JPH0335868B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for detecting a vertical synchronization signal starting point from a television signal.

In conventional detection circuits of this type, the level of the television signal is determined using a certain level close to the pedestal level as the threshold level, a composite synchronization signal is extracted, and this is leak-integrated to detect the starting point of the vertical synchronization signal. was. However, with this circuit, the S/N of the input television signal is poor,
When ghosts and the like were present, the starting point of the vertical synchronization signal could not be detected correctly in some cases.

Figure 1a shows NTSC with good S/N and no waveform distortion.
The video signal and a part of the waveform of the vertical retrace period are shown, and the signal is binarized by level judgment using a level slightly smaller than the pedestal level Vp as the threshold level, that is, the composite synchronization signal waveform. Figure b shows the result of leakage integration of this composite synchronization signal, and figure c shows the result of leakage integration of this composite synchronization signal. At this time, if the time constant of the leakage integration is appropriately selected, the position of the vertical synchronizing signal is given for each field as shown in FIG. 1c, and the starting point to of the vertical synchronizing signal can be obtained.

On the other hand, FIGS. 2a and 3a show waveforms of a video signal and part of the vertical line period when a ghost exists in the input television signal. In Figure 2 a, the ghost amplitude ratio is 0.5 and the delay time is 12μsec.
In the case of an in-phase ghost, Fig. 3a shows the case where the ghost amplitude ratio is 0.5, the delay time is 12 μsec, and an out-of-phase ghost. If a ghost exists in this way, the pedestal level Vp for the minimum value Vmin
may vary, so even if the level is determined at a certain threshold level, a correct composite synchronization signal may not always be obtained. Here, Figure 2 a
If an in-phase ghost occurs, the composite synchronization signal waveform obtained by level judgment will be as shown in figure b, and the influence of the ghost will remain; however, if this is leak integrated, it will be as shown in figure c, and the vertical synchronization signal waveform will be as shown in figure b. starting point of the signal
to is obtained similarly.

However, when an anti-phase ghost like that shown in Figure 3a occurs, when the level is determined, the composite synchronization signal shown in Figure 3b cannot be obtained. With such a waveform, not only the starting point of the vertical synchronizing signal but also information indicating the vertical retrace period cannot be obtained. This problem is caused by attempting to reproduce the composite synchronization signal using a constant threshold level despite the DC level fluctuations caused by ghosts.

The present invention has been made in view of the above points, and
The purpose is to provide a circuit that can reliably detect the starting point of a vertical synchronizing signal even when the S/N ratio is poor or ghosts are present.

The present invention compares a signal value near a vertical synchronizing signal of an input television signal with a variable reference level, detects the time width of an output pulse of the comparing means, and detects the time width of a waveform distortion such as a ghost. The threshold level is the reference level that matches the time width from the start point of the vertical sync signal of the television signal to the next change point when there is no vertical sync signal of the input television signal, and the threshold level is around the vertical sync signal of the input television signal. It is characterized by detecting the point where it first crosses as the vertical simultaneous signal starting point.

In other words, focusing on the fact that the level of waveform distortion components such as ghosts is generally lower than the level of the main signal,
By generating a threshold level that crosses the vertical synchronization signal start point, which is the main signal, but does not cross waveform distortion components such as ghosts, and performs level judgment based on this threshold level, the vertical synchronization signal start point is It detects stably and accurately without being affected.

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 4 is a block diagram showing the configuration of a vertical synchronization signal start point detection circuit according to an embodiment of the present invention, and FIG. 5 is a signal waveform diagram showing its operation. In Fig. 4, input terminal 1 has, for example, an NTSC signal as shown in Fig. 5a.
A television signal is input. This television signal is applied to the first comparator 2 and the gate pulse generator 3, and in the comparator 2, the gate of FIG. During the pulse, it is compared with a variable reference level provided by variable reference level generator 4. And this first
The output pulse of the comparator 2 is sent to the time width detection circuit 5.

