GB1583421A - Secam television receiver - Google Patents

Description

(54) S.E.C.A.M. TELEVISION RECEIVER (71) We, N. V. PHILIPS' GLOEILAM PENFABRIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it IS to be performed, to be particularly described in and by the following statement: The present invention relates to a television receiver for a S.E.C.A.M. signal comprising a clamping device for colour reference levels, a S.E.C.A.M. bandpass filter having a bell shaped filter characteristic, and a generator intended for periodically inserting reference signals of the resting frequencies fo(B-Y) and fo(R-Y) into the chrominance signals by means of switching means, at instants corresponding to an invisible portion of the picture which reference signals after demodulation are used for a level stabilisation by the clamping device.

In the S.E.C.A.M. process the blanking levels in each of the two colour difference signals B-Y and R-Y (frequently called "black levels") correspond to the resting frequencies fo(B-Y) and fo(R-Y) of the sub-carrier of the chrominance signal D'B and D'R.

In the standardized signals transmitted by the transmitter, the resting frequencies fo(B-Y) and fo(R-Y) are transmitted alternately every four micro-seconds at each trailing edge of the horizontal line synchronizing pulse; because of the response time of the bell filter circuit and the demodulators the voltages obtained by demodulating these resting frequencies during such a brief period of time are not suitable for use as a reference level for an accurate damping after demodulation.

The French Patent Specification 1,565,237 proposes the insertion of the appropriate resting frequency signals into the chrominance signal during line blanking periods in order to obtain reference levels after demodulation which are suitable for clamping purposes. This insertion is performed either after the change-over switch, by means of two separate oscillators, or before the change-over switch and after the bell filter, by means of a single oscillator whose frequency is changed-over. Proper operation of such a system is only rarely guaranteed.

The invention provides a television receiver for a S.E.C.A.M. signal comprising a device for clamping the colour reference levels a S.E.C.A.M. bandpass filter having a bell shaped filter characteristic and a generator for periodically inserting reference signals of the resting frequency fo(B Y) and fo(R-Y) into the chrominance signals by means of switching means, at instants corresponding with an invisible portion of the picture, which reference signals after demodulation are used as reference levels for a level stabilisation by the clamping device, characterized in that the switching means are disposed between at least an out ut of the signal generator for the resting frequencies fo(B-Y) and fo(R-Y) and selec- tive filter means provided between at least one chrominance path through which at least one of the chrominance signals (B-Y) and (R-Y) and at least one of the reference signals having one of the frequencies fo(B Y) and fo(R-Y) passes.

Applicants have ascertained that when the reference signals are inserted after the S.E.C.A.M. bandpass filter the values of the reference levels obtained after demodulation vary with the harmonic content of the reference signals.

The fact that the chrominance signals and the reference signals may passed through the S.E.C.A.M. bandpass filter makes the reference levels after demodulation insensitive to the degree of harmonics of the reference signals, thus rendering each adjustment of these levels unnecessary. Consequently it is possible to use so-called "coincidence" or "quadrature" demodulators and integrated circuits, which are very sensitive to the degree of harmonics.

When "coincidence" or "quadrature" demodulators are used, any zero adjustment thereof becomes unnecessary, which, consequently, excludes any drift of the colour balance of a displayed picture.

Inserting signals of the frequencies fo(B-Y) and fo(R-Y) before the selective circuit means which may be the S.E.C.A.M.

bandpass filter, allows, in particular, the use of an integrated circuit oscillator without inductive elements, whose output voltage may have a high harmonic content, because of the subsequent filtering action which is the case with an oscillator which supplies square-wave or triangular output signals; in addition, this results in transmission characteristics which are the same for the chrominance signals and for the reference levels, thus enabling the highest possible accuracy in the demodulation of said signals.

The switching means may be arranged, in an advantageous manner, between the resting frequency generator and the input of the S.E.C.A.M. bandpass filter, preferably rendered conductive during a fraction of "field blanking" periods in order to prevent phase jumps between the chrominance signal and the reference signal from becoming visible as coloured fringes in the lefthand portion of the image.

The resting frequencies fo(B-Y) and fo(R-Y) are preferably obtained by means of a programmable dividing circuit from an oscillator whose frequency is controlled by the horizontal scan frequency.

