LMSEchoCancellationStream< T > Class Template Reference

Echo cancellation with adaptive LMS filtering for microcontrollers. More...

#include <LMSEchoCancellationStream.h>

Inheritance diagram for LMSEchoCancellationStream< T >:

Public Member Functions
	LMSEchoCancellationStream (Stream &in, size_t lag_samples=0, size_t buffer_size=512, size_t filter_len=32, float mu=0.001f)
	Constructor.
virtual void	addNotifyAudioChange (AudioInfoSupport &bi)
	Adds target to be notified about audio changes.
virtual AudioInfo	audioInfo () override
	provides the actual input AudioInfo
virtual AudioInfo	audioInfoOut ()
virtual int	available () override
virtual int	availableForWrite () override
virtual bool	begin ()
void	cancel (const T *rec, const T *play, T *out, size_t len)
	Process echo cancellation on arrays of samples. Self-contained: feeds the playback reference and the recorded signal in lock-step, so it can be used directly without going through the Stream write()/readBytes() pair.
virtual void	clearNotifyAudioChange ()
	Deletes all change notify subscriptions.
virtual void	end ()
virtual void	flush () override
bool	isNotifyActive ()
	Checks if the automatic AudioInfo update is active.
virtual	operator bool ()
size_t	readBytes (uint8_t *buf, size_t len) override
	Read input and remove echo, using the playback signal previously buffered via write() as the reference for the adaptive filter.
virtual size_t	readSilence (uint8_t *buffer, size_t length)
	Source to generate silence: just sets the buffer to 0.
virtual bool	removeNotifyAudioChange (AudioInfoSupport &bi)
	Removes a target in order not to be notified about audio changes.
void	reset ()
	Reset the internal buffer and lag state.
virtual void	setAudioInfo (AudioInfo newInfo) override
	Defines the input AudioInfo.
void	setFilterLen (size_t len)
	Set the filter length for the adaptive filter.
void	setLag (size_t lag_samples)
	Set the lag (delay) in samples for echo cancellation.
void	setMu (float mu)
	Set the adaptation rate (mu) for the LMS algorithm.
void	setNotifyActive (bool flag)
	Deactivate/Reactivate automatic AudioInfo updates: (default is active)
void	setWriteBufferSize (int size)
size_t	write (const uint8_t *buf, size_t len) override
	Store the output signal (sent to speaker). The samples are pushed into the internal delay line so that readBytes() can later use them as the echo reference.
virtual size_t	write (uint8_t ch) override
virtual void	writeSilence (size_t len)
	Writes len bytes of silence (=0).

Protected Member Functions
T	filterSample (T mic_sample)
	Estimate and subtract the echo for a single microphone sample, using the oldest filter_len samples currently buffered (i.e. the playback signal delayed by lag samples) as the adaptive filter's reference, then update the filter taps (NLMS-free, plain LMS).
virtual int	not_supported (int out, const char *msg="")
void	notifyAudioChange (AudioInfo info)
void	pushPlayback (T sample)
	Push a playback (speaker) sample into the delay line, evicting the oldest sample once the window exceeds filter_len + lag so that the reference always reflects the most recent playback history.
void	refillReadBuffer ()
	Refill small read buffer (e.g. 8 bytes) to avoid single byte reads when calling read()

Protected Attributes
int	_timeout = 10
float	adaptation_rate = 0.01f
size_t	buffer_size
Vector< float >	filter
size_t	filter_len
AudioInfo	info
bool	is_notify_active = true
size_t	lag
Vector< AudioInfoSupport * >	notify_vector
Stream *	p_io = nullptr
Vector< T >	ref_vec
RingBuffer< T >	ring_buffer {0}
RingBuffer< uint8_t >	tmp_in {0}
RingBuffer< uint8_t >	tmp_out {0}
int	write_buffer_size = MAX_SINGLE_CHARS

Detailed Description

template<typename T = int16_t>
class audio_tools::LMSEchoCancellationStream< T >

Echo cancellation with adaptive LMS filtering for microcontrollers.

This class implements echo cancellation using an adaptive FIR filter (LMS algorithm). It estimates the echo path and subtracts the estimated echo from the microphone input. write() buffers the playback (speaker) signal into a sliding delay line; readBytes() pulls mic samples from the wrapped Stream and filters each one against that delay line. cancel(rec, play, out, len) is a Stream-free alternative that feeds both signals explicitly in lock-step (useful for testing or non-duplex pipelines) and is verified to produce output identical to the write()/readBytes() path.

