Echo canceller system

Details Echo canceller system Echo canceller system

1997-06-27

2004-04-27

Kizou, Hassan (Department: 2662)

Multiplex communications

Duplex

Transmit/receive interaction control

C370S535000, C370S410000, C370S360000

Reexamination Certificate

active

06728223

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates in general to the field of telephone networks and communications, and more particularly to echo cancellation in telecommunications systems.
The presence of reflected voice signals or “echo” in telephone lines is a well-known phenomenon. Modern telephone systems employ echo cancellers at various points in a transmission system to eliminate such undesirable reflected voice signals. An early example of an echo canceller is described in U.S. Pat. No. 3,500,000, issued Mar. 10, 1970.
Hybrid circuits are a common source of impedance mismatch that gives rise to the signal reflection problem that may be heard as an echo of the speaker's own voice. In addition to hybrid circuits, telephone systems have other inherent sources of reflection and signal feedback that can give rise to undesirable echo transmissions. For example, speaker phones and “hands-free” mobile phones can acoustically couple or “feedback” a portion of the sound from the phone's loudspeaker into its microphone. Conventional echo cancellers can eliminate undesirable echoes from any such additional sources, when the echo signals are correlated, as well as from the ordinary hybrid circuit.
To facilitate an understanding of the echo phenomenon, reference is made to
FIG. 1
showing a simplified transmission system of the prior art, which is designated generally by reference numeral
10
. The system
10
is shown connecting telephone A to telephone B through a network N. Phone A is connected by line
11
to a hybrid circuit H
A
which in turn is connected to an echo canceller E
A
by line
12
. The echo canceller E
A
is connected to the network N by line
13
. Similarly, phone B is connected through hybrid circuit H
B
and echo canceller E
B
to the network N via lines
14
,
15
and
16
. The lines
11
and
14
typically consist of conventional two-wire subscriber loops (or “local loops”) through which analog voice signals are conducted in both directions. The hybrid circuits H
A
and H
B
separate the two-way voice signals on lines
11
and
14
to provide separate transmit and receive signals on the respective pairs of the four-wire lines
12
and
15
. A hybrid circuit can be part of the subscriber's equipment or part of the phone company's equipment.
Whether an echo is perceptible, and therefore objectionable, depends upon the delay from original transmission to receipt of the reflected signal. In the example of
FIG. 1
, if a party using phone A is speaking, the signal must travel the distance from phone A to hybrid circuit H
B
on the opposite side of the network N and be reflected back to phone A. To prevent the return of such echo signal to phone A, echo canceller E
B
superimposes an inverted copy of the echo signal on the line
16
to cancel the actual echo signal reflected by hybrid circuit H
B
. The echo canceller E
B
senses the duration for transmission from it to hybrid H
B
and reflection back to precisely time the cancellation function. Thus, the party speaking into phone A will not hear any annoying echoes. Similarly, echo canceller E
A
may be employed to remove the echo of speech transmitted by phone B and caused by signal reflection at hybrid circuit H
A
.
More recently, digital transmission has become commonplace in telecommunications networks. As a result, sophisticated digital echo cancellers have been developed to subtract out echoes caused by reflections at various points in the transmission system. Such digital echo cancellers are well known in the art, an illustrative example being described in U.S. Pat. No. 5,418,849.
In addition to transmitting digitized voice signals, telephone systems are being used increasingly for digital data transmission, as when computers communicate with each other. A telephone technology known as Integrated Services Digital Network (ISDN) provides uniform standards and protocols for computers to send and receive digital data through the twisted-pair copper wires of the conventional local loop at relatively high transmission rates compared to “modem” technology. An important application for ISDN technology is to provide a relatively high-speed connection to the Internet via the two-wire local loop of a conventional telephone. Unlike digital voice transmissions, ISDN data transmissions do not require echo cancellation. Conventional digital echo cancellers must be disabled so that they can pass ISDN data and other digital data transmissions without applying echo cancellation.
One end of a digital transmission system is depicted in the simplified block diagram of FIG.
2
and designated generally by reference numeral
20
. A phone A used by a “near-end talker” is connected to a HYBRID circuit by a conventional subscriber loop
21
for sending and receiving analog voice signals. The HYBRID circuit provides separate communication paths
22
and
23
for “send” and “receive” signals, respectively. A conventional device known as a CODEC (coder-decoder) converts analog signals on send line
22
to digital signals on send line
24
, and converts digital signals on receive line
25
to analog signals on receive line
23
. A digital echo canceller
26
communicates with the telephone network (not shown) via send line
27
and receive line
28
. Pulse-code-modulated (PCM) signals are communicated on lines
24
,
25
,
27
and
28
in accordance with network standards. The network interconnects the near-end talker using phone A with a far-end talker (not shown).
Echo canceller
26
is employed to eliminate the echo of the far-end talker's voice reflected on send path
22
by the HYBRID circuit. The far-end talker's voice signal is received on line
28
by the echo canceller
26
, sensed internally and passed through as an output on line
25
. From the signal on line
28
the echo canceller
26
estimates an echo signal expected to be returned on line
24
. The echo canceller
26
then subtracts the estimated echo signal from the actual echo signal. The resulting signal, which may include some “residual” echo, is further processed internally by the echo canceller
26
to produce an essentially echo-free output on line
27
.
In the United States, a digital multiplexing system is employed in which a first level of multiplexed transmission, known as T
1
, combines 24 digitized voice channels over a four-wire cable (one pair of wires for “send” signals and one pair for “receive” signals). The conventional echo canceller
26
of
FIG. 2
is shown operating on a single PCM voice transmission line prior to multiplexing (or “muxing”) for network transmission. The digital coding produced by the CODEC on line
24
provides 8,000 samples per second of the analog signal on line
22
, each sample being represented by an 8-bit binary number. Thus, the transmission rate on line
24
is 64,000 bits per second (64 kbps).
The conventional bit format on the T
1
carrier is known as DS
1
(i.e., first level multiplexed digital service or digital signal format), which consists of consecutive frames, each frame having 24 PCM voice channels (or DS
0
channels) of 8 bits each. Each frame has an additional framing bit for control purposes, for a total of 193 bits per frame. The T
1
transmission rate is 8,000 frames per second or 1.544 megabits per second (Mbps). The frames are assembled for T
1
transmission using a technique known as time division multiplexing (TDM), in which each DS
0
channel is assigned one of 24 sequential time slots within a frame, each time slot containing an 8-bit word.
Transmission through the network of local, regional and long distance service providers involves sophisticated call processing through various switches and a hierarchy of multiplexed carriers. At the pinnacle of conventional high-speed transmission is the synchronous optical network (SONET), which uses fiber-optic media and is capable of transmission rates in the gigabit range (in excess of one billion bits per second). After passing through the network, the higher level multiplexed carriers are demultiplexed (“demuxed”) back

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