JPH0432858Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention provides a descrambling decoder and a television broadcasting receiver that enable the reception of pay television broadcasting in television broadcasting receivers such as television receivers and VTRs. The present invention relates to an interface device that is suitably connected.

(b) Conventional technology In Canada, VHF low and high bands
In addition to regular television broadcasts in the UHF band,
Mid, Super, and Hyper bands.
CATV broadcasts are being carried out, but in recent years, a paid system has been adopted that allows only subscribers to receive these CATV broadcasts. This pay system is a system in which a television signal is scrambled and sent from the CATV station side, and then descrambled and received by a decoder installed on the receiving side.In particular, the configuration of the receiving side is as follows:
Conventionally, CATV signals were routed to specific channels (for example, 3
A CATV converter for converting to a channel) signal was provided separately from a decoder for descrambling (descrambling) and a television receiver.
As explained in the issue, recently the configuration shown in Figure 4 has been adopted. That is, the one in FIG. 4 is an all-channel tuner 1 that can receive all TV channels including the above-mentioned CATV bands.
An interface device 3 connected to the tuner and VIF (video intermediate frequency) circuit 2 is provided in the television receiver 4, and a decoder 5 is connected to the interface device 3. In this case, tuner 1 is connected to the antenna and CATV cable, and the IF (intermediate frequency) signal output of this tuner is connected to the first input terminal of interface device 1.
_I1 , and is up-converted into a third channel RF (high frequency) signal by the first mixer 6 in this device 3, and sent from the first output terminal _O1 to the input terminal I of the decoder 5. The decoder 5 demodulates the RF signal directly or once into a baseband signal, then descrambles it and outputs it from the output terminal O as a normal three-channel signal. This three-channel signal is given to the second mixer 7 in this device through the second input terminal _I2 in the interface device 3, where it is again down-converted to an IF signal and sent to the VIF signal via the second output terminal _O2 .
It is sent to circuit 2.

FIG. 5 shows a schematic internal configuration of the interface device 3. As shown in FIG. In the figure, 8 is an attenuator, and 9 is a video carrier frequency of 45.75MHz.
An IF selection circuit passes the IF signal and blocks other unnecessary signals; 6 is the aforementioned first mixer for up-converting to three channels; and 10 is a filter for passing three channels. 11 is a 107 MHz oscillation circuit, and 12 is an amplifier that amplifies the output of the oscillation circuit 11 and supplies it to the first mixer 6. 13 is a non-tunable amplifier connected to the second input terminal _I2 ;
14 is the aforementioned second mixer for down-converting to an IF signal, and 15 is an IF tuning amplifier.

In addition, in the scrambling method adopted particularly in Canada, the CATV station side scrambles the synchronization signal part of the video signal after AM modulation by amplitude compression, etc., and also scrambles the synchronization signal part of the audio signal after FM modulation. AM modulated with a pulse corresponding to the above and sent together with the scrambled video signal, the receiving side detects the AM modulated audio signal and reproduces the above pulse, and uses this pulse to descramble the above video signal. I try to do this. Therefore, in this case, the decoder 5 provided on the receiving side is generally composed of circuits 16 to 21 shown in FIG.

Now, the interface device 3 and decoder 5 are configured as described above, but in particular, the IF signal selection circuit 9 in the interface device 3 must suitably pass only the IF signal from the tuner 1. The following characteristics are required.
That is, (a) it can sufficiently prevent leakage of the 3-channel signal from the tuner, (b) it can sufficiently attenuate the local oscillation signal from the tuner and its harmonic signals, and (c) it can sufficiently attenuate the image of the upper adjacent channel from the tuner.
(d) The interface device 3 must have a predetermined frequency characteristic so that the overall frequency characteristic from the first input terminal I ₁ to the second output terminal O ₂ is substantially flat. .

Here, the reason (a) and (b) above are required is that each of these signals generates a beat with the regular three-channel signal created within the interface device 3. Regarding the condition (c), if the descrambling is of the type described above, the video signal of the upper adjacent channel entering the decoder in FIG. 6 will be detected by the AM detection circuit 18 in the decoder. This is because the descrambler circuit 21 will malfunction. Furthermore,
Regarding the condition (d), since the interface device 3 and decoder 5 are inserted between the tuner 1 and the VIF circuit 2 as shown in FIG. 4, assuming that the frequency characteristic of the decoder 5 is flat, This is because unless the overall frequency characteristics of the interface device 3 from terminals I ₁ to O ₂ are substantially flat within the video signal band, the image quality will deteriorate.

Since the IF signal selection circuit 9 in the interface device 3 is required to have the above-mentioned characteristics, conventionally, the video carrier of the upper adjacent channel is connected to an IF filter configured in a π-shape by a coil and a capacitor. The IF signal selection circuit 9 was constructed by connecting ceramic filters in parallel, each having a frequency as a trap point. The frequency characteristics of the entire IF signal selection circuit in this case are as shown in Figure 7, where ₁ in the figure is the trap point corresponding to the video carrier frequency of the upper adjacent channel of the ceramic filter, and ₂ and ₃ are the trap points, respectively. Each trap point corresponds to the video carrier frequency of the three channels and the local oscillation frequency of the A channel by the trap circuit provided in the trap circuit.

(c) Problems to be solved by the invention By the way, in the conventional IF signal selection circuit as described above, as shown in Fig. 7, a trap point occurs due to the resonant impedance of the ceramic filter.
₁ cannot be made sufficiently large, and the frequency characteristics in the range from the audio carrier frequency s to the video carrier frequency p become a _{single peak} characteristic. The overall frequency characteristic up to this point was as shown in Figure 8, and it was not possible to make it flat. In addition, ceramic filters have low impedance for high frequencies of 100MHz or higher, so
Another drawback was that the local oscillation signal of the tuner in the band and its harmonic components could not be sufficiently blocked.

