JPH02224571A - Clamp pulse control circuit - Google Patents

Abstract

PURPOSE:To attain clamping not affected with over or deficient adjustment by extending the pulse width of a clamp pulse at unlocking so as to apply quick synchronization and decreasing the pulse width of the clamp pulse at locking. CONSTITUTION:A clamp pulse generating circuit 14 generates two kinds of clamp pulses, long and short pulses based on a horizontal timing pulse. While an unlock signal is outputted from a clock recovery distribution circuit 6, the clamp pulse with a long width is selected and an input video signal is clamped in a quick clamp time constant equivalently. Thus, the locking is attained in a short period. After the end of locking, when no unlock signal is outputted from the circuit 6, the clamp pulse with a short width is used. As a result, the input video signal is clamped so as not to be affected by over adjustment or deficient adjustment caused at the left and right edge of a horizontal synchronizing signal. Thus, the production of hue change disturbance of a reproduced picture is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a clamp pulse control circuit, and in particular to a clamp pulse control circuit.
SE decoder, MUS E-NTSC
The present invention relates to a clamp pulse control circuit that supplies clamp pulses to a clamp circuit in the input stage of a down converter.

(b) Conventional technology The multiple sub-Nyquist sampling and encoding method (MUSE method) was proposed by NHK as a method for band-compressing high-definition video signals and transmitting them using broadcasting satellites, and has been receiving awards.

This method uses a single channel of satellite broadcasting (bandwidth 27M).
In order to transmit a high-quality video signal at a frequency of 8.1 MHz), this high-definition video signal is compressed by a band compression encoder to a bandwidth of 8.1 MHz.
This converts the video signal into a band-compressed video signal (MUSE signal).

The MUSE method is introduced in the following documents.

(a) NHK Technical Research, Vol. 39, No. 2, 1988, No. 172, pp. 18 (76) to 53 (11l), Ninomiya,
Hibaku, Izumi, Agreed, Iwadate, "Development of the MUSE method" (b) Magazine published by Nikkei McGraw-Hill, "Nikkei Electronics, November 2, 1987 issue, N11433"
, pp. 189-212, Ninomiya, "Transmission method for high-definition broadcasting using satellites" MU S EJ The band compressed video signal (MUSE signal) by this MUSE method is a signal like the one shown in Figure 4, as shown in the above literature. It is an attempt to split.

This MUSE signal has a clamp level signal indicating an intermediate level of the video signal amplitude multiplexed on a specific line for each field. Further, the horizontal synchronization signal is positive polarity synchronization as shown in FIG. 5, and is line inverted. In addition, #1 to #1
2 indicates a sampling point.

In the band compression decoder (MUSE decoder), a horizontal timing signal (horizontal clamp pulse) is used to clamp the horizontal synchronization portion based on the clamp level signal at a stage before the AD conversion circuit to set the horizontal synchronization portion to the clamp level.

Figure 6 shows a conventional band compression decoder (MUSE decoder)
A block diagram of the input circuit is shown. As is clear from the figure,
The band-compressed video signal input to the video signal input terminal (1) is DC amplified by the first buffer amplifier (2). The amplified output is input to a second buffer amplifier (4) via a DC cut capacitor (3), and the output is digitized in an AD conversion circuit (5).

In addition, the clock regeneration distribution circuit (6) receives a frame synchronization signal, a horizontal synchronization signal, and other signals from the digitized signal.
Synchronize with these or sample clock, (16,2MH
z) The frequency-divided output of the resample clock and the timing signal are reproduced and supplied to each circuit of the band compression decoder.

The clamp level calculation circuit (7) calculates the clamp level based on the AD conversion data of the clamp level signal period and derives the clamp voltage. Clamp switch (8)
is a signal synchronized with the horizontal sync signal (horizontal clamp pulse)
The circuit is closed by connecting the previously formed clamp voltage to the resistor (9
) and a DC cut capacitor (3) to perform clamping.

(10) is a MUSE decoding circuit, which demodulates the original high-quality video signal from the digitized MUSE signal and outputs it. (11) is a high-definition monitor display.

By the way, this clamp circuit also serves as an energy diffusion signal removal circuit. That is, the MUSE signal is FM modulated and used as a satellite broadcast.

In satellite broadcasting, an energy diffusion signal (triangular wave) is superimposed to prevent energy from concentrating on a specific frequency. FIG. 7 shows an example of an energy spread signal in the band-compressed video signal described above. The energy spread signal is a triangular wave with a frequency of 30 Hz and an amplitude corresponding to a frequency deviation of 600 KHz.