Here, the gate pulse generator 3 is configured as shown in FIG. 6, for example. In Fig. 6, input terminal 10
A television signal input to the A/D converter 11 is converted into a digital signal. This digitized input signal is compared by a first comparator 12 with the contents of a first memory 13 storing the values of past input signals, and when an input signal smaller than the past input signal appears. , the contents of the memory 13 are rewritten with the new input signal. Therefore, the minimum value is always stored in the memory 13. This minimum value is written to the second memory 14 approximately every 70 μsec, and at the same time the contents of the first memory 13 are cleared. That is, the maximum value is written into the first memory 13. In this way for example about
Detection and holding of the minimum value is repeated every 70μsec. The timing controller 15 controls the first and second memories 13 and 14. Then, the minimum value ε read from the second memory 14 is added by the adder 16 to generate a threshold level. This threshold level and the input television signal are input to the second comparator 1.
7, and the magnitude relationship is “0”,
It is output as “1” information. This “0”, “1”
The information is assumed to be "0" when the input television signal is greater than the threshold level, and "1" when the opposite is true. This "0", "1" information string is given to a pulse interval detector 18, and the appearance interval between "1"s is detected. This interval is about 60 μsec during the video signal period, but becomes about 30 μsec in the equalization pulse portion and the vertical synchronization signal portion during the vertical retrace period. Therefore, if this interval is 25 μsec or more and 35 μsec or less, it may be determined that it is a vertical retrace period. The signal indicating the vertical retrace period extracted in this way passes through a suitable delay circuit 19 and is output to the output terminal 2 as a gate pulse indicating the vicinity of the vertical synchronizing signal shown in FIG. 5b.
Extracted from 0.

Now, there is a delay time in the input television signal.
If we consider the case where an anti-phase ghost with a duration of 12 μsec and an amplitude ratio of 0.5 exists, its waveform will be as shown in FIG. 5a. In this waveform, G represents a ghost component. When the reference levels shown as l ₁ , l ₂ , l ₃ , l ₄ , l ₅ in FIG. 5a are given from the variable reference level generator 4 to such a waveform, the first comparator 2 The output pulses are as shown in FIG. ₅ , c 1 , c ₂ , c ₃ , c ₄ , and c ₅ , respectively. Among these output pulses, the time width matches the time width from the start point of the vertical synchronization signal of the television signal to the next change point in the absence of ghost-like waveform distortion (27.3 μsec for NTSC signals). c ₂ and c ₃ . The time width of the output pulse c ₁ when the reference level is l ₁ is greater than 27.3 μsec. Furthermore, the time widths of the output pulses c ₄ and _{c 5 when} the reference levels are l ₄ and l 5 are smaller than 27.3 μsec. Then, when the reference level is varied, for example, by a constant value or randomly, one of the reference levels at which the time width of the output pulse of the comparator 2 detected by the time width detection circuit 5 becomes 27.3 μsec. Alternatively, using their average value as the threshold level, the second comparator 6 detects the point where the vicinity of the vertical synchronizing signal of the input television signal first crosses this threshold level as the vertical synchronizing signal starting point, and A timing signal indicating the synchronization start point is sent to the output terminal 7.
In this case, the timing signal may be obtained by extracting the front line of the output pulse of the second comparator 6 using a differentiator or the like. Further, the first and second comparators 2 and 6 may share one comparator in a time-sharing manner.

In this way, the present invention sets a reference level at which the time width of the output pulse of the comparison means matches the time width from the start point of the vertical synchronization signal to the next change point (for example, 27.3 μsec) when there is no waveform distortion of the television signal. By determining the level as the threshold level,
Since the start point of the vertical synchronization signal is detected, if a ghost has an amplitude ratio other than 1, it will be stable no matter what the phase (positive phase, negative phase) delay time and number of ghosts are in the television signal. , and can accurately detect the vertical synchronization signal starting point. Furthermore, even when the S/N of the input television signal is poor or there are level fluctuations, the vertical synchronization signal starting point can be detected stably.