Inserting the signals of the resting frequency fo(B-Y) and fo(R-Y) into the video-frequency signal during the field blanking periods enables, after demodulation, the storing of the colour reference signals which are then perfectly stable and free of noise, these voltages being reinserted at chosen instants into the demodulated colour difference signals B-Y and R-Y, thus making it possible for the clamping device to restore accurately the relative balance between the three fundamental colours of the transmitted picture by using the reinserted levels as clamping levels.

In this case the fact of controlling, by means of a single pilot signal, the insertion of the reference frequencies and the sampling of the corresponding demodulated reverence signals results in an "autosynchronizing" circuit, which means that the instants at which the reference signals are alternately sampled in the B-Y and R-Y paths always occur when the signals corresponding to the demodulated resting frequencies fo(B-Y) and fo(R-Y) are present in the relevant path: this excludes the risk of the occurrence of a clamping error.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is the block diagram of the "colour" section of a S.E.C.A.M. television receiver according to the invention, with a first embodiment of a clamping device.

Figure 2 is the block diagram of the "colour" section of a Secam television receiver according to the invention comprising a further embodiment of a clamping device.

Figures 3a and 3b show the process of inserting, by means of electric signals, the frequencies fo(B-Y) and fo(R-Y) in a receiver according to the invention.

Figure 4 is a more detailed block diagram of a preferred embodiment of a device for a receiver according to the invention.

Figures 5a and Sb show the sampling process by means of electric signals of the zero reference levels of the B-Y and R-Y paths of the device of Figure 4.

Figures 6 and 7 show the signals R-Y and B-Y, respectively, of a colour bar pattern and, in conjunction with Figure 4, the process of reinserting the zero reference levels during the line blanking periods.

Figure 8 shows the circuit diagram of a circuit for use in a receiver according to the invention.

Figure 9 shows the circuit diagram of a sampling and zero reference level reinserting circuit of one of the transmission paths of the colour difference signal of a S.E.C.A.M. television receiver according to the invention.

In Figure 1 an input terminal 1 for a video frequency S.E.C.A.M. chrominance signal is connected to one of the contacts of a first electronic commutator switch 2, which comprises a control terminal 3 receiving a switching signal of an appropriate frequency for instance the field scanning frequency.

Another contact of the commutator switch 2 is connected to the common contact of a second electronic commutator switch 4, which is provided with a control terminal 5 which receives a switching signal of an appropriate frequency for instance half the field scanning frequency.

The two other contacts of the commutator 4 are connected to a generator circuit comprising, respectively, two oscillators 6 and 7 of resting frequencies fo(B-Y) and fo(R-Y), two frequency dividers 8 and 9, and two frequency comparison circuits 10 and 11, one input of which is connected to a terminal 12 to which signals of the line scanning frequency are supplied.

The outputs of the comparison circuits 10, 11 are connected to frequency control inputs of the generators 6 and 7, respectively.

The common contact of the commutator 2 is connected to the input of a S.E.C.A.M.

bandpass filter 13 having a bell shaped response called "bell filter circuit", the output of which splits into two paths, the first path being directly connected to a first limiting circuit 14a, the second path to a second limiting circuit 14b by means of a delay line 15.

The outputs of the two limiters 14a and 14b are coupled to a change-over circuit 16, outputs of which are connected to demodulators 17a and 17b respectively, said change-over circuit comprising a control terminal 18 receiving a switching signal of half the line frequency.

The demodulator circuits 17a and 17b are coupled to a clamping circuit 28, comprising a keying signal terminal 30 and two outputs for the clamped colour difference signals B-Y and R-Y respectively. The keying signal must be supplied to the appropriate clamps at the moments the reference levels occur in the colour difference signals.

Apart from the arrangements which are specific to the invention and which will be explained further on, the operation of the circuit shown in Figure 1 is well-known. The chrominance signal applied to the input of the bell filter circuit 16 is split into two paths, one undelayed and the other one delayed for one horizontal line scan period (64 Cos). After having passed through the limiters 14a and 14b, the undelayed and the delayed signal from line to line are are directed to the appropriate output by the change-over circuit 16, thereafter demodulated in order to obtain the colour difference signals B-Y in the upper path and the colour difference signal R-Y in the lower path; these signals obtained from the demodulators 17a and 17b are applied to the clamping circuit 28 for clamping the colour reference levels in the colour difference signals.