Although the amplitude level of this energy spread signal is smaller than the amplitude level of the band compressed video signal, if this energy spread signal is not removed, a difference will occur in the brightness level. Therefore, at least this triangular wave must be compressed to the extent that the user does not notice it.

Therefore, an energy spread signal removal circuit is required. As this removal circuit, a circuit that creates and subtracts a triangular wave having a polarity opposite to that of the energy spread signal, and a clamp circuit are well known.

The waveform of the clamp circuit that operates as a removal circuit is shown in Figure 8 (
Shown in a). Although the energy diffusion signal is sloped as shown in the figure, the clamp circuit adjusts the DC level of the video signal to the clamp level from the clamp level calculation circuit every time a horizontal clamp pulse is input. Therefore, the DC level of the video signal after clamping has a brightness level difference of ΔV in one horizontal period, but this is not noticeable to the user and does not actually cause any problem.

However, in order to completely match the clamp level in the horizontal clamp pulse portion, the time constant of the clamp circuit must be set small. However, if this is done, a problem arises in that waveform distortion in the sync signal portion becomes large.

In other words, in the case of the MUSE signal, the horizontal synchronizing signal corresponding to the clamp position is a positive opening and has the shape shown in FIG. The clock regeneration and distribution circuit (6) monitors the shape of this horizontal synchronization signal and determines the phase of the regenerated clock.

Therefore, when key-clamping a signal portion having this shape, it is necessary to be careful not to cause distortion or deformation of the synchronizing signal waveform due to the clamping. Therefore,
There is a natural limit to the value of the clamp time constant.

In reality, the value of the clamp time constant is determined with the design goal of compressing the energy spread signal to such an extent that flicker cannot be detected in the reproduced video of the MUSE decoder. FIG. 8(b) shows the waveform of a clamp circuit whose time constant is selected to such a value.

The removal of this energy spread signal will be explained using mathematical formulas with reference to FIG.

If the time length of the horizontal clamp pulse is t, the amplitude change in the non-clamp period within one horizontal period of the energy spread signal is △■, and the clamp time constant is τ, then the band compressed video signal is ramped around k. The DC level of
-1/l). Assuming that the number of clamping circuits in 172 cycles of the energy spread signal is n times, V, = △V (1, -ml/r) / (1-e-+/
, )−<1). That is, in order to remove the energy spread signal, the clamp time constant τ may be set small so that the above-mentioned V is sufficiently small to speed up the response of the clamp.

Now, considering the first equation, a method other than the clamp time constant to increase the degree of amplitude compression of the energy spread signal is to increase the clamp pulse width.

In other words, an increase in the closing time of the clamp switch (8) is equivalent to a decrease in the clamp time constant, and basically by adjusting the clamp pulse width, the amplitude of the energy diffusion signal can be compressed. It is possible to bring the operation closer to the target value.

Also, the fact that the clamp time constant is small means that when the MUSE signal is input to the MUSE decoder, the input MUS
It is also possible to shorten the time it takes to reach the same state (locked state) from a state in which the DC level of the signal is not determined in an asynchronous state (unlocked state) in which the phase is not synchronized with the E signal.

In other words, if the clamp pulse width is increased instead of decreasing the clamp time constant itself, the circuit will operate better.

However, in the horizontal synchronizing signal portion of the MUSE signal shown in FIG. 5, there is a limit to the width of the clamp pulse for the following reason.

For example, the levels of pixels #1 and #11 in FIG. 5 are not determined by the standard, and the following three proposals are presented.

a, take a level of 64/256 or 192/256, respectively. b, in the case of #1, the Y signal level at the right end of the screen is taken as is, and in the case of #11, the C signal level at the left end of the screen is taken as is. In the case of C1 #1, the average value of the Y signal level at the right end of the screen and the level of #2 is taken, and in the case of #11, the average value of the C signal level at the left end of the screen and the level of #10.

In this way, the levels of pixels #1 and #11 are not constant,
Contains unstable elements.

On the other hand, the transmission matching filter provided at the output part of the MUSE encoder (not shown)
The SE signal is adjusted to exhibit frequency and phase characteristics that do not cause waveform interference between pixels. Therefore, the M
At both ends of the horizontal synchronizing signal of the MUSE signal input to the clamp circuit of the USE decoder, waveform overshoots and undershoots appear due to the transmission matching filter characteristics. In particular, the Y signal level at the right edge of the screen and the C signal level at the left edge of the screen are 647256 or 192/2.
If the value is significantly different from the level of 56, the overshoot and undershoot values themselves will also be large. For this reason, the horizontal synchronizing signal portion may be deformed as shown in FIG.

