JPH06245197A - Output frequency variable modulator for catv - Google Patents

Abstract

PURPOSE:To secure high C/N (carrier against noise) even in a CATV terminal by eliminating the influence of the noise even when plural modulators are used. CONSTITUTION:A synthesizer oscillator 36 is used as a local oscillator for a frequency conversion circuit 35 and a microcomputer 38 decides the oscillation frequency of the synthesizer oscillator 36 based on the set information of an output frequency setting switch 37. In this output frequency variable modulator for the CATV, a frequency band pass filter 44 is attachably and detachably provided through input/output connectors 42 and 43 between the wide-band amplifier 40 of a final stage and an output terminal 41. The frequency band pass filter 44 is set at 6MHz band in the case of the television of an NTSC system and is set at 300kHz band in the case of FM and noise components generated in other frequency bands are interrupted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable output frequency modulator used in, for example, CATV (cable television).

[0002]

2. Description of the Related Art Conventionally, in a CATV system, a plurality of modulators are used as shown in FIG.

FIG. 4 is a block diagram showing a schematic configuration of a CATV system. In FIG. 4, reference numeral 10 denotes a head end device, and the head end device 10 includes, for example, a CD.
Audio equipments 1 and 2 such as a player and a tape recorder, and video equipments 3 and 4 such as a video tape recorder and a video disc player are connected, and a broadcast receiving antenna 5 for receiving a general broadcast wave is connected. The head-end device 10 is composed of FM modulators 11 and 12, independent broadcast television modulators 13 and 14, a retransmission receiver 15, and a mixer 16.

The FM modulators 11 and 12 in these headend devices 10 modulate the acoustic signals input from the acoustic equipments 1 and 2 so that they can be received by a commercially available FM receiver and input them to the mixer 16.

[0005] In addition, TV modulators 13 and 14 for independent broadcasting
Converts the video signals input from the video equipment 3 and 4 into signals that can be received by a commercially available television receiver, and inputs the signals to the mixer 16. Further, the retransmitting receiver 15 amplifies a general broadcast wave received by the broadcast receiving antenna 5 to a required output level by the retransmitting receiver 10 and inputs it to the mixer 16.

The mixer 16 mixes these signals in a hybrid circuit or the like and outputs them via the CATV transmission cable 17. Since the signal transmitted by the cable 17 is attenuated on the way, it is amplified by the amount attenuated by the relay amplifier 18 and supplied to the transmission cable 17 again.

The transmission cable 17 is provided with a branching device 19 for supplying a signal to the terminal.
The terminal 9 is connected to the terminal 21 via the branch line 20 from the branch terminal 9.

The terminal 21 is provided with a two-divider 22 for dividing a CATV signal into two, one distribution terminal is connected to an audio device such as an FM receiver 23, and the other distribution terminal is a CATV terminal. TV receiver 2 via 24
Connected to 5.

The CATV terminal 24 is
In CATV transmission, the frequency band of general terrestrial broadcast waves (low channel: 90 to 102 MHz, high channel: 170 to 222 MHz, UHF: 470 to 770 M)
Hz), the TV transmission band is 90 to 550 MH
Since there are up to z, it cannot be received by a general commercially available television receiver, so a generic term is used for frequency conversion devices, receivers, etc. that enable even a commercially available television receiver to receive.

With the above-described structure, the FM receiver 23 can receive the signals FM-modulated by the FM modulators 11 and 12 of the head-end device 10, and the head-end device 10 has an independent broadcast television modulator. The signals modulated by 13 and 14 can be received by the television receiver 25. Next, the configuration of the modulator used in the above ATV system will be described. FIG. 5 is a block diagram of a conventional output frequency variable type television modulator.

