Telephonic communications – Echo cancellation or suppression – Residual echo cancellation
Reexamination Certificate
1999-12-17
2001-08-14
Tsang, Fan (Department: 2645)
Telephonic communications
Echo cancellation or suppression
Residual echo cancellation
C379S398000, C379S402000, C379S404000
Reexamination Certificate
active
06275582
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 relates to telephone line bridging methods that extract a unidirectional path containing predominantly far-end energy and transparently pass telephone line supervisory, alerting and communications signals to subsequent communications equipment without terminating the telephone line.
BACKGROUND OF THE INVENTION
For a more comprehensive discussion of the prior art, reference should be made to U.S. patent application Ser. No. 09/304,402 entitled, A Method and System for Scalable Near-End Speech Cancellation for Tip and Ring Tone Detectors which is assigned to the assignee of the present application. The
Background of the Invention
section of U.S. patent application Ser. No. 09/304,402 is herein incorporated by reference for the purpose of illustrating the state of the prior art and for supplementing the prior art discussion included herein.
The deployment of new CLASS services, such as Calling Identity Delivery on Call Waiting (CIDCW) and Call Waiting Deluxe (CWD), has created renewed interest in telephone near-end speech cancellation methods and systems. Once primarily used to enable bi-directional communications on a single twisted pair, speech cancellation systems are now needed to improve the reliability of inband tone signal detection for vertical telephone services. Vertical services, such as CIDCW and CWD, require special Customer Premises Equipment (CPE) that can reliably detect a unique inband tone signal, known as the CPE Alerting Signal (CAS). The CAS is transmitted by central office switching equipment to initiate transfer of the service-related information. Reliable tone detection is critical for CIDCW and CWD CPE because the CAS is the trigger for the CPE to enter data mode and receive the service information. CIDCW, for example, enables a subscriber who is engaged in a telephone conversation to receive the caller ID information associated with a second call on call waiting. The successful reception and display of the waiting caller's information to the subscriber requires the detection and acknowledgement of the CAS by subscriber equipment. Inband tone signal detection is complicated when resistance to speech simulations and detection of alerting signals masked by the subscriber speech are both desired, as is the case for CIDCW, CWD, and any off hook GR-30-CORE, Issue 2, December 1998, entitled “Voiceband Data Transmission Interface”, Issue 1, December 1994, services. Based on the prior art, it is known that the performance and reliability of CAS detection methods can be significantly improved by attenuating or canceling near-end speech signals in the receive channel, wherein the CAS is present, prior to signal discrimination by the CAS detector.
Of particularly high interest are a special class of near-end speech cancellation methods and systems that can transparently be inserted in series with the Tip and Ring telephone interface prior to the communications equipment or a typical communications circuit. These systems are herein referred to as Tip and Ring speech cancellation systems or circuits; these special systems differ in two ways from most traditional two-to-four wire hybrid circuits. First, Tip and Ring speech cancellation systems extract a receive signal channel without terminating the telephone line. One of their key functions is to transparently pass the telephone line supervisory, alerting and communications signals to subsequent or subtending communications equipment or circuitry. Traditional telephone hybrid circuits, on the other hand, were designed for end devices that terminate the telephone line. Second, Tip and Ring speech cancellation systems are designed to highly attenuate near-end speech signals to produce a pure receive channel commonly used to feed tone detection and data demodulation circuits. Traditional hybrid circuits, on the other hand, were designed to support bidirectional communications and engineered to only modestly attenuate near-end speech because a controlled sidetone response was desired.
One popular application for Tip and Ring speech cancellation systems is in telephone adjunct devices, such as Type 2 CIDCW adjuncts. A telephone adjunct connects between the telephone line and the customer's existing equipment, such as a standard telephone set. The value of adjunct devices is that they allow the customer to incrementally add support for enhanced services without necessitating the disposal of their existing equipment. In addition, adjuncts allow CPE manufacturers to introduce new service capabilities without incurring the expense of replacing the functionality already present in the customer's existing equipment. Because adjuncts connect in series with the telephone line, adjunct devices need to faithfully pass telephone supervisory signals (i.e., DC signals), alerting signals (ringing) and communications signals (i.e., AC signals) between the telephone line and the subsequent or subtending communications equipment. Consequently, Tip and Ring speech cancellation systems are well suited for adjuncts that employ near-end speech cancellation to improve tone signal detection.
A second popular application for Tip and Ring speech cancellation systems is in convenient front-end solutions that add enhanced service support to existing communications circuits. Faced with demand for shortened development cycles in a highly competitive environment, CPE manufacturers strongly desire to reuse as much of their existing technology as possible in new products. Because they connect directly to the telephone line and transparently pass telephone supervisory, alerting and communications signals, Tip and Ring speech cancellation systems enable CPE manufacturers to introduce enhanced services circuitry in front of their existing communications circuit designs without adversely affecting their performance or creating need for their modification.
A prior art Tip and Ring speech cancellation system
100
is illustrated in FIG.
1
. This arrangement employs the fundamental Wheatstone bridge principle as described in Lim et. al., U.S. Pat. No. 5,796,810, Aug. 18, 1998, entitled “Apparatus for Dialing of Caller ID Block Code and Receiving Call Waiting Caller-ID-Signal” (hereinafter Lim).
FIG. 1
shows a basic Wheatstone bridging circuit consisting of elements Ra
112
, Rb
113
, and R
3
114
. The multiple network option
130
that is shown in
FIG. 1
is described in my U.S. patent application Ser. No. 09/304,402, and is discussed below. The objective of the Wheatstone bridging circuitry in
FIG. 1
is to match the impedance of R
3
to that of Z
115
, the unknown impedance of a subscriber loop. In William L. Everitt's “Communications Engineering” (McGraw-Hill 1937) (hereinafter Everitt) a Wheatstone bridge is defined to be a physical circuit comprised of two parallel legs wherein each leg contains the series combination of two impedances. For the purposes of discussion, assume that Ra
112
and Z
115
, in
FIG. 1
, comprise the left leg of the Wheatstone bridge and Rb
113
and R
3
114
comprise the right leg of the bridge. Specifically, Everitt defines a Wheatstone bridge as: (1) a rhombus-like physical circuit; wherein (2) tapping of two output signals, one from the center of each leg of the bridge, points D and E in
FIG. 1
, is performed; and (3) circuit balance (i.e., no voltage difference) is achieved when the ratio of the impedances of the left leg of the bridge is equal to the ratio of the impedances of the right leg of the bridge; given that (4) the same potential or input signal is applied across both legs of the bridge. The characteristic balance condition occurs when the voltage drops across Z
115
and R
3
114
are approximately equal both in magnitude and phase.
In accordance with
FIG. 1
, if the balance network
114
Cockings Orville
Escalante Ovidio
Telcordia Technologies Inc.
Tsang Fan
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