JPS6154317B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise level detection circuit that detects a noise level contained in a television video signal.

It is well known that television video signals have very high inter-frame correlation, inter-line correlation, and neighborhood correlation, and that these correlations can be used to compress the band of the video signal, improve the S/N ratio, and the like.

An object of the present invention is to provide an extremely new noise level detection circuit that detects the level of noise contained in a video signal by actively utilizing this correlation to separate the video signal from the noise. be.

FIG. 1 is a conceptual diagram for explaining the present invention, in which signals a and b are video signals separated by one frame or one line, and the video signals and components themselves are exactly the same. At this time, the difference signal c does not contain any signal components, and only noise components appear. Signals (a), (b), (c) with video signal component as E(t) and noise component as n ₁ (t) n ₂ (t)
Expressing (a): E(t)+n ₁ (t) ...(1-1) (b): E(t)+n ₂ (t) ...(1-2) (c): {E( t)+n ₁ (t)} −{E(t)+n ₂ (t)} = n ₁ (t)−n ₂ (t) ……(1-3).

Since the noise components n ₁ (t) and n ₂ (t) have no temporal correlation, their root mean square value gives the average power Pn of the noise.

1/2×E [(n ₁ (t) − n ₂ (t)) ² ]=Pn……(1
- 4) (E[×(t)] is the expected value of . If the relationship between video signals separated by one frame or one line is as shown in Figure 2 a and b, the difference signal c from t ₁ to t ₂ will contain changes in signal components. become.

In this case, the portion from t ₁ to t ₂ of the difference signal c is not the noise component itself. Therefore, in the present invention, by taking advantage of the high neighborhood correlation of video signal components and the fact that the level change of the noise component is small compared to the level change of the video signal component, this part is not used as the basic value of the detection circuit. ing. That is, the noise signal level measurement according to the present invention is carried out by separating inter-frame, inter-line, and nearby portions with high autocorrelation in the video signal.

FIG. 3 is a block diagram showing the configuration of one embodiment of the present invention. In the figure, the input signal supplied from terminal 1 is a PCM signal sampled at the subcarrier frequency of the video signal. Although this sampling frequency does not satisfy the sampling theorem within the video signal band, it does not matter for the purpose of this method. This is because the purpose of this method is not to transmit or reproduce signals, but to measure the average voltage or average power of noise signals that are uncorrelated in time.To do this, it is necessary to increase the number of sampling points for averaging. This is because the influence of the sampling frequency can be sufficiently reduced.

The input signal is processed by a 1-frame delay device 2 and a subtracter 3 to remove video signal components with a high degree of frame correlation, and then processed by a delay device 4 with a 227 subcarrier period and a subtracter 5 to convert the video signal to a video signal with a high inter-line correlation. Signal components are removed. The absolute value circuits 6 and 7 are the subtracter 3,
This outputs the absolute value of the output of 5.

In this way, parts of the input video signal with low correlation and parts with high correlation are separated. The output signal of the absolute value circuit 7 passes through a squaring circuit 8 and is squared by an averaging circuit 9. Furthermore, at that time, the signal taken into the averaging circuit 9 is transferred to the averaging control circuit 10.
(described in detail later).

That is, the signal from t ₁ to t ₂ in FIG. 2c is not used as a sample point for averaging.

This ensures reliable separation of noise components from the input video signal.

The operation of each part will be explained in detail below. Figure 4 is a circuit configuration diagram for extracting the signal of the part with low correlation between frames of the input video signal, and its transfer characteristic Ff (z) is F _f (z) = 1 - e ^-z ... (2 ) and has a gain characteristic as shown in Figure 5. In other words, this circuit configuration separates the DC component of the video signal from
Components that are integral multiples of 30Hz do not appear in the output. This means that video signals with a very high degree of frame correlation are hardly transmitted. In other words, only changes in noise components and video signal components due to interframe movements appear in the output.

Changes in the video signal components due to interframe effects are further separated by the circuit shown in FIG. This circuit consists of a delay line with a period 227 times that of the subcarrier, and its transfer characteristic F _l (z) is F _l (z) = 1-e ^-227z ... (3) z: period of the subcarrier, as shown in Fig. 7. It has the gain characteristics shown in .

As can be seen from the characteristic diagram in Fig. 7, the circuit of the present invention that uses line correlation to remove video signal components has a characteristic that in the high frequency region, it is greatly attenuated at the subcarrier frequency sc, and therefore The chromaticity components in the video signal can be effectively removed. On the other hand, if we pay attention to the low frequency part, there is a large attenuation at sc/227, 2sc/227..., and these points are at sc/227 in the spectrum of the luminance component.
．． 5, 2sc/227.5..., and as a result, it is possible to effectively remove luminance components whose components are concentrated mainly in the low range. In this way, by using the delay line with 227 subcarrier cycles in this application, it is possible to simultaneously remove the chromaticity component and the luminance component with one circuit. This results in separating parts of the signal with high line correlation. As described above, noise components are output for parts of the video signal where frame correlation and line correlation are high, and changes between the noise component and the video signal component are output for other parts. However, since the degree of frame correlation and line correlation of the video signal is quite high, a large portion of this output is considered to be a noise component.

