DE2118891A1 - Demodulation device for color television receivers - Google Patents

Description

The invention relates to a demodulation system for color television receivers and, more particularly, to a demodulation system for demodulating the color television video signal, which is transmitted by means of the PAL color television system will.

In the case of the PAL color television system, which is used in European Is widely used in countries, it is known that the modulation axes of two color signal components, i. the signals of the R-Y and B-Y components, a chrominance signal (chrominance signal), which is determined by quadrature amplitude

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denmodulation of a carrier wave is formed by two color-dependent signals, phase-shifted by 90 °, one these modulation axes, namely those of the R-Y component, in alternating line periods of the video signal by 180 ° is turned over. At the same time, the color sync pulses (burst signals) for the alternating line periods are also generated line shifted alternately by + 45 °.

To that transmitted in this way according to the PAL system To demodulate color video signal, it is generally necessary, the phase of a subcarrier, that of the color signal demodulating stage of a television receiver, with the modulation axis of the R-Y signal component of the transmitted Waveform to match, their in alternating periods of the color video signal Phase changes.

For this purpose, a system has been proposed in which the phase of the for demodulating the R-Y signal component subcarrier used in the color television receiver is reversed by 180 ° for alternating line periods of the video signal so that the modulation and demodulation axes coincide. In the proposed system, however, there is a phase indicator stage in the color television receiving circuit required to reverse the phase of the

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Carry out auxiliary carrier.

According to the present invention, an improved system of the above type is to be provided in which a phase indicator level is not required. In the system according to the invention, the chrominance signal and / or the bursts, both of which are contained in the transmitted waveform, in every other Line period of the video signal phase-rotated using the horizontal synchronizing signal or synchronous switching signals generated with this in such a way that the modulation axis of the R-Y component signal (carrier-frequency R-Y color difference signal), which is received by the television receiver can be brought into register with the demodulation axis of the sub-carrier used in the receiver to transmit the color television information by means of the television receiver to represent.

Preferably according to the invention is a demodulation system for use in a color television receiver for demodulating of the color television video information transmitted according to the PAL system, which consists of the following parts consists: means for reversing the phase of the composite color signal in every other line period of the video signal in Depending on the horizontal synchronization signal or a control signal synchronized with it, a demodulation stage, to which the chrominance signal is fed to

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Demodulating the R-Y color signal, a gating stage (gating circuit) for separating the burst signal (color sync pulse) from the composite color signal, and an APC color synchronizer that receives the burst signals are fed from the gating stage in order to generate a subcarrier to be fed to the demodulating stage, its demodulation axes with the modulation axes of the R-Y component signal which is also to be fed to the demodulation stage are in coincidence.

According to a preferred feature of the invention, the means for reversing the phase of the composite color signal cause two modes of operation of the system according to the invention. In both operating modes, the color difference signal can of a predetermined polarity can be advantageously obtained so that accurate demodulation of the R-Y color signal and thereby a realistic color representation using the PAL television broadcasting system receives.

Embodiments of the invention are described below explained in more detail with reference to the drawings.

Fig. 1 shows the block diagram of a demodulator circuit for a color television receiver according to a preferred embodiment of the invention.

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FIG. 2 schematically shows various vector representations of the ones used to explain FIG. 1 Signals.

FIG. 3 shows in a block diagram another preferred embodiment of the demodulating stage according to FIG Invention.

4 shows a further preferred embodiment of the demodulating stage in a block diagram.

5 schematically shows various vector representations of the signals used to explain FIG.

FIG. 6 shows the circuit diagram of a practical embodiment of the arrangement according to FIG. 3.

Reference is first made to FIGS. 1 and 2. With a first video signal amplifier 1 is a second video signal amplifier 2 connected in series, from which a luminous signal Y (luminance signal) can be obtained. With 3 is a separation stage for the composite color signal (FarbaustastSynchronsignal) referred to, which consists of a burst signal (color synchronous pulse) and a chrominance signal (chrominance signal) consisting of a composite color signal (Color condition synchronous signal) from the composite,

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also separates the video signal containing luminance signal Y which is broadcast according to the PAL color television system. 4 and 4 ¹ denote bandpass amplifiers, 5 and 5 ¹ denote demodulation stages for the signal components RY and BY. With 7 and 7 ¹ are color synchronization stages with automatic phase control referred to, which can be of conventional design and are referred to below as APC stages.
i

