CA1105607A - Transmission system for colour television signals - Google Patents

Abstract

ABSTRACT:
A transmission system for a colour television signal, in particular a system for recording on and re-producing from a record carrier. The chrominance signal is divided into two separate colour signals, which in a line-sequentially alternating fashion are modulated on two separate colour carrier waves. The transmitted or recorded colour television signal can simply be transformed into both a standard PAL and a standard SECAM signal. Owing to the line sequential alternation of the colour signal, colour errors which may arise owing to a mutual difference of the transfer charac-teristics for the two colour signals are automatically, compensated for in the case of reproduction via a PAL
receiver.

Description

The invention relates to a transmission system for a colour television signal, in particular in a system for recording on and reproducing from a record medium, in which method a transmission signal is generated having a first signal component comprising ]uminance information, a second signal component comprising a first colour carrier wave modulated with a first colour information component and a third signal component comprising a second colour carrier wave modulated with a second colour information component, which second and third signal components are arranged to occupy separate frequency bands.
Such a transmission system, which is Eor example known rom Netherlands Patent Application No. 6,603,605 of P. Thomson published June 20, 1966, has the advantage that it is highly insensitive to timing errors introduced during transmission. This is of particular interest when a colour television signal is recorded on and reproduced from a record carrier, for example a magnetic tape or an optically readable video disk. In said systems timing errors are inter alia introduced by speed variations of the record carrier and in the case of the video dlsk also by the presence of the eccentricity. When in such recording systems the chrominance signal is recorded in known manner as a quadrature signal, i.e. on two 90 ~ '.

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~56~7 phase-shifted carrier waves of the same frequency, which are amplitude modulated with two colour signals, said timing errors will show up in the reproduced chrominance signal as a hue error, which is very disturbing.
In a transmission system of the kind mentioned in the preamble said timing errors have a far less disturbing effect because the signals which are modulated on separate carrier waves, in particular frequency-modulated carrier waves, are substantially less sensitive to said timing errors than a quadrature-modulated signal as mentioned previously. ``~
However, a drawback of a system as mentioned in the preamble is that it is very sensitive to variations in the transfer function of the system. A variation in the transfer characteristic, which may result in the second and third signal component being no longer subject to the same transfer function? causes a hue error in the reproduced chrominance signal, which as previously stated, is highly disturbing.
It is an object of the inventlon to proyide a transmission system of the type mentioned in the pre-amble, which mitigates said problem whilst the said ad-vantages are maintained. For this, the invention is characterized in that the first and second colour information components are constituted by a first and the second colour signal in a line~sequentially interchanged fashion. ~
The step according to the invention first of -all ensures that during the reproduction from a record -3- ~-~ , ~ 5S~7 ~¦ PIrN 7~9 3.2.197~

carrier, which is provided with a colour television sig-nal which is thus composed, an effective use is made of the averaging of the colour over two lines which is ef-fected in a colour television receiv~r adapted for re-i 5 producing a colour t01evision signal in accordance ~ with the PA~ standard. Owing to s~id averaging a :~i possible hue error arising from differences in the ~¦~ transfer function for the second and third signal ¦ components is autornaticall~ compensated for. The same ¦ 10 applies to differences in noise and bandwidth of thetransmission channels for said second and third sig-nal components. Moreover, the same record carrier j can also be played via a playing apparatus adapted to the SECAM colour television system without said differences in the transfer function giving rise to `~ hue errors.
Moreover, the system according to the invention is highly suited to be employed with pre recorded record carriers, i. R . reco:rd carriers which have already been __ provided ~ith a programtne by the manufacturer. In this respect, the video disk, which at present is in the focus of interest, lS an obvious application. Of course, it is of advantage when such a pre-record0d record car-rier can be distributed as widely as possible. In Europe this presents the problem that t~o different colour te-levision standard~ are ~eing used, namely PAL and SECAM, .~ ' ` . .
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5 6 ~7 P~ 79~?~
. .3-2-1976 , ¦ 80 that a pre-recorded record carrier which is speci-¦ fically adapted to either of said standards cannot be 1 played without any further p:rovisions in those areas ¦ where the other standard is used. The transmisslon sys-tem descri~ed is universal in this respect, because the second and third signal component may contain two ele-mentary colour signals, from which with the aid of cod-ing circuits in the playing equipment a colour tele-: vision signal in accordance with the P~L or SECAM
~ 10 standard can easily be obtained. This means, that for ¦ the entire PAL-S~CAM territory it is sufficient to use onQo and the same record carrier without bhis Xaving any : : significant, particularly price-increasing consequences ~ with respect to the playing equipment.
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Finally, it has been Pound that the transmis-¦ sion system according to the invention is particularly ¦ ~ advantageous. in the case of said video disks, namely~3 ¦ . in the case of slow-motion or quick-motion reproduc-¦ tion of a recorded programme, which will be explained ! 20 hereinafter, -. The invention will now be described with reference to the Figures, in which 1 Fig 1 shows a frequency spectrum of the l transmitted television signal in a transmission sys , Z5 tem accordlng to the invention, .~ Fig. 2 shows a diagram in explanation of ~! said spectrum, ,~ : , ~ -' ' , ' , , .
.,.,~ , i ~ , . , . . ; . : , :. ~

7 ~ 56~7 3.2.1976 Fig. 3 shows a first embodiment of a recording ~ or transmission apparatus, ¦ Fig. 4 shows a first embodiment of a reproduc-¦ ing or receiving apparatus for use in a system according to -the invention, Figs. 5 and 6 show -two diagrams to explain ~ .
the operation, Figs. 7 and 8 sho~ two alternatlve embodiments of the recording or transmission apparatus for use in a system according to the inventiont Fig. 9 shows a diagram to explain the opera-tion thereof, Flg. 10~hows an alternative embodiment of the reproducing or receiving apparatus for~use in a system according to the invention~ whilst Fig. 11 shows a diagram pertaining thereto and Fig. 12 schematically shows an embodiment of a lo~ic ci~cuit applied on th2 reproducing apparatus according to the invention.
Figure 1, by way of example, shows a frèquency spectrum of a colour television signal as may be trans-mitted in accordance with the transmission system ac-~ ~ cording to the invention. The luminance signal is !
j frequency modula~ed on a carrier wave ~y and covers a frequency sweep of fs (synchronizing pulsej - 6.7 MHz to fw (pea~ white) = 8~4 MHz. It is assumed that the first slgnal componont E , which in addition ' ; - 6 -.

