Variable band-stop / notch filters

Question

For a feedback suppression algorithm I am looking to implement variable gain / rejection / frequency notch or band-stop filters. 
 On other words, I need to be able to control the center frequency and amount of rejection dynamically. 
 What would be the best way to implement this on an ADAU 1761?

KJBob · Accepted Answer

Hello Roman, 
 Your final application will influence which of several approaches might work for you. I'll try to describe these methods with their advantages and limitations. 
 Second-Order IIR 
 These are by far the most versatile and capable, but also the most difficult to control in real-time: 
 
 Problem is, to adjust these filters at runtime you'll need an external means of control -- either SigmaStudio via the USBi, or a microcontroller in your final product. This is because the five IIR coefficients needed to drive these filters have calculations too complex for a SigmaDSP to do on its own. So instead you have your microcontroller calculate the needed coefficients for a desired response, then have it safeload them into the DSP's parameter memory via I2C or SPI bus. I attached a spreadsheet with formulas to calculate coefficients for various filter types -- it's fairly straightforward to translate them into C code for your microcontroller. To troubleshoot your own calculations, enter them into the IIR Coefficient version of the filter, as with the notch filter shown below: 
 
 Once you've calculated your coefficients, safeloading them has been covered in various forum topics -- for example, https://ez.analog.com/message/61175#61175 
 State-Variable Filter 
 This is the only SigmaStudio filter for which the DSP figures its own coefficients, thus allowing filter control to originate within a signal flow. It has simultaneous lowpass, highpass, and bandpass outputs. It's possible to create a notch by subtracting its bandpass output from the original signal, but tricky to obtain a deep notch this way. 
 Communications-Grade Notch 
 If your application is in communications with a narrow voice bandwidth, you can borrow an idea used in radio since the 1930's -- it's far easier to move the incoming signal's frequency than to move the filter's frequency. See an example of this technique at https://ez.analog.com/message/173545#173545 
 Best regards, 
 Bob

Electronic Design Services - PartnerZone

Products In This Thread

Variable band-stop / notch filters

Top Replies