DE2210307C3 - Color television receiver for the PAL system with synchronous demodulators and a phase correction device - Google Patents

Description

The invention relates to a color television receiver for 2in transmission systems such as the PAL system, in which two color signals are simultaneously related to a quadrature balanced modulation of a color carrier to mutually perpendicular modulation axes, one of which in consecutive lines is reversed by 180 °, with synchronous demodulators for demodulating the chrominance signals on hand of reference carriers having a reference phase corresponding to the modulation axes and with a phase correction device, which phase errors the reference carrier determines and the phase via an error signal the reference carrier is twisted.

The demodulation of the chrominance signal to derive the two color signals requires the use of two reference carriers that maintain a fixed phase relationship with the received color carrier, though one of the reference carriers carry out a phase change of 180 ° between successive lines got to. In order to automatically establish this phase relationship, the color television signal contains according to the PAL system a color burst whose phase during successive lines compared to a reference phase alternates between +45 and -45 °. The color sync signal is used in the color television receiver as Phase reference for generating the two reference carriers. The one in the transmission system between the chrominance signal and the burst occurring shifts and those in the receiver between the Chrominance signal and the reference carrier occurring

However, shifts can lead to a deterioration in the fidelity of the demodulated color signal.

It is known (DT-PS 1252731) to eliminate a phase error to delay the signal of one line by one line duration and then the signal of the next Line to be superimposed in which the same phase error due to the line-by-line phase aging The phase error is included with the same absolute angular value, but with the opposite sign Although this balances out, there is a certain reduction in resolution in the vertical direction of the image and also an error in color saturation which becomes larger the larger it is corrected phase error is. It also provides the delay line a cumbersome component.

It is also known (DT-AS 1 904 528), in the sense of the type of color television receiver mentioned at the beginning take the phase error from the signal and take it into account during demodulation via an error signal, so that the delay line is avoided of the component that has not been switched and that has already been demodulated share the low-frequency components with one another multiplied and from this a controlled variable proportional to the phase error is formed. This controlled variable is however not only the phase deviation, but also the respective amplitude of the chrominance signal component used proportional, which means that the regulation can be continuously controlled back and forth.

To avoid the amplitude dependence, it is known in further circuit complications, instead of the Multiplication product is the quotient of the two demodulated, freed from the higher-frequency components Form color signals. This requires a cumbersome and imprecise circuit, since the Division is carried out by forming the difference between the logarithms of the signal voltages.

In contrast, the invention is based on the object of the phase error correction without additional Carry out sources of error with a circuit with a simple function.

This object is achieved according to the invention in that the phase correction device at least two phase detectors that are selectively activated as a function of the instantaneous phase of the chrominance signal contains that compare the phase of the chrominance signal with the phase of a subcarrier and from the output signal of which the error signal is derived after a temporal averaging, this Subcarriers each have a fixed phase derived from one of the reference carriers.

By dje invention, the phase of one of the signals, in particular the phase of the reference carrier, is corrected to compensate for the relative error between the phase of the color sync signal or reference carrier and the phase of the chrominance signal (B- Y) axis of alternating chrominance signal obtained. The 6c invention proceeds in such a way that it demodulates the phase of the chrominance signal in comparison to a fixed phase which is related to the reference carrier phase, with the alternating part falling out on average and only the phase-constant part and a possibly phase-constant component of the phase-alternating part becomes visible in the time-averaged demodulation result. The phase-constant component of the phase-alternating portion indicates the error. It has, however, includes, depending on the demodulation and instantaneous phase of the chrominance signal, a sign that no adequate provision for the direction of the required correction phase rotation this respect, two phase detectors with separate detection areas are at least required, for example, the range of the positive and the negative (B - represent y ^ -axis, and in which the same polarity of the determined component has a phase rotation in respectively different sense result or there are four phase detectors present which are in pairs set in operation and the pair of the area of the positive or negative (R - Operate the Y) axis.