A composite color signal, the vector representation of which is indicated at a in FIG. 2 and which contains the burst signal, is fed to ^{the two bandpass amplifiers 4 and 4 1.} These feed the chrominance signal contained in the composite color signal to the corresponding demodulation stages 5 and 5 ¹ . A combination of parallel phase rotation stages 8 and 9 with 180 ° or 0 ° phase rotation and an electronic switching device 10 is arranged between the separation stage 3 and one of the bandpass amplifier 4, so that the phase of the composite color signal fed to the bandpass amplifier 4 in every second line period can be reversed. The arrangement 10 can be controlled in such a way that it alternately switches the phase rotation stage 8 and 9, which rotate by 180 ° and 0 °, in alternating line periods in the path of the composite color signal. The switching

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device 10 is controlled by switching signals which have half the horizontal frequency and their square wave form is indicated at d in FIG. These switching signals are generated by a switching signal generator 12, for example a multivibrator that is synchronized with a horizontal synchronizing signal (horizontal deflection signal) or an output of a horizontal frequency signal generator 11, this being a suitable signal such as e.g. generates a flyback pulse that is synchronized with the horizontal sync signal is synchronized, and the waveform of which is indicated at c in FIG.

The horizontal synchronizing signal (line synchronizing signal) or the other signal used, such as a return pulse synchronized with the line synchronization signal, is fed to the switching signal generator 12 in order to generate the switching signals that control the switching process the switching device 10 controls in successive line periods. According to the present invention are however, two modes of operation, namely mode I and mode II, are possible for the device according to the invention, depending on whether the switching signals that are fed to the switching device 10 and the switching on either the 180 ° or 0 ° rotating phase rotation stages 8 and 9 in the path of the composite color signal, with the even or odd components of the

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Zeilensynchronlsiersignals are synchronized. In each Case and regardless of the mode of operation used in each case, this should be done by the demodulation stage 5 for the R-Y signal can be an R-Y color difference signal. For the purpose of description it is assumed that that the operating mode I is the one in which the switching device 10, the 180 ° rotating phase rotation stage 8 in the path of the composite color signal switches in the presence of a switching signal which corresponds to the odd-numbered Components of the line synchronization signal switches, while in mode II the switching device 10 the 180 ° rotating phase rotation stage 8 switches into the path of the composite color signal when the there are switching signals synchronized with the even-numbered components of the line synchronization signal.

Operating mode I is first described below as an example. In operating mode I, the switches Switching device 10 alternately the 180 ° and 0 ° rotating phase rotation stages 8 and 9 during successive Line periods of the video signal as shown at d in Fig. 2, curve A indicating that the switching means 10 is in the position in which the 180 rotating stage 10 is in the path of the composite color signal is switched, while curve b indicates that the switching device 10 is in the position in which the rotate by 0 °

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de level 9 is switched on. It can be seen that here the component signals of the composite color signal, the corresponding to the odd-numbered lines are reversed in phase, while those corresponding to the even-numbered lines Signal components are allowed to pass without phase reversal. A vector representation of the two component signals is given at e in Fig. 2 with the R-Y modulation axes of the chrominance signal all directed downwards are.

A combination of the two color component signals is produced by the bandpass amplifier 4 of both the R-Y demodulating stage 5 and the burst blanking stage 6 is supplied. This burst blanking stage 6 allows the burst signals to pass through, when a control pulse is supplied, the waveform of which is indicated at c in Fig. 2 and that with the line synchronizing signal is synchronized. The vector representations of the blanking stage 6 allowed through in this way burst signals are shown at f in FIG. Thereafter, the burst signals of a phase detector circuit 13 of the APC stage 7 supplied.

The APC stage 7, which can be of known construction, contains the phase detector stage 13 »a filter 14, which works simultaneously as an integrating stage, a reactance element 15 and an auxiliary carrier oscillator stage 16, where-

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all of these parts are connected in series. In this APC stage 7, the burst signals that are passed are transmitted accordingly f or f 'in FIG. 2 by the phase detector 13 with the output of the subcarrier oscillator 16, and the output signals of the phase detector 13 are then the filter circuit 14 is supplied. The output signals from the phase detector 13 are integrated in the filter circuit 14 and then from the filter circuit 14 to the reactance element 15 as control signals fed. The output of the reactance element 15 is used for phase control of the output signal of the subcarrier oscillator 16 is used, in this way the vector representation of the subcarrier from the APC stage 7 of the R-Y demodulation stage 5 is controlled so as to match that of the R-Y color component signal as mentioned above which is fed directly from the bandpass amplifier 4 to the R-Y demodulation stage. Accordingly are the output signals from the APC stage 7, which in turn are fed to the R-Y demodulation stage, all in correspondence with the downward - (R-Y) axis as indicated at g in FIG.

