JPH03296368A - Horizontal synchronization changeover circuit - Google Patents

Abstract

PURPOSE:To eliminate distortion of a video image by providing a changeover switch through which a composite video signal passing through a low pass filter is selected for period other than a gate pulse and inputted to a synchronizing separator circuit and a composite video signal before passing through the low pass filter is inputted as it is to the synchronizing separator circuit. CONSTITUTION:An analog switch D selects a signal S2 after passing through a low pass filter B or a signal S1 before passing through the low pass filter B based on a gate pulse VG. A signal S3 is obtained at a contact by the changeover and the signal S3 is inputted to a poststage synchronizing separator circuit. The signal S3 inputted to the synchronizing separator circuit is a signal in which the signal S2 after passing through the low pass filter B for a peripheral period (period of gate pulse VG) of a horizontal synchronizing pulse is replaced with the signal S1 before passing through the low pass filter B. Thus, synchronizing separation is implemented in the synchronizing separator circuit in a way that the waveform of a horizontal synchronizing signal included in the composite video signal S1 is not distorted.

Description

[Detailed description of the invention] Ll rise! On the moon! The present invention relates to a video reproducing device, and more particularly to a synchronization circuit for a video reproducing device such as a television receiver or a video monitor, which performs synchronization separation within the device from an input composite video signal.

Shimika 1! When extracting a synchronization signal from a composite video signal in a television receiver or the like, if there is noise that can become a pseudo synchronization signal in a portion other than the synchronization signal period, a malfunction will occur during synchronization separation. Therefore, in order to avoid the influence of such noise, the signal is usually passed through a low-pass filter as shown in Figure 4 with a cut-off layer wave number of about 500 KHz to remove frequency components higher than a certain frequency. . for example,
When the input composite video signal has a waveform as shown in FIG. 5(a), the composite video signal passed through the low-pass filter becomes as shown in FIGS. 5(b) and 5(c). The composite video signal passed through this low-pass filter is input to a synchronization separation circuit, and a horizontal synchronization signal is extracted based on a predetermined threshold as shown in FIG. 5(C).

The horizontal synchronizing signal included in the composite video signal has a pulse-like waveform and normally contains frequency components in a wide range, but by passing it through the low-pass filter, the high frequency components are removed, and the ), the horizontal synchronization signal causes a phase lag (waveform rounding). Incidentally, the level of the composite video signal may fluctuate due to a sudden change in the average video level (APL), and in this case, the threshold value (slice level) of the synchronization separation circuit relatively fluctuates. Therefore, when the level of the composite video signal fluctuates, the timing of the horizontal synchronization signal after synchronization separation (hereinafter referred to as "horizontal synchronization output") also fluctuates. For example, in FIG. 3(a) showing the waveform of the horizontal synchronization signal portion of the composite video signal, the threshold value is (T
If the horizontal synchronization output changes from (HI) to (TH2), the horizontal synchronization output changes from (VHI) to (VH2), and the timing of the horizontal synchronization output changes.

This timing variation causes image distortion in the horizontal direction on the screen.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a horizontal synchronization switching circuit that solves these problems and makes it possible to perform synchronization separation without damaging the waveform of a horizontal synchronization signal included in a composite video signal.

In order to achieve the above-mentioned object, the horizontal synchronization switching circuit of the present invention has a horizontal synchronization switching circuit in which a composite video signal is passed through a low-pass filter and then inputted to a synchronization separation circuit to separate vertical and horizontal synchronization signals. a gate pulse generation circuit that generates a gate pulse whose pulse period is a period that is within the line erasing period and includes the period of the horizontal synchronizing pulse; and a low-pass filter that generates a gate pulse based on the gate pulse except for the period of the gate pulse. a selector switch that selects a composite video signal that has passed through and inputs it to the sync separation circuit, and inputs the composite video signal that has passed through the low-pass filter as it is to the sync separation circuit during the gate pulse period. The structure is as follows.

Work-1- According to such a configuration, a gate pulse for horizontal synchronization signal switching is generated, and based on this gate pulse, the composite video signal that has passed through the low-pass filter and the composite video signal that has not passed through the low-pass filter are switched. It can be changed with a changeover switch. As a result, the horizontal synchronization signal portion of the composite video signal that has passed through the low-pass filter is replaced with the signal before passing through the low-pass filter, and the signal obtained by this replacement is input to the synchronization separation circuit to perform synchronization separation. .

1 case of “Remains” An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing a horizontal synchronous switching circuit embodying the present invention together with a shoulder circuit, FIG. 2 is a waveform diagram for explaining the operation of the horizontal synchronous switching circuit of this embodiment, and FIG. FIG. 2 is a diagram showing the relationship between the threshold value and the horizontal synchronization output in the subsequent stage synchronization separation circuit.

