Signal distribution system
BACKGROUND OF THE INVENTION 
     The invention relates to a head end for a signal distribution system, comprising a space division switching network for selectively switching wide-band input signals to a plurality of outputs, to which a multiplexing arrangement is connected for forming at an output thereof a composite signal which is thereafter used for the signal distribution. 
     The invention also relates to a receiver suitable for processing the composite signal generated by the head-end of the distribution system, the receiver comprising a demultiplexing arrangement for deriving at least one wide-band signal from the composite signal. 
     Such a head-end and such a receiver for a signal distribution system are inter alia used in combination with light transmission through optical fibres. The signal-to-noise ratio of an optical transmission path is such that in practice, it is usually preferable to use digital modulation methods having a small number of modulating levels. In addition, in the space division switching network (matrix), cross-talk occurs between the different signals. This cross-talk can be counteracted by reclocking the digital signal streams after switching by the matrix. This, however, can only be achieved in a simple way with mutually synchronous digital signal streams. 
     Mutually synchronous digital signal streams can be time-division mutliplexed, as a result of which a digital signal stream having a high bit rate is produced. In practice, this bit rate may easily exceed the capacity of the transmission channel for the transmission of television video signals. Thus, the bandwidth - distance characteristic number for a graded-index fibre is at present approximately 700 MHz.Km. For subscribers lines having a length of 5 Km, such as occur in the telephony network, this results in a bandwidth of approximately 140 MHz. A time-division-multiplex of more than two digital television-video signals therefore would not fit in this frequency band. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to enable the provision of a head-end for a signal distribution system of the above-described type for the transmission of several wide-band signals in digital form, which need not be mutually synchronized, in a comparatively narrow frequency band. A further object of the invention is to enable the provision of a receiver for recovering at least one of the wide-band signals. 
     The invention provides a head end for a signal distribution system as set forth in the opening paragraph characterized in that the wide-band input signals are encoded, prior to their supply to the space-division switching network, into a plurality of parallel mutualy asynchronous digital signal streams and that the multiplexing arrangement comprises means for the mutual quadrature amplitude modulation of a carrier by two digital signal streams and means for adding in the base-band a third digital signal stream to the quadrature modulated signals for forming the composite signal for the signal distribution. 
     The invention further provides, a receiver as set forth in the second paragraph, characterized in that the demultiplexing arrangement comprises at least one synchronous amplitude demodulator connected to a local synchronized carrier source for frequency and phase-synchronous demodulating at least one of the quadrature modulated signals for the recovery of one of the digital signal streams and means for the digital-to-analog conversion of the recovered digital signal stream. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows a block schematic diagram of a head-end for a signal distribution system in accordance with the invention; 
     FIG. 2 is an illustration of the frequency spectrum of the composite output signal of the head-end shown in FIG. 1; and 
     FIG. 3 shows a block schematic diagram of a receiver in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The head-end for a signal distribution system shown in FIG. 1, which may, for example, be used to distribute several television signals, comprises a set of demodulators 2-1, 2-2, . . . 2-N for several (N) video signals and a set of demodulators 3-1, 3-2, . . . 3-N for several (N) associated sound signals connected to a receive aerial 1. 
     The video signals are converted into digital signals by means of a plurality of analog-to-digital converters (A/D converters) 4-1, 4-2, . . . 4-N. In the example described here, these A/D converters are ternary-delta-modulators having a symbol rate of 70 MBaud, whose frequency band does not extend to further than 42 MHz. Its frequency spectrum is illustrated by the curve V1 in FIG. 2. 
     The sound signals are converted into digital signals by means of a plurality of A/D converters 5-1, 5-2, . . . 5-N. In this example these A/D-converters produce binary output signals at a bit rate of 2 Mbit/sec. These output signals are FSK-modulated in a plurality of FSK-Modulators 6-1, 6-2, . . . 6-N on a carrier having a carrier frequency of 45 MHz. The spectrum of the modulated signal is illustrated by curve S1 in FIG. 2. 
     The FSK-modulated signals are added to the associated digital video signal in a plurality of summing means 7-1, 7-2, . . . 7-N. The sum signals are applied to inputs I1, I2, . . . IN of a space-division switching network 8 (matrix), which forms part of a subscriber unit 9. The subscriber unit 9 further comprises a control unit 10 for controlling the matrix 8, which control unit is connected to the incoming channel 11-1 of a transmission path 11 for receiving the control information. Under the control of the control unit 10, matrix 8 establishes selected connections between the inputs I1, I2, .....IN and three outputs 01, 02 and 03. 
