Network echo canceller for integrated telecommunications...

Details Network echo canceller for integrated telecommunications... Network echo canceller for integrated telecommunications...

2001-09-06

2004-05-18

Jung, Min (Department: 2663)

Multiplex communications

Duplex

Transmit/receive interaction control

C370S290000, C379S406050, C379S406080

Reexamination Certificate

active

06738358

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to signal processors and echo cancellers. More particularly, the invention relates to a network echo canceller for integrated telecommunications processing.
BACKGROUND OF THE INVENTION
Single chip digital signal processing devices (DSP) are relatively well known. DSPs generally are distinguished from general purpose microprocessors in that DSPs typically support accelerated arithmetic operations by including a dedicated multiplier and accumulator (MAC) for performing multiplication of digital numbers. The instruction set for a typical DSP device usually includes a MAC instruction for performing multiplication of new operands and addition with a prior accumulated value stored within an accumulator register. A MAC instruction is typically the only instruction provided in prior art digital signal processors where two DSP operations, multiply followed by add, are performed by the execution of one instruction. However, when performing signal processing functions on data it is often desirable to perform other DSP operations in varying combinations.
An area where DSPs may be utilized is in telecommunication systems. One use of DSPs in telecommunication systems is digital filtering. In this case a DSP is typically programmed with instructions to implement some filter function in the digital or time domain. The mathematical algorithm for a typical finite impulse response (FIR) filter may look like the equation Y
n
=h
0
X
0
+h
1
X
1
+h
2
X
2
+ . . . +h
N
X
N
where h
n
are fixed filter coefficients numbering from 1 to N and X
n
are the data samples. The equation Y
n
may be evaluated by using a software program. However in some applications, it is necessary that the equation be evaluated as fast as possible. One way to do this is to perform the computations using hardware components such as a DSP device programmed to compute the equation Y
n
. In order to further speed the process, it is desirable to vectorize the equation and distribute the computation amongst multiple DSP arithmetic units such that the final result is obtained more quickly. The multiple DSP arithmetic units operate in parallel to speed the computation process. In this case, the multiplication of terms is spread across the multipliers of the DSPs equally for simultaneous computations of terms. The adding of terms is similarly spread equally across the adders of the DSPs for simultaneous computations. In vectorized processing, the order of processing terms is unimportant since the combination is associative. If the processing order of the terms is altered, it has no effect on the final result expected in a vectorized processing of a function.
One area where finite impulse response filters is applied is in echo cancellation for telephony processing. Echo cancellation is used to cancel echoes over full duplex telephone communication channels. The echo-cancellation process isolates and filters the unwanted signals caused by echoes from the main transmitted signal in a two-way transmission.
Echoes are part of everyday life. Whenever we speak, we hear our own voice transmitted through both the air and our bodies. These echoes have a short latency, arriving at our ears within a tenth of a millisecond. Our minds automatically filter short-latency echoes so we do not notice them. We are so used to hearing these echoes as sidebands that when they are removed artificially, we notice their absence. Therefore, a certain amount of short-latency echo is desirable. However, the long-latency echoes experienced in modern telephony networks are not desirable.
Echoes are common in telephony equipment. They are caused by electrical reflections from nearly any impedance mismatch as well as by acoustical coupling between loud speakers and microphones. These echoes do not cause auditory problems until their delay (or ‘latency’) increases to roughly 30 ms or more.
Typically, echoes are not a serious issue in local telephone connections. However, in long-distance telephone connections, echoes become increasingly serious as their latency increases. As a result, a significant amount of signal processing is needed in a telephony-processing subsystem to eliminate the effect of echoes.
With the exception of speaker telephones (which are prone to echoes), most acoustical echoes can be controlled by careful design of the telephone handset. In contrast, electrical echoes are far harder to prevent and are caused by virtually any impedance mismatch in the telephone communication circuit.
Referring now to
FIG. 8
, a typical prior art telephone communication system is illustrated. A telephone, fax, or data modem couples to a local subscriber loop
802
at one end and another local subscriber loop
802
′ at an opposite end. One source of impedance mismatch is from the cable impedance in the local subscriber loop
802
. Local subscriber loops
802
vary in length from a few hundred feet to about 25,000 feet, so there is always some mismatch with the constant impedance terminations at a central office.
Each of the local subscriber loops
802
and
802
′ couple to 2-wire/4-wire hybrid circuits
804
and
804
′. An even greater source of impedance mismatch is caused by 2-wire/4-wire hybrid circuits
804
and
804
′. Hybrid circuits
804
and
804
′ are composed of resistor networks, capacitors, and ferrite-core transformers. Hybrids circuits
804
and
804
′ convert the 4-wire telephone trunk lines
806
(a pair in each direction) running between telephone exchanges of the PSTN
812
to each of the 2-wire local subscriber loops
802
and
802
′. The hybrid circuit
804
is intended to direct all the energy from a talker on the 4-wire trunk
806
at a far-end to a listener on a 2-wire local subscriber loop
802
at a near end. Impedance mismatches in the hybrid circuit
804
results in some of the transmitted energy from the far-end being reflected back to the far-end from the near-end as a delayed version of the far-end talker's speech. As little as a 30 millisecond (msec) round-trip delay in the echo back to the far end is perceptible. Round-trip delays of 50 msec or more are objectionable and should be reduced or eliminated.
Echoes
810
′ are formed when a speech signal from a far end talker leaves a far end hybrid
804
′ on a pair of the four wires
806
′, and arrives at the near end after traversing the PSTN
812
, and may be heard by the listener at the near side. A small portion of this signal is reflected by the hybrid
804
at the near end, and returns on a different pair of the four wires
806
to the far end and arrives at the hybrid
804
′ delayed by a period of time referred to as the “echo tail length”. The talker at the far end hears this reflected and delayed small portion of his speech signal as an echo. Echoes can occur at each talking end as each person switches from being a talker to a listener. In traditional telephone networks, an echo canceller is placed at each end of the PSTN in order to reduce and attempt to eliminate this echo.
In general, several things contribute to an echo: (i) energy reflection due to impedance mismatches; (ii) a sufficiently large roundtrip delay between a talker's transmitted signal and its reflection; and (iii) poor echo attenuation occurring at the hybrid (i.e. low Echo Return Loss). There are two major causes for increased round-trip delay: (I) propagation delays and (II) digital signal processing algorithmic delays. Propagation delays are caused by the circuit length from talker to listener and transit time over satellite links. The digital signal processing (DSP) algorithmic delays are caused by one or more of the following: Conversion delays between analog to digital and digital to analog; signal processing ordinarily performed to enhance signal quality; signal transcoding such as that performed in digital wireless telephony equipment for Code-division multiple access (CDMA), Global system for mobile communications (GSM) and Personal Communication

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