~ ~56~7 `~

to said frequency sweep also contains the first-order sidebands, covers a bandwidth of 4.2 - 11.4 MHz. The frequency spectrum furthermore contains a second slg-nal component El, which as a modulation of the first colour carrier wave Fl contains a first colour signal, `
and a third signal component E2, which as a modulation of a second colour carrier wave F2 contains a second - colour signal. Said second and third signal components occupy separate frequency bands~ for example frequency bands of 1.2 MHz aroand the two carrler frequencies of 1.8 and 3.2 MHz respec~ively. For completeness a fourth signal component Eg is shownj which consists of a carrier wave F which is frequency modulated wlth the audlo signal.
First of all, it is to be noted that the in-vention is by no means limited to the positions of the various signal components within the available bandwidth selected in Figure 1. As far as the principle of the invention is concerned the location of said signal components within the frequency spectrum is of no importance at all. As an example, the second and third ;
signal components may alternatively be situated within the frequency band covered by the first signal component, as is for example described in Netherlands Patent Specification 106~695 of N.V. Philips published March 15, 1963, or one of said signal components may be situated beIo~ and the other above the frequency band covered by said first signa] ~
component. -~' ': "
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6~7 PHN 79~
3-2-1~76 Furthermore, it is to b~ noted that the method of modulation of the first and second oolour carrier F1 and F2 by the two colour signa:Ls is not an essentlal step of the invention. For exarnple in thls respect an amplitude modulation or a frequency modulation may be ~ considered. For other reasons, _ ter alia the greater ,~ .
noise immunity, said colour carrier waves are pref`erabIy frequency-modulated, which is the reason why further in the description only this frequency-modulation will be discussed.
Finally, it is to be noted that the compo-sition of the colour signals is not of` direct import-ance, although the use of colour difference sig~nal (R-Y) , .
and (B-Y) is most advantageous. Therefore, said t~o ~ : ~
colour difference sign~s (R-Y) and (B-Y) will be : .
selected hereinafter as modulating signals for the two colour carrier waves F1 and E2 In accordance with the invention the two signal component~ E1 and E2 no longer continuously oontain the same colour signal, but ~ese two colour signals are added to the two colour carrier waves F1 and F2 as a modulation signal in a line-sequentially alternating fashion. In Fig. 2 *his is schematically represented for (R-Y) and (B-Y) colour signals, which Eigure shows as a function of` the line number Ln which colour signal is included in the second signal 3~ component E1 and which colour signal in the third sig-:!:

_ 8 -5~ P~IN 7996 ~ - 3.2.1976 ' nal component E2. Thls shows that everey alternate line the signal component E1 comprises the -two colour signals R - Y) and (B Y) and the signal component ~2 a~ter-nately contains the same colour signals but in phase opposition.
~ igure 3 shows a f`irst embodiment of an ap-paratus, with which such a colour television signal can be obtained at the transmitter side or in the re-.~ .
cording equipment for a record carrie3r. Said apparatus first of all comprises a transcod0r 2, which splitssa co1lour television signal which is applied to a ter-minal 1 into a luminance signal Y, and two colour difference signals (R - Y) and (B - Y), The circuit arrangement of said transcoder is of oourse det0rmlned by the composition of the colour -television slgnal ap-plied to the terminal 1. If said colour television sig-.
- nal is a signal composed in accordance with a standard (for example PAL) said transcoder 2 may for example correspond to the decoding device employed in receivers for the relevant standard colour television signal. However, any known transcoder which is capable of supplying the three slgnals (R - Y), (B - Y) and Y `~-- may be used.
3 The ex-tracted luminance signal Y at ter-minal 6 is frequency modulated in an ~M modulator 14, resulting in the ~irst;;signal compcnent Ey. The two !~, oolour components (R - Y) and ~B - Y) ar0 applied to g _ .. . ., . .

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21.5.75 : . .

~: the two input terminals 1~ and 5 of a commutator 30 Said , .
commu-tator 3 comprises a dual interlocked switch, ~lich ~ can assume two positions, viz. a first posi-tion in w~h I a connection is established between the input terminal ~ii 5 4 and an output terminal 10 on the one hand, and an input terminal 5 and an outpu-t terminal 1l on the ~' other hand, and a second position in which a connection ~¦ is established between the input terminal 4 and the output termi.nal 11 on the one hand and the input -ter-minal 5 and the output -terminal 10 on the other hancl.
Said switch, whi.ch in practice will of course take the form of any known electronic switch, is contro:~ed by ~¦ a control signal at a control terminaL 8 of the commu-¦ . tator 30 With the aid of a detector 9 said control sig-':
nal i9 derived from the horizontal synchronizing pulses ¦ in the television signal, which are .for example taken from.an output terminal 7 of the transcoder 2, and which control signal is constituted by a symmetrical square-wa~e signal of half the line frequency. This ensures tha-t the commutator 3 is controllecl in such a way . that at its output terminals 10 and 11 the colour : ~ .
signals (R - Y) and (B - Y) are available in phase opposition every altcrnate line, in accordanGe with the diagram of Fig. 2. The signal at terminal 10 is applied to a frequency modulator 12, in which said alternating colour signal is modulated on a carrier wave F1, resulting in the.second signal component E1.
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, ~r . ~ 10 .

