We have been approached by one of our customers with the following. "Hello, I am reviewing how the Filter in the CHUM software is applied. I understand that there are three types of filters for the profile filter. The Median, Running Average, and Delay.
With regards to the Median and Running Average, are both applied when a Filter is used?
Is one applied first, and then the other is applied to that same data that the first filter has been applied?
For example, if a Filter of 2 was used, is the Median determined first, then the average of the median value?
Also, what filter is the minimum acceptable value?
A filter of zero has data that is 'spikey', but I do not want to use a filter at all so as not to miss defects. Any assistance would be appreciated."
Answer
There is a Signal filter and a Profile filter The signal filter clears the noise from the signal
The graph below shows a signal before the filter is applied to it (gray) and after it is applied (green).
It is visible that the lower frequency noise at the left side of the signal was significantly reduced, allowing an accurate FAT picking.
The Profile filter
The way the filtering is done in CHUM is as follows There are three profile filters
The order of filtering is
1 - Delay
2 - Median
3 - Running Avg.
The data is passed through all three filters, one after the other, in that order. Filtering strength can be set from 0 to 5
When the filter value is 0 - No filtering is performed.
When the filter value is 5 - Maximum filtering is performed.
We recommend using a minimal filter value. If possible, keep it at zero. Higher values are, however, unavoidable sometimes to present the results. In any case, we recommend not to use values higher than 2.
An example of the type of noise each filter removes can be seen below F=0 - No profile filter, shows the signal profile with noise. 3 types of noise Noise 1 - removed by the Median filter
Noise 2 - removed by the Running Average filter
Noise 3 - removed by the Delay filter
F-1 - Low filter strength (removes most of the profile noise)
F=5 - Maximum filter strength - "Irons" the signal profile
It is good that one does not want to miss any defects, and indeed, it might be a good practice to analyze the data without a filter and to set it to a reasonable value for reporting. This way, you can have a deep analysis and an easy-to-explain report. You can read more about the CHUM profile filters in its online manual.