Strengths:

• Tiny and simple: O(filter_len) work per sample, no FFT, trivial memory footprint – a reasonable fit for small microcontrollers with a short, simple echo path (e.g. a small speaker directly coupled to a nearby mic).
• The delay line is a real sliding window (fixed-size ring buffer that evicts as it fills), so it keeps adapting indefinitely rather than stalling after some fixed number of samples.

Weaknesses:

• Plain LMS with a fixed step size (mu), not NLMS – the right mu depends on signal amplitude and echo strength and needs per-deployment tuning; too large diverges, too small converges too slowly to track a changing echo path.
• No double-talk protection: if near-end speech happens while the speaker is playing, plain LMS can't tell that apart from "the echo path changed" and may adapt in the wrong direction (contrast with MDFEchoCancellationStream's two-path foreground/background scheme).
• Only models a short, simple echo path well: filter_len must cover the actual echo delay in samples, and cost grows linearly with it – not the right tool for a long, reverberant echo tail.
• No sample-rate/timing compensation: write() (speaker) and readBytes() (mic) must be driven at the same pace for the delay line to stay time-aligned; there is no internal resampling or drift correction for a mismatched full-duplex pipeline.

Constructor & Destructor Documentation

LMSEchoCancellationStream()

template<typename T = int16_t>

LMSEchoCancellationStream ( Stream &  in, size_t  lag_samples = 0, size_t  buffer_size = 512, size_t  filter_len = 32, float  mu = 0.001f  )

inline

Constructor.

Parameters

in	Reference to the input stream (microphone or audio input)
lag_samples	Number of samples to delay the echo subtraction (default: 0)
buffer_size	Size of the internal ring buffer (default: 512)

Member Function Documentation

addNotifyAudioChange()

virtual void addNotifyAudioChange ( AudioInfoSupport &  bi )

inlinevirtualinherited

Adds target to be notified about audio changes.

Reimplemented in CodecNOP, EncodedAudioOutput, EncodedAudioStream, AACDecoderFDK, DecoderBasic, CodecChain, MP3DecoderHelix, MP3DecoderMAD, OggContainerDecoder, RTSPClient< TcpClient, UdpSocket >, Pipeline, and Pipeline::ModifyingStreamAdapter.

audioInfo()

virtual AudioInfo audioInfo ( )

inlineoverridevirtualinherited

provides the actual input AudioInfo

Implements AudioInfoSupport.

Reimplemented in JupyterAudioT< T >, MozziStream, TimerCallbackAudioStream, EncodedAudioStream, PureDataStream, AdapterAudioOutputToAudioStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, and InputMerge< T >.

audioInfoOut()

virtual AudioInfo audioInfoOut ( )

inlinevirtualinherited

provides the actual output AudioInfo: this is usually the same as audioInfo() unless we use a transforming stream

Reimplemented in MP3EncoderShine, PureDataStream, PWMAudioOutput< PWMDriverT >, ChannelFormatConverterStreamT< T >, ChannelFormatConverterStream, NumberFormatConverterStreamT< TFrom, TTo >, NumberFormatConverterStream, FormatConverterStream, Pipeline, ResampleStream, and ResampleStreamT< TInterpolator >.

available()

virtual int available ( )

inlineoverridevirtualinherited

Reimplemented from Stream.

Reimplemented in AudioKitStream, AudioLoRa, URLStreamBufferedT< T >, URLStreamBufferedT< ICYStream >, VBANStream, AudioStreamWrapper, FaustStream< DSP >, AudioMP34DT05, I2SCodecStream, MiniAudioStream, StdioStream, TfLiteAudioStream, A2DPStream, ESPNowStream, HammingFEC< bytecount, block_t >, HLSStreamT< URLStream >, HttpRequest, ICYStreamT< T >, URLStream, BufferedTaskStream, NRF24Stream, ReedSolomonFEC< bytecount, additional_bytes >, SPIAudioSlave, UDPStream, USBAudioDeviceBase, AnalogAudioStream, AudioEffectStreamT< T >, AudioEffectStream, Equalizer3Bands, Equalizer3BandsPerChannel, I2SStream, ReformatBaseStream, TimedStream, MemoryStream, RingBufferStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, BufferedStream, ConverterStream< T >, MeasuringStream, ProgressStream, Throttle, InputMixer< T, SumT >, InputMerge< T >, CallbackStream, FilteredStream< T, TF >, FilteredStream< int16_t, int16_t >, ChannelFormatConverterStreamT< T >, ChannelFormatConverterStream, NumberFormatConverterStreamT< TFrom, TTo >, NumberFormatConverterStream, CatStream, QueueStream< T >, QueueStream< uint8_t >, DynamicMemoryStream, FadeStream, FrequencyDetectorAutoCorrelation, FrequencyDetectorZeroCrossing, Pipeline, Pipeline::ModifyingStreamAdapter, QualityAnalysisStream, VolumeStream, FileLoopT< FileType >, FileLoopT< File >, AudioFFTBase, AudioBLEClient, AudioBLEClient, AudioBLEServer, and AudioBLEServer.

