DTMF signalling. It affects many types of signalling
METHOD AND APPARATUS FOR DETECTING systems which must operate in the presence of noise.
CONTROL SIGNALS Historically, talk-off has not presented a significant
obstacle to the initiation of telephone calls by DTMF
FIELD OF THE INVENTION 5 signalling. This is because, for example, people do not
The present invention relates generally to receivers usually speak into their telephone handsets (or other
of control signals in a communications system, such as wise intentionally subject telephone circuits to signifi
dual-tone multi-frequency receivers in a telephone net- cant noise) when dialing. However, if DTMF signalling
work, and, more particularly, to the use of spectrum is to be used in the presence of noise (e.g., voice, as
estimation techniques in the detection of such signals. 10 when adding a party to an on-going teleconference),
BACKGROUND OF THE INVENTION conventional DTMF receivers may exhibit significant
rates of talk-off. Thus, talk-off may be especially trouAs a result of tests demonstrating that push-button blesome for DTMF si m occurring during a teletelephones satisfied consumer desire for faster easier phone call, when voice, music or other signals are likely dialing, the American Telephone and Telegraph Com- "f, iT3 r ^ « J» J J pany (AT&T®) began development of its TOUCH- £J>!J>Te&ef BecaUS ° °f talk"off' I*TM? advanced TONE signalling system in the early 1950s. Push-button DTMF signal-controlled features and functions accessiTOUCH-TONE signalling was originally devised as a ble dunnS a cal1 may be improperly initiated or displacement for the once-common rotary-dial pulse rected> thereby hindering or preventing their beneficial signalling system. Today, TOUCH-TONE signalling 20 use
controls not only the switching associated with setting SUMMARY OF THE INVENTION up an ordinary telephone call, but also many advanced
features and functions, such as multiple-party telecon- A method and apparatus for reliably detecting conferencing (where, e.g., TOUCH-TONES are used to trol signals and reducing the frequency of false control control the addition of a party to an on-going telecon- 25 signal identification, such as that causing talk-off, are ference). provided by the present invention. The invention operThe TOUCH-TONE signalling system uses combina- ates to apply tapers (or data windows) to received sigtions of "tones" to form control signals representing nals and to determine how well transforms of the ta"dialed" digits. The system provides eight tones (or pered signals match models of expected control signals, signal frequencies) divided into two groups—one con- 30 The ... invention is appiicabie t0 the detection of taming four lower pitch tones, and the other containing commumcation system control signals comprising one
four higher pitch tones. Each digit of a telephone num- „, _„„ f,„„„^„„,, „, , „„ = „ „„ t „,
. ", , ^ , or more frequency components such as, e.g., control
ber is represented by two simultaneously generated . , . . r -ji
rt r J , c. Ljv- signals comprising one or more sinusoidal components tones—one tone from each group. Since each digit is , , ~TM. , „ , , y represented by a pair of tones selected from multiple 35 28 DTMf ^als call progress signals (e.g., nng tones, the TOUCH-TONE system is referred to as the and busy sl^ls and dlal tones)> supervisory audio tone dual-tone multi-frequency (DTMF) signalling system. sl§nals <used m> e-§- cellular telephony), and the The group of lower pitch tones of the DTMF system CCITT (International Telephone and Telegraph Conincludes tones of frequencies 697, 770, 852 and 941 Hz. sultative Committee) No. 5 signalling system, among The group of higher pitch tones includes tones of fre- 40 others).
quencies 1209, 1336, 1447 and 1633 Hz. A total of six-? In an illustrative embodiment of the present inven
teen DTMF signals can be produced by combining one tion, digital signals which may contain DTMF signals
tone from each group. For example, the DTMF signal to be detected, as well as other signals, are received and
representing the digit "five" on a conventional tele- sliced into time segments which may be overlapping,
phone keypad comprises simultaneously generated 45 To each of a plurality of copies of a segment is applied
tones at 770 Hz and 1336 Hz. Conventional telephone 0ne of a plurality of tapers, such as one of the Discrete
keypads are capable of generating 12 of the 16 possible Prolate Spheroidal Sequences (DPSS). Discrete Fou
DTMF signals (representing digits 0-9 and symbols # rier Transforms of these tapered segments are deter
and *), while specialized keypads can generate all 16 mined md combined to form one or more estimates of
DTMF signals (the conventional 12 plus special sym- 50 tfae distribution of si d w a function of fre.
bols A, B, C, and D). To identify a digit, a DTMF „„„„„„ A „f w+ a* rvrnre • -a I A I
, !. x / ^ , ^ • V r ■ quency. A ratio of best-fit DTMF sinusoidal model
receiver (or detector) must determine the frequencies t J i • • r J c , ril.
T^-r>TT-. ■ , . -A , r energy to model misfit energy is formed for each of the
present in a DTMF signal and associate with such fre- __°:L . ., . . . .... . . ^,
Quencies the digit they represent. DTMJ ^nusoida^ frequencies, f the value of the ratio
Typically, DTMF signals are not communicated over 55 ds a threshold, there is likely to be a sinusoid at the
dedicated, noise-free channels. Rather, they are often DTMF ... associated with the quotient and
transmitted over ordinary telephone circuits-the same not elsewhere. If sinusoids are found at certain pairs of
circuits which carry other signals, such as voice, music, frequencies (and not elsewhere), DTMF signals may be
or data. If these other signals have certain characteris- identified. Additional tests concerning, e.g., tone duratics in common with DTMF signals, a DTMF receiver 60 tion and spacing, may be included in the DTMF signal
may falsely identify them as DTMF signals. False iden- identification procedure.
tification may occur when a signal carded over a tele- BRmF DESCRIPTION OF THE DRAWINGS phone circuit—for example, a person's voice—includes
one tone (or frequency component) from each of the FIG- 1 presents an illustrative telecommunication upper and lower DTMF tone groups. 65 system comprising a plurality of individual telephone
The problem of falsely identifying voice, music and networks,
other signals as DTMF signals is known as talk-off. The FIG. 2 presents an illustrative control function for the
talk-off problem is not limited to systems which employ telephone network of FIG. 1.