1. Field of the Invention
The present invention relates to analog communication systems. In particular, the present invention relates to analog communication systems, such as telephony systems, wherein analog control signals are sent in voice band.
2. Description of the Related Art
An analog communication system is a communication system wherein information is transmitted as a continuous-time signal. One such analog communication system is an analog telephony system.
Within analog telephony systems, it is necessary for communication system control information to be transmitted between communications devices. Such information can be used to control switching, to alert a telephone user that he or she has a call waiting, or to display caller identification information. One way in which such control information is transmitted within telephony systems is referred to as xe2x80x9cin-band signaling.xe2x80x9d
In an analog telephony system, analog electrical signals utilized within the telephony system are directly translated to mechanical signals of identical frequencies by use of a transducer such as a telephone speaker. The mechanical signals from the speaker are perceived as sound xe2x80x9ctonesxe2x80x9d by a human telephone user. The electrical signal frequencies which directly translate to mechanical signals (tones) which humans can hear are referred to as xe2x80x9cvoice bandxe2x80x9d frequencies, since what a human usually hears over a telephone is the human voice.
When control information signals are sent in the range of electrical frequencies which will translate to tones humans can hear, such signaling is referred to as xe2x80x9cvoice-bandxe2x80x9d or xe2x80x9cin-band signaling.xe2x80x9d One specific scheme in which this is currently done is known as Dual Tone Multi-Frequency (DTMF) signaling. xe2x80x9cDual Tonexe2x80x9d implies that any two frequencies used will be in-band, Multi-Frequency implies that more than one frequency per tone is possible.
A problem which commonly occurs when in-band signaling tones, single or multiple, are used to transmit information, is the accurate estimation of the time duration for which the signal is present. For example, in DTMF tones used in standard telephony applications for push-button signal reception, signal durations in the range 20-40 ms are specified as the minimum-to-maximum requirement in various National and International Standards. Outside this range, the signal can be accepted or rejected as a true signaling tone, depending upon the accuracy of measurement specified by a particular Communications Administration.
The current trend is for more and more information to be conveyed via DTMF signals. A relatively recent example of such increase in information to be conveyed via tone detection is the group of services known as Caller Identity Deliver on Call Waiting (CIDCW) which provides caller identity information to the subscriber for calls that are call waited.
As more and more information is conveyed via DTMF signals, the allowable signal tolerances are dropping. That is, system performance requirements are rising. One example of such rising performance requirements is set forth in Bellcore Special Report SR-TSV-002476, Issue Dec. 1, 1992, which sets forth a series of six very stringent performance criteria, all of which must be passed for DTMF systems to be deemed xe2x80x9cBellcore Standard Compliant.xe2x80x9d
One set of xe2x80x9cBellcore Standard Compliantxe2x80x9d performance criteria relates to tone detection. A Bellcore Compliant system must be able to detect tones within a very narrow frequency tolerance, and a tightly controlled time window. For example, for the two DTMF tones 2130 Hz and 2750 Hz, the Bellcore performance criteria are as follows: Lower Tone: 2130 Hz+/xe2x88x920.5%; Upper Tone: 2750 Hz+/xe2x88x920.5%; Dynamic Range: xe2x88x9214 to xe2x88x9232 dBm per tone; Power Differential within Dynamic range: 0 to 6 dB between tones. The specified duration for the DTMF signal to be accepted as a control signal by Customer Premises Equipment must fall within a time window 75 to 85 ms duration.
The current generation of DTMF detectors cannot meet such foregoing noted required levels of accuracy in a cost-effective manner. Current DTMF detectors tend to bandpass filter the analog signals at the specified frequencies, and then ensure that such frequencies exceed a preset threshold. Due to the analog nature of such systems, there generally needs to be one specific analog filter for each tone. Furthermore, such analog filters tend to drift and are very difficult to keep within Bellcore compliances. Also, filters which have sharp enough cutoffs to meet the +/xe2x88x920.5% frequency tolerance are typically expensive to build and maintain. Furthermore, this difficulty in filter sharpness translates into difficulty in telling when the frequency started and stopped, which makes it difficult to meet the stringent Bellcore 75-85 msec window.
It is therefore apparent that a need exists for a method and system which will efficiently and cost-effectively allow the accurate detection and measurement of analog signals, where such signals are composed of one or more analog frequencies of defined duration, and where such signals must have, for each frequency composing the signal, the power associated with that frequency concentrated within a very narrow bandwidth and the time of duration of that frequency contained within a very narrow time window.
It has been discovered that a method and system can be produced which will efficiently and cost-effectively allow the accurate detection and measurement of analog signals, where such signals are composed of one or more analog frequencies of defined duration, and where such signals must have, for each frequency composing the signal, the power associated with that frequency concentrated within a very narrow bandwidth and the time of duration of that frequency contained within a very narrow time window. In the method and system, an analog signal is received. A stream of data samples is created from the received analog signal. Based on the stream of data samples, a duration is calculated for one or more analog frequencies contained within the received analog signal. The duration is calculated for the one or more frequencies by utilizing a calculated signal energy for each of the one or more analog frequencies. The calculated signal energies for each of the one or more analog frequencies are used to determine a number of frequency-specific data samples. The number of frequency-specific data samples are then utilized with a sampling rate to calculate the duration of the each of the one or more analog frequencies. The one or more calculated durations of each of the one or more analog frequencies are utilized to determine whether the analog signal, composed of the one or more analog frequencies of defined duration, is present.
The foregoing summary is illustrative and is intended to be in no way limiting. Other aspects, inventive features, and advantages of the present invention as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.
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