JP3166874B2 - Automatic gain control method for digital color demodulation. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control method for digital color demodulation in which the level of a demodulated color difference signal can be automatically controlled in units of 3 dB, that is, ^21/2 times, in digital color demodulation. .

[0002]

2. Description of the Related Art An NTSC color television signal has a luminance signal Ey and a chrominance carrier Fsc having two color difference signals Er-.
A carrier chrominance signal Ec that has been subjected to quadrature two-phase modulation by Ey and Eb-Ey is superimposed, and a transmission signal Em of NTSC color television is expressed by the following equation. In the following description, the chrominance signal before transmission and the demodulated chrominance signal are both represented by Er-Ey and Eb-Ey, and the chrominance signal is also represented by Ec.

Em = Ey + Ec = Ey + a · (Er−Ey) · cos (ωt) b · (Eb−Ey) · sin (ωt) (1) where a = 1 / 1.14 = 0.877 b = 1 / 2.03 = 0.493 ω = 2πF (F is the frequency of the color carrier Fsc) Ey, Er-Ey, and Eb-Ey are the three primary color signals E
Each of r, Eg, and Eb is represented by the following equation.

Ey = 0.299Er + 0.587Eg + 0.114Eb (2) Er−Ey = 0.701Er−0.587Eg−0.114Eb (3) Eb−Ey = −0.299Er−0.587Eg + 0.886Eb ... (4) In the equation (1), when the amplitude of Ey is 0 to 1,
m so that the amplitude of m falls within the range of -0.33 to 1.33.
Different coefficients a for the two color difference signals Er-Ey and Eb-Ey
And b are multiplied. The maximum value of Em is yellow (Er = Eg
= 1, Eb = 0) and cyan (Er = 0, Eg = Eb =
According to 1), the minimum value of Em is red (Er = 1, Eg = Eb)
= 0) and blue (Er = Eg = 0, Eb = 1), and the range of Er-Ey and Eb-Ey is defined as follows.

| Er-Ey | ≦ 0.701 (5) | Eb-Ey | ≦ 0.886 (6) CCIR which is a digital television studio standard
In Recommendation 601, two color difference signals having different dynamic ranges as represented by Expressions (5) and (6) are obtained.
Digital Cr and C having the same dynamic range
It is specified that the signal should be normalized to b signal.

| Cr | ≦ 0.5 (7) | Cb | ≦ 0.5 (8) Therefore, the levels of the demodulated color difference signals Er-Ey and Eb-Ey are changed to the levels of the digital Cr and Cb signals. Level conversion for normalization is required. Digital Cr, Cb
From the equation (1) and the equations (5) to (8), the level conversion coefficients K ₁ and K ₂ for the signal are as follows, respectively.

K ₁ = 1.14 × 0.5 / 0.701 ≒ 0.813 (9) K ₂ = 2.03 × 0.5 / 0.886 ≒ 1.145 (10) In such NTSC color television signals,
The chrominance carrier has a relatively high frequency of 3.58 MHz, while the luminance signal band of the broadcast wave is 4.2 MHz. For this reason,
In the transmission system, when the high-frequency component is attenuated, not only does the resolution due to the luminance signal decrease, but also the demodulated chrominance signal level decreases with the decrease in the carrier chrominance signal level.
The saturation of the reproduced image is reduced, that is, the image is light in color.
In particular, when the vicinity of the color carrier component is attenuated, the density of the color is perceived more remarkably than the level of the resolution, so that the correction of the demodulated chrominance signal level is important.

[0008]

Conventionally, in digital color demodulation, when the demodulation color difference signal level is automatically controlled in accordance with the detected color burst level,
There were two ways. The first method is to multiply a level correction coefficient for two types of demodulated color difference signals using a multiplier. Although this method has an advantage of enabling fine automatic gain control, it has a disadvantage that it is not suitable for IC since a relatively large-scale multiplier is required. In the second method, level correction for two types of demodulated color difference signals is performed by bit shift in 6 dB, that is, in units of 2 times. Although this method has an advantage of not causing an increase in circuit scale, it has a drawback that only coarse automatic gain control in units of two can be performed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an automatic gain control method for digital color demodulation which can automatically correct a demodulated color difference signal level finely by adding a small circuit.

[0010]

SUMMARY OF THE INVENTION An automatic gain control method for digital color demodulation according to the present invention is used to normalize two demodulated color difference signal levels in order to eliminate the above-mentioned disadvantages in a digital color demodulation circuit. ^Note that the ratio of the two level conversion coefficients is 2 ^1/2 (K ₂ ≒ 2 ^1/2 ·
K ₁ ), by simply bit-shifting each level conversion coefficient or exchanging the level conversion coefficients with each other,
The automatic gain control of the demodulated color difference signal level is performed.