In summary, the invention is based on the idea that the carrier phases are regulated in such a way that the chrominance signal alternating from line to line around the (B-Y) axis occurs on average symmetrically to this axis Way symmetrically oppositely polarized phase detector voltages formed, which are balanced against each other. If the weight is unequal, the phase correction device restores the symmetry around the (B- Y7 axis.

More details, advantages and training of the invention emerge from the primary claims of the following description of exemplary embodiments and reference to the drawing. It shows

F i g. 1 is a block diagram of a demodulation circuit a known color television receiver,

F i g. 2 is a block diagram of a demodulator circuit according to an embodiment of the invention,

F i g. 3 to 6 vector diagrams to illustrate the mode of operation of the circuit according to FIG. 2,

F i g. 7 is a block diagram of a demodulator circuit according to another embodiment of the invention and

F i g. 8, 9a and 9b vectorial and history diagrams to illustrate the mode of operation of the circuit according to FIG. 7th

In all figures, the same reference symbols are used to denote the same parts

F i g. 1 shows a known color demodulation circuit with a bandpass amplifier 1, which is connected to a delay element 2, which has a delay period equal to a line duration.The chrominance signal coming directly from the amplifier 1 and the chrominance signal coming from the delay element: t 2 are shared on the one hand with an adder 3 and on the other a subtraction stage 4 leads to then get the (B-Y) and the (R- Y) -Signalkomponenti from the adding stage 3 and the subtracting stage 4. The (R- V> component of subtract 4 here is subjected to a phase reversal between successive lines. The (B- Y) component obtained becomes a (B- ! ^ - demodulator 5 and di ( (R- Y) - component a (R- V> demodulator 6 conducts trains. From a reference carrier generator 7, the demodulators 5 and 6 conduct respective reference carrier trains, which are related to a color synchronous signal, since the chrominance signal contains the (R- Υ) · demodulator 6 fed reference carrier towards the (BV ^ demodulator 5 fed reference carrier in quadrature phase and reverses its phase for each line. The mode of operation of the circuit shown is known and therefore does not need a

to be described individually. It should be shown briefly, however, that the delay element 2 feeds the adder 3 with a chrominance signal whose (R-Y) component has a reversed polarity, while at the same time the adder 3 feeds a chrominance signal with a non-reversed (R- Y ^ component to each other extinguishes and the pure (B-Y) component appears at the output of adder 3.

F i g. 2 shows a color demodulator circuit according to the invention. The signal of the bandpass amplifier • amplifier 1 is fed directly to the two demodulators 5 and 6 for the (BY) and the (RY) components and is also fed to a phase shifter 14, which is put into operation on a detector voltage that is set by a Phase error checking circuit is derived, which consists of blocks 16-27. The phase shifter 14 generates a phase-modified color sync signal for the operation of the reference carrier generator 7. Gate circuits 16, 17 and 18 are actuated by the output signal of the (B- Y) demodulator 5, i.e. from the (B-Y) -S \ gna \ open at a given operating level corresponding to different output levels of the (B- YT signal, which are shown at ei, ei, - ei, - ei in Fig. 3; a chrominance signal is divided according to its phase position, as it is by four Areas A, B, C and D are shown in FIG. 3, for example. The output signals of the gate circuit 16, 17 and 18 are fed to phase detectors 19, 20 and 21 respectively phase shifters 22 and 23 are supplied to the reference carrier for the (RY) - axis or for - (RY) - to form the axis of the reference carrier for the (RY y-axis is the phase detector 19 and the reference carrier for the. - (R- Y> axis pull the phase detectors 20 and 21 rt.

