JPS635954B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an RF modulation circuit added to a VTR, video disc player, etc.

(b) Prior art When a baseband television signal reproduced by a VTR or the like is provided through the antenna circuit of the television receiver in order to display the same on the screen of the television receiver, the It is necessary to relay an RF modulation circuit that converts the television signal into an RF signal. The RF modulation circuit, which is this repeater, may be built into the VTR or the like, or it may be a separate product from the VTR, etc., with a name such as an RF converter or RF adapter.
In either case, the basic components, as shown in FIG. 1, include a first input terminal 1 for inputting a composite video signal containing video information, and an audio signal containing audio information corresponding to the video information. a video signal modulator 3 that receives the composite video signal from the second input terminal 2 and the first input terminal 1, amplitude modulates the first carrier wave with the composite video signal, and outputs a modulated video signal; An audio signal transmission system 5 having an audio signal modulator 4 which receives an audio signal from the second input terminal 2 and frequency-modulates a second carrier wave with the audio signal.
and a mixing circuit 6 that mixes the output of the video signal modulator 3 and the output of the audio signal transmission system 5, and outputs the RF signal for television signal transmission, which is the output of this mixing circuit 6, to the output terminal 7. That's what I do.

By the way, in this RF modulator, it is known that AM-PM conversion occurs when the carrier wave is amplitude-modulated and amplified using a composite video signal, which causes buzz in the output of the television receiver. . AM that is unnecessary in the modulation stage that amplitude modulates the carrier wave with the composite video signal and the subsequent amplification stage, etc.
PM conversion occurs and causes a phase change. In particular, the synchronizing signal pulse portion is accompanied by a rapid phase change. Since this phase change is equivalently considered to be the FM component, the video carrier wave undergoes a sudden frequency change in the sync signal portion. On the other hand, normal television receivers use an audio intercarrier reception method, in which the difference signal (4.5MHz in the NTSC system) obtained by mixing audio and video carrier waves is FM-detected and the audio signal is demodulated. . Therefore, a change in the video carrier frequency is relatively the same as a change in the audio carrier wave. Therefore, changes in the video carrier frequency in the synchronization signal portion are demodulated as unnecessary pulses of the FM detection output, resulting in a buzz.

Normally, the problem with buzz is mainly the harmonic components of the vertical synchronization signal, but in television audio multiplex broadcasting, the double harmonic of the horizontal synchronization signal (31.5KHz) occurs in the sub audio channel (16 to 47kHz), which is significant. It becomes a buzz disturbance.

In order to prevent buzz interference based on such AM-PM modulation, in transmitters that output commercial broadcast radio waves, AM-PM is installed between the video carrier wave oscillator and the modulated amplifier, as shown in Japanese Patent Application Laid-Open No. 47-36906. AM by inserting a conversion compensation circuit and controlling it with the video signal.
-The phase modulation component caused by PM conversion is canceled out by applying phase modulation of the opposite polarity to the carrier wave. However, this AM-PM conversion compensation circuit has an extremely complex configuration, and is used only in home VTRs.
It is difficult to apply this method to RF modulation circuits attached to video disc players.

(c) Purpose of the invention The present invention has been made in view of the above points, and has a simple structure yet provides effective buzz compensation.
It attempts to provide an RF modulation circuit.

(d) Structure of the Invention The present invention is designed to cancel unnecessary audio components produced when unnecessary AM-PM modulation components output together with a modulated video signal are applied to an intercarrier television receiver. Characterized by applying a composite video signal to an audio signal transmission system
It relates to RF modulation circuits.

If the composite video signal applied to the conventional RF modulation circuit is a signal like the one shown in Figure 2a, which captures a white scene, that is, a signal with a horizontal synchronization signal H and a 100% white signal W, intercarrier television is used. Even when no audio signal is being transmitted, the unnecessary audio component N shown in FIG. 2b is output to the audio demodulation output of the receiver. This is an unnecessary audio component based on the AM-PM modulation component, and corresponds to the horizontal synchronization signal as described above. The delay T is caused by a de-emphasis circuit included in the audio demodulation circuit of the television receiver. The present invention focuses on the fact that this unnecessary audio component corresponds to the rate of change in the amplitude of the video signal, that is, the differential value of the video signal, and conversely adds the differential signal of the video signal as an opposite phase to the audio signal input side. This is an attempt to eliminate the buzz.

(d) Embodiment FIG. 3 shows an embodiment of the RF modulation circuit of the present invention, and the same elements as those in FIG. 1 are given the same reference numerals and their explanation will be omitted.

