JPH0377694B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an impedance matching method for a television intercom, and particularly to a transmission cable and an impedance matching method for a television intercom.
This type of system performs impedance matching with each channel of a video signal and upstream and downstream audio signals connected to both sides of the transmission cable.

2. Description of the Related Art As shown in FIG. 1, a so-called television intercom, which incorporates a video function into the call function of an intercom, has been known as an intercom using a frequency division multiplexing transmission system.

This TV intercom system has 1 entrance cordless unit.
A video channel that sends video signals from the surveillance camera 8 to the monitor 17 is connected between the camera 8 and the indoor base unit 2, and a video channel that sends video signals from the microphone 22 of the base unit 2 to the base unit 2, which constantly transmits f ₁ (for example, 6.5MHz).
A transmitter 19 modulates a carrier wave operating at a frequency with the downstream audio signal of the microphone and transmits the modulated signal, and a receiver 9 that extracts the audio signal from the carrier wave sent from the transmitter 19 to the speaker 11 of the slave unit. A communication channel that sends a downstream audio signal, a transmitter 10 that modulates and transmits a carrier wave that constantly operates at a frequency of f ₂ (for example, 6.0 MHz) from the microphone 12 of the handset 1 using an upstream audio signal from the microphone, and the transmitter. Part 1
It has a communication channel that sends the audio signal up to the receiver 21 of the base unit via the receiver 18 that extracts the audio signal from the carrier wave sent from the base unit 0, and connects the base unit and the slave unit with one transmission cable 3. An impedance matching section 4 for matching each channel on the slave device side and the transmission cable is installed on the slave device side, and an impedance matching section 13 for matching each channel on the base device side and the transmission cable. are provided on the main unit side.

These impedance matching sections include band pass filters 5 and 14 that pass only the frequency band of the video signal, band pass filters 6 and 16 that pass only the f ₁ frequency band, and band pass filters 6 and 16 that pass only the f ₂ frequency band. Band pass filters 7 and 15 are respectively provided to allow the signal to pass, and impedance matching is performed for each transmission cable and each channel. Here, it has become common to use a general-purpose ceramic vibrator in order to eliminate the need for adjustment of the audio signal carrier wave generation circuit and reduce costs. However, when using such a ceramic resonator, the frequencies that can be used are limited,
The frequencies f ₁ and f ₂ have very close values. Therefore, in order to perform impedance matching for each channel, the characteristics of the bandpass filters 5 and 14 must be such that only the frequency band of the video signal is passed, as shown in the selectivity characteristic diagram of FIG. As shown in the impedance characteristic diagram a, it is necessary to perform impedance matching with the transmission cable in the frequency band, and to provide high impedance for the frequency bands of other channels. The other bandpass filters 7, 15 and 6, 16 require very steep characteristics as shown in Figure 2 c, d, e, and f, especially since the frequencies f ₁ and f ₂ are close to each other. However, in order to realize this, the structure is extremely complicated, and the number of parts increases, making it expensive. In addition, when connecting multiple devices that transmit address and data pulses to the bus in the same frequency band in parallel using a signal transmission/reception bus line, connect the ground air and power supply for the termination circuit to the ground air bus line and the termination power supply, respectively. A technology that satisfies the termination conditions as a whole by connecting them with a bus line, connecting them with a plug, and mounting a terminating resistor related to the terminating circuit on the connector side corresponding to the starting and terminating devices (Japanese Patent Laid-Open No. 1983-1999) 79551) is known.
However, this type of transmission method provides a termination method when multiple I/O devices placed in parallel to the CPU are connected via a bus line, and the transmission cable and the terminals connected to both sides of the transmission cable are It is not possible to match the impedance with each channel line assigned to video signals, audio signals, etc.

The present invention was created in view of the above-mentioned difficulties, and its feature is that the video signal and up and down audio signals are frequency-divided and multiplexed transmitted between the entrance slave unit and the indoor master unit via a transmission cable. In this case, when matching the impedance between the transmission cable and each channel of the video signal and upstream and downstream audio signals connected to both sides of the transmission cable, any one of the channels and the transmission cable The purpose of the present invention is to provide an impedance matching method for a television intercom in which matching is performed only between the transmission cable and the other channels, and the input/output impedance is increased to connect each other channel to the transmission cable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the impedance matching method for a television intercom according to the present invention will be described below with reference to the drawings.

In FIG. 3, a surveillance camera 25, a monitor 39, a microphone 48 of the main unit, and a main unit 24 are connected to each other by a coaxial cable 36 having a characteristic impedance of 75Ω as a transmission cable.
Transmitter 45, slave receiver 29, speaker 28,
Microphone 32 of slave unit, transmitting unit 33, receiving unit 4 of base unit
2. The receiver 47 has basically the same configuration as the television intercom system shown in FIG.