In the present invention, in order to make the operating characteristics more reliable, an averaging control circuit is provided to select only the portions of the input video signal with high frame correlation and line correlation as sample points to be taken into the subsequent averaging circuit 9. is provided. FIG. 8 is a diagram of its circuit configuration. 256 counter 11 is the subcarrier
It is a counter that counts 256 cycles.
Further, the counter 12 is a counter that checks the number of sample points where the level of the input signal exceeds a predetermined threshold value, and this counter 12 is cleared when the counter 11 counts 256 subcarriers.
For this purpose, the input signal is compared with a predetermined threshold value in the comparator 15. Therefore, the output of the counter 12 is the number of sample points exceeding the threshold level within a fixed number of samples (256 samples).
Comparator 13 compares the number in counter 12 with a predetermined comparison number. The comparison result is counter 12
is cleared in register 14 and held for the next 256 sample periods. The output of this register 14 becomes a control signal that controls the averaging circuit 9. That is, the circuit is in constant samples of the input signal. When the number of sample points that exceed the threshold level exceeds a certain number (comparison number), the sampling circuit operates so as not to incorporate sample points in the vicinity of the sample points into the averaging circuit, as they are considered unsuitable for noise level detection.

FIG. 9 is a diagram showing the output waveforms of each part of the averaging control circuit shown in FIG. Clear pulse and register 14 clock pulse.
c is the output of comparator 13, and counter 1
It appears when the count of 2 becomes larger than the comparison number. d is an output signal of the averaging control circuit, and the input signal is not used as averaging data during periods T ₁ and T ₂ of d.

As a result, for example, the portion from t ₁ to t ₂ shown in FIG. 2c is not used as averaging data. In the averaging control circuit described above, the fact that the neighborhood correlation of the video signal is high is utilized.

Figure 10 is a block diagram of the averaging circuit, in which the input signal from the squaring circuit is input to the coefficient unit 15 (coefficient
K ₁ ), and after being multiplied by K ₁ , adder 1
6, switch 17, and register 18, and then output as a detector output. The output of the register 18 is multiplied by (1-K ₁ ) by a coefficient multiplier 19 and then added to an adder 16. Register 18 is 3.58MHz
It is shifted by the clock and acts as a delay device. The above configuration constitutes a first-order recursive filter. The transfer characteristics of this circuit exhibit those of a first-order integrator, the time constant of which is determined by the coefficient unit. That is, it is determined by the number of sample points used for averaging.

As described above, the noise level detection circuit according to the present invention can accurately measure the level of noise contained in a television video signal.

In the configuration shown in FIG. 3, the average voltage of the noise can be obtained, but if the squaring circuit 8 in FIG. 3 is excluded, the average power can be obtained in the output of the averaging circuit. Then, by converting according to the type of noise, the peak value of the noise component can be immediately obtained.

According to the present invention, it is possible to continuously monitor the quality of video signals of various video transmission systems.

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are conceptual diagrams for explaining the present invention, and FIG. 3 is a configuration diagram showing an embodiment of the present invention.
Fig. 4 is a partial diagram of Fig. 3, and is a circuit diagram for extracting the part with low correlation between frames, Fig. 5 is a diagram showing the characteristics of the circuit of Fig. 4, and Fig. 6 is a part of Fig. 3. The figure shows a circuit diagram that uses the correlation between lines to extract a low-correlation part from the output of the circuit in Figure 4, Figure 7 shows the characteristics of the circuit in Figure 6, and Figure 8 shows the characteristics of the circuit in Figure 3. FIG. 9 is a partial diagram showing the averaging circuit control circuit, FIG. 9 is a diagram showing the outputs of each part in FIG. 8, and FIG. 10 is a configuration diagram of the averaging circuit. In the figure, 1...input terminal, 2...1 frame delay device, 3, 5...subtractor, 4...227 subcarrier period delay device, 6, 7...absolute value circuit, 8
... Square circuit, 9 ... Averaging circuit, 10 ... Averaging control circuit, 11 ... 256 counter, 1
2... Counter, 13, 15... Comparator,
14...Register.

Claims (1)

[Claims] 1 A detection circuit for detecting the noise level contained in a composite television video signal, which comprises: a first separating means for separating noise components by using autocorrelation between frames of the television video signal; a second separating means for extracting a video signal component from the output of the first separating means using autocorrelation between lines; and a circuit for averaging the output of the second separating means on a time axis. wherein the first separation means includes a frame memory for receiving an input television video signal and a subtracter for taking a difference between the video signal from the frame memory and the input television video signal; a delay line having a delay time of 227 times the subcarrier period in which the separating means receives the output of the first separating means, and taking the difference between the output from the delay line and the output of the first separating means. A noise level detection circuit comprising a subtracter.