Real-Time Background Noise Cancellation of Microphone Input on Linux

I’m surprised at how little information there is about this topic, short of the majority of forums indicating it cannot be done. Having a poor quality mic, both integrated into a laptop as well as a cheap external microphone, I know my sound quality is not the best, and a fair bit of background noise can be picked up. When I discovered that audio mixing software, such as Audacity, can do a very good job of removing background noise based on a sound sample, I figured there would be a way of doing this in real-time. Although I have not found any information on doing this with Audacity (nor did I expect to), I only really found a single Ask Ubuntu forum answer with an explanation of how to accomplish this, and to my surprise (despite it not being the ‘best answer’ of that thread), it works, and quite well to boot.

Update: I have updated the script in hopes it will solve some of the problems that have been brought up. Details can be found in my update post. I have also posted it on GitHub for easier maintenance.

By utilizing SoX audio software, the microphone input is redirected through it, and output into a loopback device, which can be chosen in PulseAudio’s volume mixer application. I have composed a bash script to essentially automate the process. Before using this, SoX must be installed, and the alsa loopback device must be loaded into the kernel:

Debian/Ubuntu: sudo apt-get install sox

Arch: pacman -S sox

sudo modprobe snd_aloop

#!/bin/bash

time=5

workDir=$(mktemp -d)

noiseFile="noise.wav"

#record noise sample
record()
{
	read -p "Recording background noise. Keep quiet for $time seconds. Press Enter to start."
	sleep 0.5
	parecord -d $input $noiseFile &
	PID=$!
	sleep $time
	kill $PID
	echo "Playing back noise sample"
	paplay $noiseFile
}

cd $workDir

#detect if aloop module is loaded
#allow the user to load the module here
	if [ $(lsmod | grep -c snd_aloop) -eq 0 ]; then
		echo "ALOOP module is not loaded."
		read -p "Attempt to load the module?[y/n]" yn
	    case $yn in
	        [Yy]* ) sudo modprobe snd_aloop;;
	        [Nn]* ) exit;;
	        * ) echo "Please answer yes or no.";;
	    esac
	fi

#get list of input and output devices (with details)
	inputs=$(pactl list short sources | grep -E -v '(monitor|aloop)')
	outputs=$(pactl list short sinks | grep aloop)

#abort if devices aren't available
	if [ $(echo "$inputs" | wc -l) -lt 1 ]; then
		echo "No input devices detected. Aborting"
		exit
	fi

	if [ $(echo "$outputs" | wc -l) -lt 1 ]; then
		echo "No output devices detected. Aborting"
		exit
	fi

inputIndex=0
#allow user selection when multiple devices are available
	if [ $(echo "$inputs" | wc -l) -gt 1 ]; then
		echo "Multiple input devices detected. Select from this list:"
		echo "$inputs" | awk -F '\\s' '{print NR-1," ",$2}'
		read -p "Enter an index:" inputIndex
		exit
	fi
	inputs=$(echo "$inputs" | awk -v idx=$inputIndex '{if (NR-1 == idx) print}')

outputIndex=0
#allow user selection when multiple devices are available
	if [ $(echo "$outputs" | wc -l) -gt 1 ]; then
		echo "Multiple output devices detected. Select from this list:"
		echo "$outputs" | awk -F '\\s' '{print NR-1," ",$2}'
		read -p "Enter an index:" outputIndex
		exit
	fi
	outputs=$(echo "$outputs" | awk -v idx=$outputIndex '{if (NR-1 == idx) print}')

#load device names
	input=$(echo "$inputs" | awk -F '\\s' '{print $2}')
	output=$(echo "$outputs" | awk -F '\\s' '{print $2}')

#get input device specs
	format=$(echo "$inputs" | awk -F '\\s' '{print $4}')
	#number type
	tmp=$(echo $format | grep -o "^[fus]")
	case $tmp in
		f ) inputEncoding="floating-point";;
		s ) inputEncoding="signed-integer";;
		u ) inputEncoding="unsigned-integer";;
	esac
	#bit count
	inputBits=$(echo $format | grep -o "[0-9]*")
	#endianness
	tmp=$(echo $format | grep -o "[bl]e$")
	case $tmp in
		be ) inputEndian="-B";;
		le ) inputEndian="-L";;
	esac
	#channels
	inputChannels=$(echo "$inputs" | awk -F '\\s' '{print $5}' | grep -o "[0-9]*")
	#bitrate
	inputBitrate=$(echo "$inputs" | awk -F '\\s' '{print $6}' | grep -o "[0-9]*")

#get output device specs
	format=$(echo "$outputs" | awk -F '\\s' '{print $4}')
	#number type
	tmp=$(echo $format | grep -o "^[fus]")
	case $tmp in
		f ) outputEncoding="floating-point";;
		s ) outputEncoding="signed-integer";;
		u ) outputEncoding="unsigned-integer";;
	esac
	#bit count
	outputBits=$(echo $format | grep -o "[0-9]*")
	#endianness
	tmp=$(echo $format | grep -o "[bl]e$")
	case $tmp in
		be ) outputEndian="-B";;
		le ) outputEndian="-L";;
	esac
	#channels
	outputChannels=$(echo "$outputs" | awk -F '\\s' '{print $5}' | grep -o "[0-9]*")
	#bitrate
	outputBitrate=$(echo "$outputs" | awk -F '\\s' '{print $6}' | grep -o "[0-9]*")

#record noise sample
record
while true; do
    read -p "Do you wish to re-record the noise sample?[y/n]" yn
    case $yn in
        [Yy]* ) record;;
        [Nn]* ) break;;
        * ) echo "Please answer yes or no.";;
    esac
done

#create noise profile
sox $noiseFile -n noiseprof noise.prof

echo "Sending output to loopback device."
echo "Change recording port to  in PulseAudio to apply."
echo "Ctrl+C to terminate."

#filter audio from $input to $output
pacat -r -d $input --latency=1msec | sox -b $inputBits -e $inputEncoding -c $inputChannels -r $inputBitrate $inputEndian -t raw - -b $outputBits -e $outputEncoding -c $outputChannels -r $outputBitrate $outputEndian -t raw - noisered noise.prof 0.2 | pacat -p -d $output --latency=1msec

cd -
rm -rf "$workdir"

This script first tests to make sure the loopback devices are loaded, then records a 5 second noise sample. The sample is then played back to the user. If there should be a problem with it, such as a non-background sound leaked into it, it can be re-recorded. Once done, the input and output devices are determined, and an audio stream is directed from PulseAudio into SoX, then back into Pulse through the loopback device. From PulseAudio Volume Control, the input of recording devices can be changed to Loopback Analog Stereo Monitor, where the filtered audio stream is being directed.