JPH0522439B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal switching device in a character multiplexing television system, and particularly provides a signal switching device suitable for connection to a color processing circuit of a color television integrated into a semiconductor circuit. .

Text multiplex television is a system that sends an image signal different from the normal screen during the vertical retrace period in addition to the normal TV screen signal, which is stored once on the receiver side.
Based on the stored signals, the other image information mentioned above is reproduced and extracted as external primary color signals (red, blue, green). The viewer can switch between viewing a normal television screen and other image information extracted as external primary color signals. Examples of this type of text multiplex television system include Teletext in the UK and Antiop in France, and are currently undergoing practical testing in Japan.

The present invention will be explained by taking as an example the case of switching between an external primary color signal and a normal television screen signal as defined in Europe, particularly in France.

The amplitude of the external primary color signal currently specified in France is approximately
1V _pp analog input corresponds to white on the screen when everything is at maximum amplitude and black when everything is zero.

On the other hand, the signal used to reproduce a normal television screen is generally extracted from a semiconductor integrated circuit demodulator and has a voltage of 3 to 5 V _pp , and is amplified to drive a cathode ray tube.

Before explaining the present invention, a brief explanation of the blocks before and after the signal switching circuit of a character multiplex receiver will be given with reference to FIG.

A brightness signal applied to a brightness signal input terminal 1 is applied to a contrast control amplifier 2, the gain of which is controlled according to the voltage of a contrast control terminal 3, and then applied to adders 7, 8, and 9. On the other hand, the demodulated 3
The three color difference signals (R-Y, B-Y, G-Y) are sent from each input terminal 4, 5, 6 to adders 7, 8, 9, respectively.
are added to the output lines 10, 11, and 12 to obtain primary color signals (R, B, C). This output 10, 11, 1
The primary color signals of No. 2 pass through the signal switching circuit 13, are applied to output amplifiers 18, 19, and 20, and are sent to the cathode ray tube 2.
1 to drive the cathode ray tube 21.

The above is the signal flow when receiving a normal television signal, but in order to receive an external primary color signal such as a character multiplex signal, a switching signal is applied to the switching terminal 14 to switch the switching circuit 13. ,
Output amplifiers 18, 19, 20 output primary color signals (R, B, G) applied from terminals 15, 16, 17.
Just add it to.

In this case, the signals on output lines 10, 11, and 12 and the signals on terminals 15, 16, and 17 are connected so that the brightness and contrast of the screen are the same when switching between a normal TV screen and an external primary color signal. The pedestal potential and level must be kept constant.

When making such a switching device, considering the ease of designing the receiver, the normal TV screen signal input signal level of the signal switching circuit 13 and the output amplitude level of the normal TV screen signal input signal level of the signal switching circuit 13 should be set to the same level as that of a normal TV screen signal without this signal switching circuit 13. It is preferable to keep the same as in the case of a television receiver, and furthermore, it is preferable to operate with the input amplitude of the external primary color input signal unchanged.

Although omitted in FIG. 1, the signal switching circuit 1
3, a pedestal clamp circuit and a contrast control circuit are required to equalize the output pedestal potentials. As mentioned above, these are
Since the contrast of the screen must be controlled so that it does not change even when the screen is switched, it is necessary to create a signal switching device to which such a circuit can be easily installed.

As an example, it is possible to consider a means of amplifying the external primary color signals (R, B, G) applied to the terminals 15, 16, and 17 in advance to make them equal to the amplitude of a normal television signal. This method has the drawback that the dynamic range is narrow, and furthermore, it is difficult to install the contrast control circuit described above.

As another example, the contrast control amplifier circuit 2
It is also possible to match the levels of the primary color signals obtained on the output lines 10, 11, 12 with the external primary color signals applied to the terminals 15, 16, 17 in advance using a . In this method, the screen S/N ratio deteriorates due to the leakage current of the carrier component included in the demodulated color difference output.

SUMMARY OF THE INVENTION An object of the present invention is to provide a switching device in which the contrast does not change when switching screens in a receiver for text multiplex television signals.

The signal switching device according to the present invention receives the demodulated television primary color signal and the external primary color signal, supplies the demodulated television primary color signal to an attenuator, and changes the signal amplitude of the demodulated television primary color signal to the external primary color signal. The external primary color signal is connected to the input terminal of the first amplifier according to the signal amplitude of the signal, and the external primary color signal is connected to the input terminal of the second amplifier, and the outputs of the first amplifier and the second amplifier are connected. A switching circuit is provided to selectively supply the common load in response to an external switching signal, and the attenuator includes a first resistor receiving the demodulated television primary color signal at one end and a reference bias source at the other end of the attenuator. and a second resistor connected between the first and second resistors, the first amplifier having a first transistor having a base connected to the connection point of the first and second resistors, The amplifier has a second transistor at its base receiving the external primary color signal and connected to the reference bias source through a third resistor.