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S~ 7 The signal at terminal 11 is applied to a frequency modulator 13, in which said alternating colour signal is modulated on a carrier wave F2, resulting in the third signal component E2. Said second and third sig~
nal component El and E2 are summed with the aid of a summation device 15.
Said sum signal (El + E2~ should then more-over be combined with the first signal component E
and, as the case may be, with a sound signal E , which is added to a carrier wave F of for example 0.5 M~lz as a frequency modulation, which signal is applied to a ~
terminal 17. A simple method to achieve this is described ~;
in United States Patent No. 3,893~163 of United States Philips Corp. issued July 1, 1975. In that case the signal components (El + E2) and E of comparatively low frequency are added to the first signal component E , which has finitely steep edges, and the sum signal (E + El + E2 + E ) is applied to a limiter circuit 18. Thus~ at the output terminal 19 is obtained a squarewave signal whose frequency contains the luminance information and in which the colour sum signal (El + E2) and the sound signal Eg are contained as a pulse-width modulation, which signal is particularly suited to serve as a recording signal for an optically readable video disk.
Figure 4 shows an embodiment of the receiving or reproducing equipment for a colour television signal which is thus transmitted or recorded. From the --11~

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colour television signal applied to an input terminal 21, which signal is for example obkained from a video disk with the aid of an optical reading system, the first signal component Ey is extracted with the aid of a high-pass filter 24, from which the luminance signal Y is demodulated with the aid of an FM demodulator 27. The second and third signal components El and E2 are extracted with the aid of two band-pass filters 22 and 23, after which the colour signals are recovered from said signal components with the aid of two FM demodulators 25 and 26. Said colour signals are applied to the two input terminals 29 and 30 of a commutator 28. Said commutator is of the same design as the commutator 3 at the transmitter side, i.e. having two input ~ `
terminals 29 and 3Q and two output terminals 31 and 32 which are alternately interconnected as a function of a control signal at a control terminal 33.
Said control terminal 33 receives a control signal Erom a logic circuit 34.
Said logic circuit 34 has three input terminals 35, 36, 37, of which terminal 35 can receive a control signal from a command device 38, terminal 37 a control signal from an identification circuit 39, and terminal 36 a control signal from a detector 40 via a switch 51.
The function of the command device 38 and the identification circuit 39 will be described in more detail hereinafter. First of all, it will be indicated how the original continuous colour signal can be recovered with the aid of the commutator 28. ~or this purpose, the circuit 51 must be in the position P ~PAL) in which position the horizontal synchronizing pulses which are detected from the luminance signal Y by the detector 40 reach the input terminal 36 of the logic circuit 34. In response to this logic circuit 34 supplies a symmetrical square wave signal of half the line frequency to the control terminal 33 of the commutator 28, so that the position of the commutator switch is changed every line. The result of this is shown in the block diagram of Figure 5.
Said Figure 5 indicates for three consecutive lines n, n-l-l, n+2 of a television picture which colour signal is available at the inputs 29 and ~ ~, ~ 56~7 30 and the outputs 31 and 32 of the commutator 28 during ~he relevant lines.
The indexes near said colour information signals indicate via which signal components El or E2 said colour information has been transmitted. It is assumed that the input 29 of the commutator 2S receives the colour signal, which has been transmitted as a modulation of ~he first colour carrier Fl ;
and the input 30 receives the colour signal which has been transmitted as a ;
modulation of the second colour carrier F2. The two inputs 29 and 30 con~
sequently receive the ~R - Y~ and ~B - Y~ colour signal every alternate `
line, but in mutuaI phase opposition.
If it is now assumed that during the line n the commutator 28 ~ `~
establishes the connection pattern represented by the continuous lines, the output 31 receives the (R Y)l colour signal from input 2~ during said line -~
and the output 32 the ~B -~ Y~2 colour signal from input 30. D~lring the next line the commutator establishes the connection pattern represented by the dashed lines and output 31 consequently recel~es the ~R - Y)2 colour signal from input 30 and output 32 the ~B - Y~l colour signal from input 29.
Consequently, the output 31 o~ the commutator 28 continuously supplies the ~R - Y) colour signal, said colour signal being alternately obtained ~rom FM demodulator 25 and 26 line-sequentially. In a similar way the output 38 ~;~
now continuously supplies the ~B - Y) signal.
In order to recover a standard colour signal in accordance with the PAL standard from said two colour difference signals ~ - Y) and ~B - Y~
the two colour signals must be quadrature-modulated in known manner on a 4.43 MHz carrier wave. For this purpose, it may be desirable to add burst keying pulses to the t~o colour signals in two adder circuits 41 and 42.
Said burst keying pulses may be derived from the horizontal synchronizing pulses detected by the detector 40 and serve for obtaining in known manner, the desired burst on the backporch of the horizontal flyback period in the case of modulation of the colour signals on the 4.43 ~Hz carrier. When said burst keying impulses have already been added to the colour signal at the ~ 13 -transmitter side, said adder circuits 41 and 42 may be dispensed with, which in the Figure is indicated by the dashed connections between the outputs 31 and 32 o~ the commutator 28 and the A~-modulators 43 and 44. The desirabil-ity of generating said burst keying impulses in the receiving equipment depends on possible disturbances in the transmitted burst keying impulses.
In this respect it is to be noted that in a PAL receiver also a certain compensation is obtained in respect of possible disturbances in said burst keying impulses because of the averaging over two lines.
The colour signal ~B - Y~ provided with burst ke~ing pulses is applied to a first amplitude modulator 43 which as carrier wave receives a 4.43 MHz signal of fixed phase (- sin) from an oscillator 45. The colour signal ~R - Y) provided with burst keying pulses is applied to an amplitude modulator 4~, which also receives a 4.43 Ml-lz carrier wave signal from said oscillator 45, but which carrier wave is 90 phase-shifted relative to the carrier wave which is applied to the modulator 44 and which moreover exhibits a 180 phase alternation every other line (~ cos). By adding the thus obtained two modulated colour signals in an adding circuit 47 a chrominance signal is obtained which complies Wit}l the PAL standard and which is quad-rature modulated. Said chrominance signal is added to the demodulated luminance signal Y in an adding circuit 48, so that at an output terminal 49 a PAL standard colour television signal becomes available, which may be applied to a standard PAL-colour television receiver.
In order to illustrate the effect of the step according to the invention reference is made again to ~igure 5 and the vector diagram shown in Figure 6. The column PAL-ID in Figure 5 indicates which PAL phase is added to the (R - Y) signal of the relevant lîne in the AM modulator 44, whilst the column PAL-chroma indicates the ultimate vectorial composition of the PAL chrominance signal of the relevant line.
If it is now assumed that owing to whatever cause the transfer characteristic of the second signal component El has a higher gain factor ~356~7 ; ' than the transfer characteristic of the third signal component E2, the colour signals ~B - Y)l and (R - Y)l will always have too high a value compared with the colour signals (B - Y)2 and ~R - Y)2. The effect of said difference in gain factor will be described with reference to Figure 6.
For this it is assumed that the colour signal which is transmitted via the third signal component E2 has the correct value. During the line n the ~B - Y)2 colour signal consequently has the desired value (B-Y)nC, the upper index indicating the relevant line The ~R - Y)l colour signal, however, is too great during said line _, i.e. greater than the desired signal (R - YjnC. The result of this is that instead of the correct colour vector ~.
Ccn _ (B - Y) c + ~R - Y)nC during said line n the colour vector cn = (B ~ Y) nc + (R _ y)nl is obtained. Said colour vector cr~ e~hibits a ;~
phase error ~ relative to the desired vector Cnc, which means that there will be a hue error, which is very disturbing.
In accordance with the invention, however, the colour vector Cn+l during the line n+l is composed of the (B _ y)ln 1 colour signal and (R _ y)2n 1 r colour signal, the ~R - Y)2 signal now having the correct value (R - Y)nC 1, which for simplicity is assumed to be equal to (R _ Y)m~ and the (B - Y)l signal being too great relative to the desired value by the same factor, as ~;
was the case with the ~R - Y)l signal during the line n. The colour vector .
n 1 which corresponds to said line n+l consequently consists of Cn 1 _ ~B _ y)ln 1 _ ~R - Y)nC 1, the -sign being the result of the negative PAL phase (- cos) which prevails during said line n + 1. In a PAL receiver, however, , :
the vectorial sum of the two colour vectors of two consecutive lines is now determined whilstpreviously the vector Cn I which corresponds to the negative PAL-phase (- cos) is reflected relative to the (B - Y) axis, which sum vector is the ultimately reproduced colour signal. The vector Cn 1 is consequently first reflected relative to the (B - Y) axis, resulting in a CP n 1, which vector is then vectorially added to the vector C n which yields the vector C n + CP n 1. The Figure shows that the phase of the said sum ~;~

