PAL DEMODULATION

Abstract: In a method of demodulating a PAL chrominance signal, PAL-S demodulated color-difference signals ((R-Y)2, (B-Y)2) (a PAL-S demodulation is a PAL demodulation without an averaging over a line period) are averaged (201, 203, 204) to obtain PAL-D demodulated color-difference signals ((R-Y)1, (B-Y)1), the PAL-S demodulated color-difference signals ((R-Y)2, (B-Y)2) are applied to a vertical color transient detection (213-215) to obtain a control signal (CTRL), and PAL demodulated color-difference signals ((R-Y)0, (B-Y)0) are generated (206, 106) in response to the PAL-S demodulated color-difference signals ((R-Y)2, (B-Y)2) and the PAL-D demodulated color-difference signals ((R-Y)1, (B-Y)1) and in dependence upon the control signal (CTRL).

Description: PAL demodulation. The invention relates to a method and device for demodulation of PAL television signals, and to a PAL television receiver comprising such a PAL demodulation device. In the first TV-sets with PAL color decoders, called PAL Simple, the designers of TV-sets used characteristics of the human eye-sight for additively color merging two successive video lines on the TV-screen. These lines slightly differed in hue, due to transmission phase-errors in the color carrier, but because of the PAL-system the correct color could still be seen, when the eyes were not too close to the TV-screen. The visual effect, of line alternating differences in color on the screen, was called "Venetian blinds", also known as H/2-effect, because the alternating rate was half the line-frequency H. In a trade-off, the Venetian blinds of PAL Simple (PAL-S) decoders were preferred over the constant hue errors of NTSC decoders. Later on, delay-lines were used for color merging in the TV-set and with this PAL Delay-line (PAL-D) decoder the story of the Venetian blinds ended. The use of the delay-lines had also a negative effect on the vertical color transient resolution, but in a trade-off this was accepted as preferable over the Venetian blinds. EP-A-0,675,658 discloses an adaptive type color demodulation apparatus for PAL system, which includes a first demodulation circuit (PAL-D) for demodulating a PAL system chrominance signal between scanning lines, a second demodulation circuit (PAL-S) for demodulating the PAL system chrominance signal on a scanning line, a vertical correlation detection circuit for detecting a vertical correlation between chrominance signals which are apart by two horizontal periods, and a selector for selecting either an output of the first demodulation circuit (PAL-D) or an output of the second demodulation circuit (PAL-S) according to the detected result of the correlation detection circuit. Hue distortion of the chrominance signal which occurs during signal transmission is cancelled even if the phase distortion is large and a precise color signal is demodulated even if there is no correlation between adjacent scanning lines. By having both a PAL-S demodulator and a PAL-D demodulator, both an optimum vertical color transient resolution and the absence of Venetian blinds can be obtained. DE-A-39.31.903 shows a similar system also having two separate PAL chrominance demodulators. It is, inter alia, an object of the invention to provide a color demodulation which needs less hardware and/or calculation efforts. To this end, a first aspect of the invention provides a PAL chrominance demodulation method as defined in claim 1. A second aspect of the invention provides a PAL chrominance demodulation device as defined in claim 3. A third aspect of the invention provides a PAL television receiver as defined in claim 4. An advantageous embodiment is defined in the dependent claim 2. In a method of demodulating a PAL chrominance signal in accordance with the present invention, PAL-S demodulated color-difference signals are averaged to obtain PAL-D demodulated color-difference signals, the PAL-S demodulated color-difference signals are applied to a vertical color transient detection obtain a control signal, and PAL demodulated color-difference signals are generated in response to the PAL-S demodulated color-difference signals and the PAL-D demodulated color-difference signals and in dependence upon the control signal. Herein, a PAL-S demodulation is a PAL demodulation without an averaging over a line period. The invention advantageously only has a single demodulation instead of the two separate demodulations for simple PAL (PAL-S) and delay-line PAL (PAL-D) present in the prior art. In a preferred embodiment, a color transient detector performs a median filter operation on three successive PAL-S demodulated (R-Y) / (B-Y) color difference lines. In the circuit of EP-A-0,675,658, the detection is made on the basis of the modulated chrominance signal, so that vertical color details not thicker than a single line cannot be detected, whilst color changes without saturation changes remain unnoticed. In the circuit of the present invention, color transients are detected on the PAL-S demodulated color-difference signals, so that even those tiny details and color changes without saturation changes are correctly detected. In one embodiment of EP-A-0,675,658, the luminance signal is employed to increase the detection resolution, but this has the disadvantage that luminance changes affect the way in which the chrominance signal is demodulated. In the present invention, a high resolution color transient detection is possible without artifacts. Temporary switching to PAL-S on detection of a color transient restores the loss on vertical color definition caused by the averaging operation of PAL-D. The use of a median filter makes the detector immune for H/2 effects, avoiding false detection on Venetian blinds. PAL-D operation stays effective for color areas without color transients, while the Venetian blinds artifacts of PAL-S are covered up by the color transient itself. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a PAL-S demodulation circuit as known from EP-A-0,675,658; Fig. 2 shows a vertical color transient adaptive circuit which, together with the PAL-S demodulation circuit of Fig. 1, forms an embodiment of a demodulation circuit in accordance with the present invention; Fig. 3 illustrates an alternative element for part of Fig. 2; Fig. 4 shows an output part of a PAL television receiver in accordance with the present invention. Fig. 1 shows a PAL-S demodulation circuit 301 as known from Fig. 2 of EP-A-0,675,658, incorporated herein by reference for further details. A PAL CVBS input signal is applied to a Y/C separator which furnishes a luminance signal Y and a chrominance signal C. A subcarrier generator 302 furnishes a color subcarrier frequency signal fsc to a 90° phase shift circuit 303, and to a 0° and 180° line switching circuit 304 which also receives a line alternating signal H'. The chrominance signal C is applied to a B-Y demodulator 305, which also receives an output signal of the 90° phase shift circuit 303, to furnish a PAL-S demodulated blue color-difference signal (B-Y) 2 . The chrominance signal C is applied to a R-Y demodulator 306, which also receives an output signal of the 0° and 180° line switching circuit 304, to furnish a PAL-S demodulated red color-difference signal (R-Y) 2 . The PAL-S demodulated red and blue color-difference signals from the PAL-S demodulator of Fig. 1 are each applied to a vertical color transient adaptive circuit as shown in Fig. 2. In Fig. 2, the input color-difference signal is applied to PAL-S input S of a fader 206 thru a compensating delay 200. The input color-difference signal is also applied to a cascade connection of two line (IH) delay circuits 201, 202. The input signal and the output signal of the line delay circuit 201 are averaged by an adder 203 and a halving circuit 204, and the average (a PAL-D demodulated color-difference signal (R-Y) j or (B-Y) 1 ; respectively) is applied to a PAL-D input D of the fader 206 thru a compensating delay 205. The non-delayed (OH) input color-difference signal, the once delayed (IH) color-difference signal at the output of the line delay 201, and the twice delayed (2H) color-difference signal at the output of the line delay 202, are applied to a median filter 213 for furnishing a median signal M. A comparator 214 compares the median signal M to the non-delayed signal OH. The comparison result diff is squared by means of a multiplier 215 to form a control signal CTRL for the fader 206. The control signal CTRL is indicative of the magnitude of the output signal diff of the comparator 214. Alternatively, an absolute value circuit can be used instead of the squaring circuit 215. In the fader 206, the control signal CTRL is applied to a limiter 211 to obtain a fading coefficient k. The fading coefficient k is applied to a - input of a subtracter 212, the + input of which receives a 1, to furnish a complementary fading signal 1-k. The PAL-S demodulated input signal of the fader 206 is applied to a multiplier 207 for multiplication by the fading coefficient k. The PAL-D demodulated input signal of the fader 206 is applied to a multiplier 208 for multiplication by the complementary fading coefficient 1-k. Output signals of the multipliers 207, 208 are averaged by an adder 209 and a halving circuit 210 to obtain an output color difference signals (R-Y) 0 or (B-Y) 0 . Fig. 3 shows an alternative 106 for the fader 206 in which a hard switch is effected between the PAL-S demodulated signal and the PAL-D demodulated signal. Circuit description: The median filter 213 gets the 0H-, IH- and 2H- delayed signal on its inputs, where H stands for the period time of the received (R-Y) or (B- Y) color difference signal. On its output the median-value M can be found. S is the PAL- Simple signal and except for some propagation time compensation by delay circuit 200, S is equal to OH. D is the PAL Delay signal. D is the average of OH and IH and is time compensated by delay 205. In the switch option of Fig. 3, the output signal is equal to S or D. In the fader option of Fig. 2, the output signal is a mixture of S and D. The time compensation delays the signals S and D to the correct moment of decision by the switch 106 or the fader 206. The switch 106 or the fader 206 is controlled by the control signal CTRL, where CTRL = (diff)- > and diff is the difference between the median M and OH. Circuit operation: Suppose we have the switch option of Fig. 3 instead of the fader option of Fig. 2. Example 1: When the input signal has no vertical change in color and no H/2, then: M = OH = IH = 2H and the signals diff and CTRL are zero. This means that D is on the output and we have PAL-D (normal PAL) operation. Example 2: When the input-signal has no vertical change in color, but it has H/2, then: OH = 2H which are not equal to IH, and M = OH = 2H. This means that diff is still zero and we have normal PAL operation like in Example 1. Example 3: When the input-signal has a vertical change in color and we have no H/2, we see that OH is not equal to IH = 2H. In this case, diff differs from 0 and CTRL sets the switch 106 to S and we have PAL-S (simple PAL) operation. Example 4: When we have a vertical change in color and we have H/2, then: OH is not equal to 2H which is not equal to IH. In this case, the signal that has the median value is passed over to M and