% pacmd load-module module-null-sink sink_name=NullSink
% gst-launch-1.0 pulsesrc device=... ! pitch pitch=2 ! pulsesink device=NullSink
% pavucontrol

Null sink as a loopback device

At first, create a null sink to output the pitch-shifted audio to.

% pacmd load-module module-null-sink sink_name=NullSink

As written in the document, a null sink have a monitor source.

All sinks have a corresponding “monitor” source which makes the null sink a practical way to plumb output to input.

So, we can use it as a loopback device. i.e. It means to output pitch-shifted audio to the null device and use the monitor device as a recording device.

Apply filter on GStreamer

To check the microphone device name, execute pacmd list-sources | grep name.

% pacmd list-sources | grep name
        name: <alsa_output.usb-XXXX_USB_PnP_Audio_Device-XX.analog-stereo.monitor>
        name: <alsa_input.usb-XXXX_USB_PnP_Audio_Device-XX.mono-fallback>
        name: <alsa_output.pci-XXXX_XX_XX.X.analog-stereo.monitor>
        name: <alsa_input.pci-XXXX_XX_XX.X.analog-stereo>
        name: <NullSink.monitor>
        name: <alsa_output.pci-XXXX_XX_XX.X.hdmi-stereo-extraX.monitor>

Device names ending in .monitor means monitor devices, not actual microphones. In this example, following devices are actual microphones:

  • alsa_input.usb-XXXX_USB_PnP_Audio_Device-XX.mono-fallback
  • alsa_input.pci-XXXX_XX_XX.X.analog-stereo

Here, let’s apply a pitch filter to the first device (USB-connected microphone).

% gst-launch-1.0 pulsesrc device=alsa_input.usb-XXXX_USB_PnP_Audio_Device-XX.mono-fallback ! \
    pitch pitch=2 ! \
    pulsesink device=NullSink

Of cause, replace device=... with microphone device name of your environment.

FYI: source device and sink device can be dynamically reconfigurable by PulseAudio Volume Control as describe on the next section.

Application-specific audio device

PulseAudio can control playback device and recording device per application. Change the recording device to null sink for the app you want to use. It can change dynamically!

For example, you can change the recording device for Skype app by clicking the button (highlighted with red box) on PulseAudio Volume Control.

Screenshot of PulseAudio Volume Control

Choose the null sink, and then pitch-shifted voice will sent to Skype app.