The arrangement of Figure 1 operates as follows: by means of the commutator 2 the input of bell filter circuit 13 is connected to terminal 1 during the field trace periods, that is to say for the periods between the square-wave pulses of Figure 3a; during the field blanking periods the clock circuit input is connected to the common contact of the commutator 4, the state of said last being controlled by a signal of half the field frequency (Figure 3b) which enables the obtention of, for the duration of one field the frequency fo(B-Y) from the generator 6 and during the next field the frequency fo(R-Y) from the generator 7 (Figure 3c).

The resting frequencies fo(B-Y) and fo(R-Y) which must be inserted in the input signal of the bell filter 13 during the field blanking periods have a value of 4250 kHz and 4406,25 kHz, being 272 and 282 times the line frequency respectively. The corresponding dividing factors of the dividers 8 and 9 are 4 250 000/15 625 = 272 and 4 406 250/25 625 = 282.

The output frequencies of the oscillators 6 and 7 are compared via the dividers 8 and 9 with a signal of the horizontal scanning frequency which is applied to the comparators 10 and 11 through the terminal 12, which signals may either originate for instance from a line synchronizing circuit or from a horizontal scanning generator. The control voltages obtained from the outputs of the comparators 10 and 11 adjust the output frequencies of the oscillators 6 and 7 to the exact values required for the frequencies fo(B-Y) and fo(R-Y).

Thus, the resting frequencies fo(B-Y) and fo(R-Y) are injected alternately in the chrominance signal during the field blanking periods (Figure 3d).

In the device of Figure 1 the signals of the resting frequencies fo(B-Y) and fo(R-Y) are supplied bv the oscillators 6 and 7, which are controlled by a closed loop of the horizontal scanning frequency. It is obvious that other types of stabilised oscillators and open loops can be used, such as, for example, quartz-stabilised oscillators.

Furthermore, as already specified previously, the oscillators 6 and 7 can, without any drawback, supply waves of any form, square-wave or triangular, such as those produced by certain astable arrangements in integrated circuits, the use of which may be particularly economical.

In Figure 2, in which the same reference numerals have been used as in Figure 1, one of the contacts of the commutator 2 is connected to the output of the variable frequency oscillator 21 which is coupled via a programmable divider 22 to one of the inputs of the comparator 10, an input for a signal controlling the dividing factor of said circuit being connected to the terminal 5 receiving a signal of half the field frequency.

In the embodiment of the device shown in Figure 2 the result is obtained by utilising a variable frequency oscillator and a programmable divider, so that the electronic commutator 4 of Figure 1 can be dispensed with.

The frequency of the oscillator 21 of the controlled variable frequency is compared, via the programmable divider 22, with the line frequency applied to comparator 10 by terminal 12; the voltage supplied by the comparator modifies alternately the frequency of the oscillator 21 between the val ues fo(B-Y) and fo(R-Y), the ratio of the divider 22, controlled by terminal 5, alternately being 272 or 282.

It should be noted that inserting the frequencies fo(B-Y) and fo(R-Y) is performed 'immediately before the frequency of oscillator 21 is modified; in this manner the frequency of oscillator 21 is modified; in this manner the inserted frequency has at its dis- posal a full field period for perfect stabilisa tion.

In Figure 4, in which the same reference numerals are used as in Figures 1 and 2, the detailed circuit diagram of the preferred embodiment of the device according to the invention comprises an amplifier 23 for compensating the attenuation of the delay line 15 as well as a circuit 19 which performs sampling and reinsertion operations. The outputs of the demodulator circuits 17a and 1 7b are resepectively coupled to two sampling circuits 24a and 24b which are followed by two reinsertion circuits 25a and 25b, the junction between them being connected to two storage capacitors 26a and 26b, which are connected to ground 27.

The reinsertion circuits 25a and 25b, each comprising an input connected to a terminal 20 of the line blanking signals, are coupled to the matrixing and clamping circuit 28 by which in this case the colour difference signals B-Y and R-Y are mixed with a luminance signal Y fed to an input terminal 29 and are clamped by means of a keying signal fed to an input terminal 30. The circuit 28 supplies clamped colour signals blue (B), red (R) and green (V) for controlling the guns of the picture tube at its outputs.

The control input of a bistable multivibrator 31 is connected to terminal 3 receiving field blanking signals. One of the outputs of the complemcntary states of the multivibrator 31, constituting the terminal 5 of the Figures 1 and 2, is connected to the control input of the programmable divider 22 which forms part of the generator, already described, generating the frequencies fo(B-Y) and fo(R-Y).

