Audio filtering for radio communications using ADAU1700/1

    Hello John,

    I had searched back issues of the ARRL's QST Magazine for DSP projects but could not find any for SigmaDSP.  They have a new book on Arduino ham radio projects which may be of interest to you.  Here's a few example projects related to peaking and notch filters to help get you started in your design, as well as demonstrate some of the crazy-fun stuff SigmaDSP can do.

     The -1701's toolbox includes a State-Variable Filter whose frequency can be adjusted by a control input pin.  Thus it's easy to adjust this filter with a pot in a self-boot system.   This example filter is adjustable from 100 Hz to 3 KHz, which you can customize by the DC and gain values as labeled.  The State-Variable Filter block includes the bandpass output used here, as well as lowpass and highpass outputs.  No notch filter, however.  You can subtract the filter's bandpass output from the input audio to make a notch, but it's hard to produce a deep notch this way.

     The General Second-Order Filter has a notch option that works very well.  These filters have no control input pins.  Thus, in a self-contained system you'll need to have your Arduino calculate the filter's five coefficients based on the desired notch frequency, then safeload them via SPI to your -1701.  Shown below is two General filters, one set up by SigmaStudio as a notch, and the other set by typing the needed coefficients for the same notch. calculated from a spreadsheet that's attached (see its rightmost column).  You could have your Arduino make the same calculations to get your coefficients.  By the way, if you would like to learn about how these DSP filters work and how to calculate their coefficients, see the book ADI makes available for download at The Scientist & Engineer's Guide to Digital Signal Processing | Learning and Development | Processors and DSP | Analog D…

     Perhaps at least at the beginning, you would prefer not to get involved with calculating and safeloading coefficients, instead using only the pots on your test board.  The example below makes an adjustable notch by shifting the frequency of the incoming audio before filtering with a fixed notch filter, then demodulating the result.  This works because the usual SSB bandwidth is about 3 KHz, giving us some room to modulate upwards.  In my example the fixed notch is at 12 KHz, the working frequency often used with sound card software radios.

     This project modulates the incoming audio to upper-sideband with a variable 9 KHz -- 11.9 KHz carrier, notch filters the result at 12 KHz, then demodulates back to audio.  Though quite convoluted, it uses only about 30% of your available program instructions, leaving room for more features.  As a bonus, since the adjustable carrier is a VCO, we can have the notch filter track the offending whistle so the operator need not constantly adjust for drift.  A quick listening test shows it works quite well.

     Have fun with your project!

     Best regards,

     Bob

    Hello John,

    I had searched back issues of the ARRL's QST Magazine for DSP projects but could not find any for SigmaDSP.  They have a new book on Arduino ham radio projects which may be of interest to you.  Here's a few example projects related to peaking and notch filters to help get you started in your design, as well as demonstrate some of the crazy-fun stuff SigmaDSP can do.

     The -1701's toolbox includes a State-Variable Filter whose frequency can be adjusted by a control input pin.  Thus it's easy to adjust this filter with a pot in a self-boot system.   This example filter is adjustable from 100 Hz to 3 KHz, which you can customize by the DC and gain values as labeled.  The State-Variable Filter block includes the bandpass output used here, as well as lowpass and highpass outputs.  No notch filter, however.  You can subtract the filter's bandpass output from the input audio to make a notch, but it's hard to produce a deep notch this way.

     The General Second-Order Filter has a notch option that works very well.  These filters have no control input pins.  Thus, in a self-contained system you'll need to have your Arduino calculate the filter's five coefficients based on the desired notch frequency, then safeload them via SPI to your -1701.  Shown below is two General filters, one set up by SigmaStudio as a notch, and the other set by typing the needed coefficients for the same notch. calculated from a spreadsheet that's attached (see its rightmost column).  You could have your Arduino make the same calculations to get your coefficients.  By the way, if you would like to learn about how these DSP filters work and how to calculate their coefficients, see the book ADI makes available for download at The Scientist & Engineer's Guide to Digital Signal Processing | Learning and Development | Processors and DSP | Analog D…

     Perhaps at least at the beginning, you would prefer not to get involved with calculating and safeloading coefficients, instead using only the pots on your test board.  The example below makes an adjustable notch by shifting the frequency of the incoming audio before filtering with a fixed notch filter, then demodulating the result.  This works because the usual SSB bandwidth is about 3 KHz, giving us some room to modulate upwards.  In my example the fixed notch is at 12 KHz, the working frequency often used with sound card software radios.

     This project modulates the incoming audio to upper-sideband with a variable 9 KHz -- 11.9 KHz carrier, notch filters the result at 12 KHz, then demodulates back to audio.  Though quite convoluted, it uses only about 30% of your available program instructions, leaving room for more features.  As a bonus, since the adjustable carrier is a VCO, we can have the notch filter track the offending whistle so the operator need not constantly adjust for drift.  A quick listening test shows it works quite well.

     Have fun with your project!

     Best regards,

     Bob