Next, the present invention will be explained in more detail with reference to the drawings.

FIG. 2 is a block diagram of one circuit of the signal switching circuit 13 shown in FIG. 1, that is, a portion that switches one of the three primary color signals. The circuit block in Figure 2 is 3
This constitutes the actual circuit of the signal switching circuit 13 shown in FIG.

In FIG. 2, a normal television signal is input to an input terminal 22 (corresponding to the output lines 10, 11, 12 from the adders 7, 8, 9 in FIG. 1), and an attenuator 23 and an amplifier. Switching circuit 2 through 24
Connected to one input of 7. On the other hand, the external primary color signal is input to the terminal 25 (input terminals 15, 16, 1 in Fig. 1).
7), passes through an amplifier 26, and is connected to the other input of the switching circuit 27. Switching circuit 2
The output 28 of 7 is connected to a power amplifier (output amplifiers 18, 19, 20 in FIG. 1).

Here, the attenuator 23 has an attenuation ratio selected to be equal to the amplitude of the external primary color signal that applies the signal at the terminal 22 to the terminal 25, and furthermore, the attenuator 23 has an attenuation ratio that is equal to the amplitude of the external primary color signal that applies the signal at the terminal 22 to the terminal 25.
have the same payoff.

By adopting such a configuration, the leakage of the carrier component is attenuated together with the image signal by the attenuator 23, so deterioration of the S/N ratio of the screen can be avoided, and since the input signal amplitude is reduced, it becomes easier to obtain the Danami range.

Next, one embodiment of the present invention will be specifically described using FIG. A conventional television signal is applied to input terminal 22 and is applied through resistor 29 to the base of transistor 33. Further, the base of the transistor 33 is connected to a reference bias 31 through a resistor 30. On the other hand, the external primary color signal has a terminal of 25
and connected to the base of transistor 35. The base of the transistor 35 is also connected by the reference bias 31 through the resistor 32.
Bias is given. Transistors 33, 35
Resistors 34 and 36 having the same resistance value are connected between the emitters and the contacts, respectively. One of the common emitters 37 and 38 of the bi-differentially connected transistors 37 to 40 is connected to the collector of the transistor 33, the other 39 and 40 is connected to the collector of the transistor 35, and the collectors of the transistors 38 and 40 are commonly connected and serve as a common load. 41 and an output terminal 42. The other end of the common load 41 and the collectors of the transistors 37 and 39 are bonded to a power source 43. The bases of the transistors 37 and 40 are connected to a terminal 44, and the bases of the transistors 38 and 39 are connected to a terminal 45, and the terminals 44 and 45 operate as input signal switching terminals.

By adopting the configuration of FIG. 3, when the terminal 44 potential is higher than the terminal 45 potential, an external primary color signal appears at the terminal 25, through the transistors 35 and 40, and across the common load 41. Conversely, when the potential at terminal 45 is higher than the potential at terminal 44, the normal television signal applied to terminal 22 is divided by resistors 29 and 30, applied to transistor 33, passed through transistor 38, and passed through common load 41. Appears on both ends. At this time, if the division ratio of resistors 29 and 30 is matched to the amplitude ratio of terminals 25 and 22, the signal amplitudes applied to the bases of transistors 33 and 35 will be at the same level, and since resistors 34 and 36 are equal, The signal appearing at the output terminal 42 also has the same level.

Furthermore, the DC potential appearing at the output terminal 42 is
Since the base potentials of transistors 33 and 35 are equal when they are equal, it is very easy to design a pedestal clamp circuit.

The above configuration is a direct-coupled amplifier, so
This signal switching device is suitable when integrated into a semiconductor circuit.

[Brief explanation of the drawing]

FIG. 1 is a simple block diagram illustrating the signal switching unit of a text multiplex television, FIG. 2 is a block diagram showing a signal switching device using the present invention, and FIG. 3 is an embodiment embodying the present invention. FIG. 1...Brightness signal input, 4-6...Demodulated 3
Color difference signal input, 13...Signal switching circuit, 15-1
7... External primary color signal input, 21... Braun tube,
23...Attenuator, 24, 26...Amplifier.

Claims (1)

[Claims] 1. In a signal switching device that switches and outputs a demodulated television primary color signal and an external primary color signal, the demodulated television primary color signal is supplied to an attenuator to change the signal amplitude of the demodulated television primary color signal to the external primary color signal. This is connected to the input terminal of the first amplifier according to the signal amplitude of the signal, and the external primary color signal is connected to the input terminal of the first amplifier.
a switching circuit connected to the input terminal of the amplifier, and selectively supplying the outputs of the first amplifier and the second amplifier to a common load in response to an external switching signal; and a second resistor connected between the other end of the resistor and a reference bias source; a first transistor connected to a junction of a first and second resistor, the second amplifier receiving the external primary color signal at a base and connected to the reference bias source through a third resistor; A signal switching device comprising a second transistor.