JPS5945702A - Video signal modulation circuit - Google Patents

Abstract

PURPOSE:To improve the general purpose properties of an RF modulator IC and to obtain inexpensive product, by adjusting externally the modulation of a video mdulation circuit of a VTR, so as to cope easily with different modulating modes such as positive and negative modulation. CONSTITUTION:A video signal is clamped to a sink chip level at a clamp circuit 4 and applied to a base of a transistor (TR)8 of a balanced modulation circuit 6. A bias input voltage determined by a position of a variable terminal of a variable resistor 24 is impressed to the base of a TR10. When a carrier input is given from a signal source 33 between a base of TRs 26, 32 and a base of TRs 28, 30, a modulation output having a modulation determined by said bias input is extracted from a terminal 38. The output having a desired modulation is obtained by changing the variable resistor 24.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal modulation circuit, and more particularly to a modulation circuit that can select and set negative modulation or positive modulation, and can adjust the degree of modulation.

Generally, the R of home video tape recorder (V”FR)
In the video modulation circuit incorporated in the F (high frequency) modulator IC, the degree of modulation is fixed, and its value can also be adjusted from the outside. 1. It does not have an eel configuration. For example, the video modulation form of the Japanese system is negative modulation, and the degree of modulation is set at around 75% of the total amount. Therefore, if it is necessary to adjust the modulation degree, the video modulation circuit setting δ
1 change is required.

Furthermore, when comparing the broadcasting systems of Japan, the United States, West Germany, etc. and France, the modulation formats are different, with the former having negative modulation and the latter having positive modulation.If the modulation degree is fixed, a single video modulation circuit is required. It is difficult to adapt to these modulation forms.
It is. Therefore, in manufacturing, in order to adapt to the market, it is necessary to incorporate a video modulation circuit compatible with each modulation type into the RF modulator IC as an IC, which is extremely troublesome and has drawbacks such as being expensive. there were.

The present invention provides a video signal modulation circuit that allows the degree of modulation to be adjusted externally and is configured to not only finely adjust the degree of video modulation but also to be compatible with either a negative modulation method or a positive modulation method. It is an object.

Embodiments of the invention will be described in detail with reference to the drawings. FIG. 1 shows an embodiment of a video signal modulation circuit according to the present invention. In the figure, a video signal is given to input terminal 2,
This video signal is clamped to a sync level in a clamp circuit 1-84 and then input to a balanced modulation circuit 6. This balanced modulation circuit 6 is provided with first and second transistors 8, IO, and these transistors 8.10
The emitters of are commonly connected via a resistor 12, and constant current sources 13 and 14 are individually connected between each emitter and a reference potential point.

That is, the video signal is applied to the base of the first transistor 8, while a bias adjustment circuit 16 whose bias input can be adjusted from the outside is installed at the base of the second transistor 10.

Bias adjustment time t? of this embodiment? Reference numeral 816 includes a buffer circuit 18 formed inside the IC together with the transistor IO, and a bias input terminal 20 is formed at the input terminal of this buffer circuit 18 as an external terminal of the IC. In this embodiment, this bias input terminal 2
0 is connected to a variable terminal of a variable resistor 24 connected between the voltage application terminal 22 and the reference potential point, and a desired noise input can be given by variably adjusting the position of this variable terminal. It is designed so that it can be used. A power supply voltage Vcc is applied to the voltage application terminal 22.

The collector of the first transistor 8 is connected to the emitters of a pair of transistors 26 and 28, and the collector of the second transistor 10 is also connected to the emitters of a pair of transistors 30 and 32. ing. The bases of each transistor 26.32 are connected in common, and the bases of transistors 28.30 are also connected in common, between the bases of these l-transistors 26.32 and the bases of I-transistors 28.30. teeth,
A signal source 33 that provides a balanced signal as a 1m9 wave is connected to the U.
9 has been done. Further, the collectors of the transistors 26.30 are connected in common and connected to the voltage application terminal 36 via the resistor 34, while the collectors of the transistors 28.32
An output terminal 38 is formed on the collector of the output terminal 38, and is connected to the voltage application terminal 36 via a resistor 40. Similarly to the voltage application terminal 22, the voltage Vcc is applied to the voltage application terminal 36.

The operation will be explained based on the above configuration. The power supply voltage Vcc is applied to each of the voltage application terminals 22 and 36, and the bias input terminal 2 is applied depending on the position of the variable terminal of the variable resistor 24.
When a constant bias voltage input to 0 is applied, the balanced modulation circuit 6 becomes operational. In this state, a carrier wave input is applied from the signal source 33 between the base of the transistor 26.32 and the base of the transistor 28.30,
When a video signal is applied to the input terminal 2, a modulated output is sent to the output terminal 3 based on the degree of modulation determined by the bias input.
It is taken out from 8.

Here, when setting the modulation degree arbitrarily, use the variable resistor 2
If the variable terminal No. 4 is moved to the power supply side or the reference potential point side, a desired degree of modulation can be obtained.

Further, in such a bias setting, when modulating a negative modulation method or a positive modulation method, a bias voltage Va or vb corresponding to these modulation methods is set at the bias input terminal 20 by the variable resistor 24. If the video signal shown in FIG. 2A is applied to the input terminal 2, and the bias voltage Va is set to the bias input terminal 2o, then N
A modulation form of the TSC system is obtained, and the negative modulation output shown in FIG. 2B can be taken out from the output terminal 38.

On the other hand, the video signal shown in FIG. 2A is applied to the input terminal 2,
A bias voltage vb (Vb<Va
), the SECAM modulation format is obtained,
A modulated output of positive modulation shown in FIG. 2C can be taken out from the output terminal 38.

In addition, even if each modulation form is set, by slightly moving the position of the variable terminal of the variable resistor 24 and adjusting the bias voltage, the modulation degree for each modulation form can be finely adjusted and the optimum modulation degree can be obtained. It is something that can be obtained.

In this way, it is possible to unify the video modulation circuit that was conventionally formed for each different modulation format, and it is possible to obtain each modulation format by adjusting its bias input.
An RF modulator IC compatible with all broadcasting systems such as negative modulation and positive modulation can be formed regardless of the difference in modulation format, and σL usability of the product can be obtained. Moreover,
It is extremely convenient because the modulation degree can be easily finely adjusted from the outside and the optimum modulation degree can also be easily set.

In this embodiment, the bias input is set by the variable resistor 24, but the same effect can be expected even if two or more fixed resistors are used to form a predetermined bias voltage instead of the variable resistor 24.

As explained above, according to the present invention, not only fine adjustment of the modulation degree but also different modulation forms such as negative modulation and positive modulation can be easily supported, and the versatility of the RF modulator IC can be improved. It is possible to provide products that are both cost-effective and inexpensive.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the video signal modulation circuit of the present invention, and FIG. 2 is an explanatory diagram showing its operating waveforms. 6... Balanced tunnel circuit, 8... First transistor,
lO: second transistor, 16: bias adjustment circuit. Figure 1 Figure 2

Claims (1)

[Claims] a first 1-range transistor to which a video signal is applied to the base; a second transistor whose base is connected to the emitter in common via a resistor, and whose base can be set with a desired bias; A video signal modulation circuit comprising a bias adjustment circuit that applies a bias voltage to the base of the second 1-transistor and is capable of adjusting its value.