In FIG. 5, 30 is a video input terminal, 31
Is an audio input terminal. The video signal supplied to the video input terminal 30 is amplitude-modulated by the video IF modulator 32 into a video IF signal, which is supplied to the video / audio mixer 34. Also, the audio signal supplied to the audio input terminal 31 is
The frequency is modulated by the FM modulator 33 into an audio IF signal, which is supplied to the video / audio mixer 34. The video / audio mixer 34 mixes the video IF signal and the audio IF signal and outputs them to the frequency conversion circuit 35.

A local signal is supplied to the frequency conversion circuit 35 from a synthesizer oscillator 36 which is a local oscillator, and the sum or difference with each IF signal is obtained to obtain a required output frequency. Here, 38 is a microcomputer, which operates to determine the oscillation frequency of the synthesizer oscillator 36 based on the setting information of the output frequency setting switch 37.

The video / audio signal converted to the required output frequency by the frequency conversion circuit 35 is a wide band filter 39 (within the maximum variable frequency, for example, 90 MHz to 550 MHz).
90 MHz to 550 MHz when variable up to MHz
After the unnecessary components are removed by the bandpass filter (1), the signal is amplified to a required output level by the broadband amplifier 40, and the output terminal 4
It is output from 1. Next, the conventional output frequency variable type FM
A block diagram of the modulator is shown in FIG. This FM modulator is the same as the television modulator in that only the video IF modulator 32 and the video / audio mixer 34 of the television modulator in FIG. 5 are not provided.

[0014]

In the conventional CATV modulator, a wide band amplifier is required for all the amplifiers to obtain a required output level. Also, this amplifier is usually 30
It has an amplification degree of -40 dB and naturally not only generates thermal noise but also amplifies noise. Moreover, CAT
Uniform noise is generated in the entire V transmission band (for example, from 90 MHz to 550 MHz). Therefore, when a plurality of modulators are used as shown in FIG. 4, noise is also proportional to the number of used units, and the C / N (carrier to noise) of the output of the mixer 16 deteriorates. For example, the C / N of the modulator alone is AdB
If the same modulator is used for 10 units, noise component N0
Is represented by "N0 = 10log n, where n is the number of units used",
"N0 = 10 log 10 = 10 dB".

Therefore, the C of a modulator having a C / N of AdB
/ N deteriorates as "C / N = AdB-10 dB". Therefore, even if the performance of the modulator alone is designed well,
If multiple units are used, the performance will be the same and the high quality image cannot be supplied to the terminal.

The present invention has been made in view of the above circumstances, and even if a plurality of units are mixed and used in a mixer, there is no influence of noise, and a high C / N (carrier to noise) is ensured even in a CATV terminal. An object of the present invention is to provide a variable output frequency modulator for CATV.

[0017]

A variable output frequency modulator for CATV according to the present invention is a CATV modulator which uses a synthesizer oscillator and whose output frequency can be varied by an output frequency setting switch. A feature is that a band-pass filter provided as possible is connected.

[0018]

Function: A frequency band pass filter provided detachably from the outside (6 MHz band, FM in case of NTSC TV, FM
In this case, the noise component generated inside the modulator of other frequency band can be blocked by connecting (300 KHz band), and even if multiple modulators are used, C / N is not deteriorated and high. A high quality image can be supplied to the terminal.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a variable output frequency type television modulator according to the present invention. Codes 30 to 4
Since each circuit section indicated by 1 has the same configuration as the conventional modulator shown in FIG. 4, the same reference numerals as those in FIG. 4 are given and detailed description thereof is omitted.

Therefore, in the present invention, the wide band amplifier 40
The frequency band pass filter 44 is detachably provided between the output terminal 41 and the output terminal 41 via the input / output connectors 42 and 43. The frequency of the bandpass filter 44 is set according to the channel output from the modulator, and the band is set to 6 MHz when the television broadcast is the NTSC system. When the wide band amplifier 40 is provided in multiple stages, a frequency band pass filter 44 may be connected between the stages.

