FR2632149A1 - Method of broadcasting television programmes of the MAC/PACKET type and receiver for such a programme - Google Patents

Abstract

The broadcasting is carried out in the form of a signal containing, for each useful line of the television frame, a baseband time-division multiplex composed of a multiplex of analogue chrominance and luminance signals strictly conforming with the MAC/PACKET standards and of a digital burst of sound and data. The digital burst is reduced to a duration which frees for each line a period of at least 4,5 mu s occupied by an analogue line-synchronisation signal compatible with the PAL or SECAM line-synchronisation signal.

Description

Method of broadcasting television programs from tvDe MAC / PAOUET and receiver for such a program.

 The invention relates to the broadcasting of television programs of the so-called MAC / PACKET type, that is to say in the form of a signal comprising, for each useful line of the frame, a time base time multiplex having a burst digital sound and data and a multiplex of analog chrominance and luminance signals.

 The invention is of particular interest for the broadcasting of programs by a signal essentially conforming to the D-MAC / PAQUET and D2-MAC / PAQUET standards, and especially in the second case, where the binary elements (bits) of the bursts are transmitted at a rate of 10,125 Mbits per second, which allows transmission over terrestrial networks by amplitude modulation with residual sideband (called MA-BLR or AM-VSB).

 The signal conforming to the D2-MAC / PACKAGE standard comprises a 64-bit frame synchronization word inserted in line 625 and line synchronization words at the start of each line. There are already integrated decoding circuits intended to receive the D2-MAC signal in digitized form, then to carry out the synchronization, the demultiplexing of the components, the decoding of the sounds, the luminance and the chrominance and the output of the line 625. which contains service information. In particular, there are known decoding circuits which carry out line synchronization from the frame synchronization word alone, which makes superfluous. line synchronization words.

 The absence of an analog line synchronization signal in the signal conforming to the D2-MAC standard raises difficulties in the case of transmission over wireless or wired narrowband networks, where the transmission takes place in MA-BLR. The alignment of the beams is difficult to achieve. It is no longer possible to adopt an interference method using a variable offset between the analog line synchronization signal and the start of the image signal, that is to say the method currently used in France for Pay TV.

The conditional access mode according to the standard
D2-MAC is performed by a process called "component rotation" with single or double cut. In the case of double breaking, the luminance signal and the chrominance signal are each cut at a point chosen from among several points at locations determined from line synchronization (ensured by decoding the line synchronization word or at using a counter from the frame synchronization word), the cut-off point being fixed by the output value of a pseudo-random binary sequence generator, frequently denoted by the abbreviation PRBS.

 The invention aims in particular to provide a method of broadcasting signals carrying a television program essentially meeting the D-MAC standard or especially D2-MAC / PACKET, making it possible to broadcast the program on a wireless or wired network intended for conventional PAL or SECAM television signals. The invention also aims to obtain this result by preserving the possibility of implementing a conditional access mode in accordance with the D-MAC or D2-MAC / PACKET standard without requiring any significant addition to the receivers intended to receive program signals in strict accordance with the D2-MAC / PACKAGE standard.

For this purpose, the invention proposes a method of broadcasting television programs in the form of a signal comprising, for each useful line of the television frame, a time base band multiplex consisting of a multiplex of analog chrominance signals. and luminance strictly in accordance with standards
MAC / PACKET and a digital burst of sound and data, characterized in that said digital burst is reduced to a duration freeing for each line a period of at least 4.5 ijs occupied by an analog line synchronization signal compatible with line synchronization signal in PAL or SECAM.

 Reducing the duration of the digital sound and data burst (that is, the number of bits transmitted on each line) does not degrade the image signal. Its only consequence is the reduction in the number of sound channels that it is possible to transmit simultaneously. In practice, it is still possible to transmit a single high quality stereo channel, which is generally sufficient in the case of a radio or cable network, since the programs are then intended for users of the same language. The advantage of D-MAC or D2-MAC, namely the broadcasting of a speech signal in several languages between which the spectator can choose, is much less decisive in this case of a terrestrial network than in that of broadcasting. direct via satellite.

 The invention also provides a receiver capable of receiving not only programs broadcast strictly according to the D-MAC or D2-MAC / PACKAGE standard, but also programs broadcast by the process defined above, characterized in that it comprises, in more than a conventional integrated decoding circuit (such as ITMA's DMA 2270 circuit) and possibly a descrambling circuit (such as ITT's DMA 2275 circuit), a synchronization extraction, window generation circuit masking and adjustment of signal levels.

