FR2509559A1 - Digital DPSK TV signal secrecy transmission method - uses pseudo random changes in carrier frequency, frequency agile pass-band filtering and line synchronisation decoder - Google Patents

Abstract

The digital tv signal transmission technique includes the shaping of digital data derived by sampling the analogue video signal, and quantifying this into n bits. Successive data are written into a memory, and subsequently extracted line by line in a time shorter than that taken to write the line. The extracted data is differentially coded in order to perform DPSK phase modulation of a high frequency carrier wave. The carrier frequency is changed periodically other time intervals which are a multiple of the line period using p distinct values taken in random order. The signal thus produced is then subject to a pass-band filter operating in a selected range corresponding to that of the DSPK modulated carrier. It is then fed to a power amplifier for transmission, the processing technique giving high security against unauthorised interception.

Description

METHOD FOR THE DIGITAL TRANSMISSION OF VIDEO IMAGES OF THE TYPE
TELEVISION AND CORRESPONDING TRANSCEIVER
The invention relates to a method of digital transmission of television type video images and to a transceiver implementing this method.

It is known to transmit digital data in differential phase modulation, known as DPSK according to the abbreviation corresponding to the Anglo-Saxon designation "Differential Phase-Shift
Keying ". This transmission technique makes it possible to get rid of the reference phase which it is necessary to know in the case of a PSK phase modulation to decode the message and recover it in the event of transmission interruptions.

 It is also known in the radar field and also in telecommunications, to use the technique known as frequency agility or frequency diversity. The transmission pulse is replaced by a group of successive pulses corresponding to the different expected frequencies. Frequency agility provides effective protection against interference by widening the frequency spectrum.

 The object of the invention is to ensure over-the-air transmission of television video images with maximum discretion, great security and with great difficulty in intercepting and decoding by third parties.

To solve this problem, the planned techniques are used jointly, the frequency changes bringing in a wide band a great resistance to jamming, and the modulation
DPSK allowing coding.

 An object of the invention is to provide a receiver-transmitter which proceeds, after transmission, after digital formatting of the video signal, to its storage in a buffer image memory. The data are extracted line by line for a duration shorter than the line period and processed by differential coding for a DPSK phase modulation of a high frequency carrier. The carrier frequency is modified on each line or after a certain number of lines using a predetermined number of distinct frequencies, the successive appearance of which is made random. On reception, after filtering in a corresponding selective band and transposition of frequency, the signal is processed by DPSK demodulation, decoding of the image synchronizations, restitution of the bit clock and of the video display signal.

The particular features of the invention will appear in the description which follows, given by way of example with the aid of the appended figures which represent:
- Fig. 1, a general diagram of the transmission part of a digital transmission device according to the invention;
- Fig. 2, a diagram of the differential encoder used;
- Fig. 3, waveforms illustrating the operation of the transmitter input converter stages;
- Fig. 4, a diagram illustrating a random sequence formation of the different transmission frequencies;
- Fig. 5, a diagram relating to the frequency diversity part of the transmitter;
- Fig. 6, a general diagram of the reception part of a digital transmission device according to the invention; and
- Fig. 7, a diagram of a frequency acquisition circuit used on reception.

 Referring to FIG. 1, the video generator 1 of television images (T.V. camera, video recorder, etc.) transmits the analog video signal SV to an analog-digital converter 2.

A separator circuit 3 is used if necessary, when the signal from the generator 1 is a composite signal, to derive the line synchronization signals SL and frame ST. Circuit 2 consists of a sampling circuit 4 of the analog signal SV at the point rate, and a quantization circuit 5 with n bits per point.

The synchronization of these circuits is produced respectively by the signal SP at the point cadence TP and the bit clock signal SB (cf.

Fig. 3). A time base circuit 6 produces the signals SB, SP as well as the synchronization signals also called line suppression SL and frame ST. The amplitude of the video signal is thus translated for each point by a word of n bits; 2 amplitude levels can thus be distinguished by this coding between the limit values corresponding to black and white. The operation of these circuits is known and recalled for information by the
Fig. 3, where we considered n = 3, i.e. 8 distinct levels coded 000 to
111.

 The video signal thus translated in digital form S I is entered into a recording and reading memory 7, such as a RAM memory, which constitutes a buffer image memory. On reading, the data is extracted line by line by removing the words of n bits from the successive points of a line, but this removal is carried out at a higher rate than that TB of bit, so that the reading time d 'a line is less than the line TL part presented for writing. However, the following line is only extracted at the end of the line period duration TL of the signal SV.

