Telephonic communications – Supervisory or control line signaling – Signal receiver
Reexamination Certificate
1999-07-27
2004-08-24
Isen, F. W. (Department: 2644)
Telephonic communications
Supervisory or control line signaling
Signal receiver
C379S399020, C708S312000, C708S405000, C708S406000, C370S345000
Reexamination Certificate
active
06782095
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and an apparatus for detecting and discriminating between various types of electronic signals. More specifically it relates to telephone call processing and is particularly applicable to multi-frequency (MF), dual-tone multi-frequency (DTMF), call progress tone (CPT) and MF-R2 tone receivers specifically in the area of telephone networks.
BACKGROUND OF THE INVENTION
Telephone service providers increasingly supply a wide variety of options and features to subscribers, such as call waiting, three-way calling, credit card calling among many others. All these services are implemented to provide customers with conveniences and facilities that were unavailable a few years back. In order to achieve communication between various units of a telephone network and with users and hence provide these services, telephone systems require various types of control signals. These signals include tones which are used to convey information to the system from the user or, alternatively, to inform the user of the current status of a telephone transaction. Tones may also be used to communicate between switches and other parts of the telephone network itself.
The correct detection of a tone is crucial to the functioning of the telephone network since the latter relies on them for operations such as dialling, billing and coin identification. Users also rely on these tones for information such as busy, dialling and dial tone. As a concrete example, automatic redial when faced with a billing signal and recognition of an incoming fax would not be possible without accurate tone recognition.
Four categories of signalling are commonly used each with its own specifications and purposes. They are multi-frequency (MF) tones, dual-tone multi-frequency tones (DTMF), international MF-R2 tones and call progress tones (CPT). There are other signalling sequences that are not mentioned here since their purposes are similar in nature to the four signalling conventions mentioned above. Two separate devices are involved in the tone communication process: the transmitter, which creates and propagates the tones, and the receiver, which receives and decodes them.
The correct detection of a tone by the receiver implies that the signal originating from a transmitter station is accurately decoded by the receiver as being the transmitted tone. Correct detection by the receiver of a digit encoded using any signalling methods also requires both a valid combination of frequencies and the correct timing element. A valid combination of frequencies implies that the receiver is able to discern that there is only the specified frequencies present and that these frequencies are located at or at least within reasonable distance of the nominal values. Furthermore the receiver is also able to verify within reasonable accuracy that the amplitude, twist and other characteristics of the tone conforms to certain pre-determined values. The receiver should also be able to obtain within reasonable accuracy the duration of a tone in order to determine if it is valid when compared to pre-determined duration requirements such as inter-tone gaps, cadence and other temporal specifications.
Detection of tones is a problem that has been addressed in the past. For example Bennett et al., U.S. Pat. No. 5,311,589 assigned to AT&T Bell Laboratories, describes a method to process DTMF and CPT tone using the Goertzel algorithm and a logical processing stage. The contents of his document are incorporated herein by reference.
Tone receiver systems have been developed in many parts of the world and, although it is difficult to describe a standard tone receiver architecture, some characteristics are shared between many of them.
FIG. 1
illustrates a typical tone receiver system as present in the prior art. A typical tone communication system of the type depicted in
FIG. 1
generally comprises a device such as a transmitter
102
which encodes and transmits pulses serially in a communication channel
104
. The transmitter
102
could be a simple touch-tone telephone or be included as part of a telephone switch. A combination of the pulses transmitted constitutes a tone which typically has frequencies in the voice range (~180-3600 Hz). Hence, in a telephone network, the communication channel is the voice channel. At the other end of the channel a receiver
100
is connected. The receiver
100
monitors the communication channel, detects and decodes the signal and, in turn, transmits the decoded information to another device such as a controller
116
.
The receiver
100
can be separated into five functional blocks namely an anti-aliasing low-pass filter
105
, an analog to digital (A/D) converter
106
, a storage buffer unit
108
, a spectral processor
110
and a logical processor
112
. Although a few receivers still use the analog signal directly, the trend is clearly towards digitization because it can be processed by a digital computer and be implemented on an easily programmable DSP chip. Therefore, the prior system shown here uses digital signals; however, the same explanations are valid for analog receiver which use similar analog components. In certain circumstances, the incoming signal may be digital. In these cases the A/D converter
106
would not be required. Typically, the incoming signal is digitally sampled by an analog to digital (A/D)
106
converter and assembled into frames in a storage buffer unit
108
. These frames are then analysed at predetermined frequencies in order to obtain the frequency characteristics of the signal during that frame. Generally, the analysis is performed in two separate units
110
112
.
The spectral processor
110
analyses the spectral characteristics of the samples received from the storage buffer unit
108
to obtain frequency and amplitude information for each frame. This analysis is similar for all types of signals differing perhaps by the frequencies analysed. Furthermore, this operation requires large computational power and limits the system in its processing capability. Traditionally, the spectral properties of tones have been detected by means of a bank of bandpass filters, one for each possible frequency in the tone. This is shown in
FIG. 2
for the detection of a Multi-Frequency (MF) tone. The filters
200
202
204
206
208
210
may be digital or analog, and are used to estimate the energies of narrow bands of the spectrum in order to obtain a frequency representation of the signal. The bands are centred at the frequencies of interest and their width is chosen to reflect the frequency tolerance of the receiver of each analysed frequency. In the case of MF signalling, a tone is registered as present if and only if there is sufficient energy in two spectral bands. This can be verified by means of devices comprising an energy computation and a pre-determined amplitude threshold
212
214
216
218
220
222
. Another technique that can also be commonly used is to analyse the spectral characteristics of the signals involves the computation of the Discrete Fourier Transform (DFT). Typically the DFTs are computed only at the frequencies of interest and result in an estimation of energy in the frequency domain. This method is described in detail in “Discrete-Time Processing of Speech Signals” by Deller, Proakis and Hansen, Macmillan Publishing Company New York 1993 whose contents are hereby incorporated by reference. Energy estimates obtained at this stage are propagated to the logical processing stage
112
.
The logical processing stage
112
determines, based on the information obtained from the previous stage, if a valid tone has been detected by evaluating the temporal and logical characteristics in the signal. Using the computed amplitude of each frequency, a candidate tone is determined for each frame and is often compared to previous frames for continuity. For instance, in the case of MP signalling, two and only two frequencies must be above the energy threshold. In any other circumstance either the signal should be ignored or an error should be report
Fedorov Sergey
Leong Michael
Titova Galina
Zakharov Yuriy
Isen F. W.
Singh Ramnandan
LandOfFree
Method and apparatus for performing spectral processing in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for performing spectral processing in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for performing spectral processing in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3342020