Telephonic communications – Supervisory or control line signaling – Signal receiver
Reexamination Certificate
1998-04-20
2001-07-31
Kuntz, Curtis (Department: 2643)
Telephonic communications
Supervisory or control line signaling
Signal receiver
C379S142030, C379S283000
Reexamination Certificate
active
06269160
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to telecommunications, and more particularly to a system and method for the detection of dual-tone multifrequency (DTMF) signals.
DESCRIPTION OF THE RELATED ART
Telephone subscribers communicate via a vast telephone network maintained by many different telephone service providers. A telephone subscriber's communications devices are typically connected in parallel to a two-wire telephone line which links a subscriber's premises to a telephone service provider's central office. Communications devices include telephones, answering machines, speakerphones, facsimile (fax) machines, and communications modems.
FIG. 1
is a top plan view of a modem telephone
10
, the most familiar of such communications devices. Telephone
10
is also generally referred to as customer premises equipment (CPE). Telephone
10
is adapted to be coupled to a two-wire telephone line, and includes a handset
12
and a keypad
14
.
FIG. 2
is a block diagram of telephone
10
. Handset
12
includes a receiver
15
and a transmitter
16
. Receiver
15
is designed to be placed over a user's ear during use, and transmitter
16
is designed to be positioned in close proximity to the user's mouth during use. Handset
12
is coupled to a communications circuit
18
. When telephone
10
is not in use, a hookswitch
20
is in an open position, and communications circuit
18
is disconnected from the telephone line. Only a ringer circuit
22
is connected to the telephone line when telephone
10
is not in use.
When a calling party places a telephone call to a receiving party, switching equipment in the central office transmits a series of ring signals over the receiving party's telephone line. The ring signals cause the ringer circuit of the receiving party's telephone to generate sounds which alert the receiving party to an incoming telephone call. At the same time, the switching equipment sends a series of ring-back signals to the calling party to let the calling party know that the telephone call is being placed. When the receiving party enables handset
12
(e.g., by lifting handset
12
) to answer the incoming telephone call, hookswitch
20
closes, connecting communications circuit
18
to the telephone line. Switching equipment senses the closure of hookswitch
20
and completes an electrical circuit between the calling party and the receiving party. A telephone conversation takes place as communications circuit
18
couples the calling party's voice signals from the telephone line to receiver
15
, and couples the receiving party's voice signals from transmitter
16
to the telephone line.
Placing a call to another telephone subscriber using telephone
10
involves enabling handset
12
and “dialing” the subscriber's telephone number. The act of “dialing” a telephone number on telephone
10
typically involves pressing numbered buttons of keypad
14
corresponding to the subscriber's telephone number in sequence. This form of dialing is known as Touch-Tone® dialing. The Touch-Tone® dialing system has nearly replaced the older rotary pulse-type dialing system in North America. Pressing a button on keypad
14
causes communications circuit
18
to generate a dual-tone multifrequency (DTMF) signal comprising a combination of two non-harmonic tones. The DTMF signal is received by switching equipment in the telephone service provider's central office and decoded. The sequence of telephone dialed telephone numbers is used to determine which telephone subscriber is to receive the telephone call.
The frequencies of the two tones in a DTMF signal are chosen such that they are not exact integer multiples (i.e., harmonics) of any frequency greater than 1 hertz (Hz). For example, 250 Hz and 375 Hz are not acceptable dual tones because they are integer multiples of 125 Hz. On the other hand, 250 Hz and 333 Hz are acceptable dual tones because they are not integer multiples of any frequency greater than 1 Hz. Such enharmonic frequencies are used to prevent false DTMF signal detections. In addition to telephone dialing, DTMF coding is also popular in interactive control applications, such as telephone banking, electronic mail systems, and answering machines. Such applications allow a user to select options from a menu by sending DTMF signals from any Touch-Tone® telephone.
Call Waiting is a telephone service which alerts a receiving party, engaged in a telephone call with a first party, to a new incoming call from a second (calling) party. The central office transmits a subscriber alerting signal (SAS) to the receiving party, and the calling party receives the customary series of ring-back signals. The SAS may be a short voice signal interruption accompanied by a clicking sound, or a short 440 Hz tone burst. The receiving party may answer the incoming call by “flashing” the hookswitch (i.e., opening the hookswitch momentarily). The receiving party may subsequently go back and forth between the first and second parties by flashing the hookswitch. Call Waiting thus allows a subscriber to answer an incoming telephone call while involved in a telephone conversation.
Caller ID is the name given to subscriber services which provide information about a calling party to a receiving party before the receiving party would normally answer an incoming telephone call. Like Call Waiting, Caller ID is available to telephone subscribers for a small monthly fee. Caller ID information is transmitted in digital format, one bit at a time, at a rate of 1,200 bits per second. Devices which receive, store, and display Caller ID information are widely available. Caller ID equipment complies with a set of standards set forth by the Bell Communications Research Group (Bellcore). Mode 1 Caller ID transmits information about a calling party between a first ring signal and a second ring signal. Mode 2 Caller ID, also known as calling identity delivery on call waiting, allows subscribers with Call Waiting to receive calling party information while engaged in a telephone call and before answering an incoming telephone call.
When a subscriber has Mode 2 Caller ID service, is engaged in a telephone call with a first party, and an incoming call from a second calling party occurs, the central office follows the SAS with an alerting signal directed to the subscriber's Mode 2 Caller ID equipment. The term customer premises equipment (CPE) includes a subscriber's Mode 2 Caller ID equipment, and the second alerting signal is known as the CPE alerting signal (CAS). The CAS is a DTMF signal comprising simultaneously generated 2130 Hz and 2750 Hz tones. Upon detecting the CAS signal, the CPE transmits a DTMF acknowledge signal (ACK) which indicates a readiness to receive Caller ID information. Central office equipment waits for the ACK signal before transmitting the Caller ID information. In order to detect the CAS, communications circuit
18
of telephone
10
includes a CAS detection system
24
.
Applying telephone engineering terminology to the above situation, the first party is the “far-end” party and the receiving party is the “near-end” party. In order to prevent the far-end party from hearing the Caller ID signals, the central office equipment mutes (i.e., silences) signals being received and transmitted by the far-end party for a period of time beginning prior to generation of the SAS and CAS alerting signals and ending after the Caller ID information is transmitted. Similarly, the CPE is required to silence the signals being received and transmitted by the near-end party for a period of time beginning after the detection of the CAS signal and ending after the Caller ID information is transmitted.
FIG. 3
is a block diagram of a prior art embodiment of CAS detector system
24
including a CAS detector
26
coupled to a receiver output port of a hybrid circuit
28
. The receiver output port of hybrid circuit
28
is also coupled to receiver
15
. Hybrid circuit
28
also has an input port connected to transmitter
16
. During use o
Advanced Micro Devices , Inc.
Conley Rose & Tayon PC
Eng George
Hood Jeffrey C.
Kuntz Curtis
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