The S2 data extracted bit by bit for each point is applied to a differential phase encoder circuit 8 With a delay line, for example electroacoustic, the diagram of this circuit can correspond to FIG. 2. Signal S2 is delayed by
TF + TB in line 8 A to form the output S20 and is only delayed by TF to form the intermediate output S21. The duration TB corresponds to the bit period, for example of 25 ns. Outputs
S20, S21 supply the differential amplifier 8B. The duration TF will be greater, for example 1 micro-second, to reduce the effects of thermal variation. The output S3 of the encoder 8 has
thus a binary state when there is no change of state of S1 from one bit to the next, and the other binary state during a transmission 0 to 1 or
i to O of S1. The signal S3 constitutes the modulation signal of a
HF carrier to produce a DPSK modulated HF signal; Incorpo
ration of periodic and invariable synchronization data
line and frame is obtained by using, for example, a small annex reading memory 9, where the signals SL and ST are written with particular predetermined words and which are advantageously constituted by pseudo-random codes. The modulator stage is shown in 10.

 According to the invention, the HF carrier S4 applied to the DPSK 10 modulator periodically changes frequency. The HF generator is composed of a frequency synthesizer circuit 11 which is controlled by a digital signal S5 produced by a key generator circuit 12.

The synthesizer circuit is designed to produce a certain number p of different carrier frequencies F1 to Fp. These frequencies are produced from a base frequency FB of an oscillator applied to frequency multipliers to obtain the different frequencies Fj = FB X Kj. The oscillator is advantageously constituted by that with high stability of the time base circuit 6 and the base frequency can correspond to the bit frequency of the signal SB. The control signal S5 is digital, composed for example of a word of u = 3bits to control at most eight distinct frequencies: FI = 000, F2 = 001,
F3 = 010, F4 = 011, F5 = 100, F6 = 101, F7 = 110 and F8 = 111.

According to the invention, the respective frequencies are controlled randomly - by the key generator 12. One method consists in forming the successive control words S5 by means of a pseudo-random code. Thus, in the simple case previously considered and for a pseudo-random code CPA represented by way of example in FIG. 4, we obtain successive frequencies
F2, F6, F4, .F7, ... indicated, without pre-established order.

 The periodicity of the carrier frequency change can be that of frame or multiple of that of line, according to the desired discretion. To ensure high discretion, change at each line is preferred. The CPA code can be obtained by a reaction shift register assembly or be written to a memory, and the words S5 are extracted therefrom in order to control the synthesizer 11.

 Fig. 5 shows an embodiment of the assembly 11-12. The code CA is entered in the reaction shift register 20. The link SS having the number u of desired bits is transmitted to the demultiplexer circuit 21 which distributes it over p outputs at u bits. Each output controls a switch 22 j via an exclusive AND circuit 23 and any additional circuits 24. The case shown corresponds to code 011, ie Fj = F4 of the example considered.

 The pseudo-random codes used in the memory 9 differ from that used in the key generator 12.

After modulation, the signal 56 is transmitted to the air, preferably through a tunable filter with digital control 13. The control word is advantageously chosen, the same 55 as that of the synthesizer. The tunable filter 13 has a reduced bandwidth adapted to the transmission frequency Fj and thus makes it possible to increase the signalXlbruitcEn e VHF mission ratio, the assembly includes a frequency transposition by beating in a mixer I with a local wave S7 originating from an oscillator 15 The other figured circuits the transmitter are the power amplifier stage 16 and the transmit antenna 17
The free time available between the end of a line extraction from memory 7 and the start of extraction of the following line is used to control, if necessary, a frequency switching of the synthesizer 11, and to produce the synchronization signals SL and ST in digital form of pseudo-random codes.

The time base 6 makes it possible, from a quartz clock, to produce the various synchronization signals of the transmitter and of the video generator 1
The synthesizer is capable of easily forming the various frequencies with very short switching times of the order of 10 microseconds.

 The output amplification 16 can be a solid circuit or a traveling wave tube, depending on the power to be transmitted.

 FIG. 6 represents a diagram of a receiver equipped with means of decoding and decoding suitable for processing the high frequency wave transmitted by the transmitter described above.