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~ S~ 7 vector is entirely correct compared with the desired colour vector Cc, so that no hue error will result. The only error which remains is an error in the colour saturation, as said sum vector is greater than twice the desired colour vector 2Cc, but this is considerably less disturbing than a hue error.
The transmission system according to the invention moreover has additional advantages, which will be discussed in more detail with reference to the previously described embodiment of Figure 4. It has already been ; stated that the transmission system according to the invention is parti-cularly advantageous for pre-recorded record carriers, because in the reproducing apparatus a colour television signal suitable for a PAL receiver or for a SECAM receiver can be obtained in a very simple manner. The `
generation of a PAL colour television signal has already been described hereinbefore, so that this will not be discussed any further.
To obtain a SECAM colour signal it suffices to set the switch 51 to the position S. A SECA~ colour signal comprises the (R - Y) and ~B - Y) colour signal alternately from line to line. When switch 51 is set to the position S, the logic circuit 34 will no longer receive any horizontal synchronizing pulses from the detector 40, so that the control terminal 33 of the commutator 2~ will not receive a control signal and the commutator switch remains in a fixed position. This means that the two outputs 31 and 32 are continuously connected to a fixed input 29 and 30. Thus, the ~R - Y) ;~
and (B - Y) colour signals will occur line-sequentially at each of said two outputs. Consequently, a line-sequential colour signal FSEcAM can be taken from one of said outputs~ for example 31, which signal is available at an output terminal 5Q. To derive a complete SECAM colour television signal from this, said colour signal ~SECA~ should merely be modulated on the standard colour carriers and subsequently be added to the luminance signal Y. In order to simplify the block diagram this obvious step has not been further elaborated.
A further advantage of the transmission system according to the i: , .,.. ;,, : :