it depends on the degree of difference between M and OH, if the switch switches to S or not. In the fader option of Fig. 2, we have a gradual change from D to S and this has some advantages and disadvantages with regard to the switch option of Fig. 3, but the choice is a matter of taste and money and has nothing to do with the principle of the present invention. Fig. 4 shows an output part of a television receiver in accordance with the present invention. A matrix circuit Mx receives the luminance signal Y from the Y/C separation circuit 300 of Fig. 1 , and output color difference signals (R-Y) 0 and (B-Y) 0 from circuits as shown in Fig. 2, to generate RGB color signals for a display D. The vertical color transient restorer circuit in accordance with the present invention restores the vertical color definition, by using PAL-S on vertical color transients and PAL-D in color areas, thereby avoiding Venetian blinds in color areas, where they are most visible. To do so, in a first embodiment, a color transient detector performs a median filter operation on three successive color lines (R-Y)n (OH), (R-Y)n-f- l (IH) and (R-Y)n+2 (2H). On a transient event, the median of (R-Y)n, (R-Y)n + 1 , and (R-Y)n+2 is not equal to (R-Y)n. Switching to PAL-S on that event, restores the vertical color definition. The same operation must also be done for (B- Y) . The use of the median filter makes the detector immune for H/2-effects, avoiding false detection on Venetian blinds "color transients" . PAL-D operation stays effective in color areas, so Venetian blinds will not occur there and PAL-S Venetian blinds are covered up by the color transient itself. The saturation of a one line colored object will increase 2 times, as it is not merged with the previous black & white video line. The color purity of the one line colored object will also improve, as it is not merged with others colors of a previous video line. The use of a median filter is not compulsory; one can also compare (R-Y)n (OH) to (R-Y)n + 1 (IH) and perform the switching to PAL-S accordingly, but that makes the circuit vulnerable to H/2. At a certain amount of H/2 the Venetian blinds will be detected as vertical color transients and that will switch the circuit to the PAL-S mode in a H/2-signal-condition, where we need PAL-D very most. A better simplification is just comparing (R-Y)n (OH) to (R-Y)n + 2 (2H), for switching between PAL-D and PAL-S. This will solve the H/2-problem, but then the detection of the vertical transient is less defined and two PAL-S lines are used on one vertical color transient introducing more Venetian blinds on the color edges. Resuming: as a consequence of the present invention, A the edge of a vertical color transient is restored; + the saturation of a one-line-colored-object will be increased by 2 times; + we have no vertical-color smear. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer.

Claims: Claims: 1. A method of demodulating a PAL chrominance signal, the method comprising the steps of: PAL-S demodulating (301) a PAL chrominance signal (C) to obtain PAL-S demodulated color-difference signals ( ( -Y) 2 , (B-Y) 2 ), where a PAL-S demodulation is a PAL demodulation without an averaging over a line period; averaging (201, 203, 204) said PAL-S demodulated color-difference signals ( (R-Y) 2 , (B-Y) 2 ) to obtain PAL-D demodulated color-difference signals ( (R-Y) j , applying said PAL-S demodulated color-difference signals ( (R-Y) 2 , (B-Y) 2 ) to a vertical color transient detection (213-215) to obtain a control signal (CTRL); and generating (206, 106) PAL demodulated color-difference signals ( (R-Y) 0 , (B-Y) 0 ) in response to said PAL-S demodulated color-difference signals ( (R-Y) 2 , (B-Y) 2 ) and said PAL-D demodulated color-difference signals ( (R-Y) j , (B-Y^ ) and in dependence upon said control signal (CTRL). 2. A method as claimed in claim 1, wherein said vertical color transient detection (213-215) includes a median filtering of non-delayed (OH), one-line-period delayed (IH), and two-line-period-delayed (2H) PAL-S demodulated color-difference signals ( (R- Y) 2 , (B-Y) 2 ). 3. A device for demodulating a PAL chrominance signal, the device comprising: means for PAL-S demodulating (301) a PAL chrominance signal (C) to obtain PAL-S demodulated color-difference signals ( (R-Y) 2 , (B-Y) 2 ), where a PAL-S demodulation is a PAL demodulation without an averaging over a line period; means for averaging (201, 203, 204) said PAL-S demodulated color- difference signals ( (R-Y) 2 , (B-Y) 2 ) to obtain PAL-D demodulated color-difference signals ( (R-Y) j , (B-Y) i ); means for applying said PAL-S demodulated color-difference signals ( (R-Y) 2 , (B-Y) 2 ) to a vertical color transient detection (213-215) to obtain a control signal (CTRL); and means for generating (206, 106) PAL demodulated color-difference signals ( (R-Y) 0 , (B-Y) 0 ) in response to said PAL-S demodulated color-difference signals ( (R-Y) 2 , (B-Y) 2 ) and said PAL-D demodulated color-difference signals ( (R-Y), , (B-Y), ) and in dependence upon said control signal (CTRL). 4. A PAL television receiver, comprising: means (300) for separating a PAL television signal into a luminance signal (Y) and a PAL chrominance signal (C); a device for demodulating the PAL chrominance signal (C) as claimed in claim 3; a matrix circuit (Mx) for generating RGB color signals in response to said luminance signal (Y) and said PAL demodulated color-difference signal ( (R-Y) 0 , (B-Y) 0 ); and means (D) for displaying said RGB color signals.

Inventors: TALENS HENDRINUS, TALENS, HENDRINUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H04N11
IPC codes: H04N9, H04N11