JP2523010B2 - Clamp pulse control circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a clamp pulse control circuit, and more particularly to MUSE.
The present invention relates to a clamp pulse control circuit that supplies a clamp pulse to a decoder and a clamp circuit at an input stage of a MUSE-NTSC down converter.

(B) Conventional technology A multi-sub-Nyquist sampling encoding method (MUSE method) has been proposed by NHK as a method for band-compressing high-definition video signals and transmitting it using a broadcasting satellite, and has been used for award.

This system is a single channel of satellite broadcasting (bandwidth 27MH
In order to transmit a high-definition video signal in z), this high-definition video signal is converted by a band compression encoder into a band compression video signal (MUSE signal) with a band of 8.1 MHz.

The MUSE method is introduced in the following documents and the like.

(A) NHK Technical Research Vol. 39 No. 2 No. 172 of 1987,
18 (76)-53 (111), Ninomiya, Otsuka, Izumi, Koshi,
Iwadate, "Development of MUSE method" (b) Magazine "Nikkei Electronics, November 2, 1987, No.433", pages 189-212, published by Nikkei McGraw-Hill
Ninomiya "Transmission system for high-definition broadcasting using satellite MUS
E ”Band compression video signal (MUSE signal) by this MUSE method
Is the signal allocation as shown in FIG. 4 as shown in the above-mentioned document.

In this MUSE signal, a clamp level signal indicating an intermediate level of video signal amplitude is multiplexed on a specific line for each field. The horizontal sync signal is positive sync as shown in FIG. 5 and line-inverted. Note that # 1 to # 12 indicate sampling points.

In a band compression decoder (MUSE decoder), a horizontal timing signal (horizontal clamp pulse) is clamped on the basis of the clamp level signal in the preceding stage of the AD conversion circuit to set the horizontal synchronizing portion to the clamp level.

FIG. 6 shows a block diagram of an input circuit of a conventional band compression decoder (MUSE decoder). As is apparent 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 the second buffer amplifier (4) via the DC cut capacitor (3), and the output is digitized in the AD conversion circuit (5).

Further, the clock reproduction / distribution circuit (6) uses the digitized signal to detect the frame sync signal and the horizontal sync signal as well as the resample clock (16.2 MHz) synchronized with these signals.
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 in the clamp level signal period to derive the clamp voltage. Clamp switch (8)
Is a signal synchronized with the horizontal sync signal (horizontal clamp pulse)
Then, the clamp voltage is closed, and the clamp voltage formed earlier is supplied to the time constant circuit formed by the resistor (9) and the DC cut capacitor (3) to perform the clamp.

(10) is a MUSE decoding circuit, digitized
The original high-definition video signal is demodulated from the MUSE signal and output. (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 satellite broadcasting.

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

The amplitude level of this energy spread signal is smaller than the amplitude level of the band-compressed video signal, but again the brightness level will differ unless the energy spread signal is removed. Therefore, at least this triangular wave must be compressed so that the user does not notice it.

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

The waveform of the clamp circuit that operates as a removal circuit is shown in FIG. Although the energy diffusion signal is inclined 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 is ΔV in one horizontal period.
There is a difference in the brightness level between the two, but this is so unnoticed by the user that practically no problem occurs.

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

That is, in the case of the MUSE signal, the horizontal synchronizing signal corresponding to the clamp position is positive polarity synchronizing and has the shape shown in FIG. The clock reproduction / distribution circuit (6) monitors the shape of the horizontal synchronizing signal and determines the phase of the reproduction clock. Therefore, when the signal portion having this shape is keyed clamped, it is necessary to take care so that the clamp does not cause distortion or deformation of the synchronization signal waveform. Therefore, the value of the clamp time constant is naturally limited.

In practice, the value of the clamp time constant is determined with the design goal of compressing the energy spread signal to the extent that flicker cannot be detected in the playback video of the MUSE decoder. FIG. 8B shows the waveform of the clamp circuit in which the time constant is selected as such a value.

The removal of the energy diffusion signal will be described by mathematical expressions with reference to FIG.

Assuming that the time length of the horizontal clamp pulse is t, the amount of change in the amplitude of the energy diffusion signal in the non-clamp period within one horizontal period is ΔV, and the clamp time constant is τ, the band-compressed video signal clamped k times The DC level Vk is expressed as the recurrence formula Vk = ΔV + Vk _-1e- ^{t / τ,} and if the above formula is converted, Vk = ΔV + (1-e ^{1kt / τ} ) / (1-e- ^{t / τ} ) . When the in clamp exemplary circuit to the energy spread signal half period is n _{times, V n = ΔV (1-} e -nt / τ) / a ^{(1-e -t / τ)} ............ (1) Become. In other words, in order to remove the energy diffusion signal, the clamp time constant τ may be set small so that the V _n becomes sufficiently small to speed up the clamp response.

