JPH0211066B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ghost removal device that uses a transversal filter and can be built into a television receiver, and particularly relates to ghost signal detection and control of the transversal filter. .

In recent years, when receiving television broadcasts,
Deterioration of received images due to various types of radio wave interference is becoming more and more problematic. In particular, the so-called ghost phenomenon, in which received images become double or triple, has become more common, mainly due to the rise in the height of buildings in urban areas. Countermeasures against this problem include the development of wall construction materials that prevent the reflection of radio waves from these buildings, higher directivity of receiving antennas, and diversity reception using horizontal stack antennas, but none of these measures are effective. It has not become widespread due to the above complexity and rising costs.

Therefore, there has been an increasing need to provide a fully automatic ghost removal system that can be built into television receivers at low cost.

First, a configuration of a television receiver and a ghost removal device in which the present invention can be implemented will be described. FIG. 1 shows a part of the configuration of the television receiver. In the figure, reference numeral 1 denotes an intermediate amplified video detection circuit which amplifies and detects an intermediate frequency modulated signal from a tuner to obtain a baseband composite video signal A. This composite video signal A becomes a signal of 0 to 4.2 MHz in the NTSC system.
In a typical television receiver, this composite video signal A is directly supplied to both the video signal processing amplifier circuit 3 and the chroma signal processing amplifier circuit 4. The ghost removal device 2 has a function of inputting the composite video signal A, removing the ghost component signal, and then supplying a ghost-free video signal B to the circuits 3 and 4. C shown in the figure is supplied to a video display device (such as a CRT) as a display signal without ghosts.

FIG. 2 is a more detailed block diagram of the portion of the ghost removal device 2 in FIG. 1. A feature of this device is that it uses a transversal filter 5.

As is generally well known, in a television transmitting and receiving system, the transfer function in the radio wave propagation system is H(S), and the transfer function in the ghost propagation system is G.
(S), the transfer function of the total signal propagation system including ghosts is H(S)·G(S). On the other hand, since the transversal filter 5 can have an arbitrary transfer function, ghosts can be removed by controlling it to have an inverse transfer function G ^-1 (S) of the propagation system due to ghosts.

The other parts in FIG. 2 are ghost filters for automatically controlling the weighting coefficient of each tap so that the transversal filter 5 has an inverse transfer function G ^-1 (S) depending on the ghost at each time. It performs operations such as detection, calculation, weighting coefficient generation and storage. In the figure, A is the input composite video signal, B
is the ghost-removed output composite video signal,
D is a synchronization signal.

Detection of a ghost in a received television signal is performed by observing the flatness of the signal in approximately one horizontal period before and after the beginning (leading edge) of the vertical synchronization signal. Ideally, this portion of the signal can be regarded as a unit step function, and if there is a ghost, distortion of the unit step function is detected accordingly. For example, the location and size of a ghost is detected at each sampling point. 8 is a timing pulse generation circuit for controlling the horizontal and vertical synchronizing signals D to extract the signals of this part.
The sampling pulse is generated based on the

The video signal B processed by the transversal filter 5 is added to the clamp/A-D converter circuit 7, and after being clamped to eliminate DC fluctuations, the video signal B is sent to the A-D converter circuit 7.
A conversion circuit converts the sampled portions A to D to obtain a digital video signal containing ghost information, which is stored in the digital memory 10.

Arithmetic circuit 9 processes the digital video signal containing ghost information stored in memory 10 and generates a signal for controlling weighting coefficients when output signals from each tap of transversal filter 5 are taken out. This part is usually configured using a microcomputer. The weighting coefficient correction circuit 6 is used to actually generate and hold a weighting coefficient for each tap based on the calculation result of the calculation circuit 9, and since the output of this calculation circuit 9 is normally a digital signal, If the weighting coefficient is an analog signal, a DA conversion circuit is required, and in that case it is also included.

In addition, since this detection → calculation → weighting is repeated many times using the so-called adaptive equalization method, the weighting coefficients are modified one after another many times, and the previous coefficients are stored in memory. 10 must be memorized. In addition, the sampling frequency of the A-D converter circuit in 7 must be at least twice the band frequency since it handles video signals, and in the NTSC system, it is 10.7MHz, which is three times the chroma subcarrier frequency.
It is recommended to use Therefore, this A-D conversion circuit is required to be high-speed. Similarly, a high-speed memory 10 is used. On the other hand, when a microcomputer is used as the arithmetic circuit 6, the speed is not so high, so the memory 10 is disconnected from the microcomputer only when the A-D conversion circuit is handling the vertical synchronization signal part. (memory access).