 The receiver comprises, a receiving antenna 30 followed by a high frequency amplifier 31 which attacks a mixer 32 receiving a local oscillation wave S10 from a circuit 33 to produce a superheterodyne detection. The signal S11 from the high frequency amplifiers 31 is preferably applied, as at transmission, through a bandpass filter 34 so as to increase the reliability and the signal to noise ratio of the receiver. The filter 34 is likewise of the tunable type and controlled from a key generator 35; the latter also controls a frequency synthesizer stage 33, possibly by the same control word S12 -as for the filter. The signal 513 transposed into an intermediate frequency is then amplified in a medium frequency circuit 36 before attacking a demodulator circuit in parallel DPSK 37 and a decoding circuit 38 of the line synchronization signals SL and frame ST.

 The circuits 33, .34 and 35 are analogous to the frequency synthesizer circuits 11, selective filter 13 and key generator 12 which were used on transmission; in particular, the key generator assembly 35 and the frequency synthesizer 33 are determined to reproduce the different frequencies according to the same pseudo-random sequence as that which was produced on transmission. The problem consists in bringing this local sequence into coincidence with the successive frequencies of the received signal. For this, a frequency acquisition logic circuit 39 is provided.

 The decoding circuit 38 identifies the codes corresponding to the line suppression periods TSL (FIG. 3) and frame suppression, and therefore the synchronization signals which surround the useful intervals of the video signal SV. The decoding of the signals SL, ST is resolved by correlation or suitable filtering of the pseudo-random codes which are used to represent these signals on transmission.

The decoding circuit 38 can consist of two electroacoustic surface wave lines, to decode the synchronization codes SL and ST without the need for a local clock. The use of electroacoustic delay lines makes it possible to pick up the signal used for the decoding before DPSK demodulation.

 The acquisition logic circuit 3,9 receives the synchronization signals S17 decoded by the circuit 38 and it controls by its output S18 the key generator 35 to effect the change to the next frequency by command of the synthesizer circuit 33.

The frequency acquisition is carried out as follows: at the start, the receiver is on standby on a frequency Fj corresponding to the initial state of the assembly 33, 34 and 35. The signal
S10 delivered by the synthesizer 33 therefore corresponds to the frequency Fj, to the amount near the medium frequency according to the principle of superheterodyne detection. When the transmission frequency corresponds to the frequency Fj, the received signal S11 is transmitted through the filter 34 centered and tuned on Fj and is detected by the mixer 32. The output S13 detected by it is amplified and limited by l medium frequency amplifier 36. The output S14 of the amplifier is applied to the loop 39, 35, 33 and 34 to pass to the next frequency and so on, insofar as the successive frequencies coincide. If the sequence does not coincide with that of transmission, that is to say when the next frequency does not coincide with that of the message received, the receiver will again be in the standby position on this next local frequency.

 FIG. 7 shows by way of example an embodiment of the acquisition logic circuit 39. It is considered that the frequency is changed on each line and that the output S17 of the decoder 38 applied to the acquisition logic 39 is constituted by the signals line synchronization. The acquisition circuit essentially comprises a counter 45 which is controlled by the line synchronization SL decoded at 38 and in which the counting of a clock signal SH delivered by an oscillator or a time base circuit 46 takes place. When the capacity of the counter is reached, it triggers a monostable circuit 47 to produce a window pulse which is recovered at the output of an ET48 circuit which receives by its second input the synchronization signal SL. The window S18 produced at output will command; the key generator 35. The local frequency of the clock circuit 46, the capacity of the counter 45, and the duration of the monostable 47 are determined so that this window is centered on the following line blanking signal and includes it. The possible decoding of a spurious signal instead of a line synchronization signal would result in the window not being coincident with the following line synchronization signal and would return the receiver to the standby and acquisition position. frequency.

 The DPSK 37 demodulator is constituted by an electroacoustic delay line comprising a tap distant from the bit duration of the output, as for circuit 8A in FIG. 2. The product in a mixer of signals from this intermediate outlet and the output signals allows, after filtering, to have the demodulated signal 15. This latter is transmitted to a synchronization circuit. bits 40 responsible for reconstructing a local bit clock allowing the video signal to be restored.

 The video signal restoration circuit 41 receives the demodulated signal S 15, the reconstructed bit synchronization signal S 16 and a signal S 17 from the decoding circuit. It allows the video signal of each line to be stored in order to restore it to its normal length, to perform digital-to-analog conversion, and to add the line and frame synchronization signals in order to reconstruct the video signal directly usable by a conventional television monitor.

