JPS62219877A - External synchronizing circuit - Google Patents

Abstract

PURPOSE:To form stable color video signals even when horizontal synchronizing period is disordered by providing a color subcarrier selector switch that selects fixed oscillation output instead of a dividing output of a master oscillation output when a burst lack detection output is generated. CONSTITUTION:In the case where external burst signals are not obtainable, a fixed oscillation output of a fixed oscillation circuit 15 is derived as a color subcarrier instead of dividing output. A crystal oscillation circuit having stable and accurate oscillation frequency (3.58MHz) is used for the fixed oscillation circuit 15. A color subcarrier selector switch 11 makes a burst lack detection output a switching control input. Accordingly, when burst signals are not inputted, the stable color subcarrier unrelated to the variation of synchronizing signals is derived from the external synchronizing circuit, and a color video camera of slave side derives stable modulated color signals unrelated to the variation of horizontal synchronizing signal.

Description

[Detailed description of the invention] (b) Industrial application fields The present invention relates to an external synchronization circuit for a video camera.

101 Conventional technology An external synchronization circuit that realizes superimposition of color video signals such as superimposition is, for example, disclosed in Utility Model Application Publication No. 54-14119.
As disclosed in the publication, a burst signal and vertical/horizontal synchronization signals are used as external inputs. However, if the video signal on the master side is a black and white video signal, no burst signal is input. For example, the external synchronization circuit of the color video camera WV-OD120 manufactured by Matsushita Electric Industrial ■ is as follows.
A multiplied output of a color subcarrier/SC is formed by a PLL circuit that uses a horizontal synchronization signal as a reference input, and a color subcarrier/SC whose phase is locked to the horizontal synchronization signal is formed. However, if a phase variation occurs in the horizontal synchronization signal of the black and white video signal, the color subcarrier frequency of the slave camera may deviate from 5.58 MHz, and there is a risk that it will not be possible to form a correct color image. For this reason, the video signal on the master side has traditionally been required to be sufficiently stable and accurate.

4. Means for Solving the Problems Therefore, the present invention provides a first method for deriving a master oscillation output at a frequency corresponding to multiplication of color subcarriers by controlling the oscillation frequency of a variable oscillation circuit using an external horizontal synchronization signal as a reference input. a PLL loop; a second PLL circuit that uses an external burst signal as a reference input to control the oscillation state of the variable oscillation circuit; a burst loss detection circuit that detects loss of the external burst signal and generates a burst loss detection output; According to the present invention, the external synchronization circuit includes a color subcarrier selection switch that selects a fixed oscillation output instead of the frequency-divided output of the master oscillation output when a burst loss detection output is generated. For example, when a monochrome video signal without a burst signal is input as an external input, the color subcarrier is used as a fixed excavation output.

(f) Example Hereinafter, the present invention will be explained according to an illustrative example. As is clear from the circuit block diagram, this embodiment uses an external horizontal synchronization signal (FB), an external horizontal synchronization signal (EE(), and an external vertical synchronization signal (EV) as reference inputs). A burst gate pulse is generated from the burst gate pulse generation circuit (1) which inputs the signal (EH). During this gate pulse generation period, the second phase comparator circuit @#
F(2) is the external burst signal (gB) and the second frequency dividing circuit (
3) The phase comparison output of 0 is input to the low-pass filter (5) via the roof selection switch (4). This low-pass output is input to a voltage-controlled variable oscillation circuit (6) to derive an oscillation output four times the color subcarrier frequency /so. The closed loop servo circuit described above forms a second PLL circuit.

On the other hand, this oscillation output is inputted into the counting input of the horizontal synchronizing signal generating circuit (7), and is reset by the external horizontal synchronizing signal (EH) to form the internal horizontal synchronizing signal 1. Vertical synchronization signal generation circuit that uses this internal horizontal synchronization signal circle as a counting input (
8) forms the internal horizontal sync signal closed while being reset to the external vertical sync signal (EV). On the other hand, the color subcarrier generation circuit (9) inputting the oscillation output is derived via the reset subcarrier selection switch (Ill) generated by the reset pulse generation circuit αυ whose reference input is the internal horizontal synchronization signal.

