Waveflex Caraudio

DSP A8 + REW – Biquad

Now that we have measured our speakers using the “REW + UMIK application note” you will want to simulate the filtering and equalization.
In this sample tutorial we will proceed to simulate filters & equalization on a measured subwoofer.
Open the EQ tab in the REW taskbar.



You will see your previously measured frequency response of the speaker and also have acces to the filters tab (red box).



Set the parameters of the scroll-down menus on the right hand side as shown in the screen captures hereunder. The shown settings, especially the “target settings” are informative only. You will want to adjust them to your needs.



Now you can simulate the low-pass filter of the subwoofer using the LQP filter type as shown hereunder.



Then you add your PEQ equalization.



Once you are satisfied with the result you can transfer the filters to the DSP A8.

Exporting PEQ

In the DSP A8 software, open the PEQ tab of the selected channel.
Now enter the filters with rising frequency for the sake of logic.
The result should look like the screen capture hereunder.



Exporting the filters

  1. Biquad : Some background
  2. A biquad (for biquadratic) filter is a 2nd order analog filter (IIR) where all the parameters can independently be altered.

    The biquad is the basis of all analog filters (PEQ, LP, HP, etc.) you can find on digital audio gear. When, for instance, the user sets a parametric filter with the usual parameters (frequency, gain, Q) the DSP will internally implement a biquad.

    The parameters of the biquad filters are values that will define the action of the filters both in the frequency and time domain. The All pass filters being the exception as they have a flat frequency response and therefore only alter the time domain.

    There is a total of 6 parameters : a0,a1,a2,b0, b1 and b2

    Given this formula it is possible to generate any – well almost any – type of filter.

    The WaveFlex Caraudio products allow you to directly implement this type of filter by simply entering the coefficients in the control software. For the DSP A8 for instance, you can configure up to 13 biquad blocks per output channel and a total of 10 blocks on the input (5 per channel). In the filter tab these biquad-cells can be cascaded in series and allow you to create, for instance, steep filters (8 cells of 12dB/octave each = 96 dB/octave).

    There are multiple ways to generate the needed values.

      • An Excel-Spreadsheet available for free download on the WaveFlex-Caraudio website (advanced users)
      • Simulation tools like for instance REW

  3. Exporting biquad from REW
  4. Let’s go back to REW to see how to export the values in a suitable format for the DSP A8.

    Open the EQ filters tab in the EQ menu and select DSP A8 in the list of available equalizers if you haven’t done that yet.

    Click on Filter tasks on the right hand side and then on Send filter settings to equalizer to export the values. As the equalization is already stored in the DSP, so you will have to un-tick PEQ to keep the LPQ of the LP filter only.

    Let’s have a closer look at the data of such a file:

            biquad1,
            b0=1.0,
            b1=0.0,
            b2=0.0,
            a1=0.0,
            a2=0.0,
            
            biquad2,
            b0=1.0,
            b1=0.0,
            b2=0.0,
            a1=0.0,
            a2=0.0,
            
            biquad3,
            b0=1.0,
            b1=0.0,
            b2=0.0,
            a1=0.0,
            a2=0.0,
            
            biquad4,
            b0=1.0,
            b1=0.0,
            b2=0.0,
            a1=0.0,
            a2=0.0,
            
            biquad5,
            b0=0.0000077070635876,
            b1=0.0000154141271752,
            b2=0.0000077070635876,
            a1=1.9921324351416319,
            a2=-0.9921632633959822
            

    Each block of 6 lines each starting with biquad corresponds to 1 filter cell.
    Our generated filter is contained in biquad5, just like in the REW control panel (line 5)

    Here we find the data mentioned before, we are missing the a0 value only. In fact the value a0 being always the same value, there is no need to fill it every time, the DSP A8 software will do it for us.

    Make sure you use the dot (“.”) for the decimal point and a comma (“,”) at the end of each line but the last one!

    The blocks that are not used can be clearly identified as their value is either 0 or 1.

  5. Export Biquads from the Excel –Spreadsheet
  6. You can download the Spreadsheet on WaveFlex-Caraudio.fr The spreadsheet allows you to generate the values of the most common biquad-filters (LPF, HPF, PEQ, AllPass, Notchn HS and LS). The spreadsheet is based on the work of the DSP manufacturer but has been modified to output the data in the expected format for the DSP A8 so that you can simply copy/paste them.

    This is what it looks like and how to use it

    Enter the input data (frequency, Q, and the sample frequency – 96 KHz in the case of the DSP A8) to allow the spreadsheet to calculate the parameters.



    Always make sure that the filter status is stable (orange box).

    You can now simply copy the values of the green area and paste them in the DSP A8 software.
    To simplify the task when working with the spread-sheet you may create a *.txt file and copy/paste the generated values into it, similiar to the file REW puts out.
    Don’t forget to change the numbering of the biquad when adding multiple filter (biquad1,biquad2, etc.

  7. Import Biquad into the DSP A8
  8. It is recommended to disconnect the remote out of the DSP during this operation. In fact, an incorrect implementation of the values might generate noise on the output!

    It is recommended to disconnect the remote out of the DSP during this operation. In fact, an incorrect implementation of the values might generate noise on the output!

    Open the DSP A8 software and open the X-over TAB of the wanted channel and select advanced mode.



    Open the *.txt file your created, copy all the text to the clip-board, then in the X-over tab select all (ctrl-A) and paste (ctrl-c) the data from your text file.

    By clicking on the process button the filters will be applied and will modify the response curve.



    The biquad cells are now implemented.

    Proceed the same way for adding other/more filters using a different *.txt file. Make sure you keep the incrementation (biquad1, biquad2, etc) at the beginning of each block.