In this way, this circuit device can be constructed mainly of digital circuits including a microcomputer, and since the transversal filter 5 also has variable integration using a solid-state delay line such as a CCD, it can be used for TVs at relatively low cost. It can be built into the digital receiver.

Next, the means for detecting ghost information by the AD conversion circuit 7 will be explained based on the signal waveforms shown in FIG. In FIG. 3, a is a baseband composite video signal output from the transversal filter 5 in FIG. 2, of which a leading edge portion of the vertical synchronization signal is shown. Determination as to whether or not there is a ghost signal is performed by using the leading edge portion of the vertical synchronization signal indicated by a circle as a reference, and detecting the presence or absence of distortion occurring before and after the leading edge portion of the vertical synchronizing signal.

The sampling pulse b indicates a pulse generated by the sampling pulse generation circuit 8 in order to extract the video signal during the detection period, and only this portion of the video signal digitized by the A-D conversion circuit 7 is stored in the memory 10 by DAM operation. Store. If there is no ghost signal, the period of this sampling pulse b is flat except for the rising edge of the leading edge of the vertical synchronizing signal. If there is a ghost signal between ±1/2 water deflection period from the leading edge, waveform distortion is detected somewhere within the period of sampling pulse b, which is at the rising edge of the leading edge of the vertical synchronization signal. Possibly a ghost signal.

An enlarged view of the rising portion of the leading edge of this vertical synchronization signal is shown in c. In the case of an NTSC television signal, the video signal bandwidth is at most 4.2MHz, so when you zoom in, the same part rises slowly as shown.

On the other hand, when digitizing such a signal as mentioned above, the sampling clock must be at least twice the frequency of the band, but here, for convenience, the frequency is three times that of the chroma subcarrier (approximately
10.7MHz) clock signal is used. What is extracted from this clock at regular intervals is d in the figure, and in this figure, the rising part is about 3
Clocks will be included. The sampling clocks are numbered A to K from the left as shown in the figure. The leading edge portion of the vertical synchronizing signal of the video signals a and c at each sampling point A to K is sampled by the AD conversion circuit 7 and converted into a digital signal. To simplify the explanation, a digital signal diagram of the result of converting each sampling value into 4 bits is shown in e. The signal e that has been A-D converted in this way corresponds to the original video signals a and c, and includes all the information contained in the video signals. Actually, the illustrated A-K
In addition, there are a number of clock pulses between them, which provide a sampled and digitized signal over the entire period of sampling pulse b. All of them are stored in memory 10 by DMA operation. In this way, the frequency of the sampling clock is 10.7MHz, which is faster than the operating speed of the microcomputer for the arithmetic processing circuit 9.
The operation causes the data to be stored directly in the memory 10.

The microcomputer of the arithmetic circuit 9 then reads the digital video signal from this memory 10 and processes it in the following order.

(1) Average the information of several sampling pulse periods b.

(2) Take the difference between them.

(3) Weight each tap of the transversal filter 5.

By repeating these operations 1 to 3, the ghost signal is gradually erased. This corresponds to finding the transfer function G(S) of the ghost propagation system using the adaptive equalization method, as described above.

FIG. 3f is a differential signal obtained by subtracting the previous signal at each sampling point B-K of the digitized signal e. Originally, this is performed after the signal e is averaged by 1 as described above, but for the sake of convenience, it is shown without averaging. In the difference signal shown in f, negative values are shown in "2's complement" representation. Looking at this, points F and G show large positive values, which can be recognized as rising portions of the leading edge. Further, 2 at point B, -2 at point C, 1 at point E, -2 at point I, +2 at point J, and -3 at point K can all be recognized as waveform distortion, that is, a ghost. Therefore, the weighting coefficient of each tap of the transversal filter 5 corresponding to each of these points may be set in a direction that eliminates distortion.