A band pass filter 26 connected to the surveillance camera 25 is used to prevent upstream and downstream audio signal components of frequencies f ₁ and f ₂ from being output to the coaxial cable 36, and a band pass filter 26 connected to the monitor 39 is used to prevent upstream and downstream audio signal components of frequencies f 1 and f 2 from being output to the coaxial cable 36 Reference numeral 38 is for preventing upstream and downstream audio signal components of frequencies f ₁ and f ₂ from being added to the monitor, and both are general filters that satisfy only the characteristics shown in b in FIG. 2. Transmitter 45 of base unit
A band pass filter 44 connected to the transmitter 33 of the slave device and a band pass filter 34 connected to the transmitter 33 of the slave unit.
These filters are for removing spurious signals and are general filters that satisfy only the characteristics shown in f and d in FIG. 2, respectively. A bandpass filter 41 connected to the receiving section 42 of the base unit has an upstream audio signal frequency f ₂
This is to remove frequency components other than
This is a general filter that satisfies the characteristics shown in d in the figure. A bandpass filter 30 connected to the receiving section 29 of the handset is used to remove frequency components other than the downlink audio signal frequency _f1 , and is a general filter that satisfies the characteristics indicated by f in FIG. 2.

Impedance conversion circuit 2 interposed between the bandpass filter 26 of the slave unit and the coaxial cable 36
7 is configured so that its output impedance Z ₂₇ has the same impedance as the characteristic impedance of the coaxial cable 36 in the entire frequency band, and does not have a frequency characteristic as in the conventional case. The impedance conversion circuit 31 interposed between the bandpass filter 30 and the coaxial cable 36 is configured so that its input impedance Z ₃₁ becomes a sufficiently high impedance that can be ignored with respect to the characteristic impedance of the coaxial cable 36. The impedance conversion circuit 35 interposed between the bandpass filter 34 and the coaxial cable 36 is configured so that its output impedance Z ₃₅ is sufficiently high as to be negligible with respect to the characteristic impedance of the coaxial cable 36. In addition, since the transmitter 33 including the uplink transmission output circuit generally has a low output impedance, its impedance
Z ₃₃ or Z ₃₄ has low impedance. In this case, the impedance conversion circuit 35 can be configured with one series resistor. Impedance conversion circuits 37, 4 interposed between the bandpass filters 38, 41, 44 of the base unit and the coaxial cable 36
0 and 43 are configured in the same manner as the impedance conversion circuits 27, 31, and 35 of the corresponding slave devices. With the above configuration, in order to match the impedance of each channel and the coaxial cable, it is sufficient that the composite impedance Z ₂₃ or Z ₂₄ of each channel matches the characteristic impedance of the coaxial cable. In this case, Z ₃₁ and Z ₃₅ are large enough to be ignored compared to Z ₂₇ , so Z ₂₃ is Z ₂₇
It is equal to , and can be matched with a coaxial cable. Similarly, Z ₄₀ and Z ₄₃ are sufficiently large to be ignored compared to Z ₃₇ , and Z ₂₄ is equal to Z ₃₇ , matching with the coaxial cable.

FIG. 4 shows a concrete example of an impedance matching circuit when a general ceramic filter is used as a bandpass filter. The input impedance of the video channel is 75Ω to match the characteristic impedance of the coaxial cable 36. Also ceramic filter
Since the matching impedance of CF ₃ , CF ₄ , CF ₇ , and CF ₈ was 470 Ω, the input impedance of each transmitter and receiver was 470 Ω, and the impedance of each ceramic filter on the coaxial cable side was 390 Ω. By adding the impedance of 75Ω, the matching becomes approximately 470Ω. As shown in FIG. 5, the circuit and impedance characteristics of this ceramic filter are several kilo ohms for frequencies outside the passband, and the impedance is negligible for 75 ohms.

Input (output) of each channel as another example
The impedance is sufficiently higher than the characteristic impedance ΥΩ of the coaxial cable, and the combined impedance is ΥΩ.
A similar effect can be obtained by setting the value to Ω.

As is clear from the above embodiments, according to the present invention, the impedance of the television intercom that frequency-divides the video signal and upstream and downstream audio signals and multiplexes them for transmission between the entrance slave unit and the indoor master unit via the transmission cable.・When performing matching, it is no longer necessary to perform matching for each channel, and it is possible to allocate the matching between the transmission cable, the video signal connected to both sides of this transmission cable, and the upstream and downstream audio signals without using expensive filters. This has the desirable effect of being able to match the impedance of each channel.

[Brief explanation of drawings]

Fig. 1 is a conceptual diagram of a conventional intercom system including a video function, Fig. 2 a to f are characteristic diagrams required for each band pass filter of the intercom system of Fig. 1, and Fig. 3 is a television intercom system according to the present invention. FIG. 4 is a conceptual diagram embodying a part of FIG. 3, and FIGS. 5 a and b are diagrams of filters used in the television intercom system of FIG. They are a circuit diagram and an impedance characteristic diagram. 36...Transmission cable, 23...Entrance slave unit, 2
4...Indoor base unit, _f1 ...Down carrier wave (downstream audio signal), _f2 ...Uplink carrier wave (upstream audio signal), _Z27 ,
Z ₃₇ ……Video signal channel impedance,
Z ₃₁ , Z ₃₅ , Z ₄₀ , Z ₄₃ ... Impedance of up and down audio signal channels.

Claims (1)

[Claims] 1. In an impedance matching method for a television intercom in which video signals and up and down audio signals are frequency-divided and multiplexed and transmitted via a transmission cable between an entrance slave unit and an indoor base unit, the transmission cable and both sides of the transmission cable are In matching the impedance with each channel of the video signal and up and down audio signals connected to the
The impedance of the television intercom is characterized in that matching is performed only between any one of the channels and the transmission cable, and the other channels and the transmission cable are connected with high input/output impedance. Matching method.