[0011]

According to the present invention, 3 dB, that is, without increasing the circuit scale of the digital color demodulation circuit, that is,
Automatic gain control of the demodulated chrominance signal level is possible with a fineness of ^21/2 times.

[0012]

FIG. 3 shows, in the automatic gain control method for digital color demodulation according to the present invention, a coefficient obtained by combining a level conversion coefficient and a level correction coefficient for a demodulated color difference signal level with respect to the level correction amount of an input color burst. It is a thing. First, n is a positive integer value, and the level of the input color burst is -3 (2n-1) d with respect to the standard level.
B, that is, when expressed by 2 ^{− (n−1 / 2)} times, the level correction amount for the demodulated color difference signal level is 3 (2n−1) dB,
That is, the correction coefficient is 2 ^{n-1 / 2} . In this case, the coefficient obtained by combining the level conversion coefficient and the level correction coefficient for the demodulated color difference signal level is equal to the two demodulated color difference signals E.
For r-Ey and Eb-Ey, the level conversion coefficients K ₂ and K ₁ shown in equations (10) and (9) are 2 ⁿ⁻¹ , 2
ⁿ times. Also, the level of the input color burst is 3 (2n-1) dB relative to the standard level, that is, 2
^When expressed by ^{n-1 / 2} times, the level correction amount for the demodulated color difference signal level is -3 (2n-1) dB, that is, the correction coefficient is 2- ^{(n-1 / 2)} . In this case, a coefficient obtained by combining the level conversion coefficient and the level correction coefficient for the demodulated chrominance signal level is expressed by the equations (10) and (9) with respect to the two demodulated chrominance signals Er-Ey and Eb-Ey. ^Are respectively 2 ^-n and 2- ^(n-1) times the level conversion coefficients K ₂ and K ₁ . In any case, at the level almost the normal level of the input color burst, only bit shift interchanged level conversion coefficients K ₂ and K ₁ for the two color difference signals Er-Ey and Eb-Ey when the level correction is not required Means that level correction and level conversion can be performed simultaneously. Next, when m is an integer and the level of the input color burst is 6 mdB relative to the standard level, that is, 2 ^m times, the level correction amount for the demodulated chrominance signal level is −6 mdB, that is, the correction coefficient is 2 ^-m . In this case, a coefficient obtained by combining the level conversion coefficient and the level correction coefficient for the demodulated chrominance signal level is expressed by Expressions (9) and (10) with respect to the two demodulated chrominance signals Er-Ey and Eb-Ey. It is 2 ^−m times the level conversion coefficients K ₂ and K ₁ . In this case, at the level almost the normal level of the input color bursts, level correction at the level conversion coefficients K ₂ and K ₁ for the two color difference signals Er-y and Eb-y when the level correction is not required only bit shifting And level conversion can be performed at the same time.

FIG. 1 is a block diagram of a digital color demodulation circuit showing a first embodiment of the present invention. In the figure, 10 is an A / D conversion circuit, 20 is a Y / C separation circuit, 30 is a color demodulation circuit, 31 and 32 are multipliers, 33 and 34 are low-pass filters (LPF), 35 is a burst gate, 36 Is a phase difference detection circuit, 37 is a phase correction amount calculation circuit, 38 is a phase generation circuit, 39 is a local color subcarrier generation circuit, 40 and 50 are level conversion multipliers, 60 is a level correction amount calculation circuit,
70 is a level conversion coefficient generation circuit, 80 is a level conversion and level correction coefficient switching circuit, and 90 is a level conversion and level correction bit shift circuit. A is an analog NT
An SC signal input terminal, b is a digital Y signal output terminal, and c and d are digital Cr and digital Cb signal output terminals.