FIG. 4 shows an example of the transversal filter 5 having a feedback configuration and its correspondence. This filter 5 includes an adder 51 and a unit delay element 52.
and each weighter 53 corresponding to the point H and thereafter. For example, “-3” of the differential signal at point K
If "+3" is used as a weighting coefficient in the direction of eliminating distortion, the weighter 53 at point K operates, outputs a signal that cancels out the distortion, adds it to the adder 51, and adds it to the input signal. and output it. By repeating such operations, the transversal filter 5 can be provided with a transfer function G(S) of the propagation system due to the ghost signal.

However, in such an adaptive equalization method, the system becomes unstable due to one-time discrimination of weight amount and reference time, and the transversal filter 5 having a feedback structure often causes an oscillation phenomenon. One of the causes of the oscillation is when a large distortion occurs at a particular sampling point due to external noise, and incorrect weighting is applied in response to the large distortion. Furthermore, if the recognition of the rising point is deviated, weighting will be repeated at deviated positions throughout, resulting in oscillation. Furthermore, especially in the case of the transversal filter 5 having a fieldback configuration as shown in FIG. 4, once oscillation occurs, recovery becomes difficult. In practice, if such an oscillation phenomenon occurs, the screen becomes difficult to see, making it impossible to realize a fully automated system.

Therefore, an object of the present invention is to realize a device system that can predict this oscillation phenomenon in advance and recover from it.

The way to do this is to check the weight each time. That is, as shown in FIG. 5, the sum of the weights of each tap is calculated by the adder 11, and when the sum exceeds a certain predetermined value, the weighting coefficient determination operation is restarted from the beginning. , or stop adding weight for that episode.
To this end, the weighted signal based on the difference signal f is added by the adder 11 for the taps after point H, and each time the weighting coefficient is corrected, it is checked whether it exceeds a predetermined set value. It is monitored by the detection circuit 12. If the set value is exceeded for some reason, the detection output of the detection circuit 12 is added to the arithmetic circuit 9, and the weighting is canceled for that time or the weighting is restarted from the beginning. This setting value is set in advance to a value that will cause oscillation if more weight is applied.

By doing this, even if a large distortion occurs at a specific sampling point due to external noise, normal operation is possible without any oscillation phenomenon occurring. That is, if the set value is exceeded, the weighting for that time is canceled, so that excessive distortion will not be added as it is and lead to oscillation.

Calculating and storing the sum of weighting coefficients in this way can be easily carried out in an apparatus having an arithmetic circuit 9 using a microcomputer, such as the present apparatus. The effect is actually much more remarkable than the case where this means is not implemented.

As described above, according to the present invention, in a device in which a video signal is passed through a transversal filter and the weighting coefficients of the taps are digitally controlled using a microcomputer or the like, the sum of the weighting coefficients of each tap is set to a predetermined value. By not adding the weighting coefficient signal to the tap of the transversal filter and preventing the weighting operation when the value exceeds the value, it is possible to prevent unstable operation that may cause oscillation due to the mixing of noise. It is possible to obtain a superior device.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a television receiver that can use a ghost removal device according to an embodiment of the present invention, Fig. 2 is a block diagram of the ghost removal device portion, and Fig. 3 explains its operation. FIG. 4 is a circuit diagram of an example of a transversal filter used in the device, and FIG. 5 is a block diagram of the main parts of the device. 5... Transversal filter, 6... Multiple coefficient correction circuit, 7... Clamp/A-D conversion circuit, 8... Timing pulse generation circuit, 9... Arithmetic circuit, 10... Memory, 11... Adder , 12
...Detection circuit.

Claims (1)

[Claims] 1 A transversal filter which receives a baseband video signal extracted from a received television signal and a weighting coefficient signal of each tap, and outputs a video signal from which ghost components contained in the television signal have been removed. , a sampling clock having a frequency that is more than twice the band of the video signal, and a pulse for extracting a reference signal for ghost detection from the video signal are input, and the sampled portion is converted from analog to digital. An analog-to-digital converter that outputs a digital video signal, a memory that stores the digital video signal, and a weighting coefficient of the transversal filter that detects ghosts from the digital signals from the memory and removes the ghosts. It also includes an arithmetic circuit that calculates the sum of weighting coefficients of the total taps, and prohibits updating of each tap weighting coefficient of the transversal filter when the sum of weighting coefficients exceeds a predetermined value. A ghost removal device characterized by comprising a weighting coefficient correction circuit.