The invention will be better understood on reading the following description of a particular embodiment, given by way of non-limiting example. The description refers to the accompanying drawings, in which
FIG. 1 is a diagram showing the waveform, during transmission, of a broadcast signal differentiating in particular from a signal according to the D2-MAC standard by the presence of an analog synchronization signal,
FIG. 1A, similar to FIG. 1, shows a variant,
FIG. 2 schematically shows the general constitution of a specific product for a receiver making it possible to receive both a D2-MAC signal and a signal having the waveform shown in FIG. 1,
- Figure 2A, similar to Figure 2, shows a variant applicable to a signal having the waveform shown in Figure lA
FIG. 3 shows how the circuit of FIG. 2 can be added, in the form of an additional integrated circuit, to the DMA circuits 2270 and 2275.

The diagram of Figure 1, where the time scale is not respected for clarity, shows the waveform of one of lines 1 to 623 of a D2-MAC signal modified in accordance with a mode of realization of the invention, as transmitted over a terrestrial network. We will see later that this waveform must be modified before being applied to the input of a decoder
Classic MAC.

Each useful line (lines 1 to 623) successively comprises an analog line synchronization signal, a burst of digital data, an alignment level, and finally the samples of the chrominance signal and the luminance signal: the alignment level and the luminance and chrominance multiplex can be absolutely identical to those of a signal
D2-MAC / normal PACKAGE, as described for example in the brochure "Specification of the D2-MAC system /
PACKAGE ", September 1985, Ministry of Posts and Tele communications and Broadcasting.

In the particular case shown in Figure 1, the signal successively presents
- a front bearing has a line synchronization signal, having a duration of 4 clock periods at 20.25 MHz, at black level
- the falling edge b of the line synchronization signal towards the synchronization foot (3 clock periods)
- the line synchronization signal base, consisting of a continuous level of 300 mV below the black level at transmission, represented by the front bearing a (90 clock periods)
- the rising edge d of the line synchronization signal (3 clock periods)
- a level e at black level after the line synchronization signal (4 clock periods)
- a transition f with the start of the digital duobinary burst of data (4 clock periods)
- the data burst g (98 clock periods), symmetrical with respect to the transmission black level and which, for this reason, has a peak-to-peak amplitude which practically cannot exceed 400 to 500 mV, so as not to not reach the foot of the synchronization level (-300 mV in the illustrated case)
- a transition h between the burst of duobinary data and the alignment level (6 clock periods)
- MAC j alignment level, at +350 mV above the transmission black level (15 clock periods)
- a transition k between alignment level and analog multiplex (11 clock periods)
- chrominance signal 1 (349 samples);
- a transition m between chrominance and luminance
- the luminance signal n (697 samples)
- a weighted transition p between the end of the luminance signal and the front plateau of the line synchronization signal.

 The durations k to p are those provided by the D2-MAC standard, which ensures full compatibility of the analog multiplex with that for which the existing decoding circuits are provided. On the other hand, the first part of the line is modified and allows the signal to be transmitted without difficulty over the terrestrial networks, by adopting a peak-to-peak amplitude for the luminance signal and the chrominance signal of +700 mV, above the transmission black level the total amplitude of the signal is 700 + 300 mV = 1 V, i.e. the usual transmission value on these networks. By reducing the data burst to 98 clock periods, that is to say at 4.8395 p5, there are 90 clock periods at the synchronization foot for the line synchronization signal, that is to say 4.4444 ijs. If we take into account the transitions, we arrive at a duration of approximately 4.5 ps which represents practically a minimum still acceptable to guarantee synchronization.

 When the decoding circuit is designed to reconstruct line synchronization from the frame synchronization word and by itself, it is not necessary to keep the 6 bit word for line synchronization of the D2-MAC / PACKET standard. , which further reduces the loss of sound signal and data bits caused by the implementation of the invention.

 In this case, the calculation shows that the data transmission capacity which remains after amputation of the duration of 4.5 ps necessary for the transmission of the line synchronization signal is sufficient to allow the broadcasting of a high quality stereo channel. .

 In the case illustrated in FIG. 1, the period g assigned to the data represents 98 clock periods, ie 50 bits per line and 31,150 bits per image of 623 useful lines. At a rate of 751 bits per packet, there are 41 packets per image and 359 reserve bits.

These 41 packets per image correspond to a rate of 1025 packets per second, whereas the transmission of a stereo sound channel in HQ / QI1 coding requires 64,000 11-bit samples per second. By matching a packet to 64 11-bit samples, we arrive at a requirement of 1000 packets per second, so that there remain 25 packets per second for possible scrambling and access control information.

 If it is desired to improve the resistance of the signal to transmission noise, it is possible, on condition of modifying the transmitters, to adopt the signal waveform shown diagrammatically in FIG. 1A, which also makes it possible to simplify the specific circuit for adapting a conventional D2-MAC decoder because this circuit then receives a form of signal conforming to the standard and does not have to modify it. In the case illustrated in FIG. 1A, the data bits are transmitted symmetrically with respect to the MAC black level: this requires modifying the transmitters on the terrestrial network, so as to prevent their alignment being done on the data burst. In return, the amplitude of the data signal can be increased since it is no longer hampered by the need not to reach the line synchronization foot. The amplitude can in particular be brought to 560 mV. From peak to peak for respect the ratio of 8/10 provided by the MAC standard between data signals and analog multiplex. The same gain in amplification will then make it possible to arrive at normalized values.

