# Enhancement of Ultrasonic Guided Wave Signals Using a Split-Spectrum Processing Method
## 2 Split-Spectrum Processing (SSP)
### SSP Filter Bank Parameters  
The SSP filter-bank parameters were first investigated by trial-and-error for NDT applications as they were processed. However, this was not very practical for field inspection as there are typically large amounts of data to analyse; therefore, researchers sought to find the optimum values for NDT methods. Hence, the optimum values have been proposed, developed and examined for UT techniques. However, these values are not suitable for GWT due to the long duration and narrow bandwidth of the signal that operates in the kHz range, whereas traditional UT operates in the MHz range. Therefore, further investigation was required to find an optimum value suitable for GWT. As a result, the rules and key factors of parameter selection for conventional UT have been reviewed and then modified for use in GWT. Figure 3 illustrates the scheme of SSP filtering. The parameters that need to be quantified are listed below with a brief description:  
1. The total bandwidth for processing (B); this needs to be large enough such that the reflections of the signal from features in the specimen are constant across this range, and the reflections from coherent noise vary. If the bandwidth is too large, then it may cause the features to be lost, as at least one of the filter outputs will not contain the feature signal. Hence it reduces the spatial resolution in the processing. In general, narrowband waveforms were used as the excitation signals to reduce the effect of unwanted wave modes, and to suppress the dispersion effect in the GWT response. Hence, the bandwidth of the transmitted signal could be employed as the total bandwidth of processing. 2. The filter separation (F); is the distance between the sub-band filters. [PERSON] et al. [16] claimed that the optimum spectral splitting could be attained by using the frequency-sampling theorem, whereby the spectrum of a time-limited signal can be reconstructed from sample points in the frequency domain separated by 1/T Hz, where T is the total duration of the signal. Note that the Gaussian filter is employed for calculating the filter bank in practice, due to its simplicity, whereas the Sinc function was utilised for actual calculation. Thus, the filter separation could be calculated as F = 1/T. 3. The sub-band filter bandwidth (B\({}_{\mathrm{Bilt}}\)) is the width to be used for each filter in the filter bank. It was recommended [15; 16; 17] that the value of the sub-band filter bandwidth needs to be set at three to four times the filter separation. It should be noted that a bandpass filter could reduce the temporal resolution of the signal. This is because reducing the bandwidth of a time-limited signal will increase its duration. This means that the SSP filter bank needs to be selected precisely, otherwise it could lead to a reduction in temporal resolution, as the pulses that correspond to reflections from features spread out in time and mask one another. Moreover, the correlation between  
Figure 2: Split-spectrum processing (SSP) block diagram. adjacent sub-bands could be affected by the overlap of the filters. This means the correlation increases with an increase in overlaps. On the other hand, little or no overlap could lead to loss of information. It is notable that the noise in adjacent filters needs to be uncorrelated and the features should be correlated. Hence, the overlap needs to be selected somehow to minimise the correlation between coherent noise regions in adjacent sub-bands without losing information. 4. The filter crossover point (\(\delta\)); or a cutoff frequency at the edges, is a boundary in the frequency response at which the energy flowing through the structure starts to decrease rather than passing through. 5. The number of filters (N); is the number of sub-band filters that is required to be selected in order to enhance the SNR and spatial resolution by correlating the signal of interest and minimising the correlation between the coherent noise region in the adjacent sub-bands signal.