FR2596228A1 - Method and device for modulating television signals, in particular of the D2-MAC/Packet type - Google Patents

Abstract

Method of frequency modulation on the basis of a modulating video signal with time-division multiplexing of an analogue part and a digital part for each line, with transmission of the continuous component of the modulating signal, characterised in that: a) with each line, from the modulating signal are extracted a time window (COM) demarcating the digital part of the signal and a clamping window demarcating the clamping period provided with each line outside of line 625 of the frame blanking; b) the modulating signal is base band processed in order to clamp it during the said clamping window and carry out an energy spreading with predetermined period, and possibly a pre-emphasising of the signal; c) the processed signal is applied to the input of a voltage-control oscillator which provides the modulated signal on an output; d) a fraction of modulated signal is sampled for the duration of the time window (COM) and is subjected to discrimination and filtering to obtain an error signal from which automatic frequency control of the oscillator is carried out. The circuit is in particular usable to modulate a signal of the D2-MAC/PACKET type.

Description

Method and device for modulating television channels. including TV2 D2-MAC / Paauet
The present invention relates to the frequency modulation of television signals and, in particular, the frequency modulation from a video modulating signal with time multiplexing of an analog part and a digital part for each line, with transmission of the DC component of the modulating signal.

The invention finds a particularly important application in the modulation of signals of the type
D2-MAC / Pack, intended for direct broadcasting by satellite. A complete description of the D2-MAC / Packet signals can be found in the standard "specification of the D2-MAC / Packet system" of September 1985, published by the State Secretariat in charge of communication techniques.

 However, to understand the invention, it suffices to remember that television signals of the X-MAC / Packet type (X designating one of the variants envisaged) use two levels of time multiplexing of the different types of information to be transmitted. The digital information is firstly multiplexed into packets, each packet transporting data belonging to a single service component.

A second level of multiplexing gives rise to a global signal comprising the digital train and the information relating to the image, as well as control information.

 The originality of D2-MAC / Packet is in particular to use a time base band multiplex in which the digital part is coded in duobinary. This baseband signal must be subjected to a pre-emphasis and the digital part must be subjected to energy dispersion before modulation of a carrier.

Digital and analog signals pass through the same channel in the modulator.

 The particular constitution of such a signal does not allow it to apply current frequency modulation techniques to it. In particular, the need to transmit the DC component of the modulating signal implies that the DC component of the signal applied to the input of the voltage-controlled oscillator is restored. It is also necessary that the energy dispersion signal is applied with a synchronism determined with respect to the line scanning period and this result can only be achieved by means of synchronization extraction.

The present invention aims to provide a method and a device meeting these requirements. To this end, the invention proposes in particular a frequency modulation method from a video modulating signal with time multiplexing of an analog part and a digital part for each line, with transmission of the DC component of the modulating signal, characterized in that
a / extracting from the modulating signal, on each line, a time window (COM) framing the digital part of the signal and a clamping window framing the clamping period provided for each line outside line 625 of the weft suppression,
b / the modulating signal is processed in baseband in order to subject it to clamping during said clamping window and to carry out an energy dispersion at a predetermined period, and possibly a pre-emphasis of the signal,
c / the processed signal is applied to the input of a voltage-controlled oscillator which supplies the modulated signal on an output,
d / a fraction of the modulated signal is taken for the duration of the time window (COM) and it is subjected to discrimination and filtering to obtain an error signal from which an automatic frequency control of the oscillator is carried out.

 The invention also provides a device for implementing the method defined above.

This device may in particular comprise a frequency modulator from a video modulating signal with time multiplexing of an analog part and a digital part for each line, with transmission of the direct component of the modulating signal, characterized in that He understands
a / a synchronization extraction circuit making it possible to extract, from the modulating signal, and for each line, a time window framing the digital part of the signal,
bl a baseband processing circuit for the modulating signal comprising means for superimposing on the signal a periodic energy dispersion signal,
c / a UHF oscillator with automatic frequency control receiving on its control input the processed signal and supplying at its output the modulated signal,
d / means for sampling the modulated signal, comprising a switch activated for the duration of the time window (COM) and,
e / a low pass discrimination and filtering circuit providing, from the sampled signal, a signal of frequency errors and applying this signal to an automatic frequency control input of the UHF oscillator.

