JPS5816392B2 - Modification adapter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses a character demodulator to assemble code or pattern signals of characters, figures, etc. that are multiplexed and sent during the vertical blanking period of a television signal into a character signal, and converts this into a character signal. The object of the present invention is to provide a teletext receiving adapter for displaying images on a cathode ray tube of a normal television receiver, which is simple and does not cause a risk of deterioration in image quality.

In order to display character signals sent superimposed during the vertical blanking period of broadcast television radio waves on the video screen of a normal cathode ray tube, it has conventionally been necessary to use the codes of characters, figures, etc. sent during the vertical blanking period. A method is used in which a character demodulator assembles a character signal, mixes it with a detected video signal, modulates it again, and receives it with a general television receiver.

In this method, the video signal passes through a detection circuit, so there is a delay in the phase of the color signal, a decrease in the amplitude, or an increase in S/N, DG,
It has the disadvantage that image quality tends to deteriorate due to DP characteristics and the like.

The present invention focuses on the fact that characters can be displayed on a cathode ray tube by switching an intermediate frequency signal with a character signal, and converts the video signal in the state of an intermediate frequency signal into a character signal without passing it through a detection circuit. By switching with
This provides an adapter that does not require amplification, modulation, etc.

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a waveform diagram thereof.

In Fig. 1, television radio waves received by an antenna 1 are frequency-converted by a tuner 2, selectively amplified by a wideband intermediate frequency amplification circuit 3, a part of the intermediate frequency signal is added to a video detection circuit 41/c, and its output is The color subcarrier oscillation circuit 5 of 3.58 MH4 is controlled to obtain a pulse signal as shown in FIG. .

This pulse signal C is given an appropriate delay and applied to the switching circuit 7, and the output of the intermediate frequency amplification circuit 3 is shown in FIG.
By switching the video intermediate frequency signal as shown in
The waveform is as shown in the figure.

During this switching, if the audio carrier signal included in the intermediate frequency signal is also switched at the same time, a buzzing sound will be produced when this signal is received.

This buzzing noise can be solved by adding a series resonant circuit tuned to the audio carrier wave to the intermediate frequency transmission line of the switching circuit 7, or by connecting a parallel vibration circuit 8 tuned to the audio carrier wave between the intermediate frequency transmission line and ground. This can be prevented.

Since the audio signal is usually FM modulated, generation of buzz can be prevented unless there is extreme attenuation.

The intermediate frequency signal thus switched is applied to a frequency conversion circuit 10 together with the output of the local oscillator 9 for frequency conversion, and a VHF television radio wave is obtained at an output terminal 12 via a band amplifier 11.

When this radio wave is received by another television receiver, the video signal during the switching period is white, so the brightness increases only in that part on the receiving screen, which becomes the shape of letters. displayed on the screen.

In the embodiment shown in FIG. 1 described above, when frequency converting the television signal received by the tuner 2 to the same channel at the output terminal 12, the output of the local oscillator of the frequency conversion stage of the tuner 20 is directly transmitted to the local oscillator 90 of the subsequent stage.
It can be used instead, and by doing so, the circuit can be omitted and beat interference can be reduced, so it is effective for receiving a specific channel.

As described above, the present invention switches a received television signal at an intermediate frequency and adds a character signal to it, and unlike the conventional method in which the character signal is mixed with a video signal detected by image detection, the video signal is Since there is no need to demodulate the video signal, the circuit can be simplified and the deterioration in image quality caused by the demodulation of the video signal can be eliminated.

Further, since an output signal of the frequency of television radio waves is obtained at the output terminal 12, this signal can be received as it is with any other receiver and a screen with characters displayed on the screen can be viewed.

Effects such as this can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is an explanatory diagram of its operation. 3...Intermediate frequency amplification circuit, 6...Character demodulator, 7.
...Switching circuit, 10...Frequency conversion circuit.

Claims (1)

[Claims] 1. A tuner that receives television radio waves on which teletext signals are multiplexed during the vertical blanking period and converts the frequency into intermediate frequency signals, a video detection circuit that detects a part of this intermediate frequency signal, and its detection output. A character demodulator that extracts a teletext signal in a vertical blanking period from a teletext signal and generates a pulse signal present in a horizontal scanning period for displaying characters corresponding to the teletext signal; A teletext receiving adapter comprising a switching circuit that switches signals, and a frequency conversion circuit that mixes the switched intermediate frequency signal and local oscillation output to produce an output signal at the frequency of television radio waves.