 One or the other of the signal constitutions which has just been described lends itself to scrambling of the image signals by rotation of components with double cut or single cut.

 Lines 624 and 625 will have a separate constitution from those shown in Figures 1 and 1A.

 Line 624 will be transmitted in the same way as the other lines and may have the content provided for by the D2-MAC / PACKAGE standard except as regards the presence of the signal at the synchronization level.

Line 625 is used to transmit the frame synchronization word, which must be kept, and the frame counter, which is an essential element for descrambling when using the line rotation technique. So that the frame synchronization word can be transmitted in accordance with the D2 standard
MAC / PACKET, the synchronization level signal (period a to c in Figure 1) can be omitted. The absence of this level of synchronization on a single line does not pose a practical problem. If however we wish to keep the level of line synchronization on all lines, it is possible to give line 625 a constitution incorporating a signal to the analog synchronization level by moving the frame synchronization word, which simply implies specific processing of line 625 on reception, before application to a conventional decoding circuit.

 FIG. 2 schematically shows additions which it suffices to make to an existing D2-MAC / PACKET decoding circuit "general public", in the form of a specific circuit, to implement the invention in a receiver when the signal is of the type shown in figure 1.

 The specific circuit 9 shown in FIG. 2 receives a signal having the waveform shown in FIG. 1 via a UHF demodulator 10. It can be considered that the circuit 9 has three channels placed in parallel. One of the channels includes a circuit 12 which receives the demodulated signal and extracts the burst of duobinary digital data therefrom. For this, the extraction circuit 12 receives a window signal from a time base 13 which will be discussed again below. The successive bursts are applied to a de-interleaving circuit 14, then to a deinterlacing circuit 16. The output of the deinterlacing circuit 16 is constituted by the data packets which are sent to a MAC decoding circuit, either directly or by l '' through a descrambling circuit in the event of a scrambled signal (in the case of a conditional access network).

 The time base 13 must generate a window for extracting bursts exact to the nearest bit. For this, it receives a clock signal H and synchronization ticks Sy coming from the MAC decoding circuit and allowing the decoding circuit to be put back into phase.

The time base can be constituted by a counter incremented by the clock signals H and resynchronized periodically by the synchronization top Sy.

 A second channel is assigned to the reconstruction of the analog multiplex (MAC part of the signal) conforming to the D2-MAC standard and which can therefore be applied to a decoding circuit provided for signals conforming to this standard. This second channel successively comprises an alignment circuit 18 for the duration of the data burst, a "blackening" circuit 20 intended to substitute a black level for the synchronization signals and the data burst of each line. The output signal of the blackening circuit 20, consisting of the analog multiplex sequences (MAC signal) having a peak-to-peak amplitude of 700 mV separated by periods of black (at a level 350 mV higher than the black level of transmission) is applied to an adapter circuit 22 intended to bring the amplitude of the MAC signal to a value of 1 V from peak to peak.

 The last channel processes the digital part of the line signal. The alignment circuit 18 also belongs to this channel which then comprises a circuit 26 for extracting the data bursts and an adapter circuit 28 intended to bring these bursts to a peak-to-peak amplitude of 800 mV. It can be seen that the extraction circuit 26 receives data from the output of the alignment circuit 18 and takes the bursts of data therein for the duration of window signals coming from a synchronization extraction and window generation circuit. 24.

 The adapter circuits 22 and 28 of the output of the last two channels attack an adder 30, the output of which is constituted by a regenerated D2-MAC signal, directly applicable to a decoder or descrambler circuit intended to process signals conforming to the D2-MAC standard.

The circuit which extracts the synchronization and generates the windows framing the bursts is provided to detect the appearance of the synchronization levels and then generate
a window signal, applied to the blackening circuit 20 for durations corresponding to those of the synchronization level and of the burst of duobinary digital data
a window signal applied to the extraction circuit 26, of duration corresponding to that of the data burst
- an alignment control pulse applied to the alignment circuit 18.

 An error corresponding to one or two clock periods on the generation of these windows is of no consequence, because on the first channel the extraction of the saves is carried out with an accuracy lower than the bit, t fact that it is controlled by the time base 14.

 In the case where the waveform is that shown in FIG. 1A, the specific circuit can be simpler than that shown in FIG. 2 and in particular have the principle constitution shown in FIG. 2A, where the components corresponding to those of FIG. 2 are designated by the same reference number. The third way of figure 2 becomes useless. The blackening circuit 20a carries the aligned signal at the black level +350 mV only during the synchronization stage.