Now, when considering the first expression, there is a method of taking a large clamp pulse width as a method other than the clamp time constant for increasing the degree of amplitude compression of the energy diffusion signal.

That is, lengthening the closing time of the clamp switch (8) is equivalent to shortening the clamp time constant, and basically the clamp pulse width is adjusted to reduce the amplitude of the energy diffusion signal. It is possible to bring the action closer to the target value.

Also, the small clamp time constant means that when the MUSE signal is input to the MUSE decoder, the DC level of the signal is not fixed in the asynchronous state (unlocked state) where the phase is not synchronized with the input MUSE signal. It is also possible to shorten the time to reach the (locked state).

That is, the circuit operates well if the clamp pulse width is increased instead of decreasing the clamp time constant itself.

However, in the horizontal sync signal portion of the MUSE signal shown in FIG. 5, the clamp pulse width is limited for the following reason.

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

Take levels 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 it is, and in the case of # 11, the C signal level at the left end of the screen is taken as it is. c. # 1
In the case of, the average value of the Y signal level at the right end of the screen and the level of # 2, and in the case of # 11, the C signal level at the left end of the screen and # 10
Take the average value with the level of.

In this way, the pixel levels of # 1 and # 11 are not constant,
It contains unstable elements.

On the other hand, the transmission matching filter provided in the output part of the MUSE encoder (not shown) is adjusted so as to show the frequency / phase characteristics such that waveform interference does not occur between each pixel of the MUSE signal to be transmitted. Therefore, waveform overshoot and undershoot due to the characteristics of the transmission matching filter appear at both ends of the horizontal sync signal of the MUSE signal input to the clamp circuit of the MUSE decoder. In particular, when the Y signal level at the right end of the screen or the C signal level at the left end of the screen shows a value greatly different from the level of 64/256 or 192/256, the overshoot and undershoot values themselves also increase. Therefore, the horizontal sync signal portion may be deformed as shown in FIG.

Since 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 with a normal program source, the overshoot and undershoot values of the waveform appearing near # 1 and the above The overshoot and undershoot values of the waveform appearing in the vicinity of # 11 are usually uncorrelated at the left end portion and the right end portion of the horizontal synchronizing signal.

In FIG. 5, the 128/256 portion is assumed to be the DC level of the horizontal synchronizing signal portion, and if there is no waveform distortion between # 2 and # 10, the DC level during this period is equal to 128/256. Therefore, it is possible to add a direct current to the clamp circuit of FIG. 6 without any problem.

However, as described above, at both ends of the horizontal sync signal of the MUSE signal input to the clamp circuit, uncorrelated overshoots and undershoots occur at the left end and the right end as described above. Strictly speaking, the DC level between # 2 and # 10 does not match 128/256.

Especially in the case of the screen as shown in FIG.
Since the state in which the direct current level between the two is constant deviates for several tens of lines, the signal direct current level after clamping deviates as a result.

The MUSE signal is a TCI format signal, and fluctuations in the DC level of the signal after clamping mainly interfere with the change in color tone of the reproduced image. In the case of the screen shown in FIG. 10a, as shown in FIG. 10b, the disturbance that the color tone of the reproduced screen changes in the vertical direction occurs.

In order to avoid this, the clamp pulse width may be narrowed so as not to be affected by the overshoot and undershoot generated at the left end portion and the right end portion of the horizontal synchronizing signal portion. The problem that the transition time from the lock state to the lock state becomes long occurs.

(C) Problems to be Solved by the Invention The present invention has been made in view of the above points, and it is possible to quickly make a transition from the unlocked state to the locked state and to overshoot at both ends of the horizontal synchronizing signal portion. It realizes a good clamp circuit that is not affected by undershoot.

(D) Means for Solving the Problems The present invention provides two types of long and short widths (that is, a “long” clamp pulse width has a length including the periods # 1 to # 11 of the horizontal synchronizing signal, and Clamp pulse generation circuit for generating a clamp pulse having a "short" clamp pulse width (at least corresponding to a range where there is no effect of DC level change due to overshoot or undershoot occurring at the left end portion and the right end portion of the horizontal sync signal portion) (14) and a clamp pulse switching circuit (16) for switching and outputting the two types of clamp pulse outputs of the clamp pulse generating circuit (14) depending on the presence or absence of an unlock signal generated in the clock regeneration distribution circuit (6). And a clamp pulse control circuit that appropriately selects the above-mentioned two types of pulses at the time of unlocking and uses them as a clamp pulse. Than it is.

