JPS6249662B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency modulation circuit suitable for integrating a video signal recording circuit of a home video tape recorder.

A conventional integrated video signal recording circuit has a problem in that it is not possible to insert a filter for removing unnecessary high frequency components generated by clipping the video signal. This will be explained in detail below using figures.

FIG. 1 shows part of a conventional integrated video signal recording circuit. 1 is an input terminal, 2 is a clamp circuit, 3 is an FM modulator, 4 is a clip circuit, 9 is an output terminal, and 10 is an AGC circuit, and the area within the broken line indicates the integration range. The operation of this circuit will be explained below. A video signal controlled to a constant amplitude is passed through the input terminal 1 and the AGC circuit 10 to the clamp capacitor 11.
The synchronizing signal is inputted to the clamp circuit 2 via the synchronous signal, and the leading end of the synchronizing signal is fixed at a constant DC potential, and then inputted to the base of the transistor 5. Since the oscillation frequency of the FM modulator 3 is proportional to the collector current of the transistor 5, the output terminal 9 receives the signal modulated by the video signal.
You can get FM signal.

In addition, a pre-emphasis circuit is configured with a transistor 5, a capacitor 11, and resistors 7 and 8.
A video signal with an overshoot in the high frequency range is obtained at the collector of the transistor 5, and both the positive and negative sides are clipped by the clip circuit 4, thereby preventing overmodulation.

The oscillation frequency f of the FM modulator 3 can be expressed as f=kI ₀ /C (k is a constant), where I ₀ is the collector current of the transistor 5 and C is the capacitance of the external capacitor 15. collector current
The resistor 7 that determines I ₀ needs to be an external resistor in consideration of temperature characteristics. Although there is an advantage in that this extraction terminal is also used as a pre-emphasis circuit, it is not possible to insert a filter to remove unnecessary harmonic components generated when the video signal is clipped by the clip circuit 4.
These unnecessary harmonic components, especially in video signal recording circuits with low FM allocation, cause a folded spectrum in the FM signal, which causes moire etc. to appear on the screen when played back, causing a loss of image quality. . Therefore, it is a big problem that a filter for removing unnecessary harmonic components cannot be inserted.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-described drawbacks of the prior art and to provide an FM modulation circuit using an integrated circuit in which a filter for removing unnecessary harmonic components can be inserted through one pin.

In order to achieve the above object, the present invention uses the emitter of the transistor to which pre-emphasis and clipped video signals are supplied as an external terminal, so that the filter for removing unnecessary harmonic components can be used as an extraction terminal for other functions. Connect to.

FIG. 2 shows a block diagram of an embodiment of the present invention. Blocks or elements with the same symbols as in FIG.
shall mean blocks or elements having similar functions. 12 constitutes a pre-emphasis circuit together with a capacitor 11 and a resistor 16, and the area within the broken line is the integration range. The signal flow will be explained using FIG.

A video signal whose amplitude is controlled to be constant via an input terminal 1 and an AGC circuit 10 is input to a clamp circuit 2 via a clamping capacitor 13, where the leading end of the synchronization signal is clamped and input to a pre-emphasis circuit 12. The video signal whose high frequency has been emphasized by the pre-emphasis circuit 12 is further clipped by a clip circuit 4 for both positive and negative overshoots, and is input to the base of a transistor 5. FM
The oscillation frequency of the modulator 3 is proportional to the collector current of the transistor 5, as in the conventional circuit. The video signal at the base of transistor 5 contains unnecessary harmonics generated by clipping, but they are removed by a band rejection filter consisting of capacitor 13 and inductance 14 connected to the emitter of transistor 5, and the harmonic components are removed. does not become a modulated wave.

As with the conventional circuit, the resistor 7 needs to be an external resistor in consideration of temperature characteristics, and this lead terminal can be used for connecting a filter for removing unnecessary harmonic components as shown in the figure.

Therefore, according to the present invention, it is possible to insert a filter for removing unnecessary harmonic components generated by clipping the video signal by increasing only one pin overall.

As described above, according to the present invention, it is possible to insert a filter for removing unnecessary harmonic components with an increase of one pin compared to the conventional circuit.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional circuit.
FIG. 2 is a block diagram showing one embodiment of the present invention. 5...Transistor, 13...Capacitor, 1
4...Inductance.

Claims (1)

[Claims] 1. A pre-emphasis circuit that receives a video signal and outputs a pre-emphasized video signal, a clip circuit connected to the pre-emphasis circuit, a transistor whose base end is connected to the clip circuit, and a transistor connected to the collector of the transistor. 1. A frequency modulation circuit, characterized in that an FM modulation circuit is provided within an integrated circuit, and a band rejection filter is connected to the emitter of the transistor via an external terminal of the integrated circuit.