Echo Canceller, part of Voice Engine project
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Makefile for Raspberry/Debian
Latest commit 276d82f Aug 25, 2018

ec - Echo Canceller

The project enables Acoustic Echo Cancellation (AEC) on Linux. It supports PC, Raspberry Pi, ReSpeaker Core V2 and Pi-like devices.

It's a part of voice-engine. The goal is to make an open source smart speaker for daily use. It uses Linux ALSA API and SpeexDSP's AEC algorithm.


sudo apt-get -y install libasound2-dev libspeexdsp-dev
git clone
cd ec

The commands will create ec and ec_hw. For devices without hardware audio loopback, ec is used. Otherwise, ec_hw is used. The hardware audio loopback means that audio output is captured by extra ADC and sent back as input audio channel.

ec for devices without hardware audio loopback

For devices without hardware audio loopback, we need a way to get audio output data. ec will be audio output broker reading audio data from a named pipe and playing it. Meanwhile it will record audio and remove the playing audio from the recording, and then output it to another named pipe. The audio stream diagram is like:

+-------------------+  +-------------------+
|  PulseAudio       |  |  PulseAudio       |
|  module-pipe-source  |  module-pipe-sink |
+----------+--------+  +-------+-----------+
           |                   |             +------------+
           |                   |             |ALSA        |
           v                   v             |file plugin |
    +------+-------+    +------+-------+     +----+-------+
    |    FIFO      |    |    FIFO      |          |
    |/tmp/ec.output|    | /tmp/ec.input| <--------+
    +------+-------+    +------+-------+
           ^                   |
           |                   |
           |                   |
           |                   |
        +--+--+                | playback
        | AEC | <--------------+
        +--+--+                |
           ^                   |
           |                   v
    +------+-------+    +------+-------+
    |  hw:x,y      |    |  hw:x,y      |
    |  plughw:x,y  |    |  plughw:x,y  |
    +--------------+    +--------------+

To use ec:

  1. Run ./ec -h to show its command line options

  2. Run arecord -L and aplay -L to get audio devices' name

  3. Select audio input/output devices ./ec -i {input device name} -o {output device name} -c {input channels}

    # terminal #1, run ec
    ./ec -h
    ./ec -i plughw:1 -o plughw:1 -s
    # terminal #2, play 16k fs, 16 bits, 1 channel raw audio
    cat 16k_s16le_mono_audio.raw > /tmp/ec.input
    # terminal #3, record
    cat /tmp/ec.output > 16k_s16le_stereo_audio.raw

    ec reads playback raw audio from the FIFO /tmp/ec.input and writes processed recording audio to the FIFO /tmp/ec.output

Use ec with ALSA file plugin

ALSA's file plugin can be used to configure the FIFO /tmp/ec.input as the default ALSA output. Just copy asound.conf to ~/.asoundrc to apply the configuration.

pcm.!default {
    type plug
    slave {
        format S16_LE
        rate 16000
        channels 1
        pcm {
            type file
            slave.pcm null
            file "/tmp/ec.input"
            format "raw"

Use ec with PulseAudio

PulseAudio has module-pipe-sink and module-pipe-source which can be used to configure /tmp/ec.input and /tmp/ec.output as the default audio output and input.

PulseAudio (version < 12) will create /tmp/ec.input and /tmp/ec.output. If the two FIFO already exist, PulseAudio will fail, we should delete the two FIFO (rm /tmp/ec.input /tmp/ec.output) and then launch PulseAudio. It means we must start PulseAudio before we start ec. For example, use the following commands to setup PulseAudio and ec.

pacmd load-module module-pipe-sink sink_name=ec.sink format=s16 rate=16000 channels=1 file=/tmp/ec.input
pacmd load-module module-pipe-source source_name=ec.source format=s16 rate=16000 channels=2 file=/tmp/ec.output
pacmd set-default-sink ec.sink
pacmd set-default-source ec.source

./ec -i plughw:0 -o plughw:0

We can also use PulseAudio's configuration file to load module-pip-sink and module-pipe-source. Just replace /etc/pulse/ with

If you have installed PulseAudio but want to disable it, we need to disable PulseAudio's autospawn feature by adding autospawn=no to ~/.config/pulse/client.conf or /etc/pulse/client.conf, and then run pulseaudio -k

ec for Raspberry Pi

The sound from the on-board audio jack of the Raspberry Pi has serious distortion, do not use the on-board audio jack!

  • ReSpeaker 2 Mic Hat for Raspberry Pi

    The delay between playback and recording is about 200. Try ./ec -i plughw:1 -o plughw:1 -d 200

ec_hw for devices with hardware audio loopback

For devices such as ReSpeaker Core V2, ReSpeaker 6 Mic Array for Pi and ReSpeaker Linear 4 Mic Array for Pi, audio output is captured as one of the input channels. The audio stream diagram is simpler.

                       +---------+          +-------+         +------+
                       |         |          |       |         |      |
                       |         |          |       |         | FILE |
Microphones ---------->|         |--------> |  AEC  |-------> | FIFO |----> Audio Input
                       |   ADC   |          |       |         |      |
                       |         |          +-------+         +------+
                       |         |             /
             __________|         |____________/
            |          +---------+   Loopback Channel
            |          +---------+
            |          |         |
Speakers <-------------|   DAC   |<------- Audio Output
                       |         |

To use ec_hw:

  1. Run ./ec_hw -h to show its command line options

  2. Run arecord -L to get audio input devices' name

  3. Select the audio input device ./ec_hw -i {input device name} -c {input channels} -l {loopback channel} -m {mic channel list}

    # terminal #1, run ec
    ./ec_hw -h
    ./ec -i plughw:1 -c 8 -l 7 -m 0,1,2,3
    # terminal #3, record
    cat /tmp/ec.output > 16k_s16le_4_channels.raw

    ec_hw uses channel 7 as playback audio, remove the playback from channels 0,1,2,3 and writes processed audio to the FIFO /tmp/ec.output




  • The ring buffer implementation is from PortAudio
  • SpeexDSP provides the excellent open source AEC algorithm
  • Using named pipe for I/O is inspired by snapcast