~ 156~7 invention is obtained ~hen a disc is used as a record carrler, although it is also applicable to the use of a record carrier in the form of a tape.
~or further details on the use of an optically ~coded video disk as a record carrier, reference is made to United Sta~es Patent Specification 3,85~,015 ~PHN.6225). Generally, the video information is recorded on such a video ~;
disk in a spiral track, in such a manner that per revolution of said disk "
exac~ly one full television picture is recorded. In order to realize a stationary picture with the aid of such a video dislc, it is sufficient to ; move the scanning spot with which the informatlon is read one track pitch back after every revolution of said disk, so that each time the same tele-vision picture is reproduced. rn the case of reproduction of a signal record- ~ , ed in accordance with the PA~ colour television standard this gives rise to a problem owing to the alternating phase of the PA~ colour signal, see United States Patent No. 3,974,519 of United States Philips Corp. issued August 10, 1976. Since a television picture in accordance with the PAL standard contains 625 lines, the first and last line of a PAL colour television signal always have the same PAL phase. However, this means that when realizing a stationary picture the line-se~uential alternation of said PAL phase is always interrupt-ed at the beginning of the picture, so that the reproduced colour signal may ~;
be disturbed.
When using the transmission system according to the invention this presents no problem, as the alternatlng PAL phase is not added to the colour signal until in the read apparatus, namely during the modulation of the (R - Y) colour signal on the standard chrominance carrier (4.43 MHz~ with alternating phase (+ cos) in the modulator 44. However, in order continuous-ly to maintain the ~R - Y) colour signal at the output 31 of the commutator 28 in the case of a stationary picture, the commutator switch should perform an extra switching cycle upon every revolution of the video disk. The first and the last line of one and the same recorded television picture contain the same colour signal, for example ~R - Y), because of the odd number of 516~7 :: :

lines, so that without additional switching the output 31 ~ould contain the (R - Y) colour signal during one picture period, the ~B - Y) colour signal during the next picture period etc. Because of the additional change-over of the commutator switch at picture frequency, this is prevented.
Said additional change-over can be accomplished very simply with the aid of the command device 38. Said command device 38 will already be provided in the reading equipment and control the desired movements of the scanning spot for realizing of said stationary picture. Consequently, it is very simple to cause said command device 38 to apply an additional pulse to the input 35 of the logic circuit 34 during each return of the scanning spot, so that said logic circuit 34 will supply an additional changing pulse to the commutator 28. It will be evident that everything that has been said in ;, respect of stationary pictures also applies to other devlating reproducing speeds, such as slow-motion or reverse-motion pictures.
~igure 4 finally shows an identificatlon circuit 39. Said identi-' fication circuit 39 serves to ascertain in the case of PAL reproduction ; whether output 31 of the commutator actually carries the (R - Y~ colour signal and output 32 the (B - Y) colour signal. Should these be interchanged owing to an incorrect alternation phase of the commutator switch, said identifica-tion circuit 39 with the aid of an identification signal which is included in the video signal, for example burst keying pulses, supplies an additional pulse to an lnput 37 of the logic circuit 34, so that the commutator switch performs an additional change-over. ~hen the transmitted burst keying pulses are used as identification signal, and if the disturbance-free burst keying ; pulses are to be added, erase circuits should be included before the adding circuits 41 and 42 in order to erase the transmitted burst keying pulses.
Figures 7 and 8 show two alternative embodiments of an apparatus with which a PAL-standard colour television signal can be transformed into a colour television signal as transmitted with the system according to the invention. ~;

- 18 _ '~, 56~17 ` ' ''-`' For the apparatus of;~igure 7 it has: been assumed that the chrominance component of the colour television signal according to the PAI.- ~ `
standard is applied to an input terminal 51. Said chrominance signal conse-quently contains a quadrature-modulated chrofninance signal on a 4.43 MHz colour carrier. In order to derive the two colour signals (R - Y) and ~B - Y) from this, an oscillator 52 is provided which supplies two carrier waves, derived from the colour burst, ~ith a frequenc~ of 4.43 MHz, but which are ~`
mutually 90 phase shifted ~sin and cos). One of said carrier ~aves is ;
applied to a phase inverter 53 ~hich line-sequentially changes the phase of the carrier wa~e by 180~. Said phase alternating carrier wave (+ cos) and the fixed carrier wave ~sin) from the oscillator 52 are applied to two inputs 54a and 54b of a commutator 54, whose outputs 54c and 54d are respectively connected to two amplitude demodulators 56 and 57 (inputs 56b and 57b respec- `
tively), to which moreover the original chrominance signal from terminal 51 is applied ~inputs 56a and 57a respectively~. In a similar way to the phase inverter 53, said commutator 54 receives a control signal at its control input 55 from a detector 62, which detects the line pulses from the luminance in-formation of the PAL colour television signal applied to a terminal 69, so that both the phase inverter and the commutator are switched line-sequentially, whilst by operation of said detector 62 it is assured that the phase inverter 53 assumes the correct position in accordance with the PAI. phase of the applied chrominance signal.
If it is assumed that the commutator 54 is in the position re~
presented by the continuous line~ the ~B - Y) colour signal is derived at the ; output of the AM demodulator 56, and ~ia a lo~-pass filter 58 becomes ava able at a terminal 60. Simultaneously, at the output of AM demodulator 57 the ~R - Y) colour signal is obtained~ ~hich signal becomes available at a terminal 61 via a low-pass filter 59. During the next line via the commutator 54 the cos-signal of the 4.43 ~Hz carrier is applied to the ~U~ demodulator 56, so that at terminal 60 the ~R - Y~ colour signal is o~tained, whilst at the .
, ~56( 17 same time the ~B ~ Y) colour signal becomes available at terminal 61. Thus, at the two terminals 60 and 61 the two desired colour signals ~R - Y) and (B - Y) become alternately available line-sequentially, to be transmitted via separate fre~uency bands by modulating them on two different colour carriers Fl and F2.
` Figure~8 shows a second embodiment of an apparatus for obtaining the desired colour signals from a PAL-standard chrominance signal for the ~ transmission system according to the invention~ Correspondlng elements are ;; denoted by the same reference numerals as in Figure 7. The PAL-chrominance `"
. 10 signal is again applied to an input term mal 51, which is now connected to a delay line 63, which dela~s said chrominance signal by one line period. The delayed chrominance signal is applied both directly and via an inverter 64 : : :
; to the two inputs 65a and 65b of a commutator 65, which at its control input 66 receives a symmetrical squarewave signal which is supplied by the line ~ ~`
pulse detector 62, so that said commutator is changed over line-sequentially.
ln order to explain the operation of said apparatus reference is made to the block diagram of ~igure 9. Said diagram shows the signals which ,~ appear at different points of the apparatus for five consecutive lines L ~
The PAL chrominance signal at terminal 51 i5 represented as U ~ V and U - V
alternately so as to denote the alternating PAL phase, U being the (B - Y) colour signal modulated on 4.43 MHz, and ~ being the (R - Y) colour signal modulated on 4.43 ~Hz. The indexes now denote the line during which said chrominance signal is applied. The output signals A and B of the commutator 65 will contain the chrominance signal which has been delayed by one line period, the polarity changing every line by operation of the inverter 64. The output signal A is added to the undelayed chr~minance signal in an adding circuit 67, resulting in the chrominance signal indicated in column 67, and the signal B is added to said undelayed chrominance signal in an adding cir-cuit 68, resulting in the signal indicated in column 68, in which it is assumed that the colour components of the two consecutive lines differ only ~ 56C~7 ~