The analog NTSC signal input from the terminal a is converted by the A / D conversion circuit 10 into a digital NTSC signal.
Converted to a signal. The digital NTSC signal is Y / C
In the separating circuit 20, the luminance signal Ey and the modulated chrominance signal Ec are separated. The digital luminance signal Ey is output from the terminal b. On the other hand, the modulated color signal Ec is
Input to 0. The modulated chrominance signal Ec is multiplied by the orthogonal two-phase chrominance subcarriers generated by the local chrominance subcarrier generation circuit 39 in the color demodulation multipliers 31 and 32, respectively. At this time, the two frequency components of the chrominance subcarrier generated in the multipliers 31 and 32 are removed by the LPFs 33 and 34, so that the two color-demodulated color difference signals Er-
Ey and Eb-Ey are obtained. Such a color demodulation circuit 3
In the case of 0, the phase of the orthogonal two-phase color subcarrier generated from the local color subcarrier generation circuit 39 needs to have a predetermined phase difference from the input color burst. Therefore, the input color burst is color-demodulated by the color demodulation circuit 30 to obtain 2.
The two color difference signal components are extracted by the burst gate 35. The phase difference between the input color burst and the local color subcarrier is obtained in the phase difference detection circuit 36 from the ratio of the extracted two color difference signal components. From this phase difference, the digital phase correction amount is calculated in the phase correction amount calculation circuit 37. After adding or subtracting the phase correction amount, the phase generation circuit 38 generates a digital phase. Based on this digital phase, a local color subcarrier generation circuit 39
Generates orthogonal two-phase local color subcarriers. When the level correction is not required, the two color-demodulated color difference signals Er-
Ey and Eb-Ey are multipliers 40 and 50 for level conversion.
, The digital Cr signal is converted into a digital Cb signal by multiplying by a level conversion coefficient. The digital Cr signal and the digital Cb signal are output from the terminals c and d via the bit shift circuit 90 as they are. In such digital color demodulation processing, the level of the demodulated color difference signal is automatically corrected and controlled in accordance with the level of the input color burst as follows.

Since the color burst is an Eb-Ey component, the level correction amount is calculated in the level correction amount calculation circuit 60 from the amplitude level of the color burst extracted by the burst gate 35. At this time, in order to accurately calculate the amplitude level of the input color burst, the phase difference between the input color burst and the local color subcarrier needs to be zero. Reference the output of 36. Level change 換係 number generation circuit 7
0 is the level conversion coefficient K shown in equations (9) and (10).
To generate a ₁ and K _2. The coefficient switching circuit 80 for level conversion and level correction switches the level conversion coefficient to be input to the multipliers 40 and 50 based on the output of the level correction amount calculation circuit 60 and the value in FIG. Then, the bit shift circuit 90 bit-shifts the outputs of the multipliers 40 and 50 based on the output of the level correction amount calculation circuit 60 and the value of FIG.

FIG. 2 is a block diagram of a digital color demodulation circuit showing a second embodiment of the present invention. In the figure, 41 and 51 are coefficient ROMs for level conversion, and 81 is a coefficient switching circuit for level conversion and level correction.

FIG. 2 shows that the circuit scales of the level conversion multipliers 40 and 50 in FIG. 1 are comparatively large, so that these multipliers are connected to the level conversion coefficient ROM 41 in which the multiplication result of the level conversion fixed coefficient is written. 51 to reduce the size of the circuit. The operation up to digital color demodulation is the same as that in FIG. 1, so detailed description is omitted, and only the level conversion and level correction operations will be described in detail below. When the level correction is unnecessary, the two color difference signals Er-Ey and Eb-Ey color-demodulated by the color demodulation circuit 30 are used.
Is directly passed through one of the switches of the level conversion and level correction coefficient switching circuit 81 to obtain the level conversion coefficient RO.
It is input to the M41 and 51, and the level conversion coefficients K ₁ and K ₂ represented by the formula (9) and (10) are multiplied, are converted respectively into a digital Cr signal and the digital Cb signal.
The digital Cr signal and the digital Cb signal are output from the terminals c and d via the other switch of the coefficient switching circuit 81 for level conversion and level correction as it is, and further via the bit shift circuit 90 as it is. In such digital color demodulation processing, the level of the demodulated color difference signal demodulated according to the level of the input color burst is automatically corrected and controlled as follows. The level conversion and level correction coefficient switching circuit 81 includes a level correction amount calculation circuit 60.
And the demodulated color difference signal Er-E based on the output of FIG.
The level conversion coefficients ROM 41 and 51 to be multiplied with y and Eb-Ey are switched. Then, the bit shift circuit 90 compares the output of the level correction amount calculation circuit 60 with that of FIG.
And the level conversion coefficient ROMs 41 and 51 based on the
Is bit-shifted.

Thus, in any of the embodiments,
In order to automatically correct the demodulated color difference signal level in accordance with the level of the input color burst, a level correction amount calculating circuit, a coefficient switching circuit,
Only by adding a bit shift circuit, a multiplier having a relatively large circuit scale is not newly required for level correction.

For convenience of explanation, the color television signal is an NTSC signal. However, the digital signal of another color television signal such as a PAL signal having exactly the same signal level superposition relationship between the two color difference signals and the luminance signal. It is apparent that the automatic gain control method for digital color demodulation according to the present invention can be applied to the color demodulation processing.