It is now assumed that a chrominance signal comes in that is only in area A. In this case, only the gate circuit 16 is open and applies the chrominance signal to the phase detector 19. If the chrominance signal and the reference carrier applied to the (B- Y) demodulator 5 are in the correct phase relationship, this can be represented by symmetrical positions of the chrominance signal with respect to the (β-YT axis during successive lines, as shown in FIG. 4 is shown at / and h so that the output signal of the phase detector 19 in successive lines is equal in height and opposite in polarity as shown in Fig. 4 at j and / A time constant which is more than twice the line duration is applied, so the output signal of the filter is NuIL

However, if the chrominance signal is the result of a transmission distortion, due to zero point shifts in the receiver, etc., offset clockwise in such a way that phase positions g and / in Fig. 4 occur in successive lines, then the output signal of the detector in successive lines corresponds in height and polarity to those at Jt or .m, so that in this example the output signal of the filter 24 is a negative voltage. For a shift in the counterclockwise direction, a positive voltage occurs as the output signal. In Fig. 4, the detector axis of the detector 19 is shown at T on the left in the figure

If it is then assumed that a chrominance signal lying in area B is fed in, then only the gate circuit 17 switches to transmission and the chrominance signal is passed to the phase detector 20. If the chrominance signal and the reference carrier applied to the (B- ^ demodulator 5 are in the correct phase relationship, then the chrominance signal is symmetrical with respect to the (B- y) axis during successive lines, as shown at η and ρ in FIG. 5 is such that the output signal of the phase detector 20 during successive lines of equal magnitude and opposite polarity, so that the output signal of the filter 24 is zero. If a shift in the chrominance signal clockwise to as q by phase angles and r in F i g 5, the detector output signal is unequal in successive lines in the manner shown at υ and ν, and a negative voltage occurs at the output terminal of the filter 24. A positive voltage occurs for a counterclockwise shift. that in Fig. 5, which illustrates the operation of the detector 20, because of the comparison with the phase of the auxiliary supplied to the detector 19 sluggishly reversed phase, the detector axis is reversed as indicated by an arrow denoted by - T

For a chrominance signal a) in the areas C and D , the gate circuit 18 opens and applies the chrominance signal to the phase detector 21. If the chrominance signal and the reference carrier applied to the (B- ^ demodulator 5 are in the correct phase relationship, the signal appears in successive line periods in the areas C and D, so that the output signal of the detector 21 is of the same height and opposite polarity during successive lines and a filter 25, which is constructed in the same way as the filter 24, emits the output signal zero. If a phase shift occurs and the chrominance signal only appears in one of the two areas C and D, a positive or negative voltage is applied during every second line the output terminal of the detector 21 and thus also generated as the output signal of the filter 25

The output signal of the filter 25 is used to control a switch 26, which is thus included in the circuit is that it routes the output of filter 24 directly to phase shifter 14 when the output of the filter 25 is zero and the output signal of the filter 24 via an inverter 27 to the phase shifter 14 conducts when the output signal of the filter 25 is a positive or negative voltage

It is assumed that the chrominance signal occurs only in area C during one line and does not appear in area D in the next line A or at b appears in area B (FIG. 6). This signal is therefore detected by the phase detector 19 or 20, depending on whether it appears at a or b , and it then appears as the output signal of the filter 24. Since such an output signal, when it is fed directly to the phase shifter 14, is of the opposite polarity to the polarity of the desired control signal, it is fed via the inverter 27 and thus reversed in polarity before it arrives at the phase shifter 14 by the amount in this to control the phase shift communicated to the burst signal. Although not shown, the phase shifter 14 or the reference carrier generator 7 may contain a burst gate or a burst gate.

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With the circuit shown, the phase of the reference carrier that is fed to the (B-Y) or (R- V / demodulator 5 or 6) can be automatically regulated in the correct relationship to the chrominance signal for each phase position of the same.

