JPH0965320A - CATV receiver - Google Patents

Abstract

Kind Code: A1 Abstract: When extracting a key pulse used for releasing a scrambled video signal of a synchronous compression method implemented in a cable television, an unnecessary component such as a video component is prevented from being mixed and taken out. A second audio intermediate frequency signal is input to a key pulse detection circuit 22 of a receiving device. The second audio intermediate frequency signal is a signal having a beat frequency of the video intermediate frequency signal and the first audio intermediate frequency signal, and has a lower frequency than the first audio intermediate frequency signal. For example, 4.5M for NTSC system
Hz. This second audio intermediate frequency signal is passed through the bandpass filter 21 to prevent the mixing of unnecessary components. The key pulse is separated and extracted by the key pulse detection circuit 22 based on the signal that has passed through the band pass filter. Based on the extracted key pulse, the descrambling control unit 10 controls the descrambler 3 as in the conventional case to restore the compressed horizontal synchronizing signal to an appropriate level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CATV (cable television) receiver, and more specifically, to extracting a key pulse used for releasing a scrambled video signal of a synchronous compression method which is mainly implemented in a cable television. Regarding

[0002]

2. Description of the Related Art As described above, in CATV, a scrambling broadcast of a synchronous compression system is carried out as a means of converting to pay broadcasting. This scramble system sets the level of the horizontal sync signal in the video signal to 6 dB (1/2) of the specified level.
It is to be compressed and transmitted. Then, in order to be able to release this scrambled signal on the receiving side, a key pulse for descrambling is superimposed on the frequency-modulated audio signal wave by amplitude modulation. Therefore, the receiving side separates and extracts the key pulse from this audio signal, and restores the horizontal synchronizing signal level to an appropriate level based on this pulse. This releases the scramble.

Conventionally, the scrambled signal is released on the receiving device side by the circuit configuration shown in FIG. FIG. 2 shows the main part of the intermediate frequency circuit related to descrambling.
In the figure, the received RF signal is frequency converted into an intermediate frequency signal by the converter 1. The intermediate frequency signals are a video intermediate frequency signal (VIF) and an audio intermediate frequency signal (SIF-this is referred to as a first audio intermediate frequency signal), the frequency of which is 58.7 in the former NTSC system, for example.
5MHz and the latter is 54.25MHz. This intermediate frequency signal is branched into two, and reaches the descramble circuit 3 for descrambling as one path through the amplifier 2 as the video intermediate frequency signal system. The descramble circuit 3 descrambles under the control of the descramble controller 10 described later. The descrambled signal is sent to the video SAW filter 5 via the amplifier 4. Same SAW filter 5
Is a filter that passes the video intermediate frequency signal (VIF). The video intermediate frequency signal from the SAW filter 5 is sent to the intermediate frequency (IF) detection circuit 6.

Further, as the other path of the intermediate frequency signal from the converter 1, it reaches the audio SAW filter 8 via the amplifier 7 as the first audio intermediate frequency signal system. Same SA
The W filter 8 is a filter that passes the first audio intermediate frequency signal (SIF). The first audio intermediate frequency signal from the SAW filter 8 is branched to the intermediate frequency detection circuit 6 and the key pulse detection circuit 9. The intermediate frequency detection circuit 6 detects the input video intermediate frequency signal, outputs an original video signal as a baseband signal, and outputs a second audio intermediate signal as a beat between the video intermediate frequency signal and the first audio intermediate frequency signal. Generate a frequency signal. The frequency of the second audio intermediate frequency signal is, for example, 58.75 MHz in the case of the Japanese NTSC system.
It is a difference frequency of 4.5 MHz from 54.25 MHz. The second audio intermediate frequency signal (4.5 MHz) is passed through the band pass filter 11, the output of the filter 11 is detected, and the original audio signal as the baseband signal is output.

