WO1994024825A1

WO1994024825A1 - Coordinately coding and decoding a message signal

Info

Publication number: WO1994024825A1
Application number: PCT/SE1994/000338
Authority: WO
Inventors: Erik Rickard SÖDERQVIST
Original assignee: Soederqvist Erik Rickard
Priority date: 1993-04-15
Filing date: 1994-04-15
Publication date: 1994-10-27
Also published as: SE9301236D0; SE9301236L; AU6547594A

Abstract

A message signal (RFA, RFB) is transmitted from a transmitting unit (10) to at least one receiving unit (25) in a state coded by means of an encoder (13). Simultaneouly, a separate control signal (SD), generated by the transmitting unit (10), is transmitted and causes a decoder (28) in the receiving unit (25) to accomplish a coding operation on the received coded message signal (RFA', RFB'), timely coordinated with the coding operation. The control signal (SD) consists of a continous data stream, composed of frames (F) of equal length. Each frame (F) contains a predetermined frame synchronization word (FW) which defines a cyclically recurring reference time (Tref) for the transmitting unit (10) and which is utilized in receiving unit (25) to determine a corresponding cyclically recurring reference time (Tref') for this unit. At least some frames (F) also contain data which define the time (t2A, t2B) for an alteration of the operating mode of the encoder (13), related to said reference time (Tref) for the transmitting unit (10), and which are utilized in the receiving unit (25) to determine a time (t2A', t2B') for a corresponding alteration of the operating mode of the decoder (28), related to said reference time (Tref') for the receiving unit (25).

Description

Coordinately coding and decoding a message signal

The invention relates to a method for coordinately coding and decoding a TV signal or other message signal which is transmitted from a transmitting unit to at least one receiving unit in a state, coded by means of an encoder incorporated in the transmitting unit, during simultaneous transmission of a separate control signal, generated by the transmitting unit and utilized in the receiving unit to cause a decoder, incorporated in the latter unit, to accomplish a decoding operation on the received message signal, timely coordinated with the coding operation.

Such coordinated coding and decoding of a message signal is used especially in subscription TV systems in order to prevent an unauthorized utilization of the coded signal transmitted from the transmitting unit.

However, a disadvantage of the methods previously known is that the control signal is of a rather simple kind, which means that the information contained therein may rather easily be read by means of a comparatively simple pirate decoder by which the coded message signal may be decoded also at an unauthorized receiving location.

The invention has for its purpose to provide an improved method of the kind initially set forth which offers a highly increased safety against an unauthorized utilization of the transmitted coded message signal by using a new type of control signal.

The method according to the invention, proposed for the above purpose, is primarily characterized in that the transmitting unit is caused to generate a control signal, consisting of a continuous data stream, composed of subsequent frames of mutually equal length, each of which contains a predetermined frame synchronization word which defines a cyclically recurring reference time for the transmitting unit and which is utilized in the receiving unit to determine a corresponding cyclically recurring reference time for the receiving unit, at least some of said frames also containing data which define the time for an alteration of the operating mode of the encoder, related to said reference time for the transmitting unit, and which are utilized in the re¬ ceiving unit to determine a time for a corresponding alteration of the operating mode of the decoder, related to said reference time for the receiving unit.

By using a control signal of the above kind, which may be made completely asynchronous or approximately synchronous in relation to the message signal, one will obtain a strongly increased safety against an unauthorized utilization of the transmitted coded message signal. Moreover, the need for effecting a separation of sync pulses from the message signal at the receiving end, in order to make it possible to decode the received coded message signal, is eliminated. The method according to the invention may also be applied on a transmission system wherein two or more message signals are transmitted simultaneously from the transmitting unit to the receiving unit, individually coded by means of respective encoders incorporated in the transmitting unit, and said signals are decoded in the receiving unit by means of respective decoders in¬ corporated in this unit. According to the invention, in this case, the transmitting unit may be caused to generate a single control signal serving to control the decoding of all message signals.

Under certain conditions, at least some of the message signals may be coded and decoded in synchronism with each other. However, at least some of the message signals are preferably coded and decoded in non-sychro- nism with each other. In the latter case, the trans¬ mitting unit may be caused to generate a control signal, consisting of frames, at least some of which contain data which define individual times for altering the operating mode of different encoders and which are utilized in the receiving unit to determine corresponding individual times for altering the operating mode of different decoders.

The invention also relates to an arrangement for coordinately coding and decoding a TV signal or other message signal which is transmitted from a transmitting unit to at least one receiving unit in a state, coded by means of an encoder incorporated in the transmitting unit, during simultaneous transmission of a control signal, generated by the transmitting unit and utilized in the receiving unit to cause a decoder, incorporated in the latter unit, to accomplish a decoding operation on the message signal received by the receiving unit, timely coordinated with the coding operation.

