of the FFT of the spectrum. It must be greater than one and an integral power of two.
• nWindowLength is the length of the analysis window in samples. The length must be greater than one and an even number.
• bTransformToDecibel is a Boolean value that indicates whether the result of the FFT is to be transformed to the decibel scale, according to transformation x → 20 * log10(x).
• fDecibelThreshold is a very small floating point value greater than zero. Values that are less than this number are replaced with this value before any transformation to the decibel scale. (The purpose is the avoidance of value range errors. The logarithm of zero is not defined and strives infinitely towards minus for the limit value of small arguments. The same applies to the argument of the number zero, arg(0). The smallest possible value is 2.3e-308, which is equivalent to the constant cCM_MinArgLog10.)
• eWindowType defines the used window function (of the type E_CM_WindowType). A good default value is the window type eCM_HannWindow.
• eWindowType defines the used window function (of the type E_CM_WindowType). A good default value is the window type eCM_HannWindow.
• aWindowParameters contains the free parameters of selected window functions. When using eCM_KaiserWindow, the first entry defines the parameter beta; if eCM_FlatTopWindow is used, all parameters are used. See section Window functions.
• nOverlap defines the number of overlapping samples. This must be greater than or equal to zero. If the value cCM_OverlapRecommended is selected, a recommended overlap is calculated internally (see F_CM_CalculateRecommendedOverlap); the value cCM_OverlapInactive deactivates the internally used buffer and sets the value to zero.
• eScalingType enables the selection of the scaling to be used (of the type E_CM_ScalingType), in case absolute scaling is required. The default value is eCM_DiracScaling. When selecting the scaling the type of signal should be considered: either deterministic signals or wide-band signals with stochastic portion. Both types require different scalings.
• nChannels defines the number of independent channels. This must be greater than zero.

Avoiding artefacts The value of nFFT_Length must be equal or greater the value of nWindowLength. In order to avoid artefacts in the calculation, nFFT_Length should be at least 25 larger than nWindowLength. An increase in the FFT length in relation to the window length makes sense with this function block in order to avoid circular aliasing.