JPH0654324A - Agc circuit of catv terminal - Google Patents

Abstract

PURPOSE:To stabilize the AGC operation of a CATV terminal in a CATV system where the scrample signal and an NTSC signal of a synchronous com pression system are mixed. CONSTITUTION:An RF output-type CATV terminal which inputs a high frequency NTSC signal or a high frequency scrample signal and the CATV signal adding a down control signal is provided with an AGC adjusting equipment 12. A status signal which classifies and recognizes the NTSC signal and the scramble signal is transmitted from CPU 3 and adjusts an AGC voltage level fed-back to a tuner 4 so as to remove the influence of a scramble key signal which is AM-modulated into the voice signal of the scramble signal. Whether a channel selected by the tuner 4 is the scramble signal or the NTSC signal is analized by CPU 3 from channel information added in the down control signal and the signal is outputted to the AGC adjusting equipment 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AGC circuit for a CATV terminal, and more particularly to an AGC circuit for a CATV terminal attached to an intermediate frequency detection circuit of an RF output type CATV terminal.

[0002]

2. Description of the Related Art In a conventional RF output type CATV terminal, as shown in FIG. 2, a CATV signal including a high frequency NTSC signal or a synchronous compression high frequency scramble signal and a downlink control signal is input to a branching device 1. Then, the downlink control signal and the high frequency NTSC signal or the high frequency scramble signal are demultiplexed.

Descrambling (de-scrambl)
The downlink control signal including the availability information of e) is demodulated by the data demodulator 2, and the data is analyzed by the CPU 3.

On the other hand, the high frequency NTSC signal or the high frequency scramble signal is supplied to the tuner 4 and subjected to channel selection processing and frequency conversion processing. When the input is a high frequency NTSC signal, it is directly output to the sync expander 5.

Further, in the case of a high frequency scrambled signal, the scrambler is commanded by the CPU 3 to be descrambled by the operation of the synchronous decompressor 5 and converted into an RF signal.

The synchronous compression timing is AM-modulated in the audio signal, and the BFF (band-pass filter) 6 is used.
After separating the audio signal by the converter, the oscillator 8 and the frequency converter 7 convert the signal to an intermediate frequency signal, and the AM detector 9 extracts the sync expansion timing signal.

This sync expansion timing signal is waveform-shaped by the timing adjuster 10, the gate is opened by a command from the CPU 3, and the sync expander 5 expands the synchronization of the video signal.

Further, the signal is detected by the AM detector 9 and A
AG of DC voltage corresponding to the input level in the GC detector 11
It is output as a C voltage and supplied to the tuner 4 to control the level of the RF signal.

[0009]

In this conventional RF output type CATV terminal, the level of the audio signal is detected and the tuner is operated at the operation level set corresponding to the level.
I was playing C.

The audio signal of the high frequency scrambled signal of the synchronous compression system is, as shown in FIG. 3B, as shown in FIG.
Since the scramble key signal of No. 2 is AM-modulated and superimposed, the setting level for applying AGC apparently decreases, and there is a problem that the S / N is deteriorated as compared with the case of the NTSC signal.

The object of the present invention is to solve the above-mentioned problems,
The level at which AGC is applied to the scramble signal does not apparently drop even when the scramble key signal is superimposed, and NT
CATV that does not deteriorate S / N compared to SC signals
It is to provide an AGC circuit of a terminal.

[0012]

An AGC circuit of a CATV terminal of the present invention is a CATV which receives a high frequency NTSC signal or a synchronous compression high frequency scramble signal sent from a center of a CATV system and a downlink control signal.
The AGC circuit of the intermediate frequency detection circuit of the terminal discriminates between the high frequency NTSC signal and the high frequency scramble signal of the synchronous compression method, and based on the discrimination result, the apparent level increase due to the scramble key signal superposition of the high frequency scramble signal is performed. It has a configuration for setting the operation level of the AGC so as to eliminate the influence.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an AGC circuit of a CATV terminal according to an embodiment of the present invention.

