Telephonic communications – Subscriber line or transmission line interface – Hybrid circuit
Reexamination Certificate
1999-05-03
2003-09-30
Isen, Forester W. (Department: 2644)
Telephonic communications
Subscriber line or transmission line interface
Hybrid circuit
C379S391000, C379S372000, C379S394000, C379S398000
Reexamination Certificate
active
06628779
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and systems that provide suppression of near-end speech energy for applications including, but not limited to, improving the talkoff and talkdown performance of inband signal tone detection systems. In particular, the present invention describes a method and system providing interconnection between the tip and ring telephone line interface and subsequent communications equipment for the purposes of calibrating a selectable line bridging circuit and extracting a single, unidirectional path containing predominantly far end energy, wherein, near-end speech signals have been canceled. The method and system inherently provide access to on-hook service signals, such as calling party identification data transmissions.
BACKGROUND OF THE INVENTION
Echo cancellation systems are widely used in the telephone network and in station set equipment. The traditional role of echo cancellation systems in the telephone network has been to improve the quality of a transmission channel by removing unwanted signal reflections that occur at points of impedance mismatch in the communication circuit. Echo cancellers have also been employed in station set equipment, for the most part, to enable high speed, full duplex data transmission. With the introduction of new telephone services aimed at the analog residential subscriber, echo cancellers or near-end speech cancellation systems have recently become of significant importance in subscriber station sets to improve the performance of inband tone signal detectors.
Inband tone signaling schemes using combinations of discrete frequencies have long been used in the telephone system. The primary advantage of inband tone signaling is that the same spectrum that normally carries customer speech can be used to alternately transmit signal and control information. Sharing the voiceband is essential in situations where bandwidth is limited and dedicated control channels are either too costly or pose a degradation to service. Some of the most common examples of inband tone signaling used in the telephone network today include call progress signals, such as dial tone, stutter dial tone, audible ringing, busy, reorder, call waiting, etc., and Dual Tone Multi-Frequency (DTMF) signals used predominantly for dialing.
In recent years, new telephone services, such as Calling Identity Delivery on Call Waiting (CIDCW), Call Waiting Deluxe (CWD) and advanced screen telephony platforms, such as the Analog Display Services Interface (ADSI) and the Internet or Web Phone, have been deployed and require reliable Customer Premises Equipment (CPE) tone signal detection for signals sent by a Stored Program Control Switching System (SPCSS) or a far-end server. These services and platforms, encouraged by many technological advances in semiconductors, are transforming the conventional telephone set into a sophisticated, integrated communications terminal bearing a liquid crystal display and keyboard that under microprocessor, if not digital signal processor, control can track the state of a call and react to network and far-end tone signals.
All inband tone signaling systems are premised on the belief that a tone signal can be reliably detected. For Analog Display Services Interface (ADSI) Customer Premises Equipment (CPE), reliable detection of network call progress signals is necessary for the CPE to properly track the state of the call and generate internal events that are to be processed by a downloadable service script resident in the CPE. For CIDCW and CWD CPEs, reliable detection of the CPE Alerting Signal (CAS) is necessary to engage the CPE's off-hook data transmission mode for the reception of a data burst containing the calling party's number, name, location or personal identification number. For telephone answering machines and voicemail systems, reliable detection of DTMF signals is necessary to allow the subscriber to specify editing and control actions, even during playback of voice messages.
While reuse of an inband channel provides an efficient means for network-to-station set or server-to-station set signaling, significant problems related to signal recognition may be encountered by station sets attempting to detect tone signals.
Two traditional problems with inband tone signal detection are detector talkoff and talkdown.
Talkoff occurs whenever a tone signal detector erroneously accepts signal imitations produced by speech, music or noise as valid tone signals. Studies, experimentation, and field experience have all decisively confirmed that human speech can imitate some of the spectral and temporal properties of tone signals. The combination of consonants, vowels, syllables, and accent that frequently occur in an ordinary telephone conversation can cause a tone signal detector to talkoff. Ever since the first use of inband tone signaling in the telephone network, it has been a challenge designing reliable tone signal detection systems that are non-responsive to signal imitations.
Talkdown is another significant performance characteristic of tone signal detectors. Talkdown occurs whenever a tone signal detector fails to recognize a valid tone signal because it was masked or denied validation as a tone signal because of extraneous energy present on the line. In some instances, tone signals may compete with speech, music and other background noise. The presence of these complex signals distorts valid tone signals and can impair their detection.
Talkoff and talkdown are two critical performance measures for a tone signal detector. They respectively describe the detector's ability to resist signal imitations and to recognize valid tone signals obscured by speech, music or noise. Although tone signal detection has been a prevalent art in the telephone network for decades, only recently has the need for robust talkoff and talkdown performance been simultaneously required in an application. For the most part, prior art tone signaling applications, such as DTMF dialing, have benefited from environments where detector talkdown performance could be sacrificed in favor of improving talkoff performance. With the advent of CIDCW, CWD and ADSI, simultaneous robust talkoff and talkdown performance became a necessity.
Bellcore has specified CPE or station set criteria in Bellcore documents SR-TSV-002476, entitled “
Customer Premises Equipment Compatibility Considerations for the Voiceband Data Transmission Interface
”, Issue 1, December 1992, and SR-3004, entitled “
Testing Guidelines for Analog Type
1, 2, and 3 CPE Described in SR-INS-002726”, January 1995, that address the talkoff and talkdown performance of tone signal detectors for the CAS and call progress signals. The recommendations contained in these documents call for highly reliable tone signal detection. For example, SR-TSV-002476 recommends that a CAS detector respond to no more than 1 signal imitation in 45 hours of exposure to equal amounts of average level near-end and far-end telephone speech. The talkdown criteria that must be simultaneously achieved by this CAS tone signal detector for the average near-end talker on an average loop are the recognition of 99% of all valid CAS. The combination of these performance criteria makes CAS tone signal detectors that are compliant with SR-TSV-002476 arguably the most robust inband tone signal detectors ever deployed in the telephone network.
For tone signal detection systems used at a subscriber's location, signal imitations can come from both the near-end subscriber's voice as well as the voice of a far-end party. The near-end subscriber's voice is usually the dominant source of talkoff because the electrical speech level of the near-end subscriber is significantly stronger than that of the far-end. The speech signal of the far-end party is reduced by the loss on two loops, i.e., the far-end party's loop and the near-end subscriber's loop, and any intervening network loss before it appears at the near-end subscriber's station set. The near-end subscriber is also th
Farbanish Glen
Giordano Joseph
Isen Forester W.
Swerdlow Daniel
Telcordia Technologies Inc.
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