On the other hand, the vector representation of the R-Y component signal, which is fed directly from the bandpass amplifier 4 to the R-Y demodulation stage 5, also after directed downwards. As a result, as shown at h in Fig. 2, the + (R-Y) color signal from the R-Y demodulating stage can be obtained obtain.

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In operating mode II, the switching device 10 switches the phase rotation stages 8 and 9 rotating by 180 ° or 0 ° alternately in the manner indicated at d ¹ in FIG. 2 during successive line periods of the video signal. Here, the color component signals of the composite color signal, which correspond to the even-numbered lines, are reversed in phase, while the remaining component signals corresponding to the odd-numbered lines are passed through without a phase reversal. A vector representation of the two component signals of the composite color signal is shown at e ¹ in FIG. 2, the RY axes all pointing upwards. In a corresponding manner as explained above with reference to operating mode I, the blanked burst signals are obtained from the blanking stage 6 with the vector representations indicated at f in FIG. Furthermore, the phase of the subcarrier, which emanates from the subcarrier oscillator 16 of the APC stage 7, is oriented upwards in a corresponding manner, as ^{indicated at g f} in FIG. 2. It can thus be seen that the RY demodulating stage 5 produces an output signal whose vector representation indicated at h * in FIG.

So you can either in mode I or mode II, the color difference signal from a

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lh

(.J

given polarity in'advantageous way. This means that the demodulation of the R-Y color signal can be carried out very precisely, resulting in realistic color reproduction for color television broadcasts after the PAL system enables. On the other hand, since the modulation axes of the B-Y color component signal in all of the line periods of the video signal do not suffer a phase change, the demodulation of this B-Y color signal can essentially can be performed in the same manner as heretofore generally used in the NTSC color television broadcast system is common.

Fig. 3 shows another preferred embodiment of the device according to the invention, in which only one band-pass amplifier 4 is used for the two R-Y and B-Y color component signals. A combination of phase rotation stages arranged in parallel and rotating by 180 ° or 0 ° 20 and 21 and a switching device 22 connected in series with this, which between the bandpass amplifier 4 and the R-Y demodulation stage 5 is located.

In the arrangement according to FIG. 3, the burst signals, which are used for generating the subcarrier to be fed to the R-Y demodulation stage 5, in every second line

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lenperlode of the video signal is reversed in its phase, namely by means of the alternately switched on, 180 ° or 0 ° rotating phase rotation stages 17 and 18 together with the switching device 19 »whereby this takes place synchronously with the line synchronization signal before the burst signals of the APC Stage 7 are fed. On the other hand, the phase of the chrominance signal to be fed to the RY demodulating stage 5 by the bandpass amplifier 4 is reversed in every second line period of the video signal, namely by the alternately switched on, 180 ° and 0 ° rotating phase rotation stages 20 and 21 together with the switching device 22, which is controlled by switching signals which are synchronized with the signals which control the switching device 19 for the phase reversal of the burst signals in every other line period of the video signal. In this way, the modulation axes of the RY color component signal contained in the chrominance signal to be fed to the demodulating stage RY are all oriented in the same direction. In this case, too, the vector representations shown in FIG. 2 can be used, regardless of whether the two switching devices 19 and 22 are operated in operating mode I or operating mode II, in order to cause the output of the RY demodulating stage 5 to be a color difference signal of the Polarity is + (RY). However, it must be taken into account that it is also possible to use one of the switching devices 19 and 22 in operating mode I and the other in operation

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drive mode II or vice versa to operate. In this case the output of the R-Y demodulation stage 5 is a - (R-Y) color signal, which is preferable in view of the number of following amplifier stages.

Fig. 4 shows a further preferred embodiment of the device according to the invention, with the same parts as in Figures 1 and 2 are denoted by the same reference numerals. In the block diagram of FIG. 4, there is no the chrominance signal to be fed to the R-Y demodulating stage 5 in every other line period of the video signal in its Phase reversed, as in the arrangement according to FIG. 3, but instead there is a phase reversal of the subcarrier in FIG every other line period of the video signal. For this purpose is a combination of arranged in parallel to 180 ° or 0 ° rotating phase rotation stages 23 and 24 and one so that switching device 23 connected in series is provided, between the subcarrier oscillator 16 of the APC stage 7 and the R-Y demodulation stage 5. The switching function this switching device 25 is controlled by switching signals, which are synchronized with those which are used to control the switching function of the switching device 19 will. In the device according to FIG. 4, too, the modulation axes of the R-Y component can be that of the R-Y demodulation stage 5 to be supplied chrominance signal with the phase of the

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fs

Subcarrier to be matched to the Generation of the color difference signal from R-Y by the R-Y demodulating stage 5 to enable.