As shown in FIG. 1, the horizontal synchronization switching circuit (10) of this embodiment is a gate pulse generation circuit (10) that inputs a horizontal synchronization signal (VH) and outputs a gate pulse (Va) for switching the horizontal synchronization signal. and an analog switch (D) that selects one of the contacts (di) and (d2) based on the gate pulse (Va) and connects it to the contact (d3). And analog switch (D)
The contact (dl) is connected to the input end of the low-pass filter (B), the contact (d2) is connected to the output end of the low-pass filter (B), and the contact (d3) is connected to the input end of the subsequent synchronous separation circuit. has been done. Here, analog switch (D
) connects the contact (d3) to the contact (d2) during the period when the gate pulse (Vo) is at low level, and connects the contact (d3) to contact (d2) during the period when the gate pulse (Vo) is at high level (
The period of the gate pulse) connects the contact (d3) to the contact (dl).

In FIG. 1, the composite video signal (Sl) that has passed through the video signal output buffer (A) is passed through the low-pass filter (B).
and is guided to the contact (dl) of the analog switch (D).

The low-pass filter (B) is the same as that conventionally used, and is composed of a resistor and a capacitor as shown in FIG. 4, and its pass band is about 0 to 500 KHz. The composite video signal (S2) after passing through this low-pass filter (B) is guided to the contact point (d2) of the analog switch (D).

The gate pulse generating circuit (E) receives the horizontal synchronizing signal (VH) as described above and generates a gate pulse (VG) for switching the horizontal synchronizing signal as shown in FIG. 2(C). This gate pulse (Va) is a pulse wider than the pulse width of the horizontal synchronizing signal (VH), and is at a high level at least during the period of the horizontal synchronizing pulse.

However, during the horizontal scanning period (other than the horizontal blanking period), the gate pulse (Vo) is at a low level. To generate such a gate pulse, for example, a monostable multivibrator can be operated using a horizontal synchronization pulse that is one water period earlier than the horizontal synchronization pulse within the period of the gate pulse (VO) as a trigger pulse. good.

The analog switch (D) receives the gate pulse (V.

), the signal (S2) shown in FIG. 2(b) after passing through the low-pass filter (B) and the signal (Sl) shown in FIG. 2(a) before passing through the low-pass filter (B) are switched. As a result of this switching, the contact point (d3) becomes as shown in Fig. 2 (e).
A signal (S3) as shown in is obtained, and this signal (S3)
is input to the subsequent synchronous separation circuit.

As can be seen from the above explanation of the gate pulse (VO) and from FIG.
3) is the signal (S2) that has passed through the low-pass filter (B).
), the peripheral period of the horizontal synchronization pulse (gate pulse (V
This is a signal in which only the period G) is replaced with the signal (Sl) before passing through the low-pass filter (B).

Therefore, as shown in FIG. 3(b), the synchronization separation circuit performs synchronization separation without damaging the waveform of the horizontal synchronization signal included in the composite video signal (S+). That is, since the horizontal synchronization signal included in the composite video signal (S3) input to the synchronization separation circuit has no waveform rounding, the composite video signal (S3) is
The threshold value of the synchronization separation circuit changes relatively due to the change in the level of S3). For example, even if the threshold value changes from (THl) to (TH2) as shown in FIG. 3(b), the horizontal The synchronization output becomes (VHl) (VH2), and the timing of the horizontal synchronization output does not vary and remains constant. Therefore, according to the horizontal synchronization switching circuit of this embodiment, the fluctuation in the timing of the horizontal synchronization output that has conventionally occurred (see FIG.
a)) is improved. Note that in the horizontal scanning period, since the signal that has passed through the low-pass filter (B) is used as before, the effect of avoiding the influence of noise is maintained.

As explained above, according to the present invention, synchronization separation is performed without impairing the waveform of the horizontal synchronization signal included in the composite video signal, and the level of the composite video signal can be changed due to sudden changes in the average video level, etc. Even if there is a fluctuation, the timing of the horizontal synchronization output of the synchronization separation synchronization circuit does not fluctuate and remains stable. Therefore, it is possible to improve the distortion of images in the horizontal direction on the screen, which conventionally occurs due to fluctuations in the timing of horizontal synchronization output.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a horizontal synchronization switching circuit embodying the present invention together with peripheral circuits, FIG. 2 is a waveform diagram for explaining the operation of the horizontal synchronization switching circuit of this embodiment, and FIG. FIG. 4 is a circuit diagram of a low-pass filter, and FIG. 5 is a waveform diagram of a composite video signal before and after passing through the low-pass filter. (10)...Horizontal synchronization switching circuit. (B)...Low pass filter. (D)...Analog switch (selector switch). (E)...Gate pulse generation circuit. (Sl)...Composite video signal before passing through a low-pass filter. (S2)...Composite video signal after passing through a low-pass filter. (S3)...Composite video signal after passing through the horizontal synchronization switching circuit. (Va)...Gate pulse. (VH)...Horizontal synchronization signal.

Claims (1)

[Claims] (1) In a synchronization circuit that passes a composite video signal through a low-pass filter and then inputs it to a synchronization separation circuit to separate vertical and horizontal synchronization signals, the period within the horizontal blanking period and including the horizontal synchronization pulse period a gate pulse generation circuit that generates a gate pulse whose period is a pulse period; and, based on the gate pulse, select a composite video signal that has passed through the low-pass filter outside the period of the gate pulse and input it to the sync separation circuit. and a changeover switch for inputting the composite video signal before passing through the low-pass filter as it is to the synchronization separation circuit during the period of the gate pulse.