slightly. ~rom these tNo columns 67 and 68 it can be seen that at the outputs of the two adding circuits 67 and 68 onl~ one colour component is present.
Apart from the commutator 65, said circuit is in fact identical to the cir-cuit generally used in PAL receivers for averaging phase errors.
~ The chrominance signals supplied by the adding circuits 67 and i 68 are applied to the AM demodulators 56 and 57 respectively, which also respectively receive the carrier Naves C and D, which with the aid of a phase - inverter 53 and commutator 54 are derived from an oscillator 52 in an identical manner to that shown in Figure 7. This results in the colour signals indicated in the columns 60 and 61 at the terminals 60 and 61. It can be seen that the desired colour sequence is obtained again at the two terminals. The difference with the device shown in Figure 7 is that the colour signal which becomes ava:ilable during a specific line, for example (B - Y)l2 is the average of the colour signals (B - Y)l and (B - Y)2, i.e.
each time the two colour signals of two consecutive lines. Of course, this is the result of the use of the delay line 63. The advantage of the device of Figure 8 compared with that of ~igure 7 is the requirements to be imposed on tbe AM demodulators 56 and 57 will be less stringent.
Figure 10 shoNs an alternative embodiment of a reproducing appara-tus for the transmission system according to the invention, the signals which appear bein~ shown in the diagram of ~igure 11. Corresponding elements are denoted by the same reference numerals as in ~lgure 4. The colour television signal which has been read is again divided into the signal components El, E2 and Ey with the aid of band-pass filters 22 and 23 and a low-pass filter 24. Said signal components are demodulated with the aid of FM demodulators 25, 26 and 27. At the outputs of the two ~M demodulators 25 and 26 the line-sequentially alternating colour signals (R - Y), ~B - Y) etc. are obtained again. To these two colour signals the burst keying pulses are now added in the adding circuits 41 and 42. As to the alternating colour signals ~R - Y) 3~ and ~B - Y~ ~eying pulses of opposite polarity should be applied a commutator ~o~