Further, when only an external horizontal synchronizing signal (EH) and an external vertical synchronizing signal (EV) are input from the black and white video camera 2 on the i-star side, the burst signal detection circuit α2 detects the omission of the external burst signal. This paste missing detection output switches the roof selection switch (4) to form a first PLL servo loop. Therefore, the first phase comparison circuit α3 compares the phases of the external horizontal synchronization signal (EH) and the divided output corresponding to the horizontal synchronization obtained from the second frequency division circuit αφ, and uses the comparison output as the loop selection switch ( 4) and is input to a low-pass filter (5). Therefore, the oscillation is controlled by the hello bus filter output of the variable oscillation circuit (6), and by feeding back the oscillation output to the second frequency dividing circuit I, an external horizontal synchronizing signal (II!: H
) is derived from an oscillation output equivalent to four times the color subcarrier frequency, which is phase-synchronized with the color subcarrier frequency.

The horizontal synchronization signal generation circuit (7) inputs this oscillation output and derives an internal horizontal synchronization signal plane while being reset by an external horizontal synchronization signal (11iiH), and the vertical synchronization signal generation circuit (8) This internal horizontal synchronizing signal plane is input and the internal horizontal synchronizing signal M is derived while being reset by the external vertical synchronizing signal (mV).
The signal generating circuit (9) divides the oscillation output by A to derive a frequency-divided output having a frequency corresponding to the color sub medium signal.

The feature of this embodiment is that when an external burst signal cannot be obtained, the fixed oscillation output of the fixed oscillation circuit a3 is derived as a color subcarrier instead of this frequency-divided output.
The fixed oscillation circuit a31 has an oscillation frequency (!1.58MH
Since 2) is accurate and stable, it is assumed that a Listal oscillation circuit is used. As mentioned above, in order to select the fixed oscillation output instead of the frequency divided output, the color subcarrier selection switch a1
1 uses the burst loss detection output as the switching control input.

Therefore, when a burst signal is not input, a stable color subcarrier is derived from this external synchronization circuit that is unrelated to fluctuations in the synchronization signal, and the color video camera on the slave side derives a stable color subcarrier that is unrelated to fluctuations in the horizontal synchronization signal. Derive a modulated color signal. Note that if an internal synchronization signal is formed based on a fixed oscillation output without inputting the oscillation output of the PLL circuit, there is a risk that the synchronization signal will not be derived correctly if the generation timing of the synchronization signal and the reset timing deviate. . Therefore, in order to form a synchronization signal, the first P
The LL circuit becomes essential.

(l) According to the present invention, when the master side video camera derives a monochrome video signal, it is possible to form a stable color video signal even if the horizontal synchronization period is disturbed, and the effect is great. It is.

[Brief explanation of drawings]

The figure reveals a circuit block diagram illustrating one embodiment of the invention. nz-... Burst detection circuit, (4)... Loop selection switch, (111... Color subcarrier selection switch, OJ... Fixed launch circuit

Claims (1)

[Claims] (1) A first PLL loop that derives a master oscillation output at a frequency equivalent to multiplication of the color subcarrier by controlling the oscillation state of the variable oscillation circuit using an external horizontal synchronization signal as a reference input, and the above-mentioned external burst signal as a reference input. a second PLL loop that controls the oscillation state of the variable oscillation circuit; a burst detection circuit that detects the loss of the external burst signal and generates a burst loss detection output; and a second PLL loop that replaces the second PLL loop when the burst loss detection output is generated. 1st
External synchronization comprising a loop selection switch that selectively forms a PLL loop, and a color subcarrier selection switch that selects a fixed oscillation output instead of the frequency-divided output of the master oscillation output when the burst loss detection output is generated. circuit.