 Bit synchronization 5 16 can be achieved in two ways. A first solution uses a conventional phase loop including a high stability oscillator, for example a voltage controlled quartz oscillator (VCXo); a second solution uses the correlation peak resulting from the decoding of the line synchronization words SL to zero in a circuit 42 a counter by n driven by an internal clock operating on a frequency nF, F being the transmission clock frequency. The internal clock is chosen to have high stability, for example better than 10 S. Under these conditions, the phase shift of the internal clock between two lines is less than the bit width, and there is synchronization on each line. This second solution is shown in dotted lines and in this case, the circuit 40 is deleted.

Claims (4)

RESELL  1 - Method of digital transmission of television type video images, comprising on transmission:  converting the analog video signal into digital data by sampling and quantization at n bits per point;  recording successive data in memory;  Extracting data from memory at the line rate from one line to the next, but for a determined duration less than that of line for the points of a line considered;  differential coding of the data extracted for DPSK phase modulation of a high frequency carrier;  the periodic change of the carrier frequency after a duration at least equal to the line or multiple period thereof, using p distinct frequency values whose order of appearance is random, being linked to the successive values of bits of a pseudo-random code ;;  bandpass filtering in a selective band corresponding to that of the DPSK modulated carrier and at agile frequency  Power amplification and radiation emission; and comprising at the reception:  the detection of said radiation followed by preamplification;  bandpass filtering in a selective band corresponding to that of the detected carrier;  transposition of superheterodyne frequency with a local oscillation wave whose frequency varies in relation to that of the detected and filtered carrier;  DPSK demodulation of the medium frequency signal resulting from the transposition;  decoding the line and frame synchronizations included in the medium frequency signal to control the frequency variations of the local wave in correspondence and in synchronization with the detected carrier frequency;  bit synchronization rendering;  bit decision and rendering of analog video signal.  2 - Method for digital transmission of images according to claim 1, characterized in that  on transmission, the line and frame synchronization signals are replaced by a respective pseudo-random coding, then differential coded for DPSK modulation; and  on reception, the decoding of the synchronizations is obtained by correlation.  3 - Transmitter for digital transmission of television type video images, characterized in that it comprises:  a digital-to-analog converter (2) for quantizing the analog video signal (SV) to n bits per point;  a buffer image memory (7) supplied at the opening by the output of the converter for storing point by point and line by line the words of n successive bits of an image;  a read memory (9) comprising binary words corresponding to the line (SL) and frame (ST) synchronization signals;  a differential encoder circuit (8) for encoding the digital data of said memories for DPSK modulation;  a DPSK modulator (10) for modulating an HF carrier by the modulation signal delivered by the differential encoder;  a frequency synthesizer (11) for producing different distinct frequency values (F1 to Fp) of the carrier wave;  a key generator (12) for controlling the synthesizer and producing a periodic frequency change after each period of predetermined duration equal or multiple of the line period, and for producing the different frequencies without pre-established order, from a pseudo code -random;  a tunable filter (13) supplied by the output of the DPSK modulator and the tuning of which is controlled by the key generator (12) in relation to the successive emission frequencies, the output of the filter being applied through a power amplifier tlb) to an antenna (17).  4 - Receiver of a digital transmission of video images of the television type by a transmitter according to claim 3, characterized in that it comprises:  a receiving antenna (30) followed by an HF amplifier (31);  a tunable filter (34) supplied by the output of the HF sensor amplifier and whose tuning is controlled by a key generator (35) identical to that of the transmitter;  an intermediate frequency transposition circuit of the output of the filter by beating in a mixer (32) with the local wave of a frequency synthesizer (33) similar to that of emission, the output of the mixer being applied to a circuit limiting amplifier (36) ;;  a DPSK demodulator circuit (37) supplied by the limiting amplifier and the output of which is applied to a video reproduction circuit (41);  a decoding circuit (38) for line and frame synchronizations also supplied by the limiting amplifier and which controls by a first output the frequency synthesizer circuit through an acquisition logic (39), by a second output the generator circuit key and which is connected to the restitution circuit by a third output;  a bit clock signal restore circuit (40 or 42) intended for the video signal restore circuit.  9 - Receiver according to claim 4, characterized in that the acquisition logic circuit (39) comprises a counter (45) controlled by line synchronization from the decoder circuit, receiving a local clock signal (SH) and which controls by its output a monostable circuit (47) to produce a window centered on the next line synchronization pulse, and an AND circuit (48) supplied by the window and line synchronization to deliver the control signal (S18) of the generator key (35).