F i g. Fig. 7 shows another embodiment of a color demodulation circuit of a color television receiver according to the invention. The circuit differs from that according to FIG. 2 primarily in the form of the phase error detection circuit which includes a blanking circuit 3t, phase detectors 32 to 35, phase shift circuits 36 to 39, low pass filters 41 to 44, polarity detecting diodes 45 to 48, and synthesizing or merging resistors 51 to 56. The dark control circuit 31 samples a part of the chrominance signal supplied by the bandpass amplifier 1 during a line period and feeds the remaining part of the chrominance signal to the phase detectors 32 to 35. This is described in more detail below. These detectors are fed ao from the generator 7 via the phase shifters 36 to 39 with reference carriers. The degree of phase rotation in these phase shifters is such that the reference carriers fed into the detectors have phases which are symmetrical to one another with respect to the (B-Y) and (R- > 7 demodulation axes, as in a, b c and d is shown in Fig. 8. The arrangement is such that the detector output is, for example, a positive voltage when the chrominance signal and the reference carrier associated with this detector are of the same phase and a negative voltage when These output signals are passed through the low-pass filters 41 to 44 and then grounded via the diodes 45 to 48, which have the polarity shown The resistors 55 and 56, whose connection point is connected to the phase shifter 14, are connected in series via the connection points of the respective resistors that belong together n to supply a control voltage. Depending on the choice of the resistance values of the resistors 51 to 54, namely when their values are chosen so that the influence of the potential at one of the connection points on the other is negligible, the resistors 55 and 56 can also be omitted.

With regard to the operation of the circuit according to FIG. 7 it should be noted that because of the fact that in the PAL system the phase of the (R- V ^ signal for successive lines by 180 ° reverses the vector sum for two successive lines as a result either the f £ -V> signal or yields the - (BY) signal alone Assuming that a (B- V) signal component such as that shown at e in Fig. 8 is applied to each of the phase detectors, the detector generates 32 a positive voltage at the output terminal of the filter 41, while the detector 33 generates a negative voltage at the output terminal of 42. Since these voltages are of the same magnitude, the addition of the voltages with the aid of the resistors 51 and 52, which have the same resistance value, results in the result is a zero potential at the junction of the resistors. On the other hand, the detector 34 produces a negative voltage at the 3 output terminal of the filter 43, this voltage being conducted to earth through the forward biased diode 47 In the same The detector 35 generates a positive voltage at the output terminal of the filter 44, which voltage is also conducted to earth by the forward-biased diode 48. As a result, the assembly control voltage is zero.

If the signal e is shifted clockwise to the phase position j, the positive output voltage of the detector 32 decreases, while the negative output voltage of the detector 33 increases, so that a negative voltage occurs at the junction of the resistors 51 and 52. In the case of a phase shift in the opposite direction, that is to say counterclockwise into phase position g, a positive voltage is obtained at this connection point.

If a chrominance signal corresponding to the - (BY) - signal is applied, as is the case with h and F i g. 8, the diodes 45 and 46 derive the output voltage of their associated detectors so that the potential at the junction between the resistors 51 and 52 is zero. At the same time, the detector 34 generates a positive voltage at the output terminal of the filter 43, while the detector 35 generates a negative voltage at the output terminal of the filter 44. Since these voltages are of the same magnitude and the resistors 53 and 54 have the same resistance value, the connection point between the resistors has the potential zero, the eventual control voltage also having the value zero.

If the - (B- y / signal component rotated clockwise, as shown at / is shown, the positive output voltage of the detector 34 decreases, while the negative output voltage of the detector 35 increases, with the result that a negative voltage at the connection point between the Resistors 53 and 54 occurs When the -fß-V ^ signal component is rotated in phase counterclockwise to j , a positive voltage is generated at this connection point Generator 7 is used as a phase reference, applied phase rotation is used

While the (B- Y) axis component was used to derive the phase control voltage in this circuit described , the (R- V ^ signal component can also be used together with reference carriers that have different phases, for example reverting phases in successive lines, with corresponding success , as well as those derived from the output signal of the generator 7 that feeds the (R-! ^ demodulator 6).

The purpose of the dark control circuit 31 is to remove part of the chrominance signal during each line in order to enable phase control also, for example, for a color bar signal in which the sum of the (B- ^ components above each line duration is equal to zero , regardless of any Degree of phase shift that could be present. For example, if the color bar signal appears symmetrical with respect to the (B- ! ^ Direction, as shown in FIG The dark control signal need not have a narrowly defined course or a specific phase as long as it remains unchanged between successive lines

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in order to provide for the case of different saturation of different hues. The phase shifters 38 and 39 can be omitted if desired if the polarity of detectors 34 and 35 is reversed.