The Y signal level at the right end of the screen and the C signal level at the left end of the screen are considered to be uncorrelated if it is a normal program source, so the overshoot and undershoot values of the waveform appearing near #1 and the above The overshoot and undershoot values of the waveform appearing near #11 are also usually uncorrelated between the left end portion and the right end portion of the horizontal synchronization signal.

In Fig. 5, assuming that the 128/256 part is the DC level of the horizontal synchronizing signal part, and if there is no waveform distortion etc. between #2 and #lo, the DC level during this period is 128/256.
Matches 256. Therefore, direct current can be applied to the clamp circuit shown in FIG. 6 without any problem.

However, as mentioned above, uncorrelated overshoots and undershoots occur at both ends of the horizontal synchronization signal of the MUSE signal input to the clamp circuit at the left and right ends, and in reality, Strictly speaking, the DC level between #2 and #lo is 128/2.
56 does not match.

Especially in the case of the screen shown in Figure 1O, the above #2 to #1
Since the DC level between 0 and 0 remains constant for several tens of lines, as a result, the signal DC level after clamping shifts.

The MUSE signal is a TCI format signal, and fluctuations in the signal DC level after clamping mainly interfere with the color tone change of the reproduced image. In the case of the screen shown in FIG. 1O, a disturbance occurs in which the color tone of the reproduced screen changes in the vertical direction as shown in FIG. 1O.

In order to avoid this, it is possible to narrow the clamp pulse width so as not to be affected by the overshoot and undershoot that occur at the left and right end portions of the horizontal synchronizing signal portion, but in this case, the above-mentioned amplifier A problem arises in that the transition time from one locked state to another becomes long.

(c) Problems to be Solved by the Invention The present invention has been made in view of the above points, and provides a system that can quickly shift from an unlocked state to a locked state, and
Overshoot that occurs at both ends of the horizontal synchronization signal part,
This realizes a good ramp circuit that is not affected by undershoot.

(D) Means for Solving the Problems The present invention provides two types of widths, long and short (i.e., the "long" clamp pulse width is long enough to include periods #1 to #11 of the horizontal synchronizing signal. A clamp pulse generation circuit that generates a clamp pulse with a short clamp pulse width that corresponds to a range that is not affected by DC level changes due to overshoot and undershoot that occur at least in the left and right ends of the horizontal synchronizing signal part. (14) and
The clamp pulse switching circuit (16) is equipped with a clamp pulse switching circuit (16) that switches the output of two types of clamp pulses from the Cono clamp pulse generation circuit (14) depending on the presence or absence of an unlock signal generated in the clock regeneration distribution circuit (6). The object of the present invention is to provide a clamp pulse control circuit that appropriately selects the two types of pulses for locking and locking and uses them as clamp pulses.

(E) Effect According to the configuration of the present invention, while the unlock signal is being output from the clock regeneration distribution circuit (6), the clamp pulse with the longer width is selected from the two types of clamp pulses and is isometrically faster. Clamping the input video signal with a clamp time constant contributes to lock operation in a short time, and when the unlock signal is no longer output from the clock regeneration circuit (6) after locking is completed, one of the two types of clamp pulses is used. By using a short-width clamp pulse to clamp the input video signal so that it is not affected by overshoot and undershoot that occur at the left and right end portions of the horizontal synchronization signal portion, the color tone change disturbance of the reproduced image can be suppressed. This can be prevented from occurring.

(f) Embodiment An embodiment of the present invention will be described with reference to FIGS. 1 to 3. Incidentally, the same parts as those in the prior art are denoted by the same reference numerals and the description thereof will be omitted.

(14) is a clamp pulse generation circuit, and this clamp pulse generation circuit (14) generates two types of clamp pulses based on the horizontal timing pulse.

The first clamp pulse has a long pulse width that includes all of #1 to #11 of the horizontal synchronizing signal portion, as shown in FIG. Also, the second clamp pulse is as shown in Fig. 2C.
As shown in the figure, the pulse width is about ±1 to ±2 clocks (#5 to #7,
#4 to #8), and this second clamp pulse has a length that is not affected by the above-mentioned overshoot and undershoot.

(16) is a clamp pulse switching circuit, which is controlled by an unlock signal. This clamp pulse switching circuit (
16) selects and derives a long first clamp pulse when the unlock signal is input. Further, when the unlock signal is not input, a short second clamp pulse is selectively derived.

The above operation will be explained.