Next, another embodiment of the present invention will be described with reference to FIG. In FIG. 7, the NTSC video signal input to the input terminal 21 is sampled by the sample clock 31 from the timing circuit 29 and written into an analog memory 27 such as a CCD via the contact 26a of the changeover switch 26. It will be done. The period to be written is given by a waveform obtained by delaying the gate pulse near the vertical synchronization signal from the gate pulse generator 23 by the delay circuit 28. The waveform near the vertical synchronizing signal written in the memory 27 in this way is read out by the read clock 32 also provided from the timing circuit 29, and the comparator 22 outputs the reference level from the variable reference level generator 24, for example. FIG. 5 a First, a comparison is made with one of l ₁ , l ₂ , l ₃ , l ₄ , l ₅ , for example l ₁ . The output pulse of this comparator 22 is sent to a time width detection circuit 25. At this time, the switch 26 is switched to the contact 26b side at the same time, so that the output of the memory 27 is changed to the memory 26b side again.
7 is written. The time width detection circuit 25 is a comparator 2
The time width of the second output pulse is detected, and it is determined whether it is the time width (27.3 μsec) from the vertical synchronization signal start point to the next change point. Such a comparison operation is performed for the number of reference levels, five times in this case. The variable reference level generator 24 generates a threshold level for detecting the start point of the vertical synchronization signal from the reference levels l ₂ and l ₃ at which the time width of the output pulse of the comparator 22 is 27.3 μsec. To do this, for example, one of the levels l ₂ or l ₃ may be adopted as is, or their arithmetic mean l=(l ₂ +l ₃ )/ ₂ may be adopted.
And this threshold level and memory 27
The comparator 22 again compares the level of the signal read from the signal, and from this comparison result, the timing circuit 29 detects and stores the timing for giving the vertical synchronization signal start point. A timing signal indicating
The gate pulse near the vertical synchronization signal is applied to the memory 27 at the correct timing by adjusting the delay time.

According to this embodiment, the waveform near the vertical synchronizing signal of the input television signal is temporarily stored in the memory 27 before being compared with the variable reference level, generating the threshold level, and detecting the starting point of the vertical synchronizing signal. , the vertical synchronization signal can be detected within one field period, which has the advantage of shortening the time required to establish synchronization.

In the above embodiment, an analog memory is used as the memory 27, but if the input television signal is digitized and processed by an A/D converter, a digital memory such as a shift register may be used. Also random. access. If a memory that can be read multiple times, such as a memory, is used, the switch 26 as a feedback circuit becomes unnecessary.

Further, in each of the above embodiments, the case of an NTSC television signal has been explained, but the present invention
The present invention can be similarly applied to detection of vertical synchronization signals in PAL and SECAM television signals, which have similar vertical synchronization signal waveforms to those of the NTSC system.

[Brief explanation of the drawing]

Figures 1 to 3 are signal waveform diagrams for explaining the principle and problems of a conventional vertical synchronization signal start point detection circuit, Figure 4 is a block diagram of an embodiment of the present invention, and Figure 5 is its operation. FIG. 6 is a block diagram of a gate pulse generator, and FIG. 7 is a block diagram of another embodiment of the present invention. 2, 6, 22... Comparator, 3, 23... Gate pulse generator, 4, 24... Variable reference level generator,
5, 25... Time width detection circuit, 27... Memory, 29
...timing circuit.

Claims (1)

[Claims] 1. Means for generating a variable reference level, comparison means for comparing this reference level with a signal value near the vertical synchronization signal of the input television signal, and time width detection for detecting the time width of the output pulse of this comparison means. means, and the reference level when the time width detected by this means matches the time width from the start point to the next change point of the vertical synchronization signal when there is no waveform distortion of the television signal is set as a threshold level. , means for detecting a point near the vertical synchronizing signal of the input television signal that first crosses this threshold level as a vertical synchronizing signal starting point.