The embodiment results in an automatic correction of the phase of the supplied to the two demodulators Reference carrier in terms of the chrominance signal for any phase thereof, whereby from Color television receiver that can reproduce real, pure color.

For this purpose 4 sheets of drawings

Claims (11)

Patent claims: 1. Color television receiver for a transmission system such as the PAL system, in which two color signals a quadrature-balanced modulation of a color carrier at the same time with respect to mutually perpendicular Effect modulation axes, one of which in successive lines in each case around 180 ° is reversed, with synchronous demodulators for demodulating the chrominance signals on the basis of a reference phase corresponding to the modulation axes having reference carriers and with a Phase correction device which detects phase errors in the reference carrier and uses a stolen signal the phase of the reference carrier is twisted, thereby characterized in that the phase correction means selectively at least two in dependence phase detectors activated by the instantaneous phase of the chrominance signal (19, 20; 32, 33, 34, 35) contains the phase of the chrominance signal with the phase of each subcarrier compare and from their output signal after a time averaging the error signal is derived, these auxiliary carriers each being a fixed one derived from one of the reference carriers Have phase. 2. Color television receiver according to claim 1, characterized in that one of the subcarriers has the + (R- Yy phase of the reference carrier and the other subcarrier the - (Ä-V> Phas2 of the reference carrier) and that the output signal of the phase detector (19, 20) is taken as an error signal after the temporal averaging. 3. Color television receiver according to claim 1, characterized in that the further processed by the temporal averaging output signals from two phase detectors (32, 33; 34, 35), their subcarriers to the + <7? -Y> phase or - (R- Yy phase the reference carriers have symmetrical phases (a 6; c, d in FIG. 8), are averaged against each other by circuitry (at 51, 52; 53, 54) and that the output signal obtained in this way is taken as an error signal. 4. Color television receiver according to one of claims 1 to 3, characterized in that the phase detectors (19, 20) at least one of the input signals to be compared with regard to the phase each via a gate circuit (16,17) is fed, which depends on the phase of the The chrominance signal switches to passage or blocking. 5. Color television receiver according to one of claims 1 to 3, characterized in that the time averaged output signals of the phase detectors (32,33,34, 35) through diodes (45,46,47, 48) on their Polarity checked and selectively short-circuited by the respective diode. 6. Color television receiver according to claim 2 and 4, characterized in that the gate circuits (16, 17) are controlled by the demodulated (B- V / signal and have different response levels. 7. Color television receiver according to claim 6, characterized in that of three gate circuits (16, 17, 18) assigned to each phase detector (19, 20, 21) one (16) in the case of a positive demodulated (B-YyS'igna \ of a minimum level (ei ), one (17) with a negative demodulated (BV> signal of a minimum level (-ei) and one (18) with a demodulated (BY) signal lying between the minimum levels around the value zero switches to pass and that the time-averaged output signal of the to the latter Gate circuit (18) connected phase detector (21) is used to reverse the polarity of the time-averaged output signal of the two other phase detectors 8. Color television receiver according to claim 3, characterized in that four phase detectors (32, 33, 34, 35) are present, of which one of the pairs of phase detectors (32, 33; 34, 35), the time-averaged output signals of which are circuit-averaged against each other, works while the other of the couple is at rest 9. Color television receiver according to claim 8, characterized in that the auxiliary carriers for the four phase detectors (32, 33, 34, 35) are also symmetrical to the + (BY) axis or - (B-Yy axis of the reference carrier. 10. Color television receiver according to one of claims 3, 8 and 9, characterized in that between a bandpass amplifier (1) delivering the chrominance signal and the phase detectors (32, 33, 34, 35) a chrominance signal dark control circuit (31) is switched on. 11. Color television receiver according to one of claims 1 to 10, characterized in that the Temporal averaging of the output signals from the phase detectors (19, 20; 32, 33, 34, 35) Filter (24, 25; 41, 42, 43, 44) takes place, the time constant of which is greater than twice the line duration.