The invention will be better understood on reading the following description of a particular embodiment of the invention, given by way of non-limiting example. The description refers to the accompanying drawings, in which
- Figure 1 is a diagram showing the waveform of the baseband signal used for modulation,
FIG. 2 is a block diagram of a modulator according to the invention,
FIG. 3 is a block diagram of a synchronization extraction circuit usable in the modulator of FIG. 2,
FIG. 4 is a block diagram of a baseband processing circuit usable in the modulator of FIG. 2,
FIG. 5 shows a pre-emphasis network usable in the circuit of FIG. 4,
- Figure 6 is a block diagram of a possible ultra high frequency amplification and switching circuit
Figures 7 and 8 are block diagrams of a frequency doubling and transposition circuit and a CAF loop usable in the decoder of Figure 2.

 Before describing a modulator according to the invention, it may be useful to recall the constitution of the baseband signal used in the implementation of the D2-MAC / Packet standard.

Figure 2 shows the baseband waveform for an unscrambled signal. We successively find
a period a corresponding to 208 clock periods, intended for the transmission of 105 binary elements (ebs or bits) of data and synchronization.

 - after a transition period, a clamping period g corresponding to 15 clock periods, & a level of O Volt.

 - after a new transition period, a period e corresponding to 348 clock strokes, intended for the transmission of the color difference component (or chrominance).

 - after a new transition period, a period h of 696 clock cycles for the transmission of the luminance component.

 The modulator which will now be described is designed to provide a modulated carrier whose central frequency can vary in a determined range, thanks to the presence of a transposition oscillator. It can be viewed as comprising a synchronization extraction circuit 10 and a modulation chain proper consisting of a base band processing circuit 12, an adjustable attenuator 14, a voltage-controlled frequency oscillator 16, a UHF amplification and switching circuit 18, and a transposition circuit 20. A loop 22 for automatic frequency control of the oscillator 16 is provided. It is put into service by the switch of circuit 18 during the periods of presence of the digital part of the signal and includes a discriminator.

 The synchronization extraction circuit 10 (FIG. 3) receives the modulating input signal. It successively comprises a high-pass filter 24 of RC type followed by a duobinary decoder 26, formed by two comparators and an EXCLUSIVE OR circuit. The high pass filter eliminates variations in the amplitude of the limit signal. It suffices to give it a time constant of the order of 1 ps so that the frame synchronization word placed in the digital part can be recognized without risk of error. The decoder 26 is followed by a clock recovery circuit carrying out a non-linear processing on the signal coming from the decoder. This non-linear processing, which makes it possible to pass from the signal NRZ leaving the decoder to a signal RZ, provides a spectral component at a frequency of 10.125 MHz.

 Nonlinear processing is performed during periods of digital signal presence. For this, the output signal from the decoder 26 is taken by a switch 28 closed by a COM signal whose generation will be described later during the periods. The signal provided by the switch 28 is applied to a rhythm recovery circuit 30, constituted by a phase locked loop. This loop can have a simple constitution and for example peddle an oscillator formed by an ECL gate whose reaction circuit contains a quartz in series with a diode with capacity adjustable by voltage. The circuit provides on its output a spectral component at the frequency frequency 1 / T of 10.125 MHz according to the D2-MAC standard.

 The rhythm frequency is applied to the other elements of the extraction circuit 10 which comprise a second output branch of the decoder 26. This second branch comprises a flip-flop 32 and a correlator 34 for recognizing the frame synchronization word. In practice, it is sufficient that the average value of the input signal be close to O after a time of less than 4 ps to avoid errors of recognition of the frame synchronization word. Consequently, it suffices that the filter 24 has a time constancy of the order of 1 lash.