The vector representations of the burst signals and of the subcarrier generated by the subcarrier oscillator 16 are shown at f and f 'and g and g ¹ in FIG. In other words, the phase of the subcarrier generated by the subcarrier oscillator 16 is reversed in every second line period of the video signal, namely by the alternately switched on phase rotation stages 23 and 24 together with the switching device 25 »so that the subcarrier, which in every second line period of the video signal is phase shifted by 180 °, as shown at h or h ¹ in FIG. 5, the RY demodulating stage 5 can be fed. With this arrangement, too, the demodulated output + (RY) is obtained from the RY demodulating stage 5, since the two switching devices 19 and 25 are operated in the same mode of operation. Here too, however, one of the switching devices 19 and 20 can be operated in operating mode I and the other in operating mode II. In this case, the demodulated output signal - (RY) is obtained at the demodulation stage 5.

The circuit arrangement shown in Fig. 6 is a practical embodiment of the arrangement indicated in Fig. 3, where the switchgear framed by the dashed lines

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blocks correspond to the circuit blocks of FIG. 3, which are provided with corresponding reference numerals.

In the switching arrangement according to FIG. 6, the composite video signal from the first video signal amplifier 1 (in FIG. 3) is fed to an input terminal 50 of the separation stage 3. C ,. denotes a coupling capacitor with a small capacitance, for example 10 to 30 pF. T ^ is a bandpass transformer and C, a bypass capacitor. In this separation stage 3, the composite color signal of 4.43 MHz + approx. 0.5 MHz can be separated from the composite video signal transmitted by the color television broadcasting system. The composite color signal separated in this way contains the color synchronizing pulses (burst signals). The bandpass amplifier 4 comprises two amplifier transistors Tr ₁ and Tr ₂ and two bandpass transformers T ₂ and T 1. The emitter of the second transistor Tr ₂ can receive horizontal pulses which are synchronized with the line synchronizing signal, and it can be controlled in such a way that it blocks when the horizontal pulse is applied. In this case no burst signals appear at an output terminal 51 of the bandpass amplifier. Between the bandpass amplifier 4 and a block comprising the phase rotators 20 and 21 and the switching device 22, a transistor Tr _{2 is provided} for amplifying the chrominance signal (chrominance signal).

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The blanking stage 6 for the burst signal comprises a transistor Tr ^ which receives _{the horizontal pulse via a coupling capacitor C 5} and the composite color signal from the collector of the transistor Tr ₁ via a coupling capacitor C-, the latter containing the burst signals. The transistor Tr ^ works in such a way that it only conducts when a horizontal pulse is applied. The blanking stage 6 also includes a coupling transformer T. which, together with a capacitor Cg, forms an oscillating circuit which is matched to the color sync signal of 4.43 MHz.

The switching signal generator 12 comprises a bistable multivibrator with two transistors Trc and Tr ^, which are both actuated by alternately applying the horizontal pulse to their base in such a way that a square wave voltage with half the horizontal frequency appears at the output terminals 52 and 53 of the switching signal generator 12, as shown . By means of this square-wave voltage, two diodes D ₁ and D «can be brought into the conductive state alternately in each line period of the video signal. In one state, the diode D _{1 is} connected to ground via a winding I ₁ and a resistor R ₁ , while in the other state the diode D _{2 is} connected to ground via a winding 1 ¹ ^ and the resistor R ₁ , this switching process takes place in each line period of the video signal. Through this switching process, the burst signals that

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from the emitter of the transistor Try via a capacitor C _{7 to} a transformer T =, which acts as a 180 ° or 0 ° phase shifter, are reversed in their phase in every second line period, so that one secondary winding ± 2 of the transformer Tc receives burst signals whose vectors are directed either up or down. The burst signals are then _{fed to a transistor Tr Q} , which amplifies them.

Accordingly, two diodes D and D ^ and a phase shifter transformer Tg are provided to reverse the phase of the chrominance signal in every second line period of the video signal, so that a chrominance signal is obtained at a secondary coil of the transformer Tg, the RY modulation axes of which are oriented in one direction are. This signal is fed to the demodulating stage 5, which contains two diodes D ^ and Dg, two resistors R and R ^, two capacitors C _Q and Cg and a transformer Ty. It should be noted that the transformer Ty of the demodulating stage 5 is set so that the phase components of the subcarrier that _{are to be fed to the diodes D 5} and Dg can be kept phase-shifted by 180 ° from one another and that at the same time each of these phases of the Subcarrier wave is held in a fixed relationship with the RY component of the chrominance signal to be supplied to the diodes Dr and D / -.