The complementary outputs of the multivibrator 31 are connected to the first inputs of two AND-gates 32a and 32b, having two inputs, the second inputs thereof being connected to the terminal 3 for field blanking signals. The outputs of the gates 32a and 32b are connected to the sampling circuits 24a and 24b, respectively.

The sampling and reinsertion circuit 19 operates as follows: starting from the field blanking pulses (Figure 5a), the multivibrator 31 supplies at its two outputs two signals of complementary states (Figure Sb and Figure 5c), which are applied to two AND gates 32 and 32b whose other inputs receive field blanking pulses from terminal 3.

Therefore, each of the gates 32a and 32b supplies a pulse which is repeated every other field (Figure Sd and Figure Se), pulses which, applied to the circuits 24a and 24b, sample the reference voltages VRB (Figure 5f) and VRR (Figure 5g), which are the result of the demodulation of the reference frequencies fo(B-Y) and fo(R-Y) inserted by circuit 2.

The samples of the voltages VRB and VRR are stored in the capacitors 26b and 26a, respectively, and reinserted at each horizontal retrace level in path B-Y (Figure 7) and path R-Y (Figure 6) by each of the circuits 25b and 25a which are controlled via the terminal 20 by the line blanking pulses.

The reference levels VRB and VRR are thus perfectly defined at each line and independent of the degree of harmonics of oscillator 21; an adjustment of these reference levels becomes superfluous, which enables the use of integrated circuitry demodulators, of the "coincidence" or "quadrature" type.

As the reinsertion of the reference levels is effected during the line blanking periods it is possible to use the same signals as those applied to the input of the comparator 10; in these circumstances the terminals 20 and 12 can be interconnected, which is shown in the circuit of Figure 4 by means of a dashed line.

In Figure 8, in which the same reference numerals are used as in Figures 1, 2 and 4, the insertion circuit 2 comprises a positive conductor 33 and a negative conductor 34, connected to two terminals 35 and 36 of a voltage source Vb, the negative pole of the latter also being connected to ground 27.

A chain of resistors 37, 38 and 39 is arranged between the conductors 33 and 34, the junction between the first two resistors 37, 38 being connected to the base of an NPN-type transistor 40. The junction between the resistors 38 and 39 is connected to the base of an NPN-type transistor 41, the emitter of which is connected, together with the emitter of a third transistor 42, also of the NPN-type, through a resistor 43 to the conductor 34.

The collector of the transistor 40 is connected directly to the positive conductor 33, while the emitter is connected to the collectors of the transistors 41 and 42 via two resistors 44 and 45 respectively.

The bases of the transistors 41 and 42 are coupled via a capacitor 46 to the terminal 3 for the field blanking pulses, the firstmentioned base via a resistance network 47, 48, the second one directly.

The interconnected emitters of two groups of NPN transistors 49-50 and 51-52 are coupled, respectively, to terminal 1 for the video frequency chrominance signals and to the collector of an NPN transistor 53, whose base is coupled to the oscillator 21, the emitter being connected through a resistor 54 to the conductor 34.

The interconnected bases of the transistors 49 and 52 are connected to the collector of transistor 41, while the interconnected bases of the transistors 50 and 51 are connected to the collector of transistor 42.

The collectors of the transistors 49 and 51 are both connected directly to the positive conductor 33 while the interconnected collectors of the transistors 50 and 52 are connected to the emitter of an NPN transistor 55, the base of which is connected to a resistance network 56 and 57 arranged between the conductors 33 and 34.

The collector of the transistor 55 is con nected on the one hand to the conductor 33 via the bell filter 13 and on the other hand to the inputs of the limiting circuit 14a and the delay line 15.

The insertion circuit of Figure 8 operates in the following manner: the transistors 49, 50, 51 and 52 form a commutator circuit controlled by the transistors 41 and 42. In the absence of field blanking pulses at terminal 3 transistor 41 conducts and transistor 42 is cut-off; in these circumstances the transistors 50 and 51 conduct and the transistors 49 and 52 are cut-off. The result is that the chrominance signals supplied by the terminal 1 are passed via the transistors 50 and 55 to the direct and the delayed path; in contradistinction therewith the signals supplied by the oscillator 21 are directly applied to the positive conductor 33 by the transistor 51.