On the other hand, the key pulse detection circuit 9 is used for voice S
A key pulse is extracted from the first audio intermediate frequency signal from the AW filter 8. As described above, the key pulse for descrambling is amplitude-modulated and superimposed on the frequency-modulated audio signal wave. Therefore, the key pulse detection circuit 9
Detects the AM pulse and separates and extracts the key pulse. The key pulse extracted by the key pulse detection circuit 9 is sent to the scramble release controller 10 (usually a microcomputer). The descrambling control unit 10 controls the descrambling circuit 3 based on the key pulse so as to restore (expand) the compressed horizontal synchronizing signal to the normal level. As a result, the scramble signal is released.

[0006]

As described above, the key pulse required for descrambling has conventionally been separated and extracted from the first audio intermediate frequency signal (54.25 MHz) (FIG. 2). Since the first audio intermediate frequency signal (54.25 MHz) has a high frequency, the necessary narrow band filter cannot be performed. Therefore, it is inevitable that a video component is mixed in the output of the audio SAW filter 8. Since the audio component has a different modulation method (frequency modulation), there is no mixing. In this way, the key pulse detection circuit 9 receives an unnecessary video signal component in addition to the key pulse component. Therefore, in addition to the key pulse, the output signal contains unnecessary components of the above-mentioned image components. Therefore, in the descrambling operation, the undesired descrambling may not be performed due to the influence of these unnecessary components, and the image may become unstable.
The conventional method of extracting the key pulse from the first audio intermediate frequency signal has the above-mentioned drawbacks. The present invention has been made to improve the above-mentioned conventional drawbacks, and an object of the present invention is to provide a CATV receiving device that stabilizes descrambling.

[0007]

According to the present invention, a descramble circuit for descrambling a scrambled video signal of a synchronous compression system such as a cable television broadcast signal is provided in a video intermediate frequency circuit, while an amplitude of a frequency modulated audio signal is increased. A key pulse detection circuit that separates and extracts the descrambling key pulse that is superimposed by modulation,
A descrambling control unit for controlling descramble for the descrambling circuit based on the key pulse extracted by the key pulse detecting circuit is provided, and the key pulse is extracted by the key pulse detecting circuit for the video intermediate frequency signal and the first audio intermediate signal. A second audio intermediate frequency signal generated as a difference frequency from the frequency signal is passed through a band pass filter and is extracted from the passed signal.
A receiver is provided.

[0008]

The key pulse detection circuit extracts the key pulse from the second audio intermediate frequency signal. The second audio intermediate frequency signal is a signal having a beat frequency of the video intermediate frequency signal and the first audio intermediate frequency signal, and has a frequency lower than that of the first audio intermediate frequency signal. In case of NTSC system, it is 4.5MHz. The second audio intermediate frequency signal is passed through a narrow band pass filter before being input to the key pulse detection circuit to prevent mixing of unnecessary components. A key pulse is separated and extracted by a key pulse detection circuit based on the signal that has passed through the band pass filter. Based on the extracted key pulse, the descrambling control unit controls the descrambling circuit as in the conventional case, and returns the compressed horizontal synchronizing signal to an appropriate level.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
The receiver will be described. FIG. 1 is a block diagram of essential parts showing an embodiment of a CATV receiver according to the present invention. In FIG. 1, the same components as those in FIG. 2 are designated by the same reference numerals, 21 is a band pass filter (BPF) whose center frequency is the frequency of the second audio intermediate frequency signal, and 22 is a band pass filter from the same band pass filter 21. It is a key pulse detection circuit that separates and extracts a key pulse from a signal.

Next, the operation of the present invention will be described. However, since FIG. 1 is common in many parts with FIG. 2, there is a part overlapping with the above (prior art) in the description of the operation. In FIG. 1, the received RF signal is frequency converted into an intermediate frequency signal by the converter 1. The intermediate frequency signals are a video intermediate frequency signal (VIF) and an audio intermediate frequency signal (SIF-this is referred to as a first audio intermediate frequency signal), and the former frequency is the NTSC system (Japan), for example. 58.75M
Hz and the latter is 54.25 MHz. This intermediate frequency signal is branched into two, and reaches the descramble circuit 3 for descrambling as one path through the amplifier 2 as the video intermediate frequency signal system. The descramble circuit 3 descrambles under the control of the descramble controller 10 (described later). The descrambled signal is sent to the video SAW filter 5 via the amplifier 4. The SAW filter 5 is a filter that passes the video intermediate frequency signal (VIF). The video intermediate frequency signal from the SAW filter 5 is sent to the intermediate frequency (IF) detection circuit 6.

