DIGITAL TRANSMISSION OF TELEVISION SIGNALS

Abstract: Transmission of teletext in a digital television format is disclosed. The video information is transmitted in the conventional YUYV format. For teletext transmission, the U and V sample periods are used to transmit additional luminance samples. Accordingly, teletext is transmitted in YYYY format, which in the CCIR 601 system corresponds to a 27 MHz sampling frequency. Teletext recovery is much more reliable.

Description: Digital transmission of television signals FIELD OF THE INVENTION The invention relates to a method and apparatus for transmitting a television signal. Transmission is herein not only understood to mean transmission from a television transmitter to one or more television receivers, but also local transmission from one apparatus to another (for example, in a television studio), or transmission from one circuit to another in a television receiver. Transmission is also understood to mean storage on storage media such as tapes, disks, etc. The invention also relates to a method and apparatus for receiving a television signal. BACKGROUND OF THE INVENTION A known television transmission method comprises the steps of sampling the luminance component Y of an analog television signal at a luminance sampling frequency and sampling the chrominance components U and V of the television signal at a chrominance sampling frequency. The sampling frequencies of 13.5 MHz and 6.75 MHz, respectively, are often used. The samples are then transmitted in a format such as YUYV or YYUV. Usually, the samples are transmitted as digital words, but this is not essential to the invention. An analog television signal may also include auxiliary data signals such as teletext, VPS data for controlling videorecorders, closed caption data, and wide-screen signalling bits. Said data is transmitted in lines of the vertical blanking interval, the data signal waveform being shaped so as to fit within the bandwidth of the analog signal. The data signal is transmitted in the form of a luminance signal. Accordingly, in the prior art transmission system, the data signal will be sampled at the luminance frequency f usually 13.5 MHz. It has been found that reconstruction (data slicing, clock recovery) of the data from the data signal thus sampled is difficult and error-prone. OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide a method and apparatus for transmitting and receiving a television signal, in which the above-mentioned problems of the prior- art systems are alleviated. In accordance with the invention, the method of transmitting a television signal is characterized by the steps of sampling the luminance signal in data lines of the television signal at a luminance sampling frequency which is higher than the luminance sampling frequency in video lines so as to obtain additional luminance samples, and transmitting said additional luminance samples instead of chrominance samples. The invention is based on the recognition that the data signal does not comprise a chrominance component. By using the chrominance sample periods in the YUYV transmission format for conveying data signal samples, the data signal can be transmitted at a considerably higher sampling rate, without increasing the overall transmission rate. The performance of the data slicing and clock recovery operation which is applied to the sampled data signal is thereby considerably improved. In a preferred embodiment, all chrominance sample periods are used for data signal transmission. In a system having a normal luminance sampling frequency of 13.5 MHz and a chrominance sampling frequency of 6.75 MHz, the data signal can then be sampled at 27 MHz. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 show arrangements for transmitting a television signal in accordance with the invention. Fig. 3 shows a time diagram for illustrating the operation of the arrangements which are shown in Figs. 1 and 2. Fig. 4 shows an arrangement for receiving a television signal in accordance with the invention. Fig. 5 shows a television receiver for digitally processing an analog input signal in accordance with a further aspect of the invention. DESCRIPTION OF EMBODIMENTS Fig. l shows an arrangement for transmitting a television signal in accordance with the invention. The arrangement receives an analog RGB video signal and teletext data TXT. The RGB video signal is converted into an analog luminance component y and two chrominance components u and v by a conversion circuit 1. The teletext data is shaped by a waveform-shaping circuit 2. The conversion circuit 1 and waveform-shaping circuit 2 are well-known in the field of analog television transmission. The data signal d, luminance signal y and chrominance signals u and v are applied to respective sampling stages and A/D converters 3, 4, 5 and 6, respectively, which receive respective sampling frequencies from a clock-generating circuit 7. In the present example, the luminance signal is sampled at f y = 13.5 MHz, the chrominance signals u and v are both sampled at f c =6.75 MHz, and the data signal d is sampled at twice the luminance sampling frequency, i.e. f y =27 MHz. The digital luminance samples are denoted Y, the digital chrominance samples are denoted U and V, and the data signal samples are denoted D. The samples are applied to a transmission formatting circuit 8 which also receives a Data Line signal DL which indicates whether the current television line is an active video line or a teletext line. Usually, teletext lines are selected lines of the video blanking interval, but this is not essential to the invention. The formatting circuit 8 is adapted to transfer video samples to the system's output as successive sequences YUYV during television lines for which DL is low, and to transfer data samples in the format DDDD during television lines for which DL is high. Fig.2 shows an alternative embodiment of the arrangement. It is now assumed that the arrangement receives an analog composite video signal CVBS which already includes a teletext signal in selected television lines. The signal is applied to a video processor 10 which splits the input signal into its luminance component y and chrominance components u and v. They are applied to respective sampling stages and A/D converters 11-13. The sampling frequencies are now obtained from a selection circuit 14 which selects one of its inputs in response to the Data Line signal DL. If DL is low, the conventional sampling frequencies f y and f c from clock generator circuit 7 are selected. If DL is high, the sampling frequency f y =27 MHz is selected for the luminance channel, whereas the chrominance channels are not sampled. Accordingly, during video lines, the conventional sequences YUYV will be outputted by the arrangement, whereas during data lines, the sequences YYYY will be generated, each Y representing a 27 MHz sample of the data signal. Fig.3 shows a time diagram for illustrating the operation of the arrangements which are shown in Figs. 1 and 2. Waveform A shows a master clock signal which corresponds to the highest sampling frequency of the system, ie f y =27 MHz in the present example. Waveforms B and C show the outputs of two successive divide-by-two stages in the clock generator circuit (not shown in Figs. 1 and 2). Waveform D is the conventional luminance sampling clock f y = 13.5 MHz. Waveform E is the chrominance sampling clock f c =6.75 MHz for the U-channel. Waveform F is the chrominance sampling clock f c =6.75 MHz for the V-channel. Waveform G denotes the output signal of the arrangement during video lines (DL is low) in the format YUYV. Waveform H denotes the output signal of the arrangement during teletext lines (DL is high) in the format YYYY (or DDDD in the notation which is used in Fig. l). Fig.4 shows an arrangement for receiving a television signal in accordance with the invention. The digital television signal is applied to both a video processing stage and a teletext-processing stage. A significant feature of the invention is that it is not necessary to first determine whether YUYV or YYYY sequences are being received, and to apply the signal to either a video processor or a teletext processor in response to said determination. The video-processing stage is adapted to process sequences of YUYV samples and comprises a demultiplexer 40 to split the input signal into Y-samples, U-samples and V-samples. They are applied to respective D/A converters 41-43 so as to obtain an analog luminance signal y and analog chrominance signals u and v which are applied to a conventional analog video processor 44. The video processor 44 may be a digital processing circuit. In that case, the D/A-conversion is done after said processing. By applying the input signal to the teletext-processing stage via a connection 47, the digital samples constituting the data signal are concatenated. The teletext- processing stage comprises a D/A converter 45 adapted to run at a sampling frequency of 27 MHz. The analog signal d is then applied to a conventional analog teletext decoder 46. Alternatively, the teletext decoder may be a digital decoder, which is understood to mean that the process of data slicing and clock regeneration is performed by digital signal processing. In that case, D/A converter 45 can be dispensed with. Usually, teletext is transmitted in the vertical flyback interval of a television signal. The teletext decoder can be arranged to decode data only in said interval. The invention also enables transmission of teletext in lines of the active video interval. The decoder shown in Fig.4 will automatically handle this operation mode, because the decoder receives all samples during all television lines. Optionally, a code indicating the transmission of teletext in the active video interval may be transmitted. Fig.5 shows a television receiver for digitally processing an analog input signal in accordance with a further aspect of the invention. Because of the great resemblance with the arrangement shown in Fig.2, the same reference numerals are used. As will readily be understood with reference to Fig.2, the television receiver receives an analog television signal CVBS and converts the video lines thereof into digital YUYV sequences and the data lines into YYYY sequences. Instead of transmitting the samples, the YUV samples are now applied to a digital video processor 48, whereas the Y samples are also applied to a digital teletext decoder. Conventional analog television receivers with digital processing circuits 48 and 49 have a fourth A/D converter for the teletext signal. In the receiver shown in Fig.5, this converter is dispensed with. In summary, transmission of teletext in a digital television format is disclosed. The video information is transmitted in the conventional YUYV format. For teletext transmission, the U and V sample periods are used to transmit additional luminance samples. Accordingly, teletext is transmitted in YYYY format, which in the CCIR 601 system corresponds to a 27 MHz sampling frequency. Teletext recovery is much more reliable.