MUS input to input terminal (1) of MUSE decoder
The E signal is DC amplified by the first buffer amplifier (2), and then input to the second buffer amplifier (4) with high input impedance via the DC cut capacitor (3).
It is digitized by an AD conversion circuit (5). The clock regeneration/distribution circuit (6) appropriately supplies a resample clock, its frequency divided clock, timing pulses, etc. to each circuit section (not shown) based on the digitized signal. The clamp level calculation circuit (7) calculates a clamp reference level based on the output of the AD conversion circuit (5) during the clamp level signal generation period.

One of the horizontal timing signals (horizontal timing pulse) synchronized with the horizontal scanning line period from the clock regeneration distribution circuit (6) is input to the clamp pulse generation circuit (14). A clamp pulse generation circuit (14) generates two types of clamp pulses based on the horizontal timing pulse.

These two types of clamp pulses are supplied to a clamp pulse switching circuit (16). Clamp pulse switching circuit (1
6) is controlled as follows by the unlock signal generated within the clock regeneration/distribution circuit (6) when the MUSE decoder is not phase synchronized with the input MUSE signal.

That is, as shown in FIG. 3, when an unlock signal is applied from the clock regeneration/distribution circuit (6) to the clamp pulse switching circuit (16), the two types of clamps are output from the clamp pulse switching circuit (16). A first clamp pulse with a longer width among the pulses is output, and M U S
The E decoder is in phase synchronization with the input MUSE signal, and when the unlock signal from the black regeneration distribution circuit (6) is no longer applied, it outputs the second clamp pulse with the shorter width of the two types of clamp pulses. Output.

The selected clamp pulse from the clamp pulse switching circuit (16) is applied to the clamp switch (8) as an opening/closing control signal for the clamp switch (8), and the input MUSE signal is connected to the DC cut capacitor (3) and the resistor. Clamp to the clamp reference level with a clamp time constant determined by (9). At this time, the clamp time constant determined by the DC cut capacitor (3) and the resistor (9) is such that flicker disturbance does not occur in the amplitude of the energy diffusion signal even by a clamp pulse with a shorter width among the two types of clamp pulses. It goes without saying that the value should be set to a value that can be compressed to a certain extent.

As mentioned above, the present invention has the feature that two types of clamp pulses, long and short, are used depending on the operating state of the MUSE decoder, and when the MUSE decoder is in an unlocked state, a long width clamp pulse is used to quickly pull in synchronization, and the MUSE When the E-decoder is in a locked state, a short width clamp pulse is used to reproduce an image without disturbing the color tone.

In the configuration shown in FIG. 1, the functions of the clamp pulse generation circuit (14) and the clamp pulse switching circuit (16) can be easily realized at the same time by using, for example, a ROM (Read Only Memory).

(G) Effects of the Invention According to the present invention, the pulse width of the clamp pulse is long during unlocking, and the synchronization state is quickly achieved. When locked, the pulse width of the clamp pulse is shortened, and a good ramp operation can be performed without being affected by overshoot and undershoot occurring at both ends of the horizontal synchronizing signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention. FIG. 2 is a waveform diagram showing the first and second clamp pulses, and FIG. 3 is a waveform diagram showing the relationship between the unlock signal and the clamp pulse. Figure 4 is a diagram showing the assignment of the MUSE signal, and Figure 5 is a diagram showing the assignment of the MUSE signal.
A diagram showing the horizontal synchronization signal of the USE signal, Figure 6 is the conventional M
A diagram showing the USE decoder, FIG. 7 is a diagram showing the energy spread signal, FIG. 8 is a waveform diagram to explain the triangular wave removal operation, and FIG. 9 is the overshoot of the horizontal synchronization signal part.
FIG. 10, which is a diagram for explaining undershoot, is a diagram of a screen that causes color tone change disturbance. (14)...Clamp pulse generation circuit (clamp pulse generation means), (6)...Clock regeneration distribution circuit (clock regeneration means), (16)...Clamp pulse switching circuit (clamp pulse switching means).

Claims (1)

[Claims] (1) Clamp pulse generating means (14) for generating a first clamp pulse having a width corresponding to a horizontal synchronizing signal period and a second clamp pulse having a width corresponding to a period near the center of the horizontal synchronizing signal; , a clock reproducing means (6) which creates a clock signal synchronized with the input video signal and outputs an unlock signal when the input video signal is not synchronized; Clamp pulse switching means (16) that selectively outputs the first clamp pulse when the lock signal is output, and selectively outputs the second clamp pulse when the unlock signal is not output. Clamp pulse control circuit.