The present invention relates to the transmission and reception of color television signals.

It is well known that by the present color television transmission methods it is not possible to offer the user a television signal which is free of cross-color and cross-luminance defects due to interlacing of luminance and chro minance spectra. This is due to the fact that at present the color television signals are transmitted on a single channel and any attempt to eliminate said defects would bring to such complications as to discourage any approach in this sense. The difficulty could be overcome by subdividing the color television signal in two channels, but this would involve the requirement of modifying the present television standards with all the complications which would derive therefrom.

These considerations apply,however, only to transmissions of color television signals in the normal earth network.

For direct broadcasting from satellite, which is a new service which has not yet been adopted in any European country, it is possible to imagine different standards provided they are compatible with the standard rules established by WARC-77. One of these rules is that the interferences produced by the new signal on other channels must not be higher than those produced by a normal NTSC, PAL or SECAM signal. It is easily understood that once these rules are met the broadcasting from satellite, by making possible the transmission on a plurality of channels, makes it possible to furnish the users with a signal having a higher quality than the present signals.

It is an object of the present invention to provide a method of transmitting color television signals for broadcasting from satellite comprising the steps of trans mitting the television signal on two channels in different manners according to whether the system involved is compatible or non compatible.

The difference between the two systems is that in the compatible system a normal NTSC, PAL or SECAM signal is transmitted-in the first channel so that said signal can be.-received if one renounces to receive the improved signal due to the transmission on two channels, by a normal television set. In the non compatible system instead no complete NTSC, PAL or SECAM signal is transmitted in any of the two channels so that no signal is received by a normal television set unless a standard converter outside the television set is used.

More particularly, the method of transmitting color television signals for broadcasting from satellite according to the invention is characterized in that when a compatible system is used, it comprises the steps of transmitting on the first channel the complete color television signal and on the second channel the baseband chrominance R-Y and B-Y signals from 0 to about 3 MHz sequentially on alternate lines and from 3 MHz up to the limit of the video band the high frequency components of the luminance signal.

In the case of a non compatible system, the method of transmitting color television signals on two channels according to the invention is characterized in that it comprises the steps of transmitting the luminance information in the whole available band on the first channel and the R-Y and B-Y chrominance information in the whole available band on the second channel.

It is a further object of the invention an apparatus for carrying out the method.

The invention will be better understood from the following detailed description, given merely by way of example and therefore in no limiting sense, of some embodiments thereof referring to the accompanying drawings in which:

• Fig. 1 is a block diagram of a compatible apparatus according to the invention;
• Fig. 2 is a block diagram of a non compatible apparatus according to the invention with time division in the second channel;
• Fig. 3 is a graph of a signal generated in the apparatus of Fig. 2;
• Fig. 4 is a block diagram of a non compatible apparatus according to the invention with frequency sharing multiplex in the second channel;
• Fig. 5 is a graph of the spectrum of a signal generated in the apparatus of Fig. 4; and
• Fig. 6 is a graph of a signal appearing in the apparatus of Fig. 4.

Referring first to Fig. l, the transmitter of the compatible system comprises an encoder (NTSC, PAL or SECAM) 1 having three inputs for applying thereto the three Y, U and V signals and an output connected to the transmitting apparatus TX for a first channel referred to as A. This transmitter comprises also an adder 2 having 2 inputs one of which is connected to the output of a high pass filter 3 having the Y signal applied to its input, whereas the other input is connected to the output of a low pass filter 4 having the input connected to the output of a sequential switch 5 having two inputs for applying thereto the U and V signals. The output of the adder 2 is connected to the transmitting apparatus TX for the second channel referred to as B.

The signal appearing in the channel A is a normal NTSC, PAL or SECAM signal suitable for normal television sets. It can be therefore, in the receiving apparatus, VHF or UHF vestigial band modulated and sent directly to the antenna terminal of the television set. Those who desire to receive the improved quality signal must provide a special adapter capable of using both transmitted signals (that on the channel A and that on the channel B). This adapter can be connected to a television set provided with a low frequency R, G, B input.

The adapter comprises a matrix 6 having three inputs, one of which is connected to the output of an adder 7 whereas the other two are each connected to an output of a switch 8. The adder 7 has two inputs, one of which is connected to the output of a low pass filter 9 having the signal received by the re.ceiving apparatus RX for the channel A applied to its input, whereas the other is connected to the output of a high pass filter having the signal received by the receiving apparatus RX for the channel B applied to its input. The switch 8 has two inputs, one of which is connected directly to the output of a low pass filter 11 having the signal received by the receiving apparatus RX for the channel B applied to its input, whereas the other is connected to said output of the filter 11 through a delay line 12. The receiver comprises also a detector 13 of the NTSC, PAL or SECAM alternance having the signal received on the channel A applied to its input and having the output connected to a third input of the switch 8. The matrix 6 has three outputs connected to the input R, G, B of the television set.

The described apparatus operates as follows:

• On the channel A there is transmitted the NTSC, PAL or SECAM SIGNAL. On the channel B there are transmitted from 0 to about 3 MHz the baseband R-X and B-Y signals sequentially on alternate lines and from 3 MHz to about the limit of the available band in the channel the high frequency components of the luminance signal which are present in the channel A but are not usable because mixed with the chrominance signal.

The adapter according to the invention receives the luminance components from 0 to about 3 MHz from the channel A and those from 3 MHz on from the channel B and receives in addition the chrominance components from the channel B. In this manner the cross-color and cross-luminance defects are eliminated which are due to the interlacing of the luminance and chrominance spectra present either with NTSC, SECAM or PAL.

