JPH0410835A - Data terminal equipment - Google Patents

Abstract

PURPOSE:To reduce the cost and to improve the stability by generating a modulation reference signal at a carrier frequency required for modulating a reply data based on a carrier frequency of a transmission request inquiry signal. CONSTITUTION:A comparison signal 38 applied in the lock state is fed to a data signal modulator 18 as a modulation frequency signal 42 for generating an incoming data signal. Then a microprocessor 16 receives and analyzes a data signal 26, outputs a carrier control signal 23 to turn on a carrier control switch 19 when a reply is required and sends a reply data 27. The reply data 27 is received by the data signal modulator 18, in which the signal is subject to PSK modulation based on a modulation frequency signal given from a PLL circuit 35. Thus, the data terminal equipment with low cost and excellent stability is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a data communication device, and, for example, to a data terminal device used in a two-way cable television broadcasting system.

[Conventional technology]

Cable television broadcasting (hereinafter referred to as CATV), which distributes high-quality broadcast television signals received by high-sensitivity antennas to receivers in each home or office using broadband transmission lines such as coaxial cables, has become widespread. There is.

This CATV system was originally introduced as a measure to prevent viewing difficulties in areas where it was difficult to receive general television broadcasts, but recently it has been developed to take advantage of its wideband characteristics, add bidirectional functions, and connect to computers. A variety of services are provided. For example, dedicated television (CCTV) that is used not only in the field of broadcasting but also between specific persons, etc.
It has also come to be applied in the field of bidirectional individual communication.

Such a two-way CATV system is usually composed of one center station and a plurality of terminal stations.

The center station sequentially checks the presence or absence of transmission requests (hereinafter referred to as polling) for these plurality of terminal stations.

), and if it is present, the terminal station is instructed to start transmission, and if it is absent, it polls the next terminal station. Each terminal station is specified by the terminal station address information included in the polling signal from the center station, and for example, in the case of a transmission request, it transmits data,
If not, sends an EOT (end of transmission) signal.

In this case, polling, which is downstream communication from the center station to the terminal stations, uses a specific frequency to perform so-called FSX
(Frequency Shift Keying) modulation, and the response in the upstream direction from the terminal station to the center station is generally performed by so-called PSK (Phase Shift Keying) modulation based on the frequency of the oscillator possessed by each terminal station. In this way, in the CATV system, the terminal station starts transmitting data only after receiving a command from the center station.

FIG. 2 shows a terminal device in a conventional two-way CATV system. This device is equipped with a branching filter 11, and a CATV line 12 consisting of a coaxial cable etc.
is connected to a center station (not shown). The duplexer 11 is connected to a data signal demodulator 15 via an amplifier 13 and a bandpass filter 14.

The device is also provided with a microprocessor 16, which is connected to the data signal demodulator 15 and to the data signal modulator 18.

This data signal modulator 18 is a carrier control switch 1
9, further connected to the duplexer 11 via an amplification carrier 21, and also connected to a carrier frequency oscillator 22. A carrier control signal 23 is manually input to the carrier control switch 19 from the microprocessor 16.

The operation of the conventional data terminal device configured as above will be explained.

When an FSX-modulated polling signal is transmitted from a center station (not shown) and input to the demultiplexer 11 as a downlink data signal via the CATV line 12, the demultiplexer 11 demultiplexes this signal to transmit the received data. Signal 25 is extracted and sent to amplifier 13. After the received data signal 25 is amplified to the required level by the amplifier 13, only the data carrier frequency is extracted by the bandpass filter 14.
The signal is input to the data signal demodulator 15. In the data signal demodulator 15, the data signal 26 is
is detected and input to the microprocessor 16. The microprocessor 16 receives and analyzes the data signal 26, outputs the carrier control signal 23 when a response is required, turns on the carrier control switch 19, and then outputs the response data 27.
Send out. This response data 27 is transmitted to the data signal modulator 1
PSK modulation is performed based on a modulation signal 28 of a predetermined frequency given from a carrier frequency oscillator 22. The data signal thus modulated passes through the carrier control switch 19 and is amplified to a required level by the amplifier 21 to become a transmission data signal 29. And duplexer 1
1, this signal is sent to the CATV line 12 as an upstream data signal, thereby completing transmission to the center station.

