JPH0112457Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a frequency conversion and burst emphasis circuit useful for converting the frequency of a carrier color signal and emphasizing a burst signal.

[Background technology and its problems]

Conventionally, when recording a video signal with a magnetic recording/reproducing device (VTR), etc., the signal is separated into a luminance signal and a carrier color signal, and the luminance signal is frequency modulated, while the carrier color signal is converted to a lower frequency. .

Fig. 1 shows a frequency conversion block circuit diagram of a carrier color signal.The carrier color signal separated from the video signal is first set to an appropriate amplitude value by a gain control circuit 1, and then converted to a coupling capacitor. The signal is input to the frequency conversion circuit 2 via C, and is converted to a low frequency by mixing it with a signal for frequency conversion. Further, during reproduction, the frequency is converted by a similar circuit block to the frequency of the original carrier color signal.

In this case, by directly connecting the gain control circuit 1 to the frequency conversion circuit 2 and omitting the coupling capacitor C, it is possible to reduce the number of external parts and pins of the circuit, but Therefore, when fluctuations in the DC level occur between the two circuits 1 and 2, the dynamic range decreases,
At the same time, there is a problem in that carrier signal leakage increases.

[Purpose of invention]

This idea solves the above-mentioned problems by creating a frequency conversion circuit that can control gain, making it easier to integrate into an IC, and reducing the number of external capacitors and pins.

[Summary of the idea]

Converting the frequency of a carrier color signal in which a color burst signal is inserted into a part of the horizontal blanking period,
and a differential amplifier whose input is supplied with the carrier color signal, and a pair of outputs of the differential amplifier, as a circuit that makes the gain for at least the color burst signal different from the gain for the carrier color signal during the scanning period. A current is converted into a voltage by a PN junction element, a double balanced differential amplifier is configured to receive this voltage, and the current source of the double balanced differential amplifier is driven by a signal for frequency conversion. , the circuit configuration is such that the current value of the current source of the differential amplifier is different at least only during the color burst signal period, and by omitting the gain control circuit, the number of pins and external parts can be reduced when integrated into an IC. This reduces the

〔Example〕

Figure 2 shows the frequency conversion and burst emphasis circuit of this invention, which supplies a carrier color signal and a signal for frequency conversion, and performs frequency conversion by forming a difference (sum) frequency between the two. , the gain can be controlled.

In this figure, the outputs of transistors Q ₁ , Q ₂ , Q ₃ forming the first differential amplifier and transistors Q ₄ , Q ₅ , Q ₆ forming the second differential amplifier are as follows.
They are connected to common load resistors R _L1 and R _L2 so as to have opposite phases to each other, forming a double balanced differential amplifier. A frequency conversion signal V ₀ (hereinafter referred to as a carrier signal) is supplied to the transistors Q ₃ and Q ₆ of this double-balanced differential amplifier, and a current source I ₀ is connected to their common emitter.

Furthermore, diode-connected transistors Q ₉ and Q ₁₀ are connected to the common input terminal of the double-balanced differential amplifier.
The output of a third differential amplifier constituted by transistors Q ₇ and Q ₈ whose load is connected to
A carrier color signal V _C is supplied to the bases of the transistors Q ₇ and Q ₈ , and a variable current source I _C is connected to their common emitters.

As is well known, this circuit is a four-quadrant multiplication circuit, so from the output terminal _T0 ,
A signal of V _0ut =K・Vc・V ₀ is output. (K indicates a scale factor) Therefore, the sum and difference frequency of the frequency _C of the carrier color signal Vc and the frequency ₀ of the carrier signal _V0 are generated in the output signal, so when converting to reduction, the difference signal is generated. When converting to a high frequency range, the frequency can be converted by extracting the sum signal using a filter or the like.

Note that the third differential amplifier includes transistors Q ₉ ,
Transistor Q ₇ whose load is the PN junction element of Q ₁₀ ,
Since _Q8 is a so-called Gilbert circuit, the linearity of the carrier color signal Vc input to this circuit is improved, and since it is connected to the variable current source Ic via the emitter resistor re, this In the frequency conversion and burst emphasis circuit, the conversion gain G can be made variable in the ratio of Ic/ _I0 by changing the variable current source Ic.