The situation described above is reversed when field blanking pulses are present at terminal 3, the transistors 42, 49 and 52 conduct and the transistors 41, 50 and 51 are cut-off; in these circumstances the chrominance signals are passed to the conductor 33 while the reference signals fo(B Y) and fo(R-Y) are passed every alternate field to the direct and the delayed paths (Figure 3d).

The buffer transistor 55, which is arranged in cascade, reduces the output impedance of the collectors of the transistors 50 and 52, thus eliminating any risk of cross-talk between the two signals owing to the parasitic emitter-collector capacitance of the cut-off transistor.

In Figure 9, the reference numerals of which are the same as those in Figures 1, 2 and 4, the sampling and reinsertion circuit 19 comprises two pairs of NPN-transistors 58-59 and 60-61, whose emitters are connected in pairs to the collectors of two other transistors 62 and 63, which are also of the NPN-type', whose respective bases are connected to the output of the gate 32b and to the terminal 20 for the line blanking signals, respectively, the emitters being connected to the negative conductor 34 via two resistances 64 and 65.

The bases of the transistors 58 and 61 are interconnected via a resistor 66, one of these bases being connected to the output of the demodulator circuit 17b and the other one to one of the inputs of circuit 28 for mixing and amplifying the colour difference signals.

The respective collectors of the transistors 58, 59, 60 and 61 are connected to the collectors of two other pairs of PNP-transistors 67, 68, 69 and 70, whose bases are interconnected in pairs, the emitters of these four transistors being directly connected to the conductor 33.

The interconnected collectors of the transistors 59 and 68 are connected via a resistor 71 to the interconnected bases of transistors 59 and 60, while the intercon nected collectors of the transistors 61 and 70 are connected via a resistor 72 to the base of transistor 61. In addition, the inter connected bases of the transistors 59 and 60 are connected to the positive plate of the storage capacitor 26b.

The interconnected collectors of the two transistors 73 and 74, which are of the PNP-type, are directly connected to the conductor 34, the emitters being connected directly to the interconnected bases of the transistors 67-68 and 69-70 respectively; the collector of another transistor 75, which is also of the PNP-type, is connected to the bases of the transistors 69 and 70, while the remitter is connected directly to the conduc tor 33.

The bases of the transistors 73 and 74 are connected directly to the interconnected col lectors of the transistors 58-67 and 60-69, respectively, while the base of the transistor 75 is connected to the terminal 20 via a resistor 76.

The sampling and reinsertion circuit 19 of Figure 9 operates in the following manner: in the absence of field blanking signals or line blanking signals at the bases of the transistors 62 and 63, these transistors and all the other transistors are cut-off and the demodulated signals of the considered path, for example the blue path, are transmitted via the resistor 66 to the input B-Y of the mixing and amplifying circuit 28.

When the signal at the output of gate 32b (Figure Sd), in the present case every alter nate field blanking pulse, is applied to the base of the transistor 62, this transistor is rendered conductive, and, consequently, also the transistors 58 and 59; these last mentioned transistors form a differential amplifier to which a "current mirror", con stituted by the transistors 67, 68 and 73 is added, while the presence of the resistor 71 ensures a complete negative feedback; because of the very high gain of the assem bly the reference voltage VRB (Figure Sf) applied to the base of the transistor 58 is sampled and is found substantially fully at the base of the transistor 59, thus charging the capacitor 26b to the value of the voltage VRB. When the field retrace pulses applied to the transistor 62 disappear, the capacitor 26b remains charged to the value VRB dur ing the duration of two fields; this voltage VRB which is thus stored, is thereafter rein serted during each line blanking interval by means of a second differential amplifier which has the same structure as the first one.

During the whole duration of the line blanking periods which render the transistor 63 conductive the reference voltage VRB obtained from the transistor 60 is therefore at the base of the transistor 61 and at the input of the circuit 28 (Figure 6).

Taking the high value of the capacitor 26b and the very weak current, obtained from the base of the transistor 60 at each line blanking period, into account the voltage VRB is substantially constant between two successive recharging processes of said capacitor.

The transistor 75 has for its object to remove the stored charges from the bases of the transistors 69 and 70 which, without this measure, would tend to continue to be conducting after the line blanking pulses have disappeared; to this end the transistor 75, which is cut-off by the positive pulses supplied by the terminal 20 is rendered conductive from the disappearance of these pulses onwards.