The other path of the intermediate frequency signal from the converter 1 reaches the audio SAW filter 8 via the amplifier 7 as the first audio intermediate frequency signal system. Same SA
The W filter 8 is a filter that passes the first audio intermediate frequency signal (SIF). The first audio intermediate frequency signal from the SAW filter 8 is sent to the intermediate frequency detection circuit 6. The intermediate frequency detection circuit 6 detects the input video intermediate frequency signal, outputs an original video signal as a baseband signal, and outputs a second audio intermediate signal as a beat between the video intermediate frequency signal and the first audio intermediate frequency signal. Generate a frequency signal. For example, in the case of Japan's NTSC system, the frequency of the second audio intermediate frequency signal is the difference frequency between 58.75 MHz and 54.25 MHz, which is 4.
5 MHz. The second audio intermediate frequency signal (4.5 MHz) is passed through the band pass filter 21 and branched to the intermediate frequency detection circuit 6 and the key pulse detection circuit 22. The intermediate frequency detection circuit 6 FM-detects the output of the bandpass filter 21 and outputs an original audio signal as a baseband signal.

On the other hand, the key pulse detection circuit 22 extracts a key pulse from the second audio intermediate frequency signal from the band pass filter 21. As described above, the key pulse for descrambling is amplitude-modulated and superimposed on the frequency-modulated audio signal wave. Therefore, the key pulse detection circuit 22 is AM
It detects and separates the key pulse. The key pulse extracted by the key pulse detection circuit 22 is sent to the scramble release control unit 10 (normally a microcomputer). The descrambling control unit 10 controls the descrambling circuit 3 based on the key pulse so as to restore (expand) the compressed horizontal synchronizing signal to the normal level. As a result, the horizontal sync signal becomes the normal level and the scramble signal is released.

[0013]

As described above, according to the present invention, in the CATV receiving apparatus for canceling the scrambled video signal of the synchronous compression method, which is mainly implemented in the cable television, the key pulse necessary for the descramble is conventionally changed. , The first audio intermediate frequency signal is separated and extracted, while the second audio intermediate frequency signal having a lower frequency than the first audio intermediate frequency signal is separated and extracted. Therefore, the band setting for extracting the key pulse is set. Optimization (narrow band)
Therefore, it is possible to reduce the mixing of unnecessary components of video components. As a result, it is possible to make the descrambling operation less likely to be affected by the unnecessary components and to make the descrambling operation stable. For example, in the past, depending on the timing, there was a case where the descrambling could not be performed due to a defect of the key pulse, whereas the method according to the present invention improves this. Such stable descrambling means that the image itself on the screen is stable. Therefore,
It can be said that the present invention can contribute to improving the performance of the CATV receiver.

[Brief description of drawings]

FIG. 1 is a principal block diagram showing an embodiment of a CATV receiver according to the present invention.

FIG. 2 is a block diagram of essential parts showing an example of a conventional CATV receiver.

[Explanation of symbols] 1 converter 3 descramble circuit 5 image SAW filter 6 intermediate frequency detection circuit 8 audio SAW filter 10 scramble release control unit 21 band pass filter 22 key pulse detection circuit

Claims (1)

[Claims] 1. A descrambling circuit for descrambling a scrambled video signal of a synchronous compression system such as a cable television broadcast signal is provided in a video intermediate frequency circuit, while the scramble is superimposed on a frequency-modulated audio signal by amplitude modulation. A key pulse detection circuit that separates and extracts the key pulse for cancellation, and a scramble cancellation control section that controls the descramble circuit to the descramble circuit based on the key pulse extracted by the key pulse detection circuit are provided, and in the key pulse detection circuit, The key pulse is extracted by passing a second audio intermediate frequency signal generated as a difference frequency between the video intermediate frequency signal and the first audio intermediate frequency signal to a band pass filter and extracting from the passed signal. CATV receiver.