1. Field of the Invention
The present invention relates to a telephone apparatus that allows the user to use a caller ID service informing him or her of the telephone number and the name of the calling party and a signal detection circuit for use therewith.
2. Description of the Related Art
In the North American countries, a caller ID service that informs the user of the telephone number and the name of the calling party has been used. In such a caller ID service, when a telephone terminal receives a call, the service informs the user of the telephone number and the name of the calling party. As a modification to the above conventional service, a new type of service has been proposed. In this new service, while a first user is communicating with a second user, if a third user calls the first user, the service informs the first user of the telephone number and the name of the third user who is call-waiting. In other words, when the first user does not know who is the third user, the first user does not know whether or not the third user is superior to the second user. When the first user receives a call waiting signal, however, and if the first user is informed who is the third user, the first user can conveniently determine to whom the first user should talk.
Such a new type caller ID service can be accomplished in the following manner.
While the first user is communicating with the second user, if the third user calls the first user, the telephone exchange sends a dual tone signal referred to as CAS (Call Alert Signal) to the telephone of the first user so as to mute the receiving speech signal. Responding to the CAS signal, the telephone of the first user sends a DTMF signal that represents "D" so as to receive information from the third user. When the telephone exchange has received the "D" signal of the DTMF signal from the telephone terminal of the first user, the telephone exchange sends data of the telephone number and the name of the third user as FSK (Frequency Shift Keying) modulated data. The telephone terminal of the first user demodulates the FSK-modulated data, decodes the data of the telephone number and the name of the third user, and displays the decoded data on the display of the telephone terminal.
In the new type caller ID service corresponding to the call-waiting signal, before the telephone exchange sends the telephone number and the name of the third user, it sends the CAS signal that causes the telephone terminal of the first user to mute the receiving speech signal. The CAS signal is a dual tone signal with frequencies of 2130 Hz and 2750 Hz. The CAS signal lasts for 80 msec. The telephone terminal that accomplishes the new type caller ID service corresponding to the call-waiting signal has a signal detection circuit that detects the dual tone signal with frequencies 2130 Hz and 2750 Hz.
Generally, as shown in FIG. 1, the signal detection circuit that detects such a dual tone signal comprises a frequency band limiting filter 102, a band pass filter 103 that extracts a frequency component of 2130 Hz, a band pass filter 104 that extracts a frequency component of 2750 Hz, a tone detection circuit 105 that detects whether or not the component of the tone signal is present, and a guard time setting circuit 106 that determines whether or not a component of the tone signal is continuously detected for a predetermined guard time that is shorter than the time period of the CAS signal.
In the new type caller ID service corresponding to the call-waiting signal, the CAS signal that is a dual tone signal with frequencies of 2130 Hz and 2750 Hz is supplied from the telephone exchange to an input terminal 101. When the CAS signal is received, the band pass filter 103 extracts a frequency component of 2130 Hz. The band pass filter 104 extracts a frequency component of 2750 Hz. The extracted frequency components are supplied to the tone detection circuit 105. When the tone detection circuit 105 detects these frequency components of the tone signal, it outputs a detection signal. Since the CAS signal lasts for 80 msec, the guard time setting circuit 106 detects the tone signal for a time period more than a predetermined guard time which is shorter than the time period of the CAS signal. A detection signal that represents the dual tone signal is outputted from an output terminal 107.
A transmitting speech signal and a receiving speech signal flow on a telephone line. Not only a signal sent from the telephone exchange, but a transmitting speech signal of the telephone terminal are supplied to the input terminal 101. The transmitting speech signal may contain a component of the tone signal by chance. When the detection signal contains frequency components of 2130 Hz and 2750 Hz and the transmitting speech signal lasts for the predetermined guard time, although the transmitting speech signal is not the dual tone signal of the CAS signal, the detection signal that represents that the tone signal is present is fed to the output terminal 107. In particular, speech signals of some users may have spectrum peaks at frequencies 2130 Hz and 2750 Hz by chance. When such users send calls, mis-detections may frequently take place.
To prevent such mis-detections, the guard time may be set to 30 msec or more. However, in the case that the guard time is set to 30 msec or more, since a transmitting speech signal is present while the dual tone signal is being detected, the sound level of the transmitting speech signal may exceed the sound level of the dual tone signal during the guard time. Thus, frequently, the signal detection is adversely affected. In the case that the guard time is set to around 5 msec, since the dual tone signal can be detected in a no-sound period between words, the probability that the signal detection is adversely affected becomes high. Moreover, as described, the probability that a component of the speech signal is detected as the dual tone signal becomes high.