A second circuit which is identical to that of Figure 9 is utilised for processing the path R-Y in order to feed the corresponding input of the mixing and amplifying circuit 28.

In the described preferred embodiment reinserting the reference levels in the colour difference signals B-Y and R-Y is effected at each line blanking interval; in theory there is nothing to prevent this operation from being effected at a lower rate, in view of the limitation during the field blanking periods.

In this case it is sufficient to energize the terminal 20 by means of signals of a repetition rate and a duration suitable for achieving the required object. In the abovementioned embodiments the reference signals were inserted in the S.E.C.A.M. chrominance signal before the bell filter circuit. At this place the colour difference signals alternately are present modulated on their carrier. Naturally it is also possible to insert the reference signals at a different point of the receiver. The modulated colour difference signal and its corresponding reference signal have to be conducted through the same selective filter then before demodulated which is additional to the bell filter circuit.

The circuits of the abovementioned embodiments have the advantage that no additional filters are needed.

The clamping periods can be chosen independent of the periods on which the reference signals are inserted if a sampling and reinsertion circuit is used. In that case it also is unnecessary to insert a reference signal in its corresponding modulated colour difference signal if all signals are conducted through the same selective filter.

If a sampling and reinsertion circuit is not used the insertion of the reference signals and the clamping of the corresponding demodulated signals have to be performed in corresponding periods.

WHAT WE CLAIM IS: 1. A television receiver for a S.E.C.A.M. signal comprising a clamping device for colour reference levels, a S.E.C.A.M. bandpass filter, having a bell shaped filter characteristic, and a generator circuit for periodically inserting reference signals of the resting frequencies fo(B-Y) and fo(R-Y) into the chrominance signals by means of switching means, at instants corresponding to an invisible portion of the picture, which reference signals after demodulation are used as reference levels for a level stabilisation by the clamping device, characterized in that the switching means are disposed between at least an output of the signal generator circuit for the resting frequencies fo(B-Y) and fo(R-Y) and selective filter means provided between at least one chrominance path through which at least one of the chrominance signals (B-Y) and (R-Y) and at least one of the reference signals having one of the frequencies fo(B Y) and fo(R-Y) passes.

2. A television receiver as claimed in claim 1, characterized in that the switching means are arranged between the generator circuit for the resting frequencies fo(B-Y) and fo(R-Y) and the input of the S.E.C.A.M. bandpass filter which filter forms the selective filter means.

3. A television receiver as claimed in any of the claims

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. at the base of the transistor 61 and at the input of the circuit 28 (Figure 6). Taking the high value of the capacitor 26b and the very weak current, obtained from the base of the transistor 60 at each line blanking period, into account the voltage VRB is substantially constant between two successive recharging processes of said capacitor. The transistor 75 has for its object to remove the stored charges from the bases of the transistors 69 and 70 which, without this measure, would tend to continue to be conducting after the line blanking pulses have disappeared; to this end the transistor 75, which is cut-off by the positive pulses supplied by the terminal 20 is rendered conductive from the disappearance of these pulses onwards. A second circuit which is identical to that of Figure 9 is utilised for processing the path R-Y in order to feed the corresponding input of the mixing and amplifying circuit 28. In the described preferred embodiment reinserting the reference levels in the colour difference signals B-Y and R-Y is effected at each line blanking interval; in theory there is nothing to prevent this operation from being effected at a lower rate, in view of the limitation during the field blanking periods. In this case it is sufficient to energize the terminal 20 by means of signals of a repetition rate and a duration suitable for achieving the required object. In the abovementioned embodiments the reference signals were inserted in the S.E.C.A.M. chrominance signal before the bell filter circuit. At this place the colour difference signals alternately are present modulated on their carrier. Naturally it is also possible to insert the reference signals at a different point of the receiver. The modulated colour difference signal and its corresponding reference signal have to be conducted through the same selective filter then before demodulated which is additional to the bell filter circuit. The circuits of the abovementioned embodiments have the advantage that no additional filters are needed. The clamping periods can be chosen independent of the periods on which the reference signals are inserted if a sampling and reinsertion circuit is used. In that case it also is unnecessary to insert a reference signal in its corresponding modulated colour difference signal if all signals are conducted through the same selective filter. If a sampling and reinsertion circuit is not used the insertion of the reference signals and the clamping of the corresponding demodulated signals have to be performed in corresponding periods. WHAT WE CLAIM IS:

1. A television receiver for a S.E.C.A.M. signal comprising a clamping device for colour reference levels, a S.E.C.A.M. bandpass filter, having a bell shaped filter characteristic, and a generator circuit for periodically inserting reference signals of the resting frequencies fo(B-Y) and fo(R-Y) into the chrominance signals by means of switching means, at instants corresponding to an invisible portion of the picture, which reference signals after demodulation are used as reference levels for a level stabilisation by the clamping device, characterized in that the switching means are disposed between at least an output of the signal generator circuit for the resting frequencies fo(B-Y) and fo(R-Y) and selective filter means provided between at least one chrominance path through which at least one of the chrominance signals (B-Y) and (R-Y) and at least one of the reference signals having one of the frequencies fo(B Y) and fo(R-Y) passes.

2. A television receiver as claimed in claim 1, characterized in that the switching means are arranged between the generator circuit for the resting frequencies fo(B-Y) and fo(R-Y) and the input of the S.E.C.A.M. bandpass filter which filter forms the selective filter means.

3. A television receiver as claimed in any of the claims 1 or 2, characterized in that the switching means conduct during a fraction of the field blanking periods.

4. A television receiver as claimed in any of the claims 1, 2 or 3, characterized in that a waveform furnished by the resting frequency generator circuit is substantially a square-wave.

5. A television receiver as claimed in any of the claims 1, 2 or 3, characterized in that a waveform furnished by the resting frequency generator circuit is substantially triangular.

6. A television receiver as claimed in any of the claims 1, 2 or 3, characterized in that the generator circuit for the resting frequency fo(B-Y) and fo(R-Y) comprises an oscillator whose output is coupled to an input of a frequency dividing circuit, the output of this dividing circuit being connected to the input of a frequency comparison circuit, another input terminal of which is coupled to an output of a generator generating signals of the horizontal scanning frequency, the output terminal of the frequench comparison circuit being connected to a frequency control input of the oscillator.

7. A television receiver as claimed in claim 6, characterized in that the frequency dividing circuit is of a programmable type and comprises a control terminal coupled to a control pulse generator for periodically adapting the dividing factor of said circuit to the values of the quotient of the resting fre

quencies fo(B-Y) and fo(R-Y), respectively, by the horizontal scanning frequency.

8. A television receiver as claimed in any of the claims 1 to 7, characterized in that the switching means are implemented in the form of an insertion circuit comprising two pairs of transistors arranged as a commutator switch provided with a first input connected to a terminal for the signals of the vertical scanning frequency, a second input connected to a terminal for the video frequency chrominance signals, a third input connected to the output of the generator for the resting frequencies fo(B-Y) and fo(R Y), and an output coupled to the input terminal of the S.E.C.A.M. bandpass filter.

9. A television receiver as claimed in claim 7 or claim 8 when dependent on claim 7 characterized in that the control pulse generator is a bistable multivibrator whose triggering output is connected to the second input of the insertion circuit, the two complementary logic state outputs of said multivibrator being connected, one simultaneously to the control input of the programmable divider and to one of the inputs of a first AND-gate, the other one to one of the inputs of a second AND-gate, the two other inputs of said AND-gates being connected to the second input of the insertion circuit, and the outputs to corresponding sampling signal inputs of a circuit for sampling and reinserting the colour reference levels in the difference colour signals B-Y and R-Y.

10. A television receiver as claimed in claim 9, characterized in that the sampling and reinsertion circuit comprises a reinsertion control signal terminal connected to a signal generator of the vertical scanning frequency.

11. A television receiver as claimed in claim 9, characterized in that the sampling and reinsertion circuit comprises a reinsertion control signal terminal connected to a signal generator of the horizontal scanning frequency.

12. A television receiver as claimed in claim 9, 10 or 11, characterized in that the sampling and reinsertion circuits both comprise two pairs of transistors arranged as dif erential amplifiers, adjacent to which there is an arrangement of the type called "current mirror" provided between the two collectors of the transistors of one of the pairs mentioned above.

13. A television receiver as claimed in any of the claims 1 to 7 characterized in that the demodulated colour difference signals B-Y and R-Y are switched, either during the duration of the visible image by an electric circuit towards a mixing and amplifier circuit for the colour signals or during the field blanking intervals, towards a storage capacitor.

14. A television receiver for a S.E.C.A.M. signal substantially as herein described with reference to the accompany ing drawings.