Telephonic communications – Echo cancellation or suppression – Using digital signal processing
Reexamination Certificate
2000-05-19
2003-07-08
Isen, Forester W. (Department: 2644)
Telephonic communications
Echo cancellation or suppression
Using digital signal processing
C379S406090, C370S292000
Reexamination Certificate
active
06590976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to telecommunications circuitry, and, in particular, to echo cancellation filters for interface units that interconnect analog and digital components, such as analog front-end circuits for modems.
2. Description of the Related Art
FIG. 1
shows a simplified block diagram of a conventional asymmetric digital subscriber line (ADSL) modem
100
that converts an existing twisted-pair telephone loop into an access path for multimedia and high-speed data communications in addition to analog voice signals. As shown in
FIG. 1
, ADSL modem
100
comprises a digital unit
102
(e.g., a digital signal processor (DSP)) configured to an analog front-end (AFE) circuit
104
, which is in turn configured to a line interface unit
106
. Digital unit
102
provides a digital transmit (TX) signal to AFE circuit
104
, which converts the digital TX signal into an analog TX signal. AFE circuit
104
provides the analog TX signal to the line interface unit
106
, which transmits the analog TX signal over the loop, while providing high-voltage, high-current electrical isolation between the loop and the terminal side of ADSL modem
100
. At the same time, line interface unit
106
receives an analog signal from the loop and provides an analog receive (RX) signal to AFE circuit
104
, which converts the analog RX signal into a digital RX signal, which is then presented to digital unit
102
.
FIG. 2
shows a block diagram of AFE circuit
104
for a conventional ADSL modem, such as ADSL modem
100
of FIG.
1
. As shown in
FIG. 2
, AFE circuit
104
has two parallel processing paths: a transmit path for the digital TX signal received from digital unit
102
and a receive path for the analog RX signal received from line interface unit
106
. The transmit path comprises:
TX digital shaping filter
202
, which digitally shapes the digital TX signal according to a specified shaping function;
TX 1:32 interpolator
204
, which upsarnples and interpolates the digital TX signal from filter
202
(for subsequent sigma-delta coding);
TX sigma-delta digital-to-analog converter (DAC)
206
, which converts the digital TX signal from interpolator
204
into a sigma-delta coded analog TX signal;
TX analog low-pass filter (LPF)
208
, which filters out high-frequency components from the analog TX signal from DAC
206
; and
TX programmable gain amplifier (PGA)
210
, which amplifies the analog TX signal from LPF
208
to generate the analog TX signal that is presented to line interface unit
106
. Analogously, the receive path comprises:
RX PGA
212
, which amplifies the analog RX signal received from line interface unit
106
;
RX analog LPF
214
, which filters out high-frequency components from the analog RX signal from PGA
212
; and
RX sigma-delta analog-to-digital converter (ADC)
216
, which converts the analog RX signal from LPF
214
into a sigma-delta decoded digital RX signal;
RX 8:1 decimator
218
, which downsamples the digital RX signal from ADC
216
;
RX 4:1 decimator
220
, which further downsamples the digital RX signal from decimator
218
; and
RX digital filter
222
, which digitally filters the digital TX signal from decimator
220
to generate the digital RX signal that is presented to digital unit
102
.
For an ADSL modem, such as ADSL modem
100
of
FIG. 1
, the TX and RX signals are present on the telephone loop simultaneously with the transmitting and receiving operations being conducted at the same time. The standard technique for separating the signals for the TX and RX paths is based on impedance matching. If the terminating impedance of the line interface unit were exactly equal to the equivalent loop impedance, then the transmit and receive signals would be processed completely independently of one another by the TX and RX paths, respectively. However, since the equivalent loop impedance can vary significantly from one loop to another, no matter how the terminal impedance is designed in the line interface unit, a perfect match will not be achieved for all applications. As a result, there may be significant leakage of the transmit signal into the receive path, also known as echo, which can adversely affect the quality of the receive signal.
One way to address the problem of echo in the RX signal is to implement adaptive echo cancellation (EC) in the digital domain (e.g., implemented within digital unit
102
). In that case, AFE circuit
104
does not have to get involved in the EC process. However, in applications with very long loops (e.g., about 10% of all loops), the ADC in the RX path of AFE circuit
104
cannot provide sufficient dynamic range to handle both a strong echo and a weak signal to allow the echo to be sufficiently canceled in the digital domain (i.e., after digitization). In that case, echo cancellation in the analog domain is needed to achieve better performance. With analog-domain EC, the echo is canceled before the ADC in the RX path. As a result, the RX ADC's dynamic range is no longer a performance limiting factor.
SUMMARY OF THE INVENTION
The present invention is directed to a scheme for training circuitry that performs echo cancellation (EC) in the analog domain, for example, for the AFE circuit of an ADSL modem. The present invention is based on EC circuitry that comprises an estimation filter that estimates, from the TX signal, the echo that will appear in the RX signal. According to certain embodiments of the present invention, the coefficients for the estimation filter are determined as a result of a two-step training algorithm. In the first step, the coefficients for the estimation filter are held fixed, while the coefficients of a path equivalent filter (i.e., a filter that is to be trained to be substantially equivalent to a combination of the EC and RX paths) are determined with the TX path disabled, white noise applied to the EC path, and no receive signal applied to the line interface unit. In the second step, the coefficients of the path equivalent filter determined during the first step are held fixed, and the output from the path equivalent filter is used to determine coefficients for the estimation filter, while applying sequences of TX signals to both the TX and EC paths with no receive signal being applied to the line interface unit. After training is complete, the path equivalent filter may be disabled and the coefficients for the estimation filter determined during the second training step are preferably held fixed during processing of real transmit and receive signals.
In one embodiment, the present invention is, in a circuit comprising (1) a transmit (TX) path configured to convert a digital TX signal into an analog TX signal; (2) a receive (RX) path configured to convert an analog RX signal into a digital RX signal; and (3) an echo cancellation (EC) path configured to generate an analog EC signal, based on the digital TX signal, to be subtracted from the analog RX signal prior to digitization, a method for training an EC estimation filter in the EC path, comprising the steps of (a) training a path equivalent filter to be substantially equivalent to a combination of the EC and RX paths, while keeping the EC estimation filter fixed; and (b) then training the EX estimation filter, with the path equivalent filter trained during step (a).
In another embodiment, the present invention is a circuit comprising (1) a transmit (TX) path configured to convert a digital TX signal into an analog TX signal; (2) a receive (RX) path configured to convert an analog RX signal into a digital RX signal; (3) an echo cancellation (EC) path configured to generate an analog EC signal, based on the digital TX signal, to be subtracted from the analog RX signal prior to digitization, wherein the EC path comprises an EC estimation filter; (4) a path equivalent filter; and (5) one or more coefficient update units. During a first training step, one of the one or more coefficient update units is configured to update coefficients in the path equivalent filter to train the path equivalent f
Agere Systems Inc.
Isen Forester W.
Mendelsohn Steve
Swerdlow Daniel
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