By using the band-pass filter 44 matched to the TV channel band as described above, noise components generated in other frequency bands can be blocked, and even if a plurality of modulators are used, C / N is not deteriorated, and high-quality video can be supplied to the terminal. In addition, the bandpass filter 44
By detachably providing, it is possible to correspond to any TV channel.

FIG. 2 shows an example in which an FM modulator is implemented. In contrast to the television modulator shown in FIG. 1, there is no video IF modulator 32 and video / audio mixer 34, and
Only the point that the band of the band pass filter 44 is set to 300 KHz is different, and other configurations and operations are the same as those of the above television modulator. Next, a specific configuration of the bandpass filter 44 detachably provided as described above will be described with reference to FIG.

As shown in FIG. 3, a filter housing 52 to which the bandpass filter 44 can be attached and detached is provided on the front surface of the modulator main body 51, and the bandpass is provided at the rear end of the housing 52. Input / output connectors 42b and 43b for signals to and from the filter 44 are provided. Input / output connectors 42a and 43a are provided at the rear end of the bandpass filter main body 44,
The band pass filter 44 is connected to the filter housing portion 5 of the modulator main body 51.
2 by inserting into the input / output connector 42a and 4
2b, 43a and 43b are fitted together. Then, after inserting the band pass filter 44 into the filter housing 52, a screw hole 53 provided in the modulator main body 51.
Then, screw the filter mounting screw 54 to the modulator main body 51.
Fixed to. Further, on the front surface of the bandpass filter 44, a handle 55 used for attachment / detachment, a name plate 56 for marking the pass frequency of the bandpass filter, etc. are provided.

[0025]

As described above in detail, according to the present invention, by using the band pass filter matched with the band of the desired TV channel or FM, the noise components generated in other frequency bands are blocked. Therefore, even if a plurality of modulators are used, the C / N is not deteriorated and a high quality image can be supplied to the terminal.

Further, by providing the band-pass filter detachably, it is possible to deal with any TV channel, and the modulator can be mass-produced. That is, even with the same type of modulator, the frequency of the synthesizer oscillator can be set by the output frequency setting switch, and since the bandpass filter corresponding to the desired channel can be used, the modulator can be mass-produced. Therefore, the production efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a variable output frequency type television modulator according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a variable output frequency type FM modulator in the embodiment.

FIG. 3 is a perspective view of an output frequency variable modulator according to the same embodiment.

FIG. 4 is a block diagram showing a configuration of a conventional CATV system.

FIG. 5 is a block diagram showing the configuration of a conventional output frequency variable television modulator.

FIG. 6 is a block diagram showing a configuration of a conventional output frequency variable FM modulator.

[Explanation of symbols]

1, 2 ... Audio equipment, 3, 4 ... Image equipment, 5 ... Broadcast receiving antenna, 10 ... Headend device, 11, 12 ... FM
Modulator, 13, 14 ... Independent broadcast TV modulator, 15 ...
Retransmission receiver, 16 ... Mixer, 17 ... Transmission cable, 1
8 ... Repeater amplifier, 19 ... Brancher, 20 ... Service line, 2
1 ... Terminal, 22 ... 2 distributor, 23 ... FM receiver, 24 ...
CATV terminal, 25 ... Television receiver, 30 ... Video input terminal, 31 ... Audio input terminal, 32 ... Video IF modulator, 33 ... FM modulator, 34 ... Video / audio mixer, 35 ...
Frequency conversion circuit, 36 ... Synthesizer oscillator, 37 ... Output frequency setting switch, 38 ... Microcomputer,
39 ... Wide band filter, 40 ... Wide band amplifier, 41 ... Output terminal, 42, 43 ... Input / output connector, 44 ... Band pass filter, 51 ... Modulator main body, 52 ... Filter housing.

Claims (1)

[Claims] 1. A CATV modulator which uses a synthesizer oscillator and whose output frequency can be varied by an output frequency setting switch, wherein a band-pass filter which is detachably provided is connected to a final stage wide band amplifier. Output frequency variable modulator for.