General purpose filter example

Worked Example

Steps in this example:

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

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

  • The 500Hz high-pass filter coefficients are:

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

  • The 4kHz low-pass filter coefficients are:

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

  • Hear the effect filtered signals when the filters are enabled

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.