Therefore, the present invention aims to eliminate such drawbacks of the conventional example and to realize an interface device that can fully satisfy each of the above-mentioned conditions (a) to (d).

(d) Means for solving the problem In the interface device of the present invention, the IF signal selection circuit that guides the IF signal from the tuner to the RF conversion circuit has two channels in the vicinity of the video carrier frequency and the audio carrier frequency, respectively. The configuration uses a double-tuned circuit that exhibits a peak.

(e) Effect According to the above configuration, the steep frequency characteristics of the double-tuned circuit can provide a sufficiently large degree of attenuation to the video IF frequency of the upper adjacent channel. In addition, the audio carrier frequency s of the above selection circuit
Since the frequency characteristic in the range from to the video carrier frequency p is downward convex, it is canceled out by the frequency characteristic (unimodal characteristic) between the terminals I ₂ and O ₂ of the interface device 3 (see Fig. 5), Terminal of the above device 3
The overall frequency characteristics between I ₁ and O ₂ can be made substantially flat.

(f) Embodiment FIG. 1 shows the main part of an embodiment of the interface device of the present invention. In this figure, parts corresponding to those in Figure 5 are given the same figure numbers and explained as follows:
First, an attenuator 8 is constituted by a capacitor C ₁ connected to the first input terminal I ₁ and a damping resistor R ₁ of the tuning circuit. Next, the capacitors C ₂ and C ₃ and the air-core coils L ₁ , L ₂ , and L ₃ form a double-tuned circuit 22 for the IF filter in the IF signal selection circuit 9, and one peak frequency of this double-tuned circuit is a is 0.5MHz higher than the audio carrier frequency s in the IF band
41.75MHz is selected, and the other peak frequency b is 1.0MHz higher than the video carrier frequency p.
It is selected as 46.75MHz. Also, capacitor
C ₄ and coil L ₄ constitute a first trap circuit 23 that resonates in parallel with the 3-channel signal at 61.25 MHz, and capacitor C ₅ and coil L ₅ constitute a mid-band A
A second trap circuit 24 is configured to resonate in parallel with the channel signal of 167MHz.

Further, 8 is a first mixer using a diode and a transformer, and 10 is a filter for passing three channel signals consisting of a coil and a capacitor. 25 is a trap circuit for preventing the 107 MHz local oscillation signal from the oscillator 11 from leaking through the first mixer 8, and is composed of a coil _L6 and a capacitor _C4 .

FIG. 2 shows the overall frequency characteristics of the IF signal selection circuit 9. As can be seen from this figure, the characteristics become steep near the video carrier frequency ₁ of the upper adjacent channel. The video IF signal component can be sufficiently attenuated. Also, since the characteristic between the audio carrier frequency s and the video carrier frequency p of the reception channel is convex downward, the frequency characteristic between the terminals I ₂ and O ₂ in Fig. 5 (in the region s to p As a result, the overall frequency characteristic between the terminals I ₁ and O ₂ in FIG. 5 becomes approximately flat in the region from s to p as shown in FIG. 3. In addition, the steep characteristics of the double-tuned circuit 22 and the characteristics of the first and second trap circuits 23 and 24 together make the gain in the high frequency region above 150 MHz sufficiently small as shown in FIG. Leakage of the local oscillation signal of the tuner 1 and its harmonic signals in the band to the interface device 3 is prevented.

(g) Effect of the invention According to the interface device of the invention, the IF
Since a double tuning circuit is used for the signal selection circuit,
3-channel signal from tuner, local oscillation signal and its harmonic signal, video of upper adjacent channel
It can sufficiently block unnecessary signals such as IF signals, and
IF is the overall frequency characteristic of the interface device.
It is substantially flattened within the signal band, and the IF signal can be suitably passed through.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing the main parts of an embodiment of the interface device according to the present invention, and Figure 2 is the IF
FIG. 3 is a diagram showing the frequency characteristics of the signal selection circuit, and FIG. 3 is a diagram showing the frequency characteristics of the entire interface device in that case. FIG. 4 is a diagram showing the configuration of the main parts of a pay TV broadcast receiving device using the interface device of the present invention, FIG. 5 is a diagram showing the schematic configuration of the interface device, and FIG. 6 is a diagram showing the schematic configuration of the tecoder. FIG. 7 is a diagram showing the frequency characteristics of a conventional IF signal selection circuit used in the above-mentioned interface device, and FIG. 8 is a diagram showing the frequency characteristics of the entire interface device in that case. 3...Interface device, 9...IF signal selection circuit, 22...Double tuning circuit.

Claims (1)

[Claims for Utility Model Registration] (1) Connected between a tuner capable of receiving a pay television broadcasting channel and a VIF circuit, and transmitting the IF signal output from the tuner to a specific channel.
In a device that converts the RF signal into an RF signal and supplies it to a descramble decoder, converts the descrambled RF signal from the decoder again into the IF signal and supplies it to the VIF circuit, the IF signal from the tuner is Connect the signal input terminal and the IF signal
An IF signal selection circuit is provided between the circuit for converting into an RF signal, and the IF signal selection circuit is configured by a double-tuned circuit that exhibits a peak near each of the video carrier frequency and the audio carrier frequency. An interface device for receiving pay TV broadcasts. (2) The specific channel is three channels, and the two peak frequencies of the double-tuned circuit are selected to be approximately 0.5 MHz higher than the audio carrier frequency and approximately 1 MHz higher than the video carrier frequency. An interface device for receiving pay television broadcasting according to claim 1.