[Problem to be solved by the invention]

As described above, in the data terminal equipment in the conventional CATV system, a carrier frequency oscillator is provided in the terminal station, and the uplink data signal is modulated based on this oscillation frequency. However, as such a carrier frequency oscillator, it is necessary to use one with good stability such as temperature characteristics and high precision. Since such oscillators are usually expensive, they have the disadvantage of increasing the price of the terminal data device as a whole.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a data terminal device that is inexpensive and has good stability without requiring an expensive carrier frequency oscillator.

[Means to solve the problem]

The invention according to claim 1 provides: (i) transmission request inquiry signal receiving means for receiving a transmission request inquiry signal for inquiring whether there is a transmission request; and (ii) the transmission request inquiry signal receiving means transmits response data output means for outputting response data in response to the request inquiry signal when it is received;
ii) The data terminal device is provided with modulation reference signal generation means for generating a modulation reference signal of a carrier frequency necessary for modulating response data based on the carrier frequency of the transmission request inquiry signal.

In the first aspect of the invention, a modulation reference signal of a carrier frequency used in modulating response data is generated based on the carrier frequency of the received transmission request inquiry signal.

In the invention according to claim 2, (1) receiving means for receiving a polling signal for inquiring about the presence or absence of a transmission request, which is sequentially transmitted from a center station of a cable television broadcasting system to each terminal device in the system; (ii> demodulation means for demodulating the polling signal received by this reception means; (iii) response data output means for decoding the data signal detected by this demodulation means and outputting corresponding response data; (1) (2) response data modulation means for modulating the response data output from the response data output means based on a predetermined modulation reference signal; Carrier 2!! A data terminal device is equipped with a phase-locked loop circuit that synchronizes with the frequency.

In the invention as claimed in claim 2, the phase-locked loop circuit is used to synchronize the carrier frequency used for varying the width of the response data to the center station with the carrier frequency of the polling signal sent from the center station. do.

〔Example〕

The present invention will be described in detail below with reference to Examples.

FIG. 1 shows a data communication device according to an embodiment of the present invention. In this figure, the same parts as in the conventional example (FIG. 2) are given the same reference numerals, and the explanation will be omitted as appropriate.

This device includes a phase detector 31, a low pass filter 32
, and a PLL consisting of VC○ (voltage controlled oscillator) 33
A (phase locked loop) circuit 35 is provided. The output side of the phase detector 31 is connected to V through a low-pass filter 32.
Connected to CO33. This phase detector 31 has two
two input terminals are provided, one of which is a bandpass filter 14.
It is connected to one of the two branched outputs. vC
The output side of O33 is connected to the other input terminal of phase detector 31 and also to data signal modulator 18. The rest of the configuration is the same as that in FIG. 2.

The operation of the data terminal device configured as above will be explained.

In this embodiment, the operation related to receiving the downlink data signal is the same as in the conventional example. That is, when a downlink data signal that has been FSX-modulated using two frequencies 1 and ω2 at a center station (not shown) is input to the duplexer 11 via the CATV line 12, this demultiplexer 11 By demultiplexing the data signal, a received data signal 25 is extracted and sent to the amplifier 13. The received data signal 25 is amplified to the required level by the amplifier 13 and then passed through the bandpass filter 14.
Only the data carrier frequency is extracted and input to the data signal demodulator 15 and the phase detector 31 of the PLL circuit 35. The data signal demodulator 15 detects the data signal 26 by performing FSK demodulation and inputs it to the microprocessor 16 .

