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Custom EQ Filter Implementation

Question asked by James29 on Oct 2, 2017
Latest reply on Oct 6, 2017 by KJBob

I am attempting to implement a custom EQ filter from an old DSP in SigmaStudio and am not having any success.

The reason for doing it this way is so that I can use the same coefficient values as before and therefore not have to update any of the other software in our system.

There is an application note from the old DSP manufacturer which describes the filter and gives its block diagram:

I've attached a diagram showing my implementation - the coefficients are just DC values and they have been calculated using the spreadsheet provided with the app note:

With my version I get exactly the same signal out regardless of the value of any of the coefficients (never any gain nor attenuation). I've tried several different frequencies and input signal levels.

My suspicion is that there might be a quirk in how the Sigma deals with the top 'double' feedback loop?
Grateful for any help or suggestions.