71 is required, whose two outputs are connected to the adding circuits 41 and 42 and whose two inputs are connected to the line pulse detector 40, which supplies both keying pulses of positive polarity and keying pulses of negative polzrity which are locked to the detected line impulse in the luminance signal y to said commutator 71. Said commutator 71 is controlled via a control input 72 by the logic circuit 34, which in its turn receives a control signal from the detector 40. Thus, the correct burst-keying pulses ; are always added to the demodulated colour signals. These two colour signals provided with keying pulses, at the outputs of the two adding circuits 41 and 42 ~columns 41 and 42 in ~igure 11~ are then applied to the two AM modulators 43 and 44 (inputs 43a and 44a~. To these two AM modulators are applied two carrier waves E and F with a frequency of 4.43 MHz, whose phase is line sequentially switched (inputs 43b and 44b). This is again effected with the aid of a 4.~3 M~lz oscillator 54 w~lich supplles two 90 phase-shifted carrier waves (sin and cos), whilst one of said carriers moreover changes polarity line-sequentially (l cost~. These two carrier wave signals are applied to a commutator 73, which at its control input 74 receives a control signal from the logic circuit 34 and thus changes over line-sequentially. As a result, the colour signals ~R - Y) and (B - Y) are always modulated on a carrier wave of the correct phase. At the output 43c and 44c of the two AM modulators line-sequentially alternating colour signals are still obtained, but they are now modulated on carrier waves with a frequency of 4.43 ~ and are there-fore represented by U and Y in analogy with ~igure 9. By adding said two colour signals in an adding circuit 47 a chrominance signal (column 47 in Figure 11~ is obtained which fully complies with the PAL standard and which when added to the luminance signal y in adding circuit 48 yields a complete PAL colour television signal YpAL at the output terminal 40.
Finally, Figure 12 schematically shows an embodiment of the logic circuit 34 together ~ith the identification circuit 39. The identification circuit 39 for example receives the colour signal from output 32 of the 35~7 commutator 28 (see F~gure 4). Said signal is applied to a switching tran-sistor 80, which at its control electrode 81 receives such pulses, derived from the horizontal synchroni~ing pulses, that said switching transistor is ; conducting during ~he time intervals in which the colour signal at output 32 contains the transmitted burst keying pulses. Ihese transmitted burst keying pulses are applied to a capacitance 82. As is known, the keying pulses which are added to the (R ~ Y) colour signal are positive and the keying pulses which are added to the ~B - Y~ colour signal are nsgative. Depending on the colour signal which is present at the output 32 the voltage across the capacitance 82 will consequently be positive or negative. Said voltage is transferred via an amplifier 83 and an integrator 8~ with an integra~ion time, of, for example, 200 line periods~ to an input of a comparator 85, whose second input is connected to earth potential. Said comparator for example has a preferred posltion which corresponds to a negative input signal, which in its turn corresponds to the presence of the ~R - Y) colour signal at the output 32 of the commutator 28. ~lo~ever, if the switching phase of said commutator 28 should not be correct and the (B - Y) colour signalshould con-sequently appear at the output 32, the signal which is applied to the com-parator 85 becomes positive, sO that said comparator changes over. With the aid of a monostable multivibrator 86 a suitable pulse is then applied to the terminal 37 of the logic circuit 34.
Sald logic circuit 3~ furthermore comprises a terminal 35, which is connected to the command device 38. The two terminals 35 and 37, via a differentiating network which consists of the capacitances 87 and 88 and the ; common resistor 89, are coupled to an amplifier 90, which supplies a pulse of the desired duration as soon as one of said terminals 35 and 37 receives a pulse. The logic circuit 34 further comprises a D flip-flop 91, whose set input S is connected to the aTnplifier 90. The outputs Q and Q of said 0 flip-flop are connected to the ~ and K inputs of a ~K flip-flop 92. The Q
output of said JK flip-flop is connected to an input of an AND-gate 94 and ,, ' ' ' ' . , 1 ~l~S6~;D7 the Q-output to an input of a NAND~gate 93. At their output inputs these two gates 93 and 94 receive the line-synchronizing pulse train supplied by ; the detector 4Q, which is also applied to the T input of the JK flip-flop 92.
; The output of the gate 93 is connected to the reset input R o~ the D flip-flop 91 and the output of the gate 94 to the T input of a JK flip-flop 95.
The J and K inputs of said JK flip-flop 95 are connected to a common terminal 96, whilst the signals at either of the t~o outputs Q and Q may ~e used as switching signals for the commutator 28.
The operation of the circuit is as follows. By feedback via the gate 93 to the reset input R of the D flip-flop 91 a stable state is obtained in which a logic 1 appears at the Q output and a logic 0 at the Q output of flip-flop 91. As a result, a logic 1 and 0 are also present at the Q and Q output respectively of the JK Elip-flop 92. The AND-gate 9~ in this state consequently transfers the llne-synchronizing pulses train from detector 40 to the T input of the JK flip-flop 95. Should the colour signal which is applied to the reproducing apparatus need to be transformed into a standard PAL signal, a logic 1 is applied to terminal 96, so that as a result of the line-synchronizing pulse train at the T input, the logic level of the two outputs Q and Q changes line-sequentially, so that the position of the commutator is also switched line-sequentially.
When subsequently one of the terminals 35 or 37 receives a pulse, the set input S of the D flip-flop receives a pulse and the state of said D flip-flop changes, i.e. the Q output supplies a logic 0 and the Q
output a logic 1. As these two outputs are connected to J and K inputs of the JK flip-flop, the state of said JK flip-flop also changes upon the next line synchronizing pulse at the T ~nput. The Q output then supplies a logic 0 to the AND-gate 94, so that the second, subsequent line synchronizing pulse from detector 40 is not transferred to the R input of the JK flip-flop 95 and said Elip-flop does not switch over its output signals. As the Q output of the JK flip-flop 9Z supplies a logic 1 to the NAND-gate 93, said second line .~ .

56 ~7 synchronizing pulse is applied to the reset input R of the D flip-flop, so that after said second line synchronizing pulse the stable state is restored.
A pulse at one of the terminals 35 and 37 thus eventually ensures that the linc-sequentially alternating control signals which becomes available at the outputs of the J~ flip-flop 95 for the commutator 28 are maintained in one state durin~ two line periods, so that said commutator 28 remains in the same state during these t~o line periods, which yields the desired phase inversion of the commutation. It ~ollows that the phase inversion of the commutation mentioned in the introduction of the specification is to be understood to include both the omission of a change-over prescribed by the line-synchronizing pulse train and the performance of an extra commutation, for example during the vertical flyback period.
It is obvious that the embodiment of the logic circuit is by no means limited to the embodiment shown. Various modifications are also conceivable in respect o~ the modulation method in which the three signal components are recorded on a record carrier. Obviously, said modulation method is not essential ~or the principle of the invention. For the purpose of illustration reference is made to the United States Patent No. 3,963,862 issued June 15, 1976 and United States Patent No. 3,962,720 issued June 8, lg76, both of United States Philips Corp.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for transforming a colour television signal in a trans-mission system, in particular in a system for recording on and reproducing from a record medium, in which method a transmission signal is generated having a first signal component comprising luminance information, a second signal component comprising a first colour carrier wave modulated with a first colour information component and a third signal component comprising a second colour carrier wave modulated with a second colour information component, which second and third signal components are arranged to occupy separate frequency bands characterized in that the first and second colour information components are constituted by a first and second colour signal in a line-sequentially interchanged fashion.