 The frequency frequency signal and the signal SYT for recognizing the frame synchronization word are applied to a time base 36 intended to supply the clamping signal corresponding to the durations ç, a square signal at 25 Hz used to achieve energy dispersion. signal, and the COM signal. The time base 36 can essentially consist of a line counter and a point counter on each line which address two memories programmed so as to supply the output signals during the scheduled periods.

The baseband processing circuit 12 can have the basic principle shown in FIG. 4. The input video signal (modulating signal) is first subjected to clamping in a gate circuit 38 which receives the "clamp" signal coming from time base 36. The signal obtained is then subjected to a pre-emphasis in a circuit 40 which will generally have a transfer function of the type
1 + i (ftf1)
H (f) = A 1- + + j (f / f2) - with A = 1 / (E f1 = 0.84 MHz f2 = 1.5 MHz
This transfer function can in particular be performed by a T-shaped quadrupole of the kind shown in FIG. 5, comprising two resistors 42 in series, the midpoint of which is grounded by another resistor 44 and an inductor 46, all of the resistors 42 and 44 being placed in parallel with an LC circuit.

 An adder network, shown diagrammatically in FIG. 4 by two resistors 48, makes it possible to add, to the pre-emphasized signal, a triangular signal of energy dispersion obtained by integration of the square signal at 25 Hz supplied by the time base 36. This integration can be carried out by a circuit 50 of conventional constitution.

 The output signal from circuit 12 drives, through the attenuator 14, which can be provided to provide adjustable attenuation ranging from 0 to 10 db, the oscillator 16 controlled in voltage. This oscillator can be of known type, having a central frequency of 230 MHz. it should only be noted that it must include two frequency control inputs, on the one hand a modulation input with transmission of the DC component, receiving the output signal from the attenuator 14, on the other hand a control input automatic frequency which cooperates with a loop which will be discussed later.

 The UHF amplification and switching circuit 18 receives the output signal from the oscillator 16. it performs two functions. On the one hand it samples the modulated carrier during the intervals corresponding to the digital part of the MAC signal, time intervals defined by the COM signal coming from the extraction circuit 10; on the other hand it provides, after amplification, the carrier modulated at 230 MHz.

 The circuit 18 can have the general constitution shown in FIG. 6 and include a separator 52 which supplies an amplification branch 54 supplying the modulated carrier applied to the transposition circuit 20. The second branch comprises an amplifier 56 followed by a ring modulator 58 constituting a high frequency switch, controlled by the signal COM coming from the time base 36. The transposition circuit 20 can take very diverse forms. In the embodiment shown in FIG. 7, it includes a frequency doubler 60 which has the role of improving the linearity characteristics of the modulator. The 460 MHz signal supplied by the doubler 60 is applied to a bandpass filter 62, which will generally be a passive filter so as to have a bandwidth at -3 db of the order of 50 MHz. Then the filtered signal is transposed to the output frequency of the modulator by heterodyning with the signal supplied by a transposition oscillator 64. The output signals 62 and 64 are applied to a mixer 66 and the output of the latter applied to a pass filter -bas 68. A bandwidth at 3 db of around 170 MHz will generally be sufficient. The local oscillator can be adjustable to allow the adoption of a modulator output frequency between 30 and 140 MHz.

 Finally, the modulator includes an automatic frequency control circuit which plays an important role, since it makes it possible to free the modulator from long-term variations in the central frequency of the voltage-controlled oscillator 16.

 The automatic frequency control loop works as follows. An error voltage is extracted from the digital part of the D2-MAC signal whose mean value is zero. The frequency modulated carrier is sampled on each line during the 10 us period of transmission of digital information using the COM signal. Then the samples are applied to a frequency demodulator; the demodulated signal is filtered with a large time constant, of the order of 10 seconds, then applied to the voltage-controlled oscillator 16.