On the other hand, the phase detector 31 of the PLL circuit 35 outputs an error voltage 39 proportional to the phase difference and frequency difference between the signal 37 manually inputted from the bandpass filter 14 and the comparison signal 38 outputted from the vC○ 33. Here, the frequency ω1 of the signal 37
The signal portion of is the target of phase comparison. However, for example, by providing a frequency selection circuit, the frequency ω1
It may be possible to select one of ω2.

The error voltage 39 has high frequency components removed by the low-pass filter 32 to become a control voltage 41, which is manually input to the VCO 33. The VCO 33 has a frequency ω according to this control voltage 39. A comparison signal 38 is output and supplied to the phase detector 31 and also input to the data signal modulator 18. Note that in the initial state, since the signal 37 is not input to the phase detector 31, the error voltage 39 becomes 0, and the VCO 33 oscillates at the initially set free running frequency.

In this way, loopback control is performed in such a direction that the difference between the frequency of comparison signal 38 and the frequency of signal 370 decreases, and these two frequencies ω1, ω. When the PLL circuit 35 matches, the PLL circuit 35 enters a so-called locked state. The comparison signal 38 in the locked state is then applied to the data signal modulator 1 as a modulation frequency signal 42 for creating an uplink data signal.
8.

The microprocessor 16 receives and analyzes the data signal 26, and when a response is required, outputs a carrier control signal 23, turns on the carrier control switch 19, and then transmits response data 27-. This response data 27 is input to the data signal modulator 18 and subjected to PSK modulation based on the modulation frequency signal 42 provided from the PLL circuit 35.

The PSK-modulated data signal passes through the carrier control switch 19 and is amplified to a required level by the amplifier 21 to become a transmission data signal 29. Then, the branching filter 11 sends this signal to the CATV line 12 as an uplink data signal, thereby completing the transmission to the center station.

Note that in this embodiment, the data signals in the downstream direction from the center station and in the upstream direction to the center station are FSK and PSK, respectively.
Although modulation is performed using SK modulation, modulation may be performed using the FSK or PSK method in both directions.

Furthermore, if an LSI element is used as the PLL circuit, vibration resistance and the like will be improved, and the reliability of the device can be further improved.

〔Effect of the invention〕

According to the invention as claimed in claim 1, since the modulation reference signal of the carrier wave frequency used in modulating the response data is generated based on the carrier wave frequency of the received transmission request inquiry signal, the temperature characteristics can be adjusted. A data terminal device with good stability can be constructed at low cost without requiring an expensive carrier frequency oscillator for compensation.

Further, according to the invention as claimed in claim 2, for example, by using an LSI element as a phase-locked loop circuit, vibration resistance etc. are improved, and a more reliable cable television broadcasting system can be constructed. There is also this effect.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a data terminal device according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional data terminal device. IX...Brancher, X2...CATV line, 15...Data signal ti adjuster, 16...
... Microprocessor, 18 ... Data signal modulator, 25 ... Downlink data signal, 29 ... Uplink data signal, 31 ... Phase detector , 32...Low pass filter, 33...vCOl 35...PLL
Circuit, 42...Modulation frequency signal.

Claims (1)

[Claims] 1. Transmission request inquiry signal receiving means for receiving a transmission request inquiry signal for inquiring whether there is a transmission request; This transmission request inquiry signal receiving means receives the transmission request inquiry signal. and a modulation standard that generates a modulation reference signal of a carrier frequency necessary for modulating the response data based on the carrier frequency of the transmission request inquiry signal. A data terminal device comprising signal generation means. 2. A receiving means for receiving a polling signal for inquiring about the presence or absence of a transmission request, which is sequentially transmitted from a center station of a cable television broadcasting system to each terminal device in this system, and a polling signal received by the receiving means. demodulation means for demodulating the data signal; response data output means for decoding the data signal detected by the demodulation means and outputting corresponding response data; and a phase-locked loop circuit that synchronizes the carrier frequency of the modulation reference signal used by the response data modulation means with the carrier frequency of the polling signal. Device.