Coded data generation or conversion – Converter compensation
Reexamination Certificate
2001-03-29
2003-02-25
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Converter compensation
C341S120000
Reexamination Certificate
active
06525681
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to telecommunication systems and, more particularly, to a DC compensation method and apparatus.
BACKGROUND OF THE INVENTION
In many modem telecommunication systems, e.g., Digital Subscriber Line (DSL) systems, a digital signal processor (DSP) at a user site processes data for transmission to and received from a telephone company central office (TCCO). The data is exchanged between the DSP and the TCCO via a transmission line.
As depicted in
FIG. 1
, a coder/decoder (CODEC)
10
is positioned between the DSP
20
and the TCCO
30
to convert digital signals from the DSP
20
to analog for transmission to the TCCO
30
over a transmission line
40
, and to convert analog signals received from the TCCO
30
over the same transmission line
40
to digital for processing by the DSP
20
. The CODEC
10
contains analog circuits which may contribute DC components to signals within the telecommunication system. If not compensated for, these DC components may result in a DC offset on a receive path after analog-to-digital conversion, signals falling outside of the dynamic range of circuits within the CODEC
10
, signal degradation, and digital representation errors. Accordingly, methods and apparatuses for compensating for these DC component are useful.
FIG. 2
depicts a typical prior art telecommunication apparatus residing at a user site. The apparatus comprises a DSP
20
for processing digital signals, a CODEC
10
for converting signals between the digital and analog domains, and a known hybrid circuit
25
for converting between unidirectional data flow and bidirectional data flow. In addition, the apparatus comprises capacitors
62
,
64
,
72
, and
74
for filtering out DC components introduced by transmit path and echo cancellation path components within the CODEC
10
and capacitors
42
and
44
for preventing DC components from flowing between the telecommunication apparatus of FIG.
2
and the transmission line
40
. Also, capacitors
82
,
84
are coupled within the CODEC
10
to remove other DC components found in the receive path within the CODEC
10
.
The DSP
20
processes data for transmission over a transmission line
40
(
FIG. 1
) to a remote processing facility, e.g., a TCCO
30
, and processes data received over the same transmission line
40
from the TCCO
30
. Also, the DSP
20
typically generates an echo cancellation signal which represents a delayed and specially filtered transmit signal to cancel remote echos on the transmission line
40
. For example, the echo cancellation signal cancels echos due to bridge taps and other types of connections on the transmission line
40
.
The hybrid circuit
25
is a well known circuit for converting between unidirectional data flowing through the CODEC
10
and bidirectional data flowing on the transmission line
40
. The hybrid circuit
25
contains circuitry to prevent transmit signals from interfering with receive signals.
The CODEC
10
comprises circuitry necessary to couple the DSP
20
to a transmission line
40
(FIG.
1
). The circuitry comprises an interface
50
, an optional amplifier (op-amp)
80
, a receive amplifier (RXAmp)/analog filter
85
, a digital-to-analog (D/A) converter
60
, a D/A converter
70
, and an analog-to-digital (A/D) converter
90
. In addition, the CODEC
10
comprises capacitors
82
,
84
for removing DC components introduced in the receive path of the CODEC
10
.
The interface
50
within the CODEC
10
organizes signals to and from the DSP
20
into a transmit path, an echo cancellation path, and a receive path. The RXAmp/analog filter
85
in the receive path amplifies a receive signal from the transmission line to remedy attenuation of the receive signal as it is passed over the transmission line
40
, and filters out frequencies which may cause aliasing (i.e., the interpretation of high frequencies as lower frequencies).
The op-amp
80
, together with the hybrid circuit
25
, reduces the effect of the AC components of the transmit signal and an echo signal on the receive path. Since the receive signal passes over the same transmission line
40
(
FIG. 1
) as the transmit signal and the echo signal, the receive signal will include images of the transmit signal and the echo signal. If the transmit signal and the echo cancellation signal of the CODEC
10
are applied directly to the input of the op-amp
80
at 180 degrees out of phase with the images of the transmit signal and the echo signal in the receive signal (in this case through the hybrid circuit
25
), the effect of the transmit signal and echo signal on the receive signal in the receive path is reduced greatly. The residual signal at the output of the op-amp
80
will contain mostly receive signal.
The CODEC
10
, through the use of the D/A converter
60
, converts digital signals from the DSP
20
to analog signals for transmission over a transmission line
40
(FIG.
1
). Optionally, the CODEC
10
contains another D/A converter
70
for converting echo cancellation signals developed by the DSP
20
from digital-to-analog. In addition, the A/D converter
90
converts analog signals received from the TCCO
30
(
FIG. 1
) via the transmission line
40
(
FIG. 1
) to digital signals for processing by the DSP
20
.
Due to inherent imperfections of analog circuits, DC components are introduced within the CODEC
10
which contribute to a DC offset after conversion by A/D converter
90
. The DC components introduced by the analog circuits within the CODEC alter the voltage level around which the resultant signals are centered. For example, if an analog signal varies between a voltage level of −1 V and +1 V for a 0 V DC component, the analog signal would vary between 0V and 2 V for a +1 V DC component, potentially resulting in saturation of the circuit to which this signal is applied. If circuits which are coupled to the analog signals are designed such that their dynamic range is between −1 V and +1 V, the dynamic range of the circuit is effectively reduced and may result in digital representation errors. Digital representation errors may occur when a signal with a DC component is converted by an A/D converter such as A/D converter
90
without correcting for the DC component.
The DC components may also result in signal degradation. Signal degradation occurs when analog circuits such as op-amp
80
and RXAmp/analog filter
85
amplify the DC component by a DC gain and introduce their own DC components. Unless neutralized, these DC components will degrade the quality of the receive signal.
Additional components (not shown) within the CODEC provide additional signal processing. An example of a CODEC
10
is found in the data sheets for the 3.3 V Integrated ADSL Over Pots CODEC produced by Texas Instruments, Inc. (part no. TLV320AD11A), incorporate fully herein by reference.
In prior art systems, such as the one depicted in
FIG. 2
, the DC components are removed through the use of AC coupling capacitors
62
,
64
,
72
,
74
,
82
,
84
which filter out DC components introduced by analog circuits. Hence, the DC components will not affect the receive signal. Eliminating typical DC components in the transmit path and the receive path, however, requires large AC coupling capacitors
62
,
64
,
72
,
74
. The AC coupling capacitors
62
,
64
,
72
,
74
need to be large enough to avoid introducing a noticeable impedance at the frequencies of interest, e.g., typically 25 kHz or higher for an asynchronous digital subscriber line (ADSL). Presently, electrolytic capacitors in the microfarad range are used to remove the DC offset in the transmit and receive paths. In addition, electrolytic capacitors may be used to remove DC components in the receive path.
The electrolytic capacitors which are presently used, however, introduce a non-desirable, non-linear distortion to the system and are too large to be fabricated on a chip. Since the capacitors are too large to be fabricated on a chip, additional printed circuit board (PCB) surface area is required to
Laturell Donald Raymond
Sindalovsky Vladimir
Agere Systems Inc.
Synnestvedt & Lechner LLP
Williams Howard L.
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