Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a method ~or the trans-
mission of digital signals over a p~th having a frequency transmis-
sion range whose upper frequency limit corresponds to the maximum
bit rate of the signals to be transmitted.
Such rnethods are known as direct current telegraphy or
direct current data transmission, for example with low signal power
level.
In direct current telegraphy it is known to provide for
full duplex traffic by means of difference or bridge circuits. How-
ever~ this traffic is limited to lower transmission bit rates thansimplex traffic allows. For high transmission bit rates, difference
or bridge circuits become ver~ expensive and for very high bit
rates ~ull duplex traLfic is practically impossible.
Therefore, traffic in the opposite direction generally
requires an additional transmission path, for example by providing
for four-wire operation.
It is therefore an object of the present invention to pro~
vide a transmission method of the above type which permits, while
utilizing the maximum bit rate determined by the upper limit frequ-
2n ency, the additional transmission of a further digital signal ata lower bit rate. A further object of ~he invention is to achieve
the above-stated objective in an economical manner.
According to the present invention, there is provided in a
method for transmitting digital signals over a transmission path
having a frequency transmission band whose upper limit frequency
corresponds to the maximum bit rate of the signals to be transmitted
the digital signals including a digital signal having a high bit
rate and having a substantial proportion of its energy spectrum
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located in the upper part of the transmission band, and a digital
signal having a low bit rate, the improvement cornprising simultan-
eously transmitting, over the transmission path, the digital signal
havlng a high bit rate in an upper part of the transmission band
and the digital signal having a low bit rate in a lower part of
the transmission band, the digital signal with the high bit rate
being coded in a code which has substantially no direct current
component.
Brief Description of the Drawings:
Figure 1 shows a functional block diagram of a preferred
embodiment of circuitry at the input and output ends of the trans-
mission path for implementing the invention.
Figure 2 shows a diagram in which the energy spectrum of
the signals are drawn versus frequency.
The solution according to the invention makes it possible,
while utilizing the highest possible bit rate determined by the
upper limit frequency of the transmission path, to effect an
additional transmission of a digital signal w~th a lower bit rate.
The invention is based essentially on recognition of the
fact that it is not absolutely necessary for the transmiss;on of
digital signals to transmit all frequencies of the transmission
range up to the upper limit frequency but rather that for a reliable
detection of the transm~tted information it is suf~icient to make
available only an upper portion of thetransmission range. Certain
codes, e,g., the amicode, contain no d,c. component and the propor-
tion of lower frequency components in the energy spectrum is very
small~ The idea of the invention is therefore to transmit the high
frequency components of a high bit rate digital signal so as to
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provide a frequency gap in the lower transmission range and to
accommodate therein a digital signal having a low b~t rate.
According to one embodiment of the invention, the di~ital
signals with the high and low bit rates, respectively, are each
coupled into the transmission path via a corresponding bandpass
filter.
The method according to the invention can be optionally
employed in integrated broadband and narrowband distributing net-
works, for example in video-telephone and television distributing
networks in which, in a favourable manner, television programs
can be selected or requested, respectively, by transmission
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in the return direction of digital signals having a low bit rate.
In integrated broadband local networks employing optical fiber trans-
mission paths, the method can be used to advantage in the low~r distribution
plane where coaxial lines are provided between the subscribers and ~he terminal
equipments of the optical transmission paths.
In Figure 1 is shown a line 1 as a transmission path with input/output
ends 2~3, which each are connected to a high-pass- and a low-pass filter 4, 5
and 6, 7, respectively. The signal Sl with a high bit rate of about 300 Mbit/s
is fed in the line end via the hp-filter 4 and the signal S2 with a low bit rate
of about 80 kbi~/s is fed in the other line end 3 via the lp-filter 7 to be
transmitted in the opposite direction. The signal with high bit rate received
at the line end 3 via hp-filter 6 as signal Sl' and the signal with low bit rate
is received at line end via lp-filter ~ as signal S2'.
In Figure 2 energy spectrum of the digital signals are shown ~,
frequency f. It is obvious that the signals with the high bit rate ~l,J ~
have no energy proportions for frequencies lower than the cut-off frequency fg
of the low pass and the high pass, respectively. The above described signals
have frequency ranges of 0,8 to 80 kHz and 3 to 300 MHz for S2, S2' and Sl, Sl',
respectively. Cut-off frequency fg has therefore an optimal value of about
1 M~lz.
If cut-off frequencies of high and low pass arechosen nearly to 3 MHz
and 80 k~z, respectively, the frequency range gap may be used for transmission
of further digital or modulated signals.
The digital signals are optimally encoded in AMI-code ~alternate
mark inversion), which is also named "bipolarsignal") wherein the binary value
O corresponds with the voltage level O and the binary value 1 is alternating a
positive and a negative voltage level. Other codes with similar energy spectrum
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like HDB 3 (high density bipolar of order 3) may be used advantageously.
In another modification the ~ method can be advantag.eou~I.y
implemen~ed by combining with the burst mode (ping-pong-system) also referred
to~time compression multiplexed (TCM) system, wherein transmission in each
direction occurs in alternating, disjoint time intervals, detailed described
by Bosik, Bell Labor., in "NTG (Nachrichtentechnishe Gesellschaft)-Fachbericht"
volume 73, pp~ 26 f.
It will be understood that the above descrip~ion of the present inven-
tion is susceptible to various modifications, changes and adaptations, and the
same are intended to be comprehended within the meaning and range of equivalents
of the appended claims.
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