Reference numeral 8 denotes an audio multiplex signal encoder which receives an audio signal from a second input terminal and outputs a known audio multiplex signal of different formats depending on the program content (stereo, bilingual, monaural). This audio multiplexed signal is applied to the audio signal transmission system 5 through resistors R ₂ and R ₃ and a coupling capacitor C ₂ , and the composite video signal from the first input terminal 1 is applied to the audio signal transmission system 5 through a capacitor C ₁ , a resistor R ₃ , and a capacitor C _{2 .} The signal is applied to the audio signal transmission system 5 through. Capacitor C ₁ is the giving circuit 9,
It constitutes a differentiating circuit with a resistor to be described later, and in the example, the value is around 30 pF. Resistance R ₁ is obtained by converting the output impedance of the encoder 8 into series and parallel, and can be expressed as an almost pure resistance.
The value is 10Ω. The resistor _R4 indicates the input impedance of the transmission system 5 and has a value of about 50KΩ. The resistor R ₂ and the variable resistor R ₃ are used to match the output level of the encoder 8 to the input level of the transmission system 5, and have values of around 100KΩ and 200KΩ, respectively. On the other hand, since the composite video signal is driven by a signal source of several tens of ohms, and the input impedance of the video signal modulator 3 is about 1K ohm, it is hardly affected by the capacitor _C1 . Therefore, when the signal shown in FIG. 2a is inputted to the first input terminal 1, a signal having the waveform shown in FIG. 4a appears at point A.

When the output point D of _the encoder 8 is set to zero and the signal shown in FIG. 2a is applied to the first input terminal ₁ as described above, the combined resistance _R {R
= (R ₁ + R ₂ ) (R ₃ + R ₄ ) / (R ₁ + R ₂ + R ₃ + R ₄ )}
The video signal is differentiated by FIG. 4b shows this differential waveform observed at point B. This waveform is multiplied by R ₃ /(R ₃ +R ₄ ) and input to a transmission system 5 including an audio signal modulator that performs positive polarity FM modulation.

When the signal from the output terminal 7 obtained under these conditions is demodulated using a standard split-carrier TV demodulator that extracts and demodulates only the audio carrier, an audio signal output as shown in Figure 4c is obtained. is obtained. Here, it can be seen that this output waveform has an opposite phase relationship with that of FIG. 2b.

As a result, it is derived from the output terminal 7 of this circuit.
When an RF signal is applied to a normal intercarrier television receiver, the unnecessary audio component shown in Figure 2b is added to the component shown in Figure 4c, which has the opposite phase, and both components cancel each other out, resulting in a horizontal Buzz caused by synchronization signals is removed or reduced.
The signal level shown in FIG. 4c used for this cancellation can be easily adjusted by appropriately selecting the capacitance value of the capacitor _C1 , and the buzz level can be easily adjusted to the lowest level.

(F) Effects of the Invention The present invention eliminates the buzz caused by the AM-PM modulation component that occurs when AM-modulating a composite video signal to form an RF signal by applying the composite video signal to the audio transmission system. The configuration is extremely simple. FIG. 5 shows a comparison between one without the circuit of the present invention (characteristic P) and one with the circuit of the present invention (characteristic Q) when audio multiplex television broadcasting is received by an intercarrier receiver. As is clear from the figure, the audio S/N improved by about 8 dB, and it was confirmed that there was a sufficient effect on hearing.

[Brief explanation of the drawing]

Figure 1 is a circuit configuration diagram of a conventional RF modulation circuit, Figures 2a and b are waveform diagrams explaining the generation of buzz due to AM-PM modulation, Figure 3 is a configuration diagram of the RF modulation circuit of the present invention, and Figure 4. a, b, and c are waveform diagrams explaining the generation of the cancellation signal, and FIG. 5 is a characteristic diagram showing the effect of the present invention over the conventional example. Explanation of main drawing numbers, 1, 2...first and second input terminals, 3...video signal modulator, 5...audio signal transmission system, 6...mixing circuit, 9...applying circuit.

Claims (1)

[Claims] 1. A first input terminal for inputting a composite video signal containing video information, a second input terminal for inputting an audio signal containing audio information corresponding to the video information, and a second input terminal for inputting an audio signal containing audio information corresponding to the video information; a video signal modulator that receives the composite video signal from one input terminal and outputs an unnecessary AM-PM conversion component together with a modulated video signal obtained by amplitude modulating a first carrier wave with the composite video signal; an audio signal transmission system having an audio signal modulator that receives the audio signal from an input terminal and modulates a second carrier wave with the audio signal, and mixes the output of the video signal modulator and the output of the audio signal transmission system. In the RF modulation circuit, the RF modulation circuit includes a mixing circuit that cancels unnecessary audio components derived based on the AM-PM conversion component when the output of the RF modulation circuit is applied to an intercarrier television receiver. An RF modulation circuit comprising: a circuit for applying the composite video signal from a first input terminal to the audio signal transmission system. 2. The RF according to claim 1, wherein the imparting circuit is a differentiating circuit connected between the first input terminal and the audio signal modulator and differentiating the composite video signal. Modulation circuit. 3. Claim 2, wherein the differentiating circuit is comprised of a capacitor connected between the first input terminal and the audio signal transmission system, and a resistor within the audio signal transmission system. RF modulation circuit described in section.