General purpose filter example

Specification

This page illustrates the process of defining an audio filter block in the XVF3610.

The example below routes a USB audio signal through a filer block and sends the output back to the USB host.

Steps to set up this configuration are:

  1. Set the stereo USB output to listen to the stereo USB input (loopback, skipping audio processing pipeline completely)

  2. Apply a stereo 500Hz high-pass and 4kHz low-pass cascaded biquad filter

  3. The 500Hz high-pass filter coefficients are:

    a1 = -1.90748889
    a2 = 0.91158173
    b0 = 0.95476766
    b1 = -1.90953531
    b2 = 0.95476766

  4. The 4kHz low-pass filter coefficients are:

    a1 = -1.27958194
    a2 = 0.47753396
    b0 = 0.04948800
    b1 = 0.09897601
    b2 = 0.04948800

  5. Enable the filter and hear the effect of the filter on a signal when the filters are enabled

Worked Example

This example assumes that the input and output sample rate is 48kHz.

First, connect the USB output to the USB input:

vfctrl_usb SET_IO_MAP 0 7 # (USB output left outputs USB input left)
vfctrl_usb SET_IO_MAP 1 8 # (As above for right channel)

Now configure the filter:

vfctrl_usb SET_FILTER_INDEX 2 (USB output left filter)
vfctrl_usb SET_FILTER_COEFF -1.90748889 0.91158173 0.95476766 -1.90953531 0.95476766 -1.27958194 0.47753396 0.04948800 0.09897601 0.04948800
vfctrl_usb SET_FILTER_INDEX 3 (USB output right filter)
vfctrl_usb SET_FILTER_COEFF -1.90748889 0.91158173 0.95476766 -1.90953531 0.95476766 -1.27958194 0.47753396 0.04948800 0.09897601 0.04948800

Now enable the filter:

vfctrl_usb SET_FILTER_INDEX 0
vfctrl_usb SET_FILTER_BYPASS 0
vfctrl_usb SET_FILTER_INDEX 1
vfctrl_usb SET_FILTER_BYPASS 0

Play a white noise source from the USB device and record the input. Use a spectrogram to show the band limited signal due to the effect of the filters. The effect should also be audible.