The present invention concerns a device for the insertion of digital packets supplied by different sources in a transmission channel having an available average flow rate at least equal to the sum of the average flow rates supplied by the sources. It has a particularly important application in the insertion of packets to form the digital multiplex of a television signal belong into the X-MAC/PACKET family.
In a X-MAC/PACKET signal, the flow of bits intended for sound and data transmission and broadcasting is distributed in packets of fixed length, each having an address of 23 bits, serving to identify the origin of the packet, and a useful part of 728 bits. The channel has a data rate with a constant average value (ranging, depending on each case, from 2,050 to 24,600 packets per second) but with a variable instantaneous value. The input unit of the output channel, consisting for example of the time division multiplexer for deformation of the multiplex intended to be addressed to the modulator, make the request for packets in accordance with its availabilities.
Each source displays data rate characteristics and insertion rules in the output channel which are specific to it. The order of insertion of the packets in the out channel must take this factors into account.
In the case, for example, of a D2-MAC/PACKET system with conditional access according to the "EUROCRYPT" standard, the digital channel may have to transmit one or more sound channels each demanding a capacity of 250 to 1778 packets per second, and whose phase shift with respect to the image must remain small (original sound and dubbing, for example), one or more high fidelity sound channels demanding a higher data rate than the above, an identification channel for the services consisting of packets having address "zero", and of packets representing the entitlement checking messages ECM and the entitlement management messages EMM. Further more, the digital channel must transmit any teletext signals in packets.
It is clear that the sources have data rates which vary from source to source and insertion rules which may also vary.
Several insertion techniques already exist.
The one described in the document called "The EBU-MAC/PACKET system for direct broadcasting by satellite", by Mertens et al, IEEE International Conference on Communications, ICC 84, vol. 1, p. 1-9 North-Holland, New-York, consists in presenting data from various sources to a multiplexer comprising a number of input buffer memories equal to the number of sources. Whenever the multiplexer is ready to form a packet, it examines the buffer memories in an order of priority which depends on the needs of service continuity, and it selects the data in a memory which contains sufficient information to make up a whole packet.
Document FR-A-2 629 972 describes a priority management device having two cascaded FIFOs. The output from the second is connected to an input of a multiplexer. The input of the first is connected to a comparator and to the other input of the multiplexer.