(E) Operation With the configuration of the present invention, while the unlock signal is being output from the clock recovery / distribution circuit (6), a clamp pulse having a long width is selected from among the two types of clamp pulses described above, and an equivalent fast clamp pulse is selected. Clamping the input video signal with a time constant contributes to the lock operation in a short time, and when the unlock signal is no longer output from the clock recovery circuit (6) after the lock is completed, it is the shortest of the two types of clamp pulses. Occurrence of disturbance of color tone change of the reproduced image by clamping the input video signal so as not to be affected by overshoot and undershoot generated at the left end and right end of the horizontal sync signal part by using the width clamp pulse. Can be prevented.

(F) Embodiment An embodiment of the present invention will be described with reference to FIGS. It should be noted that the same parts as those of the related art are designated 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 including all of # 1 to # 11 of the horizontal synchronizing signal portion as shown in FIG. 2B.
As shown in FIG. 2C, the second clamp pulse has a pulse width of ± 1 to +/- 1 around # 6 of the horizontal synchronizing signal portion.
The length is about ± 2 clocks (# 5 to # 7, # 4 to # 8), which is short, and the second clamp pulse is not affected by the above-mentioned overshoot and undershoot. .

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

 The above operation will be described.

The MUSE signal input to the input terminal (1) of the MUSE decoder is DC-amplified by the first buffer amplifier (2),
It is input to the second buffer amplifier (4) having a high input impedance via the DC cut capacitor (3) and digitized by the AD conversion circuit (5). The clock reproduction / 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) is AD during the clamp level signal generation period.
A clamp reference level is calculated based on the output of the conversion circuit (5).

One of the horizontal timing signals (horizontal timing pulse) synchronized with the horizontal scanning line period from the clock reproduction 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 the clamp pulse switching circuit (16). Clamp pulse switching circuit (1
6) is controlled as follows by an unlock signal generated in the clock reproduction 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 reproduction / distribution circuit (6) to the clamp pulse switching circuit (16), the clamp pulse switching circuit (16) outputs two types of clamp signals. Of the pulses, the first clamp pulse having the long width is output, and when the MUSE decoder is in phase synchronization with the input MUSE signal and the unlock signal from the clock regeneration distribution circuit (6) is no longer applied, the above two types of Second of the short width of the clamp pulse
The clamp pulse of is output.

The clamp pulse selected from the clamp pulse switching circuit (16) is applied to the clamp switch (8) as an opening / closing control signal of the clamp switch (8), and
The MUSE signal is clamped to the clamp reference level by the clamp time constant determined by the DC cut capacitor (3) and the resistor (9). At this time, the clamp time constant determined by the DC cut capacitor (3) and the resistor (9) does not cause flicker interference in the amplitude of the energy diffusion signal even if the clamp pulse having the short width of the two types of clamp pulses is used. It is natural to set the value so that it can be compressed to some extent.

As described above, the present invention is characterized in that two types of clamp pulse, long and short, are used properly according to the operating state of the MUSE decoder. When the MUSE decoder is in the unlocked state, the clamp pulse with a long width is used for quick synchronization pull-in.
In the locked state of the SE decoder, a clamp pulse with a short width is used to reproduce an image without disturbing the color tone.

In the configuration of FIG. 1, the clamp pulse generating circuit (14) and the clamp pulse switching circuit (16) are, for example, RO
By using M (Read Only Memory), both functions can be easily realized at the same time.

(G) Effect of the Invention According to the present invention, the pulse width of the clamp pulse is long when unlocked, and a quick synchronization state is achieved. Then, at the time of locking, the pulse width of the clamp pulse is shortened so that a good clamp operation can be performed without being affected by the overshoot and undershoot generated at both ends of the horizontal synchronizing signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing an 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. FIG. 4 is a diagram showing allocation of MUSE signals, FIG. 5 is a diagram showing horizontal sync signals of MUSE signals, FIG. 6 is a diagram showing a conventional MUSE decoder, FIG. 7 is a diagram showing energy spread signals, and FIG. FIG. 8 is a waveform diagram for explaining the triangular wave removing operation, FIG. 9 is a diagram for explaining overshoot and undershoot of the horizontal synchronizing signal portion, and FIG. 10 is a diagram of a screen which causes color tone change disturbance. (14) ... Clamp pulse generation circuit (clamp pulse generation means), (6) ... Clock reproduction distribution circuit (clock reproduction means), (16) ... Clamp pulse switching circuit (clamp pulse switching means).

Claims (1)

(57) [Claims] 1. Clamp pulse generating means 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 (14). ), A clock reproducing means (6) for generating a clock signal synchronized with the input video signal and outputting an unlock signal when the clock signal is asynchronous, and controlled by the unlock signal from the clock reproducing means (6), Clamp pulse switching means (16) for selectively outputting the first clamp pulse when the unlock signal is output, and selectively outputting the second clamp pulse when the unlock signal is not output. Clamp pulse control circuit.