2. A method as claimed in claim 1 characterized in that the first and second colour signals are constituted by colour difference signals.

3. A transmission or recording apparatus for performing the method as claimed in claim 1 comprising first means for generating the first signal component, second means for generating the second signal component and third means for generating the third signal component characterized in that the apparatus comprises a first commutator having a first and second input and a first and second output, which commutator is controlled by a control signal such as to establish a parallel and cross wise coupling between said first and second input on the one hand and said first and second output on the other hand in a line-sequentially alternating fashion and is coupled to the second and third means for obtaining the line-sequential alternation of the colour signals modulated on the first and second colour carrier wave.

4. A transmission or recording apparatus as claimed in claim 3, characterized in that the first input of the first commutator receives the first colour signal and the second input the second colour signal, whilst the first output is coupled to a first modulating circuit for modulating an applied signal on the first colour carrier wave, and the second output to a second modulating circuit for modulating an applied signal on the second colour carrier wave.

5. A transmission or recording apparatus as claimed in claim 3, adapted to a colour television signal composed in accordance with the PAL standard, characterized in that the apparatus is provided with a first and a second amplitude demodulator each having a first input, to which a chrominance sig-nal derived from a PAL colour television signal applied to an input terminal is applied, each having a second input which is coupled to the first and the second output respectively of the first commutator, and each having an output for supplying the second and the third signal component respectively, whilst the apparatus furthermore comprises an oscillator circuit for supplying two carrier waves to the first and the second input of the first commutator, which two carrier waves have a frequency equal to that of the PAL chrominance carrier wave, and a mutual phase shift which every alternate line is + 90°
and - 90°.

6. A transmission or recording apparatus as claimed in Claim 5, characterized in that the first and the second colour signal are constituted by the complete chrominance signal contained in the PAL colour television signal.

7. A transmission or recording apparatus as claimed in Claim 5, char-acterized in that the apparatus is furthermore provided with a delay means which is coupled to the input terminal for delaying the colour signal by one line period, an inverter circuit which is coupled to the output of the delay means for inverting the output signal thereof, a second commutator for establishing in a line-sequential alternation fashion as a function of a control-signal which is supplied by a control circuit, a parallel and crosswise coupling between a first and second input on the one hand, and a first and second output on the other hand, the first input being coupled to the output of the delay means, and the second input to the output of the inverter circuit, a first adding circuit with a first input which is coupled to the input terminal, a second input which is coupled to the first output of the second commutator, and an output which is coupled to the first amplitude-demodulator, and a second adding circuit with a first input which is coupled to the input terminal, a second input which is coupled to the second output of the second commutator, and an output which is coupled to the second amplitude demodulator.

8. A receiving or reproducing apparatus for performing the method as claimed in Claim 1, characterized in that the apparatus is provided with a first commutator with a first and a second input and a first and second output, which as a function of a control signal in a line-sequentially alter-nating fashion can establish a parallel and a crosswise coupling between the first and second input on the one hand, and the first and second output on the other hand, whilst the two inputs are coupled to two individual colour channels which supply the third and fourth signal components respec-tively, and the outputs to two separate output channels, the control signal being supplied by a logic circuit which as a function of a horizontal synchronizing signal which is applied to a first input can supply a square-wave control signal.

9. A receiving or reproducing apparatus as claimed in Claim 8, char-acterized in that the apparatus includes an identification circuit which is coupled to at least one of the output channels and which is adapted to detect which colour signal appears in the relevant output channel and which in the event that the incorrect colour signal is detected is capable of supplying a pulse to a second input of the logic circuit for causing a phase inversion of the alternation of the coupling between the two inputs and the two outputs of the first commutator.

10. A reproducing apparatus as claimed in Claim 8 or 9 for reproducing a colour television signal which is recorded on a disc-shaped record carrier, characterized in that the apparatus is provided with a command device for controlling the read process, which command device is also coupled to a third input of the logic circuit for causing a phase inversion of the alternation of the coupling between the two inputs and the two outputs of said first commutator so as to maintain the PAL phase-alternation in the case of a play-ing sequence of the recorded television images which differs from the recording sequence.

11. A receiving or reproducing apparatus for performing the method as claimed in Claim 1, characterized in that the apparatus is provided with a first demodulator for demodulating the colour signal which is modulated on the first colour carrier wave, a second demodulator for demodulating the colour signal which is modulated on the second colour carrier wave, a first and a second amplitude modulator, each with a first input which is coupled to the first and the second demodulator respectively, an oscillator for supplying two carrier waves with a frequency equal to that of the PAL chromin-ance carrier wave, but with a mutual line-sequential phase alternation of + 90° and - 90°, and a first commutator with two inputs and two outputs, which as a function of a control signal supplied by a logic circuit, can establish a parallel and crosswise coupling between these two inputs and the two outputs in a line-sequentially alternating fashion, the first and the second output of the commutator being respectively coupled to a second input of the first and the second amplitude modulator respectively, whose outputs are connected to an adding circuit.

12. A receiving and reproducing apparatus as claimed in Claim 11, characterized in that the apparatus comprises a first and second adding circuit to which burst keying pulses are applied via a fourth commutator whose two outputs are connected to the individual adding circuits and whose two inputs receive two pulse trains of line frequency, one of said inputs receiving pulses of positive polarity and the other input pulses of negative polarity, which fourth commutator is also controlled by the logic circuit for establishing in a line-sequentially alternating fashion a parallel and crosswise coupling between the two inputs and the two outputs, and further-more said first and second adding circuit receive the colour signals demodu-lated by the first and second demodulator as input signals, and their outputs are coupled to the first and second amplitude modulator.