1. Field of the Invention
The present invention relates to a signal distribution circuit for use in a converter for receiving cable television broadcast (hereafter referred to as a xe2x80x9cCATVxe2x80x9d converterxe2x80x9d).
2. Description of the Prior Art
Conventionally, a CATV converter is used to permit a subscriber to view a desired program by selecting one among about 60 cable television broadcast signals (hereafter referred to as xe2x80x9cCATV signalsxe2x80x9d) that are transferred across a coaxial cable in a frequency range of, for example, 70 to 450 MHz (in the case of Japan). Such a CATV converter is installed on that end of the coaxial cable that leads to a television receiver of each subscriber.
In recent years, such a CATV converter has come to have enhanced functions that allow it to be connected to a videotape recorder for timer-recording, a second CATV converter for a second television receiver, an FM tuner, and the like. To achieve this, a modern CATV converter is additionally provided with output terminals for connection to a videotape recorder, a second CATV converter, an FM tuner, and the like, and also with a signal distribution circuit for distributing CATV signals among those output terminals.
Specifically, a CATV converter is configured as shown in FIG. 14. Reference numeral 31 represents a CATV signal input terminal at which the CATV converter receives the CATV signals transferred thereto across a cable. Reference numeral 32 represents a brancher (directional coupler) for branching the CATV signals received at the CATV signal input terminal 31 into signals that flow in a through direction and signals that flow in a branch direction. The signals output from the brancher 32 in the through direction are fed, on the one hand, to a CATV signal output terminal 40 for connection to an analog tuner, and, on the other hand, through a BPF (band-pass filter) 33 having a bandwidth of 70-75 MHz or 70-130 MHz that extracts, from those signals, downstream signals consisting of FSK (frequency-shift keying) signals having that band width, to a downstream signal output terminal 41 for connection to a tuner for receiving downstream signal.
The signals output from the brancher 32 in the branch direction are fed, through a variable attenuator 34 designed as, for example, an attenuator employing PIN diodes, then through two-stage wide-band amplifiers 35 and 36, to a distributor 37 that distributes the signals into two groups of signals. Here, the gain of the variable attenuator 34 is controlled by a gain control voltage that is applied to a control terminal (not shown) thereof. One of the two groups of signals output from the distributor 37 is fed to an extractor 38 composed of a band-pass filter that extracts VHF signals. The extracted VHF signals are, together with the UHF signals extracted by an extractor 39 composed of a band-pass filter from the signals fed in via a UHF signal input terminal 43, to a UHF/VHF signal output terminal 42 for connection to a UHF or VHF tuner of a television receiver. The other of the outputs from the distributor 37 is fed to an FM signal output terminal 44 for connection to an FM receiver. In this conventional example, a second CATV converter may also be connected to the output terminal 44.
The conventional example shown in FIG. 14 and described above is an example of a signal distribution circuit used in an analog STP (set-top box). In the case of a signal distribution circuit used in a digital STP, since it is connected to, as the succeeding stage, an analog/digital tuner and to a tuner for downstream signal reception, the signals are, before being fed to them, distributed into two groups of signals.
In a signal distribution circuit used in a conventional analog STB, much care is taken to transfer CATV signals with as small a loss as possible. To adapt this distribution circuit for digital-signal handling, it needs to be so configured as to permit an analog/digital tuner and a tuner for downstream signal reception to be connected thereto; specifically, an additional distributor needs to be provided after the band-pass filter 33 to distribute the signals into two groups of signals so that one of those outputs is fed to the downstream signal output terminal 41 and the other to a terminal (not shown) for connection to an analog/digital tuner. However, additionally providing this distributor causes a loss of 3 to 4 dB in the CATV signals, and thus degrades the C/N (carrier to noise) ratio. The distribution loss resulting from the insertion of this distributor can be compensated for by reducing the loss that is caused in the through direction by the brancher 32 provided in the preceding stage. However, reducing the loss caused in the through direction by this brancher 32 increases the loss in the branch direction, and thus degrades the NF (noise figure) of the signals that are extracted from the signals flowing in the branch direction so as to be fed to a television receiver and an FM receiver.
Moreover, a signal distribution circuit used in a digital STB allows a subscriber to transmit upstream signals from his part to the CATV central station via the CATV signal input terminal 31. However, no conventional technique is known to be ready for this function. At this CATV signal input terminal 31, upstream signals need to have a level of 120 dBuV.
An object of the present invention is to provide a signal distribution circuit that has a simple circuit configuration but nevertheless achieves efficient distribution of received digital CATV signals.
To achieve the above object, according to one aspect of the present invention, a signal distribution circuit is provided with: a directional coupler for distributing input CATV signals into signals that flow in a through direction and signals that flow in a branch direction; a first extraction circuit for permitting, out of the signals output from the directional coupler in the through direction, only signals having predetermined frequencies to pass therethrough; a variable attenuator for adjusting the level of the signals output from the first extraction circuit to a predetermined level; a wide-band amplifier for amplifying the signals output from the variable attenuator; a first output terminal for connection to an analog/digital tuner for receiving CATV signals; a second output terminal for connection to a tuner for receiving downstream signals; a first distributor for distributing the signals output from the wide-band amplifier to the first and second output terminals; a filter for permitting, out of the signals output from the directional coupler in the branch direction, only VHF signals and FM signals to pass therethrough; an amplifier for amplifying the signals output from the filter; and a second distributor for distributing the signals output from the amplifier into two groups of signals.
According to another aspect of the present invention, a signal distribution circuit is provided with: a CATV signal input terminal; a first extraction circuit for permitting, out of signals received at the CATV signal input terminal, only CATV signals to pass therethrough; a gain adjustment circuit for adjusting the level of the signals output from the first extraction circuit to a predetermined level; a wide-band amplifier for amplifying the signals output from the gain adjustment circuit; a first output terminal for connection to an analog/digital tuner for receiving CAIV signals; a second output terminal for connection to a tuner for receiving downstream signals; a first distributor for distributing the signals output from the wide-band amplifier into two groups of signals and feeding one of those two groups of signals to the first output terminal; a filter for permitting only VHF signals and FM signals to pass therethrough; a second distributor for distributing the other of the two groups of signals output from the first distributor to the second output terminal and the filter; a third output terminal; a fourth output terminal; and a third distributor for distributing the signals output from the filter to the third and fourth output terminals.
According to another aspect of the present invention, a signal distribution circuit is provided with: a CATV signal input terminal; a first extraction circuit for permitting, out of signals received at the CATV signal input terminal, only CATV signals to pass therethrough; a first output terminal for connection to an analog/digital tuner for receiving CATV signals; a second output terminal for connection to a tuner for receiving downstream signals; a first distributor for distributing the signals output from the first extraction circuit into two groups of signals; a first circuit for adjusting the gain of one of the groups of signals output from the first distributor and feeding those signals to the second output terminal; a second circuit for adjusting the gain of the other of the groups of signals output from the first distributor; a filter for permitting only VHF signals and FM signals to pass therethrough; a second distributor for distributing the signals output from the second circuit into two groups of signals and feeding one of those two groups of signals to the first output terminal and the other to the filter; a third output terminal; a fourth output terminal; and a third distributor for distributing the signals output from the filter to the third and fourth output terminals.