[0020]

As described above, according to the automatic gain control method for digital color demodulation of the present invention, the demodulated color difference of the color burst when the phase synchronization between the input color burst and the local color subcarrier for color demodulation is established. The amplitude of the signal Eb-Ey component is obtained, the amplitude value is compared with the standard amplitude value of a color burst defined in the color television signal, and the level correction amount for the color demodulation signal output is calculated in units of 2 ^1/2 times. When the level correction amount is 2 ^{n-1 / 2} times where n is a positive integer, the coefficient 2 ^n-1 K ₂ is converted to the demodulated color difference signal Er-Ey.
When 2 ⁿ K ₁ is used as the demodulated color difference signal Eb-Ey, and when the level correction amount is 2 ^{− (n−1 / 2)} times where n is an integer value,
The coefficient 2 ⁻ⁿ K ₂ is added to the demodulated color difference signal Er-Ey,
^{- (n-1)} to K ₁ in the demodulated color difference signal Eb-Ey, level correction amount as an integer value m, in the case of 2 ^m times the coefficient 2 ^m K ₁
In the demodulated color difference signals Er-Ey, multiplying all respect demodulated color difference signals 2 ^m K ₂ when multiplied demodulated color difference signal Eb-Ey
The normalization coefficients K ₁ and K ₂ by 2 ^1/2 times the level correction amount
In each case, alternate control is performed and the demodulated color difference signal
Bit shift by every 2 times the level correction amount to be multiplied
Since control and control are combined, a few circuits such as a level conversion coefficient switching circuit for normalization of two demodulated color difference signal levels and a bit shift circuit are added without using a multiplier having a relatively large circuit scale. And 3 dB, ie, 2
There is an advantage that the demodulated chrominance signal level can be automatically corrected finely in units of ^1/2 . In addition, there is an advantage that the digital color demodulation circuit is suitable for use as an IC because a new circuit is slightly added.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a digital color demodulation circuit according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a digital color demodulation circuit according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between an input color burst level, a level correction amount for a demodulated color difference signal, and a level conversion and level correction coefficient based on the automatic gain control method for digital color demodulation of the present invention.

[Explanation of symbols]

 Reference Signs List 10 A / D conversion circuit 20 Y / C separation circuit 30 Color demodulation circuit 40 Multiplier for level conversion 50 Multiplier for level conversion 60 Level correction amount calculation circuit 70 Level conversion coefficient generation circuit 80 Coefficient for level conversion and correction Switching circuit 90 Bit shift circuit for level conversion and level correction

Claims (1)

(57) [Claims] When Er and Eb are primary color signals and Ey is a luminance signal, a modulated color signal Ec and a luminance signal Ey obtained by quadrature two-phase modulation of two color difference signals Er-Ey and Eb-Ey are superimposed. and the color television signal, Y / C separation, the color demodulation, when returning again to the two color difference signals, a normalization factor K ₁ so that the respective maximum amplitude is the same value in the demodulated color difference signals Er-Ey , digital chrominance demodulation automatic gain system for multiplying the normalization factor K ₂ in the demodulated color difference signal Eb-Ey
In the control method , the amplitude of the demodulated chrominance signal Eb-Ey component of the color burst when the phase synchronization between the input color burst and the local color subcarrier for color demodulation is established is determined. Then, the level correction amount for the color demodulation signal output is calculated in units of 2 ^1/2 times, and the level correction amount is 2 ^{n−1 / 2 where} n is a positive integer. In the case of double, the coefficient 2 ^n-1 K ₂ is added to the demodulated color difference signal Er-Ey by 2 ⁿ
If K ₁ is the demodulated color difference signal Eb-Ey, and the level correction amount is 2 ^{− (n−1 / 2)} times where n is a positive integer,
The factor 2 ^-n K ₂ in the demodulated color difference signals ^{Er-Ey, 2 - (n} -1) K
₁ is used as the demodulated chrominance signal Eb-Ey, and when the level correction amount is 2 ^m times, where m is an integer value, the coefficient 2 ^m
The K ₁ in the demodulated color difference signals Er-Ey, multiplied to the demodulated color difference signals 2 ^m K ₂ when multiplied demodulated color difference signal Eb-Ey
Normalization coefficients K ₁ and K ₂ to be used are increased by 2 ^1/2 times the level correction amount.
Performing alternate control for each unit and demodulation color difference signal
The bit shift is performed every 2 times the level correction amount to be multiplied to the signal.
Automatic gain control method for digital color demodulation, wherein the automatic gain control method is combined with performing shift control .