 In the case of a receiver using a decoding circuit 42 of DMA 2270 type and a descrambling circuit 32 of DMA 2275 type, the links of the specific circuit 9 can be those shown schematically in FIG. 3.

 The reconstructed D2-MAC signal is applied to the input of the descrambling circuit 32 via an analog / digital converter 34, while the packets are applied to it directly.

 The circuits 32 and 42 are connected by a bus 36 to a management microprocessor 38 receiving the information allowing the descrambling of a conditional access subsystem 40 which will not be described since its constitution remains unchanged and is not concerned by the invention.

 The descrambled MAC signal is applied, in the decoding circuit 42, on the one hand to the video processor 44, on the other hand to a circuit 46 for recovering the clock and data. This circuit supplies, from the frame synchronization word, the synchronization ticks Sy and the clock signal H necessary for the operation of circuits 9 and 32.

 From the packets received from circuit 9, the descrambling circuit 32 reconstitutes scrambled packets which are processed by a sound processor 48 in the decoder circuit 42. The latter feeds a mixing circuit 50 external to the decoder and providing at output, in the case of a waveform conforming to FIG. 1, a single pair of stereo channels.

 The processed video signals are sent by the decoding circuit 44 to an external video conversion circuit 52 comprising three digital / analog converters and outputting the analog signals of color R, G and D.

 The presence of circuits 12, 14 and 16 in the specific circuit 9 (FIG. 2) makes it unnecessary, in the event of reception of a signal modified in accordance with the invention, the circuit 56 for brushing and deinterlacing provided in the circuit DMA 2270 A switching, shown diagrammatically in FIG. 3 by a switch 58, is provided for changing the supply of the descrambling circuit 32 as a function of the type of signal received.

The circuits shown in FIGS. 2 and 3 can easily be modified to allow the reception of signals of yet other constitutions, on the condition that they comprise, for each line, an analog multiplex identical to that of a D2-MAC signal, supplemented by a synchronization and alignment signal of duration and level compatible with those of the signals
PAL or SECAM. In particular, this signal can be identical to those provided in PAL and SECAM but in this case the sound transmission capacity is reduced to a single speech channel, which is sufficient in certain cases, in particular for reporting, and allows in return, broadcasting on conventional terrestrial networks.

 The invention is not limited to the particular embodiments which have been shown and described by way of examples. On the contrary, it is susceptible to numerous variations. The durations of the different phases given in FIGS. 1 and 1A should only be considered as examples and the specific circuit constitutions shown in FIGS. 2 and 2A are by no means exclusive.

Claims (10)

 1. Method for broadcasting television programs in the form of a signal comprising, for each useful line of the television frame, a time-division multiplex in baseband composed of a multiplex of analog chrominance and luminance signals strictly in accordance with MAC / PACKET standards and a digital burst of sound and data, characterized in that said digital burst is reduced to a duration freeing for each line a period of at least 4.5 ps occupied by an analog synchronization signal of line compatible with line synchronization signal in PAL or SECAM.  2. Method according to claim 1, characterized in that the analog synchronization signal has a continuous level duration at the foot of 90 clock periods at 20.25 MHz.  3. Method according to claim 1 or 2, characterized in that the analog multiplex has an amplitude of 700 mV from peak to peak.  4. Method according to claim 1, 2 or 3, characterized in that the base of the synchronization level is at a level of 300 mV below the transmission black level, itself at 350 mV below the black level MAC.  5. Method according to claim 4, characterized in that the amplitude of the digital burst is between 400 and 500 mV from peak to peak and is symmetrical with respect to the transmission black level.  6. Method according to any one of claims 1 to 4, characterized in that the digital burst has an amplitude equal to 8/10 of the amplitude of the analog multiplex and is symmetrical with respect to the black level MAC.  7. Receiver intended to receive programs broadcast according to the D-MAC or D2-MAC / PACKAGE standard and also programs broadcast by the method according to any one of claims 1 to 6, characterized in that it comprises, in more than one D-MAC or D2-MAC decoding circuit (42), and possibly a descrambling circuit (32), a specific circuit (9) comprising at least one synchronization extraction circuit (24) and signal level alignment.  8. Receiver according to claim 7, characterized in that the extraction circuit (24) is provided for also generating a window for masking the analog synchronization signal and the digital burst of data applied to a video channel.  9. Receiver according to claim 7 or 8, characterized in that the specific circuit comprises a digital data channel in parallel with a video channel and having means (12) for extracting bursts of data, removing data from and data deinterleaving outputting the data packets, controlled by a time base (13) supplied by clock and synchronization signals coming from the decoding circuit.  10. Receiver according to claim 7, 8 or 9, characterized in that it comprises means (22,28) intended to bring the analog multiplex and the data bursts to the level provided by the MAC standard before application in decoding circuit or descrambling.