Interactive video distribution systems – Video distribution system with upstream communication – Transmission network
Reexamination Certificate
2000-03-09
2004-04-27
Srivastava, Vivek (Department: 2611)
Interactive video distribution systems
Video distribution system with upstream communication
Transmission network
C725S121000, C725S125000, C725S126000, C455S226300, C348S193000
Reexamination Certificate
active
06728968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to upward-joining-noise decreasing methods and apparatuses, and more particularly to an upward-joining-noise decreasing method and an apparatus for decreasing noise of upward signals used in two-way communication services of a CATV (cable television) transmission system.
2. Description of the Related Art
FIG. 1
is a diagram showing a conventional frequency spectrum of some ordinary upward signals used in two-way communication services of a CATV transmission system. As may be seen from this diagram, an upward-date-communication signal, which may be used in the two-way communication services such as the Internet, is transmitted within a band ranging from 30 to 36 MHz, and an upward-video signal, which may be used in a television broadcasting or the like, is transmitted within a band ranging between 36 to 42 MHz. In addition, a pilot signal, which serves to automatically adjust the gain of a transmission-line amplifier as a reference signal, is transmitted within a narrow band centered on approximately 48 MHz, and an STM (status monitoring) signal, which serves to monitor the state of a transmission-line amplifier at a center device as a response signal of a status monitor system, is transmitted within a band ranging from 54 to 55 MHz.
FIG. 2
is a diagram showing an example of the conventional CATV transmission system. As may be seen from this diagram, there are a plurality of CATV transmission lines
10
A,
10
B, . . . ,
10
X, each of which may, if necessary, be provided with a plurality of transmission-line amplifiers
12
which are respectively coupled to a plurality of subscriber terminals
14
. Upward signals transmitted by the respective transmission lines
10
A,
10
B, . . . ,
10
X are mixed at a head-end device
16
in which a mixed signal is generated therefrom and then supplied to a center device
18
.
In general, the upward-video signal for television broadcasting is transmitted to the center device
18
from a transmitting terminal (upward television modulator) provided on a CATV transmission line, for example,
10
A. In this case, since communications between the center device
18
and the transmission terminal is “one-to-one” communications, it can be performed by using only one transmission line connected therebetween. For this reason, the influence of noise upon upward signals from the transmission line is minimal and ignored.
In contrast, in a case of the two-way data communication system, in order to perform communications from subscriber terminals provided in subscriber houses, the system should be configured such that the center device
18
(one device) can cope with a number N of the subscriber terminals
14
(N devices), namely, an “one-to-N” system. In this case, since upward signals from all of the transmission lines
10
A,
10
B, . . . ,
10
X are concentrated on the center device
18
, noise contained in these upward signals are all concentrated thereon as well.
Thereby, even though the noise coming from one transmission line may be too small to be worried about, noise coming from a plurality of the transmission lines can become upward-joining noise with a level high enough to interfere with communications on these transmission lines. For this reason, there is a problem that if noise coming from one transmission line is increased during communications on this transmission line, it may interfere with communications of all the other transmission lines. Further, in general, the lower the frequency band becomes, the larger the upward-joining noise contained therein becomes. Therefore, the frequency band to be used in communications is usually restricted to be medium or high.
In order to solve the above-mentioned problem, conventionally, when noise from a certain transmission line increases, the center device
18
measures noise from each transmission line so as to identify from which transmission line the noise has increased.
Also, a transmission-line-correction method, as a method for suppressing noise generated on the transmission lines, is provided in which a status monitoring device (STM device) is used to close a gate corresponding to a transmission line on which noise is being generated so as to interrupt upward signals therefrom. The transmission line correction method determines the locations of noise sources on a specified transmission line so as to correct the specified transmission line.
The method using the STM device may be understood with reference to FIG.
3
.
FIG. 3
is a diagram showing a conventional CATV transmission system using the STM device. As may be seen from this diagram, the transmission lines
10
A,
10
B and
10
C have a transmission-line amplifier
20
provided at a junction thereof, which includes a gate with respect to these transmission lines
10
A through
10
C. And a CATV center has an STM device
24
and the center device
18
which are coupled to the head-end apparatus
16
. When the STM device
24
determines that the noise from the line
10
A is increased according to a status signal supplied by the transmission-line amplifier
20
, it controls the gate thereof so that the gate is closed only to the transmission line
10
A and opened for the transmission lines
10
B and
10
C.
Recently, a communication establishing method has been provided in which a tap-off-connection portion is provided on the side of each subscriber terminal with a gate which is closed before communications. When the subscriber terminal transmits a specific signal for communications to the tap-off-connection portion, the tap-off-connection receives the specific signal and then switches the gate ON, and communication from the subscriber terminal to the center device is thus established. Accordingly, seen from the center device, the gate of only one transmission line is switched ON at any time, and thereby an increase in noise due to a mixture of upward signals from a plurality of transmission lines can be prevented.
Further, a conventional system is provided which comprises a switching device provided immediately before the center device so as to communicate with subscriber terminals while switching transmission lines connected between the center device and the subscriber terminals. According to this system, when the center device communicates with the subscriber terminals by means of the switching device, only one transmission line can be coupled to the center device at a time, therefore preventing noise coming from a plurality of transmission lines being added together and then being supplied to the center device.
However, it takes much time and labor to determine on which transmission line the noise is increased by means of the center device. Herein, if the noise on a certain transmission line increases due to its deterioration with age, it is easy to determine the transmission line by measuring habitual noise. But, if the noise thereon is increased due to a transient cause, it is difficult and time-consuming to determine the transmission line by the above-mentioned conventional method.
With respect to the transmission-line-correction method, there is a problem that communication services on a transmission line to be corrected must be interrupted for a given time during the correction. Also, with respect to the method using the STM device, there is a problem that the two-way communication is available for the lines
10
B and
10
C and is unavailable for the line
10
A which has to be corrected separately.
With respect to the method in which the gate is provided in the tap-off-connection portion, since the gate has to be opened during communications, it is necessary to transmit/receive a gate-ON signal besides information about the communications, that is, additional information about the gate-ON signal has to be processed. For this reason, it takes time to turn the gate ON/OFF and this may result in degradation in the communication of the upward signals. Further, there is a problem that the upward-joining noise generated on each transmission line interferes wit
Abe Kaoru
Ito Yuichi
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Srivastava Vivek
Vu Ngoc
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