MpDtmfDetector Class Reference

A simple DTMF detector class that uses the Goertzel algorithm. More...

#include <MpDTMFDetector.h>

Public Member Functions
Creators
	MpDtmfDetector (const unsigned samplesPerSec, const unsigned nProcessSamples)
	Default constructor. More...
virtual	~MpDtmfDetector ()
	Destructor. More...
Manipulators
void	reset ()
	Reset the state of the detector. More...
void	setSamplesPerSec (const unsigned samplesPerSec)
	Set the sample rate. More...
void	setNumProcessSamples (const unsigned nProcessSamples)
	Set the number of samples to use to detect frequencies on. More...
UtlBoolean	processSample (const MpAudioSample sample)
	Process a sample through the detector. More...
Accessors
unsigned	getSamplesPerSec () const
	Get the sample rate. More...
unsigned	getNumProcessSamples () const
	Get the number of samples that this detector uses to determine frequencies on. More...
char	getLastDetectedDTMF () const
	Get the last DTMF tone that the detector detected. More...

Private Member Functions
void	calcCoeffs ()
	Calculate coefficients needed for the goertzel algorithm. More...
void	dtmfValidation ()
	Validate the detected frequencies detected by processSample. More...

Private Attributes
unsigned	mSamplesPerSec
unsigned	mNumProcessSamples
uint32_t	mSampleCount
double *	mQ1
double *	mQ2
double *	mR
double *	mCoefs
char	mLastDetectedDTMF

Static Private Attributes
static double	sFreqs_to_detect []
static uint8_t	snFreqsToDetect = sizeof(MpDtmfDetector::sFreqs_to_detect)/sizeof(double)

Detailed Description

A simple DTMF detector class that uses the Goertzel algorithm.

The MpDtmfDetector is a simple class that implements the goertzel algorithm for detecting multiple frequencies. It also incorporates some other checks to make sure that the DTMF tones pass other tests that are needed for working with bell standards (i.e. twist is in-range, etc).

Full Disclosure: This code is based off of the Goertzel example C code in wikipedia: http://en.wikipedia.org/w/index.php?title=Goertzel_algorithm&oldid=179514279 It has been heavily re-worked by Keith Kyzivat, but important parts may be identical.

Usage: To use this, construct it with your sample rate and an adequate value for number of samples to do the goertzel algorithm on. Then just call processSample repeatedly as you get samples. It will return /p FALSE, until /p nProcessSamples samples have been processed, after which time it will return /p TRUE. You then can get the detected DTMF using getLastDetectedDTMF(). If no DTMF was properly detected, NULL will be returned.

Constructor & Destructor Documentation

MpDtmfDetector	(	const unsigned	samplesPerSec,
		const unsigned	nProcessSamples
	)

Default constructor.

~MpDtmfDetector ( )

virtual

Destructor.

Member Function Documentation

void reset

(

)

Reset the state of the detector.

Reset all accumulators, last detected DTMF, and recalculate coefficients.

void setSamplesPerSec

(

const unsigned

samplesPerSec

)

Set the sample rate.

Parameters

[in]

samplesPerSec

- The new sample rate to use.

This resets the object state and recalculates coefficients.

void setNumProcessSamples

(

const unsigned

nProcessSamples

)

Set the number of samples to use to detect frequencies on.

This sets the number of samples that are collected before running the goertzel algorithm. The Goertzel algorithm will then be run on the collected samples.

92 is shown in wikipedia, 205 is mentioned in some mailing lists as a good choice – both are only for 8khz.

Parameters

[in]

nProcessSamples

- the number of samples that are collected before running the goertzel algorithm.

UtlBoolean processSample

(

const MpAudioSample

sample

)

Process a sample through the detector.

When getNumProcessSamples() samples are processed, a DTMF tone is either detected or not. In either case, the stored last DTMF tone detected is overwritten with the current detection value.

See also

getLastDetectedDTMF()

Returns

/p FALSE if this has not yet processed a multiple of getNumProcessSamples(), /p TRUE if this has processed a multiple of getNumProcessSamples()

unsigned getSamplesPerSec

(

)

const

Get the sample rate.

unsigned getNumProcessSamples

(

)

const

Get the number of samples that this detector uses to determine frequencies on.

char getLastDetectedDTMF

(

)

const

Get the last DTMF tone that the detector detected.

When processSample() processes getNumProcessSamples() samples, the last detected DTMF tone will be stored. Use this to access the detected tone.

If getNumProcessSamples() samples have not been processed yet, or if a tone was not detected during the last getNumProcessSamples(), then NULL will be returned here.

Returns

the last detected DTMF tone. If no tone has been detected,

void calcCoeffs ( )

private

Calculate coefficients needed for the goertzel algorithm.

These coefficients are dependent on the sample rate, so new coefficients need to be calculated whenever the sample rate changes.

From Wikipedia:

coef = 2.0 * cos( (2.0 * PI * k) / (float)GOERTZEL_N)) ; Where k = (int) (0.5 + ((float)GOERTZEL_N * target_freq) / SAMPLING_RATE));

More simply: coef = 2.0 * cos( (2.0 * PI * target_freq) / SAMPLING_RATE );

void dtmfValidation ( )

private

Validate the detected frequencies detected by processSample.

This looks at the detected frequencies, and determines if it conforms to DTMF rules as specified by bell, and others – i.e. can work well with PSTN.

Member Data Documentation

unsigned mSamplesPerSec

private

unsigned mNumProcessSamples

private

uint32_t mSampleCount

private

double sFreqs_to_detect

staticprivate

Initial value:

= 
{ 
   697, 770, 852, 941, 
   1209, 1336, 1477, 1633 
}

uint8_t snFreqsToDetect = sizeof(MpDtmfDetector::sFreqs_to_detect)/sizeof(double)

staticprivate

double* mQ1

private

double* mQ2

private

double* mR

private

double* mCoefs

private

char mLastDetectedDTMF

private