Amplitude modulation detector

Details Amplitude modulation detector Amplitude modulation detector

2001-08-20

2004-11-23

Corrielus, Jean B. (Department: 2631)

Pulse or digital communications

Receivers

Amplitude modulation

Reexamination Certificate

active

06823025

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplitude modulation detector, an apparatus and method for detecting a CAS (Customer premise equipment Alert Signal), and an apparatus and method for detecting caller identification.
2. Description of the Related Art
In general, a caller ID system called “caller identification service” is a system for transferring information to customers through telephone lines. The information may include caller information such as a caller's telephone number or a caller's name, or general information regarding topics such as the weather or traffic. However, the use telephone lines for voice communications may limit the transfer an information signal. For example, in transferring a caller ID through a telephone line, identifying a CAS lying in the same frequency band as a voice signal can be difficult. Thus, telephone operation includes a general talk mode and an information reception mode, and a signal applied only in the information reception mode can be more easily identified as a CAS.
For the caller identification service, a CAS is used to switch between the general talk mode and the information reception mode. The CAS is a dual tone signal in which two signals with different frequency values, e.g., 2.13 KHz and 2.75 KHz, are mixed according to the system standard. When a transmitting site, i.e., a central office, intends to transmit information in the general talk mode, the transmitting site sends a CAS through a telephone line to a receiving site, before transmission of the information. Here, a CID/CIDCW (Calling Identity Delivery/Calling Identity Delivery on Call Waiting) receiver at the receiving site transmits to the central office a CAS acknowledgment signal that indicates detection of the CAS. The CID/CIDCW receiver can display the caller ID on customer premise equipment (CPE) in a network. The CID/CIDCW receiver is in the CPE. Here, the receiving site transmits the CAS acknowledge signal to the central office, when changing the general talk mode into the information receiving mode.
If a receiver is in an OFF-HOOK state, i.e., a user is calling another person, a voice signal of the user may be mixed with the CAS. Accordingly, the receiving site must detect the CAS from a mixed signal including the CAS and the voice signal.
FIG. 1
is a block diagram of a conventional CAS detection apparatus. The conventional CAS detection apparatus includes a first band-pass filter (BPF)
10
, a second BPF
12
, a first frequency discriminator
14
, a second frequency discriminator
16
, and a CAS discriminator
18
. BPF
10
filters an input signal IN and passes signal components in a first frequency band, i.e., the 2.13-KHz band. BPF
10
passes signal components in a second frequency band, i.e., the 2.75-KHz band, from the input signal IN.
Frequency discriminators
14
and
16
receive the signals from BPFs
10
and
12
, respectively, and determine whether the signals have respective frequencies 2.13 KHz and 2.75 KHz, which correspond to a CAS. If a CAS is in the input signal, the filtered signals from BPFs
10
and
12
have significant amplitudes for the frequency components having frequencies 2.13 KHz and 2.75 KHz. However, if the CAS is not present in the input signal and a voice signal is presents, the filtered signals typically have frequencies other than 2.13 KHz and 2.75 KHz or small amplitudes at frequencies 2.13 KHz and 2.75 KHz. CAS discriminator
18
receives the output signals of frequency discriminators
14
and
16
, and generates a CAS detection signal CAS_DET if signals with the frequencies 2.13 KHz and 2.75 KHz are present.
The conventional CAS detection apparatus of
FIG. 1
utilizes multiple paths to detect multiple tones and identify the presence of the CAS. The multiple paths increase circuit complexity. Also, if the voice portion of the input signal includes frequency components of frequencies 2.13 KHz and 2.75 KHz, the CAS detection apparatus may generate a false CAS detection.
FIG. 2
is a block diagram of another conventional CAS detection apparatus. The conventional CAS detection apparatus of
FIG. 2
includes a first signal detector
100
, a second signal detector
150
, and a timing selector
180
.
The first signal detector
100
detects a first frequency component of a CAS in input signal IN. The second signal detector
150
detects a second frequency component of the CAS. Here, detectors
100
and
150
indicate the presence of the first and second frequency components to timing selector
180
. If the first and second frequency components remain for a predetermined time, timing selector
180
determines that the input signal contains a CAS and outputs a CAS detection signal CAS_DET.
In signal detector
100
, a guard band filter
102
passes the first frequency component filtered from the input signal. A first AC-DC converter
112
converts the first frequency component to a DC signal having a voltage proportional to the amplitude of the first frequency component. A first threshold detector
114
detects whether the DC signal from AC-DC converter
112
is at a voltage higher than a first threshold value. To distinguish noise in the input signal IN, AC-DC converter
112
and threshold detector
114
preferably operate first to provide a control signal to a limiter
104
or a post limiter filter
106
. If the voltage of the DC signal is above the first threshold value, limiter
104
amplifies the frequency component signal from the guard band filter
102
to a predetermined level. Post limiter filter
106
filters the amplified signal before another AC-DC converter
108
converts the amplified frequency component signal to another DC signal. A threshold detector
110
compares the voltage of the DC signal from converter
108
to a second threshold value. If the DC signal has a voltage higher than the second threshold value, the second threshold detector
110
sends to the timing selector
180
, a signal indicating the presence of the first frequency component in input signal IN. A process detector
150
uses for detecting a second frequency signal in input signal IN is similar to the above. Timing selector
180
detects whether the detected first and second frequency signals remain for a predetermined time and asserts a CAS detection signal CAS_DET if they do.
One drawback of the conventional CAS detection apparatus of
FIG. 2
is the use the analog circuitry to detect the frequencies and the levels of the input signal. The required analog circuitry is difficult to form in an integrate circuit. For example, the CAS detection apparatus of
FIG. 2
requires external components such as a capacitor, for detecting the frequency and the level of the CAS. The capacitor must be external to an integrated circuit because the required capacitance for the CAS detection apparatus is large. Thus, the CAS detection apparatus is difficult to implement in a single chip.
FIG. 3
is a block diagram of still another conventional CAS detection apparatus. The conventional CAS detection apparatus of
FIG. 3
includes a high tone filter
210
, a low tone filter
220
, first and second level comparators
215
and
225
, a tone detector
240
, and a guard time detector
280
.
High tone filter
210
passes only the high frequency band of the CAS from input signal IN. Then, level comparator
215
compares the filtered signal with a first internal reference and applies a signal indicating the result of the comparison to the tone detector
240
. Similarly, low tone filter
220
passes only the low frequency band of the CAS from input signal IN. Level comparator
225
compares the filtered signal with a second internal reference and applies a signal indicating the result of that comparison to tone detector
240
. Tone detector
240
receives the result signals from level comparators
215
and
225
and determines whether each level is higher than a predetermined reference level. If the output signals of level comparators
215
and
225
are higher than the predetermined reference level, tone detec

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