Note: Descriptions are shown in the official language in which they were submitted.
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A. Jessop-R J Catchpole- !.;;.:
P.J. Dyke~B.5. Farley 7-~-1-1
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A REPEATER FOR A DIGITAL TRAIISMISSIO~ SYSTEM
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Back~round of the Invention
This invention relates to repeaters for PCM (pulse
code modulation) or digital transmission svstems, and more -~
particularly to arrangements for compensating therein for -~
transmission line characteristics.
Conventional automatic line build out (ALBO) networ~s
provide variable eqùalization within a primary equalization
:
feedback control loop to correct for mainly frequency
dependent losses due to line length variations. However,
changes in cable type, e.g., paper insulated, plastics
insulated, etc., in cable characteristics and signal level
variations all e~fect the performance o~ repe~ters. The ~.
. . .
use of high efficiency line codes, such as that known as
4B3T and disclosed in U.S. Patent 3,611,141, issued
October 5, 1971, requires accurate equaIization of line
losses, especially at lower frequencies where this code :
contains more energy than codes, such as that known as
HDB3 (~igh Density B;polar 3~, whether Gr not the signals
are scrambled.
A known equalizer design for heavily multiplexed PCM
systems, e.g., 48 channels at 2.37M bit/sec, for example, is
shown in Fig. 1. This design incorporates a primary equali-
zation control loop to keep the height of the so-called "eye 1f"
diagram" constant, with the ALBO network carefully designed
to give correct equalization for a range of different line
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lengths. The equalized signal is taken from a decision point X and applied
to an ALB0 peak detector 10, which detects the peak or "height" of the eye
diagram. This peak value is amplified and fed to a diode network 11. The
output of diode network 11 is used to control the ~LB0 network equalization.
When the repeaters are manufactured a manually adjustable variable resistor
is included in network 12 to enable the repeater, once installed, to be
adjusted for the lo~ frequency content of the eye diagram
An object of the present invention is to provide an improved
arrangement in a repeater for a PCM or digital transmission system to
compensate for transmission line characteristics.
According to a first aspect of the present invention, there is
proyided a feedback control loop for an equalization network in a repeater
; of a digital transmission line~system comprising: a filter coupled to
the output of said network responsive to equalized signals at the output
of said network, and a peak detector coupled to the output of said filter
responsive to a continuous low frequency voltage output of said filter to
produce a control slgnal for controlling a controllable impedance included
in said network.
According to a second aspect of the present invention, there is
provided a repeater for a digital transmission line system comprising: an
automatic line build out circuit having a primary equalization feedback
,
control loop coupled between an output of said line build out circuit and
a irst controllable impedance means connec~ed in said line build out
circuit, said primary feedback control loop being responsive to equalized
signals at said output of said line build out circuit; and an equalizer
circuit having its output coupled to an input of said line build out cir-
cuit, said equalizer circuit having at least one secondary feedback control
loop coupled to said output of said line build out circuit, sald secondary
loop being resporlsive to said equalized signals including a filter coupled
to said output of said line build out circuit and a first peak detector
coupled to the output of said filter to produce a control signal to control
~J a second controllable impedance means connected in said equalizer circuit.
s~
In a preferred embodiment the secondary loop filter is a low pass
filter.
According to a third aspect of the invention, there is provided
a method of equalization for a digital transmission line rep~ater compri~
sing the steps of filtering equalized signals within a control loop; and
detecting peak values of said filtered equali~ed signals to provide a
control signal for controlling equali~ation in said repeater.
The above-mentioned and other features and objects of this
invention will become more apparent by reference to the following descrip-
tion taken in conjunction with the accompanying drawing~ in which:
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A. Jessop et al 7-4
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~ic~. 1 is a bloc}; diagram of the prior art equali-
zation arrangement described hereinabove under
the heading "~ack~round o~ the Invention"; and
Fig. 2 is a block diagram of a repeater equalization
arrangement in accordance with the principles of
the present invention.
escription of the Preferred Embodiment
The repeater of Fig~ 2 is basically the repeater of Fig. 1
with the addition of the secondary control loop. The equalized
si~nals rom decision point X are fed to a low-pass filter 20
which is followed by a peak detector 21. The output of peak
; detector 21 is amplified and ~ed as a control signal to a
variable impedance 22 in equalizer 13. The low pass filter 20
ensures that the secondary loop only responds to the low
frequency content of the eye diagram or control of equalizer
13. This automatic control of equalizer 13 optimizes the
equaliæation for thebest eye diagram, regardless of the cable
type or characteristic of the input cable.
While we have described above the principles of our
invention in connection with specific apparatus, it is to be
`~ clearly understood th~t this description is made only by way of
ex~ample and not as a limitation to the scope of our inventicn
as set Cor~h in the objects thereof and in the accompanying
claims.
ACH/mlW/jn
November 17, 1980
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