The configuration of the embodiment shown in FIG.
The data demodulator 2, the CPU 3, the tuner 4, the sync expander 5, the BPF 6, the frequency converter 7, and the oscillator 8
, AM detector 9, timing adjuster 10, AGC
CPU with a detector 11 and an AGC controller 12
3. The AGC detector 11 and the AGC adjuster 12 are the components directly related to the present invention.

The embodiment of FIG. 1 shows a part of an RF output type one way CATV terminal.

The CATV signal input to the branching device 1 includes a high frequency NTSC signal or a high frequency scramble signal of a synchronous compression system and a downlink control signal. The high frequency NTSC signal or the high frequency scramble signal is input to the tuner 4 and whether or not it can be descrambled. Downlink control information including information is supplied to the data demodulator 2.

The high frequency NTSC signal or high frequency scramble signal supplied to the tuner 4 is subjected to channel selection and frequency conversion processing, the high frequency NTSC signal is output as it is to the sync expander 5, and the high frequency scramble signal is CP.
The scramble is released by the scramble release command output from U3, and is converted into an RF signal by the operation of the sync expander 5.

The BPF 6 separates the audio signal, the output of which is converted to an intermediate frequency by the frequency converter 7 and the oscillator 8 and then detected by the AM detector 9 to extract the synchronous extension timing signal.

The sync expansion timing signal is subjected to waveform shaping adjustment by the timing adjuster 10, and then the sync expander 5 expands the synchronization of the video signal under the control of the CPU 3.

The above operation is almost the same as that of the conventional example shown in FIG.

Now, the detection output of the AM detector 9 is converted by the AGC detector 11 into a DC voltage of a level corresponding to the detection output, that is, the audio signal output, and this is converted into the AGC voltage as A voltage.
It is supplied to the GC switch 12.

The AGC voltage output from the AGC detector 11 is supplied to the AGC controller 12, and the level is adjusted based on the status information designated by the status signal output from the CPU 3.

When the channel selected by the tuner 4 is a high frequency scramble signal, this status signal is of a quantity that can eliminate the influence of the scramble key signal superimposed on the audio signal as described above with reference to FIG. In the example, the AGC voltage is reduced by 7 dB and is processed by the AGC controller 12 before being supplied to the tuner 4.

The condition for setting the contents of the status signal is easily determined based on the scramble assigned channel information included in the downlink control signal sent from the center and supplied to the data demodulator 2, and the descrambled and CATV terminal information. be able to.

In this way, it is possible to accurately set the AGC operation level.

[0027]

As described above, the present invention is based on NTSC.
By separating the signal from the scrambled signal and setting the AGC voltage for the tuner, RF of stable quality can be obtained.
There is an effect that a signal can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an AGC circuit of a CATV terminal according to an embodiment of the present invention.

FIG. 2 is a block diagram of an AGC circuit of a conventional CATV terminal.

FIG. 3 is a waveform diagram showing an example of a scrambled signal of a synchronous compression system.

[Explanation of symbols]

 1 Divider 2 Data Demodulator 3 CPU 4 Tuner 5 Sync Expander 6 BPF 7 Frequency Converter 8 Oscillator 9 AM Detector 10 Timing Adjuster 11 AGC Detector 12 AGC Controller

Claims (1)

[Claims] 1. A CATV which receives a high frequency NTSC signal or a high frequency scramble signal of a synchronous compression system transmitted from a center of a CATV system and a downlink control signal.
The AGC circuit of the intermediate frequency detection circuit of the terminal discriminates between the high frequency NTSC signal and the high frequency scramble signal of the synchronous compression method, and based on the discrimination result, the apparent level increase due to the scramble key signal superposition of the high frequency scramble signal is performed. A of the CATV terminal characterized by setting the operation level of the AGC so as to eliminate the influence
GC circuit.