Therefore, as shown in FIG.
This eliminates the need to provide an IC, and reduces the number of pins and external parts when integrated into an IC.

By the way, in the present invention, as shown in Fig. 3, another current source ΔIc is switched to the variable current source Ic in Fig. 2.
The switch SW is configured to be inserted in parallel by the SW, and as described later, the switch SW is opened and closed by a control pulse whose pulse width is the color burst signal period,
It is configured to simultaneously improve the S/N of the converted carrier color signal.

[Application example]

FIG. 4 shows a circuit block when the frequency conversion and burst emphasis circuit of FIG. 2 is applied to frequency conversion of a carrier color signal during recording and reproduction of a video signal. During recording, the decomposed video signal S is A synchronization separation circuit 11 separates only the synchronization signal, and a burst flag generator 12 generates a control pulse having a pulse width corresponding to the color burst signal period from the synchronization signal. On the other hand, the video signal is separated into a luminance signal V _Y and a carrier color signal Vc by the Y/C separation circuit 13, and the luminance signal V _Y is sent to the FM modulator 1.
4 is input.

The carrier color signal Vc is input together with the carrier signal V ₀ (frequency 4.27MHz) to the frequency conversion and burst emphasis circuit 15 shown in FIG. The variable current source Ic of the frequency conversion and burst emphasis circuit 15 is controlled by the control pulse from the flag generator 12, and the gain of the frequency conversion and burst emphasis circuit 15 is increased only during the color burst signal period. Then, the color burst signal converted to the low frequency is emphasized and the S/N ratio is improved.

Similarly, during reproduction, a control pulse with a pulse width corresponding to the color burst signal period is formed by the synchronization separation circuit 21 and the burst flag generator 22, and a control pulse is separated by the Y/C separation circuit 23. The variable current source Ic of the frequency conversion circuit 25 to which the carrier color signal Vc and the carrier signal _V0 of the range are input is controlled.

On the reproduction side, the control pulse is used to reduce the conversion gain of the frequency conversion circuit 25 to correct color jitter.
This ensures the operation of the APC loop.

Note that 16 and 26 are filters for extracting the conversion frequency, 17 and 27 are adder circuits, and 24 is a luminance signal.
The demodulation circuit for V _Y is shown.

[Effect of idea]

This idea drives the current source of the double-balanced differential amplifier with a carrier signal as described above, and
The differential amplifier connected to the input terminal of this double-balanced differential amplifier is driven by the carrier color signal, and the variable current source connected to its common emitter is set to a different value only during the color burst signal period to obtain the conversion gain. Since it is configured to be variable, there is no need to provide a separate gain control circuit, especially when converting the frequency of the carrier color signal, which has the advantage of reducing the number of pins and external parts when implementing an IC. have

[Brief explanation of the drawing]

FIG. 1 is a conventional frequency conversion block diagram for a carrier color signal, FIG. 2 is a frequency conversion and burst emphasis circuit diagram of this invention, and FIG. 3 is a circuit diagram showing an embodiment for controlling a variable current source. Figure 4 shows the frequency conversion and burst emphasis circuit of this invention for a VTR.
FIG. 2 is a block circuit diagram applied to a recording and reproducing circuit of the present invention. In the figure, Q ₁ to _{Q 6} are transistors forming a double-balanced differential amplifier, Q ₇ to _{Q 10} are transistors forming a differential amplifier, Ic is a variable current source, Vc is a carrier color signal, and V ₀ is a carrier. Show signal.

Claims (1)

[Scope of utility model registration request] Converting the frequency of a carrier color signal in which a color burst signal is inserted into a part of the horizontal blanking period,
and a differential amplifier whose input is supplied with the carrier color signal, and a pair of the differential amplifiers, as a circuit that makes the conversion gain for at least the color burst signal different from the conversion gain for the carrier color signal during the scanning period. Converts the output current of 1. A frequency conversion and burst emphasizing circuit, characterized in that the frequency conversion and burst emphasizing circuit is configured to be driven by a current source of the differential amplifier, and to set a current value of a current source of the differential amplifier to a different value only during at least a color burst signal period.