availableForWrite()

virtual int availableForWrite ( )

inlineoverridevirtualinherited

Reimplemented from Print.

Reimplemented in AudioLoRa, UDPStream, VBANStream, AudioStreamWrapper, ConverterStream< T >, EncodedAudioStream, FaustStream< DSP >, I2SCodecStream, MiniAudioStream, StdioStream, A2DPStream, ESPNowStream, HammingFEC< bytecount, block_t >, AbstractURLStream, URLStreamESP32, NRF24Stream, ReedSolomonFEC< bytecount, additional_bytes >, SPIAudioSlave, USBAudioDeviceBase, AnalogAudioStream, AudioEffectStreamT< T >, AudioEffectStream, Equalizer3Bands, Equalizer3BandsPerChannel, I2SStream, ReformatBaseStream, TimedStream, MemoryStream, RingBufferStream, MeasuringStream, ProgressStream, Throttle, FilteredStream< T, TF >, FilteredStream< int16_t, int16_t >, ChannelFormatConverterStreamT< T >, ChannelFormatConverterStream, NumberFormatConverterStreamT< TFrom, TTo >, NumberFormatConverterStream, QueueStream< T >, QueueStream< uint8_t >, DynamicMemoryStream, FadeStream, FrequencyDetectorAutoCorrelation, FrequencyDetectorZeroCrossing, Pipeline, QualityAnalysisStream, VolumeStream, AudioFFTBase, AudioBLEClient, AudioBLEClient, AudioBLEServer, and AudioBLEServer.

begin()

virtual bool begin ( )

inlinevirtualinherited

Reimplemented in AudioOutputWithCallback, AudioMP34DT05, I2SCodecStream, MozziStream, PureDataStream, SPDIFOutput, VS1053Stream, WM8960Stream, AdaptiveResamplingStream, AudioLoRa, ESPNowStream, USBAudioDeviceBase, VBANStream, AudioEffectStreamT< T >, AudioEffectStream, Equalizer3Bands, Equalizer3BandsPerChannel, I2SStream, AudioStreamWrapper, TimerCallbackAudioStream, FrequencyDetectorAutoCorrelation, GoertzelStream, ResampleStreamT< TInterpolator >, FileLoopT< FileType >, FileLoopT< File >, EncodedAudioStream, AudioBoardStream, AudioKitStream, MiniAudioStream, PortAudioStream, StdioStream, HLSStreamT< URLStream >, NRF24Stream, AnalogAudioStream, AdapterAudioOutputToAudioStream, TimedStream, MemoryStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, MeasuringStream, ProgressStream, Throttle, InputMerge< T >, CallbackStream, FilteredStream< T, TF >, FilteredStream< int16_t, int16_t >, VolumeMeter, AudioInputMonitor, ChannelFormatConverterStreamT< T >, ChannelFormatConverterStream, NumberFormatConverterStreamT< TFrom, TTo >, NumberFormatConverterStream, FormatConverterStream, CatStream, QueueStream< T >, QueueStream< uint8_t >, DynamicMemoryStream, Pipeline, Pipeline::ModifyingStreamAdapter, QualityAnalysisStream, ResampleStream, VolumeStream, AudioFFTBase, and EqualizerNBands< SampleT, AccT, NUM_TAPS, NUM_BANDS >.

cancel()

template<typename T = int16_t>

void cancel ( const T *  rec, const T *  play, T *  out, size_t  len  )

inline

Process echo cancellation on arrays of samples. Self-contained: feeds the playback reference and the recorded signal in lock-step, so it can be used directly without going through the Stream write()/readBytes() pair.

Parameters

rec	Pointer to received (microphone) signal samples
play	Pointer to playback (speaker) signal samples
out	Pointer to output buffer for echo-cancelled signal
len	Number of samples to process

clearNotifyAudioChange()

virtual void clearNotifyAudioChange ( )

inlinevirtualinherited

Deletes all change notify subscriptions.