The loop may have the basic principle shown in FIG. 8 and include a discriminator 70 of the delay line type having a central frequency of 230 MHz. This discriminator provides demodulated digital bursts on its output, the mean value of which is zero when the carrier is at the nominal frequency of 230 MHz but different from O in the event of drift. An active filter 72, having a time constant greater by several orders of magnitude than the duration of a frame (10 seconds for example), supplies an error voltage at its output when the oscillator 16 has a drift relative to the set frequency of 230
MHz. This error voltage is amplified by an amplifier 74, then applied to the automatic frequency control input of the oscillator 16.

Claims (10)

CLAIMS:  1. Method for frequency modulation from a video modulating signal with time multiplexing of an analog part and a digital part for each line, with transmission of the direct component of the modulating signal, characterized in that  a / a time window (COM) framing the digital part of the signal and a clamping window framing the clamping period provided for each line outside the line 625 of the weft suppression is extracted from the modulating signal on each line,  bl the modulating signal is processed in baseband in order to subject it to clamping during said clamping window and to carry out an energy dispersion at a predetermined period, and possibly a pre-emphasis of the signal,  c / the processed signal is applied to the input of a voltage-controlled oscillator which supplies the modulated signal on an output,  di a fraction of the modulated signal is taken during the duration of the time window (COM) and it is subjected to discrimination and filtering to obtain an error signal from which an automatic frequency control of the oscillator is carried out.  2. Frequency modulator from a video modulating signal with time multiplexing of an analog part and a digital part for each line, with transmission of the direct component of the modulating signal, characterized in that it comprises  a / a synchronization extraction circuit (10) making it possible to extract, from the modulating signal, and for each line, a time window framing the digital part of the signal,  b / a circuit (12) for processing the baseband of the modulating signal comprising means for superimposing on the signal a periodic energy dispersion signal,  c / a UHF oscillator (16) receiving on one of the inputs the automatic frequency control signal and on another input the processed signal, and supplying at its output the modulated signal,  d / means for sampling the modulated signal, comprising a switch activated for the duration of the time window (COM) and,  e / a low-pass filtering and discrimination circuit providing a frequency error signal from the sampled signal and applying this signal to an automatic frequency control input of the UHF oscillator.  3. Modulator according to claim 2, characterized in that the synchronization extraction circuit comprises means for developing, during the duration of each digital part, an energy dispersion control signal applied to the band processing circuit of based.  4. Modulator according to claim 3, characterized in that the synchronization extraction circuit is constituted so as to also provide a clamping window framing the clamping period, provided for each line outside the line 625 of the weft suppression , and in that the baseband processing circuit is designed to subject the modulating signal to clamping during said clamping window.  5. A modulator according to claim 4, characterized in that the synchronization extraction circuit comprises means for decoding the digital part of each line and a circuit for recovering the sampling rhythm during the duration of the time window (COM) and in that the modulator includes a correlator providing a frame synchronization signal to a generation time base  - a time window corresponding to the digital part of the signal (COM),  - a clamping signal  - and an energy dispersion signal.  6. Device according to any one of claims 2 to 5, characterized in that the baseband processing signal comprises pre-emphasis means constituted by a T-quadrupole comprising a resistance chain, in parallel with a capacitor, and an inductor connecting the middle point of the resistance chain to ground.  7. Device according to any one of claims 2 to 6, characterized in that the switch consists of a ring modulator controlled during the time window (COM).  8. Device according to any one of claims 2 to 7, characterized in that the discriminator is a delay line, designed to provide an output signal consisting of demodulated digital bursts of zero mean value when the carrier is at the nominal frequency ,  9. Device according to any one of claims 2 to 8, characterized in that it further comprises a frequency transposition circuit, comprising a transposition circuit at the output frequency of the modulator.  10. The circuit as claimed in any one of claims 2 to 9 for modulating a D2-MAC / Packet television signal, characterized in that the synchronization extraction circuit is designed to provide a clamping window of 750 ns, a time window of 10 ps and an energy dispersion signal at a frequency of 25 Hz for a frame rate of 50 frames per second.