5 ο

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2 0 9 8 A 0 / Ό 6 3 7

The phase detector 13 comprises two diodes Dr ₇ and D ₀ . two resistors R, - and Rg, two capacitors C ₁₀ and C ₁₁ and a transformer T _Q. The burst signals are fed to the diodes D _{1 and D Q} via the capacitors CLq and C _11, respectively. However, since a center tap 55 of a secondary winding of the transformer Tq is connected to ground, the _{burst signals fed to the diodes D ~ and D Q} are 180 ° out of phase with one another. The filter stage 14 comprises capacitors C ₁₂ and C ₁ - * and a resistor R. and operates to smooth the ripple of the output signal from the phase detector 13. The output signal from the filter stage 14 is fed to a DC voltage amplifier transistor Trq which is contained in the reactance element 15. A diode Dq of the reactance element 15 is designed as a diode with variable capacitance, the capacitance changing proportionally to the voltage applied _{to both sides of the diode D Q.}

The oscillator 16 for the subcarrier oscillation comprises a crystal oscillator element X and a transistor Tr ₁₀ and can be a BE pias oscillator. The output of this oscillator 16 is _{fed to the base of a transistor Tr 1} , which amplifies the output signal, the Kol lector of the transistor Tr _{11 is} connected to one end of a primary winding of a load transformer Tg. The subcarrier oscillation to be fed to the demodulating stage 5 can be obtained from the secondary coil of the load transformer T _Q.

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The APC stage 7 further comprises a phase shift element which comprises an inductance L ^ and a resistor R ₈ and is arranged between the subcarrier oscillator 16 and the phase detector 13. As a result, the phase of the subcarrier oscillation fed from the oscillator 16 to the detector 13 can be kept in a 90 ° relationship to the middle phase, of which each burst signal swings out by + 45 ° if the phase of the subcarrier oscillation to be fed to the demodulation stage 5 coincides with the modulation axes of the W RY component signal coincides.

A transistor Tr ^ p connected in series with the demodulating stage 5 amplifies the RY color difference signal, which is the output signal of the demodulating stage 5 .

The above embodiments are only to be understood as examples to illustrate the invention, and numerous modifications and configurations are within the scope of the invention for those skilled in the art. For example, can in the embodiments shown in Fig. 3 (Fig. 6) and in Fig. 4 an additional gating circuit be arranged with the same function as the blanking stage 6 between the switching device 19 and the APC stage 7. In this case, the connection between the blanking stage 6 and the combination of the phase rotation stages 17 and 18 must be established.

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211889Ί

distant, while an additional connection between the bandpass amplifier 4 and the two phase rotation stages 1 / and 18 is required.

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Claims (3)

Patent sayings Λ ^ Demodulating device for a color television receiver to receive a transmitted signal, which consists of a brightness signal, a chrominance signal formed by quadrature modulation of a carrier wave by two color signals, one of the modulation axes being rotated by 180 ° in alternating line periods of the video signal, and a color sync signal (burst- Signal), which reverses its phase in each line period, characterized by switching means for reversing the phase of the chrominance signal and the burst signals in every second line period as a function of the horizontal synchronizing signal, means for blanking the burst signals, a color synchronizing stage that blanked the burst signals are supplied and which comprises an oscillator with a frequency substantially the same as the carrier frequency, which generates an oscillation with a predetermined phase, and a demodulating stage for demodulating the chrominance signal when the switch is inputted in the direction of the chrominance signal and the output signal generated by the oscillator. - 22 - 209840/0537 2. Device according to claim 1, characterized in that the switching means consists of a first switching means for reversing the phase of the chrominance signal every other line period in dependence from the horizontal synchronizing signal and from a second switching device for reversing the phase of the burst signals exist in every second line period synchronously with the first switching device and that the color synchronizing stage the burst signals that have passed through the second switching device and the demodulation stage that through the chrominance signal passed to the first switching device will. 3. Device according to claim 1 or 2, characterized by a first switching device to reverse the phase of the burst signal in every other line depending on the horizontal sync signal, wherein the burst signals passed through the first switching device are supplied to the color synchronization stage and by a second switching device for reversing the phase of the output signal of the oscillator in each second line as a function of the horizontal synchronizing signal synchronous with the first switching device, wherein the output signal of the oscillator which has passed through the second switching device together with the chrominance signal is fed to the demodulation stage. - 23 - 209840/0537