Reimplemented in RTSPClient< TcpClient, UdpSocket >.

end()

virtual void end ( )

inlinevirtualinherited

Reimplemented in FaustStream< DSP >, AudioMP34DT05, I2SCodecStream, MozziStream, PureDataStream, SPDIFOutput, VS1053Stream, WM8960Stream, AdaptiveResamplingStream, AudioLoRa, ESPNowStream, BufferedTaskStream, URLStreamBufferedT< T >, URLStreamBufferedT< ICYStream >, USBAudioDeviceBase, Equalizer3Bands, I2SStream, AudioStreamWrapper, TimerCallbackAudioStream, FormatConverterStream, GoertzelStream, FileLoopT< FileType >, FileLoopT< File >, EqualizerNBands< SampleT, AccT, NUM_TAPS, NUM_BANDS >, EncodedAudioStream, AudioKitStream, MiniAudioStream, PortAudioStream, StdioStream, A2DPStream, HLSStreamT< URLStream >, HttpRequest, ICYStreamT< T >, URLStream, URLStreamESP32, NRF24Stream, SPIAudioSlave, AnalogAudioStream, AudioEffectStreamT< T >, AudioEffectStream, Equalizer3BandsPerChannel, ReformatBaseStream, AdapterAudioOutputToAudioStream, MemoryStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, InputMixer< T, SumT >, InputMerge< T >, CallbackStream, FilteredStream< T, TF >, FilteredStream< int16_t, int16_t >, ChannelFormatConverterStream, NumberFormatConverterStream, CatStream, QueueStream< T >, QueueStream< uint8_t >, DynamicMemoryStream, Pipeline, Pipeline::ModifyingStreamAdapter, VolumeStream, AudioFFTBase, AudioBLEClient, AudioBLEClient, AudioBLEServer, AudioBLEServer, AbstractURLStream, and AudioBLEStream.

filterSample()

template<typename T = int16_t>

T filterSample ( T  mic_sample )

inlineprotected

Estimate and subtract the echo for a single microphone sample, using the oldest filter_len samples currently buffered (i.e. the playback signal delayed by lag samples) as the adaptive filter's reference, then update the filter taps (NLMS-free, plain LMS).

flush()

virtual void flush ( )

inlineoverridevirtualinherited

Reimplemented from Print.

Reimplemented in PureDataStream, URLStreamBufferedT< T >, URLStreamBufferedT< ICYStream >, ReformatBaseStream, AudioStreamWrapper, ResampleStream, EncodedAudioStream, URLStream, BufferedTaskStream, I2SStream, MemoryStream, RingBufferStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, and BufferedStream.

isNotifyActive()

bool isNotifyActive ( )

inlineinherited

Checks if the automatic AudioInfo update is active.

not_supported()

virtual int not_supported ( int  out, const char *  msg = ""  )

inlineprotectedvirtualinherited

notifyAudioChange()

void notifyAudioChange ( AudioInfo  info )

inlineprotectedinherited

operator bool()

virtual operator bool ( )

inlinevirtualinherited

Reimplemented in A2DPStream, ICYStreamT< T >, AudioBLEStream, PortAudioStream, URLStream, USBAudioDeviceBase, I2SStream, AdapterAudioOutputToAudioStream, TimedStream, MemoryStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, Pipeline, and AudioFFTBase.

pushPlayback()

template<typename T = int16_t>

void pushPlayback ( T  sample )

inlineprotected

Push a playback (speaker) sample into the delay line, evicting the oldest sample once the window exceeds filter_len + lag so that the reference always reflects the most recent playback history.

readBytes()

template<typename T = int16_t>

size_t readBytes ( uint8_t *  buf, size_t  len  )

inlineoverridevirtual

Read input and remove echo, using the playback signal previously buffered via write() as the reference for the adaptive filter.

Parameters

buf	Pointer to buffer to store processed input (T*)
len	Number of bytes to read

Returns

Number of bytes read from input

Reimplemented from AudioStream.

readSilence()

virtual size_t readSilence ( uint8_t *  buffer, size_t  length  )

inlinevirtualinherited

Source to generate silence: just sets the buffer to 0.

refillReadBuffer()

void refillReadBuffer ( )

inlineprotectedinherited

Refill small read buffer (e.g. 8 bytes) to avoid single byte reads when calling read()

removeNotifyAudioChange()

virtual bool removeNotifyAudioChange ( AudioInfoSupport &  bi )

inlinevirtualinherited

Removes a target in order not to be notified about audio changes.