Claims: Claims 1. A method of transmitting a television signal, comprising the steps of sampling the luminance component (Y) and chrominance components (U,V) of the television signal at respective sampling frequencies (f y , f c ) and transmitting said samples, characterized by the steps of: sampling the luminance signal in data lines of the television signal at a luminance sampling frequency (f' y ) which is higher than the luminance sampling frequency (f y ) in video lines so as to obtain additional luminance samples, and transmitting said additional luminance samples instead of chrominance samples. 2. A method as claimed in Claim 1, in which the number of additional luminance samples in data lines is equal to the number of chrominance samples in video lines of the television signal. 3. A method as claimed in Claim 2, in which the luminance sampling frequency in video lines is 13.5 MHz, the chrominance sampling frequency is 6.75 MHz, and the luminance sampling frequency in data lines is 27 MHz. 4. A method of receiving a television signal in the form of samples, comprising the steps of: concatenating luminance samples (Y) in video lines of the television signal to obtain a luminance signal sampled at a luminance sampling frequency (f y ), concatenating chrominance samples (U,V) in video lines of the television signal to obtain chrominance signals sampled at a chrominance sampling frequency (f c ); characterized by the steps of: concatenating all samples in data lines of the television signal to obtain a data signal sampled at a sampling frequency (f' y ) which is higher than the luminance sampling frequency (f y ) in video lines. 5. A method as claimed in Claim 4, characterized by the steps of decoding the sampled data signal to obtain a code indicating the transmission of a data signal in video lines of the television signal, and concatenating all samples of the television signal also in video lines of the signal in response to the detected code. 6. A method as claimed in Claim 4, characterized by the steps of concatenating all samples also in video lines of the television signal to obtain a signal sampled at the higher sampling frequency (f' y ), and decoding said sampled signal to determine whether it is a data signal. 7. An apparatus for transmitting a television signal, comprising means adapted to carry out the method as claimed in any one of Claims 1-3. 8. An apparatus for receiving a television signal, comprising means adapted to carry out the method as claimed in any one of Claims 4-6. 9. A television signal in the form of luminance and chrominance samples, comprising video lines having luminance samples (Y) at a luminance sampling frequency (f y ) and chrominance samples (U,V) at a chrominance sampling frequency (f c ), characterized in that the signal comprises data lines having only luminance samples at a sampling frequency (f' y ) which is higher than the luminance sampling frequency (f y ) in video lines. 10. A storage medium on which a television signal as claimed in Claim 9 has been stored. 11. A television receiver substantially as shown in Fig.5.

Inventors: VAN GESTEL HENRICUS ANTONIUS W, VAN GESTEL, HENRICUS, ANTONIUS, WILHELMUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H04N7
IPC codes: H04N7, H04N11