It is in addition possible, if the channel B permits it, to transmit luminance components having a higher frequency than the nominal limit of the video band.

In an apparatus made according to the diagram of Fig. 1 the high pass filter 3 had a cut-off frequency f_T ≅ 3,5 MHz, the low pass filter 4 had f_T ≅ 3 MHz, the complementary high pass and low pass filters 10 and 9 respectively had f_T ≅ 3,8 MHz and the low pass filter 11 had f_T ≅ 3,2 MHz and the delay of the delay line 12 was 64 ps.

The video signal in the channel B was of a conventional type, with the exception of the line blanking at 0,35 V, inasmuch as U and V have a zero average value. The Y, U and V signals were conventional PAL signals, i.e.:

The values of the k_U and k_V coefficients can be anyway changed without departing from the scope of the invention.

E'_R, E'_V and E'_B are the γ corrected R, G and B signals respectively.

The delay line 12, since it must operate in baseband, must be made with COD (Charge-coupled devices) techniques.

In the described apparatus the sequential switch 5 of the transmitter selects, on alternate lines, U or V. The switch 8 of the receiver exchanges the inputs with one another with a line frequency so that always U and always V appears-at the output. The correct switching phase is derived from the NTSC, PAL or SECAM alternance, or from the vertical synchronism.

Referring now to the description of the apparatus embodying the non compatible system, in Fig. 2 is shown a non compatible apparatus with time division multiplex in the channel B. This apparatus is provided for the transmission on the channel A of the luminance information in the whole available band and the transmission on the channel _B of the chrominance (R-Y B-Y) information in the whole available band (by available band is meant the portion not occupied by audio subcarriers) by a time division multiplex, i.e. one of the two chrominanoe components is transmitted in the first half of the active line and the other component in the second half.

The feature of the apparatus of Fig. 2 is that it comprises in the transmitter 14 a compressor l5 and in the receiver 16 an expander 17 whose two outputs are connected to two inputs of a matrix 18.

The compressor generates the signal shown in Fig. 3.

In the 52 µs forming the "active" portion of the line there are present the signals U and V (or V and U) time compressed with a ratio 1:2. The synchronisms are those of a normal black and white video signal but, like in the case of the compatible apparatus of Fig. 1, the line blanking is shifted to the "grey" level (0,35 V) inasmuch as the U and V signals are at a 0 average value. The Y, U and V signals are those conventional of PAL already considered with reference to Fig. 1.

In both channels the shape of the synchronizing signals can be changed. In particular there can be inserted audio channels PCM transmitted with the well known "Sound-in-Syncs" system, or the duration of the line blanking (which in Fig. 3 is l2 µs) can be reduced in order to suitably increase the duration of the active portion of the line, thus widening the size of the image, which is particularly useful for large screen vision, inasmuch as a width-height ratio higher than the present (4/3) is subjectively more appreciated.

Because of the non compatible character of the system being discussed, it is also possible to increase the number of the scanning lines in order to improve the vertical definition, by reducing proportionally the horizontal one; everything without departing from the scope of the invention.

In Fig. 4 an apparatus is shown providing the non compatible version of the proposed system with frequency sharing multiplex in the channel B. This apparatus is provided for the transmission on the channel A of the luminance information in the whole available band (like the apparatus of Fig. 2) and for the transmission on the channel 3 of the chrominance (R-Y, B-Y) information in the whole available band by means of a frequency sharing multiplex, i.e. one of the chrominance components is transmitted in baseband, i.e. from 0 to about 3 MHz (better the R-Y component inasmuch as it is more influenced by noise) and the other component is transmitted on an amplitude modulated or single lateral (higher) band or vestigial band subcarrier when it is desired a lower complexity of the modulators and demodulators at the cost of a non optimal use of the available spectrum.

The feature of the apparatus of Fig. 4 is that it compri ses in the transmitter 19 a modulator 20 having the output connected to an input of an adder 21 connected with a second input to the output of a low pass filter 27 which limits the spectrum of the baseband transmitted component, and in the receiver 22 a demodulator 23 having the output connected to a matrix 24 and the input connected to the output of a high pass filter 25 which has,_in turn, the input connected to the receiving apparatus of the channel B to which also the input of a low pass filter 26 is connected having the output connected to a second input of the matrix 24.

In the case of transmission on vestigial band subcarrier the spectrum of the signal going to the TX channel B is that shown in Fig. 5.

The value of the U modulated subcarrier depends on the band f_B available in the channel B and the type of modulation (side band or vestigial). For example with f_B = 8 and vestigial band modulation the subcarrier f_C can be located in the area of 4, 8 ÷ 5 MHz.

The video signal is shown in Fig. 6.

Like in the preceding cases the synchronisms are of the conventional type, but the line blanking is raised to + 0,35 V inasmuch as the V signal is at a zero average value. A frequency burst f_C is present, situated in the same position as the conventional PAL burst. The phase of this burst is constant and equal to that of the subcarrier f_C. In demodulation the burst is used to rebuild the subcarrier f_C supplied to a conventional synchronous- demodulator.

In the receiver 22 the low pass filters 26 and the high pass filters 25 , whose response curves are diagrammatically shown in dotted lines in Fig. 6, are used for separating U and V.

While but some embodiments of the invention have been illustrated and described, it is obvious that a number of changes and modifications can be made without departing from the scope of the invention.