Reimplemented in RTSPClient< TcpClient, UdpSocket >.

reset()

template<typename T = int16_t>

void reset ( )

inline

Reset the internal buffer and lag state.

setAudioInfo()

virtual void setAudioInfo ( AudioInfo  info )

inlineoverridevirtualinherited

Defines the input AudioInfo.

Implements AudioInfoSupport.

Reimplemented in VS1053Stream, WM8960Stream, ChannelFormatConverterStreamT< T >, ChannelFormatConverterStream, VolumeStream, MiniAudioStream, PortAudioStream, I2SCodecStream, MozziStream, SPDIFOutput, AudioLoRa, AnalogAudioStream, I2SStream, TimerCallbackAudioStream, AudioBLEStream, MDFEchoCancellationStream< T, SampleType >, AudioKitStream, NRF24Stream, SPIAudioSlave, USBAudioDeviceBase, VBANStream, Equalizer3Bands, Equalizer3BandsPerChannel, AdapterAudioOutputToAudioStream, TimedStream, MeasuringStream, ProgressStream, CallbackStream, VolumeMeter, AudioInputMonitor, FormatConverterStream, FadeStream, GoertzelStream, Pipeline::ModifyingStreamAdapter, QualityAnalysisStream, AudioFFTBase, EncodedAudioStream, PureDataStream, GeneratedSoundStream< T >, GeneratedSoundStream< int16_t >, NumberFormatConverterStreamT< TFrom, TTo >, NumberFormatConverterStream, Pipeline, ResampleStream, and ResampleStreamT< TInterpolator >.

setFilterLen()

template<typename T = int16_t>

void setFilterLen ( size_t  len )

inline

Set the filter length for the adaptive filter.

Parameters

len	Length of the adaptive filter

setLag()

template<typename T = int16_t>

void setLag ( size_t  lag_samples )

inline

Set the lag (delay) in samples for echo cancellation.

Parameters

lag_samples

Number of samples to delay the echo subtraction

setMu()

template<typename T = int16_t>

void setMu ( float  mu )

inline

Set the adaptation rate (mu) for the LMS algorithm.

Parameters

mu	Adaptation rate

setNotifyActive()

void setNotifyActive ( bool  flag )

inlineinherited

Deactivate/Reactivate automatic AudioInfo updates: (default is active)

setWriteBufferSize()

void setWriteBufferSize ( int  size )

inlineinherited

write() [1/2]

template<typename T = int16_t>

size_t write ( const uint8_t *  buf, size_t  len  )

inlineoverridevirtual

Store the output signal (sent to speaker). The samples are pushed into the internal delay line so that readBytes() can later use them as the echo reference.

Parameters

buf	Pointer to PCM data sent to the speaker (T*)
len	Number of bytes in buf

Returns

Number of bytes processed

Reimplemented from AudioStream.

write() [2/2]

virtual size_t write ( uint8_t  ch )

inlineoverridevirtualinherited

Reimplemented in MemoryStream, AudioStreamWrapper, BufferedTaskStream, RingBufferStream, BufferedStream, and URLStream.

writeSilence()

virtual void writeSilence ( size_t  len )

inlinevirtualinherited

Writes len bytes of silence (=0).

Member Data Documentation

_timeout

int _timeout = 10

protectedinherited

adaptation_rate

template<typename T = int16_t>

float adaptation_rate = 0.01f

protected

buffer_size

template<typename T = int16_t>

size_t buffer_size

protected

filter

template<typename T = int16_t>

Vector<float> filter

protected

filter_len

template<typename T = int16_t>

size_t filter_len

protected

info

AudioInfo info

protectedinherited

is_notify_active

bool is_notify_active = true

protectedinherited

lag

template<typename T = int16_t>

size_t lag

protected

notify_vector

Vector<AudioInfoSupport*> notify_vector

protectedinherited

p_io

template<typename T = int16_t>

Stream* p_io = nullptr

protected

ref_vec

template<typename T = int16_t>

Vector<T> ref_vec

protected

ring_buffer

template<typename T = int16_t>

RingBuffer<T> ring_buffer {0}

protected

tmp_in

RingBuffer<uint8_t> tmp_in {0}

protectedinherited

tmp_out

RingBuffer<uint8_t> tmp_out {0}

protectedinherited

write_buffer_size

int write_buffer_size = MAX_SINGLE_CHARS

protectedinherited

---

The documentation for this class was generated from the following file:

• src/AudioTools/AEC/LMSEchoCancellationStream.h