     In this manner three arbitrary television signals can be applied to a multiplexing unit 12, which is also part of the subscribers unit 9. 
     The multiplexing unit 12 comprises two modulators 13 and 14 which produce amplitude modulated signals with a suppressed carrier. The modulators 13 and 14 are connected, via the phase shifters 15 and 16, which have a mutual phase shift of 90°, to a carrier source 17, having, in this example, a carrier frequency of 100 MHz. The output signal from output 02 is applied to modulator 13 and the output signal from output 03 to modulator 14, the two signals being mutually quadrature-amplitude modulated on the 100 MHz carrier by these modulators. 
     The signal from output 01 and the output signals of the modulators 13 and 14 are combined in a summing means 18 into a composite signal which is applied to the outgoing channel 11-2 of the transmission path 11. The spectrum of the composite signal is illustrated in FIG. 2. In this Figure the curve V2+3 illustrates the frequency position (after modulation) of the video signals received from the outputs 02 and 03. The specrum, having an upper and a lower sideband, of the associated sound signals is shown at S2+3. The frequencies of the digital video signal and the FSK-modulated sound signal from output 01 are not shifted and their spectra are again illustrated by V1 and S1. 
     By way of illustration of a practical embodiment, in which a transmission path 11 with light transmission through an optical fibre is used, the following additional information can be given with reference to FIG. 2. 
     The level of the video signal V1 is approximately 6dB below the level of the modulated video signals V2+3. The level of the sound signal is approximately 12 dB below the level of he associated video signal. At the frequency of 50 MHz, a pilot signal P is conveyed along with the other signals with a level which is approximately 20 dB below the level of the total composite signal P. The pilot signal is derived from the carrier source 17 by means of a divide-by-two divider 19 and applied to the summing means 18 for conveyance to the outgoing channel 11-2. 
     The matrix 8 is an analog space-division network in which the signals are switched-through in the analog form. During this switching-through operation, a certain degree of cross-talk occurs between the different signals. By digitally encoding the signals at the input side of the matrix 8, the cross-talk can be effectively counteracted by signal regeneration at the receiving side of the distribution system. 
     A receiver for one of the modulated signals from the composite signal of FIG. 2 is shown in FIG. 3. It comprises a phase-locked loop (PLL) 20, which generates a synchronous 100 MHz carrier from the 50 MHz pilot. A synchronous amplitude demodulator 21 is connected to this phase-locked loop 20 for frequency and phase-synchronous demodulating one of the quadrature modulated signals V2, S2, and V3, S3, respectively. A simple phase adjustment of the local carrier to be effected for each of the two possible types of receiver determines which of the two modulated signals will be demodulated. 
     The output signal of demodulator 21 is applied to a clock extraction circuit 22 and to a regenerator 23 connected thereto. From the digital video signal, the clock extraction circuit 22 derives a clock signal having a repetition frequency of 70 MHz for the regeneration of the digital video signal by regenerator 23. The regenerated digital video signal is thereafter applied to a digital-to-analog converter 24 (D/A-converter) for reproducing the analog video signal. 
     The sound component of the output signal of demodulator 21 is applied to the FSK-demodulator (DM) 26 via bandpass filter 25 which is tuned to the sound carrier frequency of 45 MHz. The demodulated digital sound signal is applied to a clock extraction circuit 27 and to a regenerator 28 connected thereto. From the digital sound signal, the clock extraction circuit 27 derives a clock signal having a clock repetition frequency of 2 MHz for the regeneration of the digital sound signal. This sound signal is thereafter applied to the D/A-converter 29 for reproducing the analog sound signal. 
     The signal regenerators 23 and 28 eliminate any cross-talk occuring between the signals in the matrix 8, while, because of the transmission of the signals on the one hand in the base band and on the other hand by means of quadrature amplitude modulation, it is not necessary to maintain synchronism between the signals. 
     It will be obvious that a receiver for the signals V1 and S1 can do without the synchronous demodulator 21 and PLL 20. 
     FIG. 3 further shows a selection arrangement 30 connected to the outgoing channel 11-1 for applying programm selection information to the control unit 10 of the subscribers unit 9. On the basis of this information, the control unit 10 establishes a connection through the matrix 8 from the selected input I1, . . . IN to the output 01, 02 or 03 which is associated with the receiver transmitting the selection information.