The characteristic features of said arrangement appear from claims 6 to 10. Below the invention is further described with reference to the accompanying drawings which, only by way of example, illustrate one possible application of the proposed method on a subscription TV system where¬ in two or more TV signals, transmitted from a trans- mitting unit to a plurality of receiving units connected thereto, are transmitted from the transmitting unit to the receiving units in a coded state in order hereby to permit said signals to be utilized only at authorized receiving locations. In the drawings:- Figure 1 shows a block diagram of the transmitting unit,

Figure 2 shows a block diagram of a receiving unit,

Figure 3 is a diagrammatical illustration of a control signal transmitted from the transmitting unit of Figure 1 to the receiving unit of Figure 2, and Figure 4 shows two time diagrams, illustrating the mutual relationship between the times for certain events taking place in one or the other of said units. The transmitting unit shown in Figure 1 and generally designated 10 has been assumed to comprise only two TV signal channels designated A and B. However, the number of such channels may be substantially larger. Each channel contains a satellite receiver 11, a modula¬ tor 12 and an encoder 13. In each receiver 11, the re- ceived radio frequency signal, which is modulated with a TV program, is demodulated to the base band, i.e. to audio and video level. The demodulated video signal V or V , respectively, and the demodulated audio signal A or A_n, respectively, are then- fed to modulator 12 which generates an uncoded radio frequency signal RF or RF , respectively, modulated with the TV program in question. This signal is suppled to encoder 13 which converts it into a coded signal RF ' or RF ' , respective- ly, which is fed to an output terminal 14 which is common to both channels A and B and which may be connected to a cable network forming part of the subscription TV system in question.

The coding operation, in each channel effected on the uncoded TV signal RF or RF , respectively, in encoder 13, may be of different kinds but may preferably consist in frequency shifts, for instance of the kind described in PCT/SE92/00737. In any case, the coding operation involves that the encoder 13 is caused to switch from one operating mode to another at short inter- vals, in order hereby to generate a coded signal RF * or RF JD' , respectively, having a successively shifting character.

The equipment serving to cause said alteration of the operating mode of the encoder 13 in each channel A and B, respectively, comprises , on the one hand, some components common to both channels and, on the other hand, some components occurring as separate com¬ ponents within each channel. The common components primarily consist of a clock 15, a first counter 16, a second counter 17 and a third counter 18, while the individual separate components within each channel con¬ sist of a sync separator 19, a register 20 and a compa¬ rator 21.

The manner of operation of the equipment above described is as follows. Clock 15 generates a continuous series of clock pulses, the frequency of which may for instance be assumed to be 1 MHz. These clock pulses are supplied to counter 16 which is a cyclic counter which for instance may be assumed to count cyclically from 0 to 19999. Counter 16 will then have a cycle length of 20 ms and a counting resolution of 1 μs . The two other counters 17 and 18 are identically equal to counter 16 and thus, they will also count cyclically from 0 to 19999 at a cycle length of 20 ms and a reso¬ lution of 1 μs. However, counters 17 and 18, which are both connected to the output of counter 16, are indi¬ vidually programmable in order to be able to count with a certain individual displacement in time in relation to counter 16. As shown in Figure 1, counter 17 and 18 may be programmed from a microprocessor 22. The output signal from the second counter 17 is continuously supplied to the input of each one of the two register 20 which belong each to one channel A or B, respectively. Each register 20 has a control input con¬ nected to the output of the appurtenant sync separator 19. Sync separator 19, which receives video signal V or V_D, respectively, from satellite receiver 11, may hereby supply a load pulse to register 20 each time a vertical sync pulse appears in video signal V or V_,, respectively. In a video signal having a frame fre- quency of 25 frames per second, i.e. 50 fields per se¬ cond, this occurs every 20 milliseconds. When the load pulse is fed to register 20, the actual count at the output of counter 17 is written into register 20. The count stored in register 20 is continuously supplied to one input of comparator 21 which, at the other input thereof, receives the output signal from the third counter 18. Hereby comparator 21 will deliver an output signal each time the count at the output of counter 18 is equal to the count stored in register 20. This output signal from comparator 21 is supplied to encoder 13 where it serves as a load pulse which may cause the encoder to load an instruction regarding a new operating mode for the encoder, previously received from micropro¬ cessor 22, into a control circuit incorporated in the encoder. If the encoder is designed in the manner described in PCT/SE92/00737, said instruction may con¬ sist in an altered value of the frequency of a local oscillator, controlled by means of a phase locking cir¬ cuit and connected to a mixer which also receives the uncoded signal RF_A or RF , respectively. The alteration in the operating mode of encoder 13 will then consist in an alteration in the frequency shift operation effected by the encoder on the TV signal passing therethrough.

In addition to the components above described, the transmitting unit shown in Figure 1 also comprises a data generator 23 and a data modulator 24. Data gene¬ rator 23 has for its purpose to generate a control data signal S_n for controlling the decoding in an authorized receiving unit of the coded TV signals RF ' and RF ' transmitted over output terminal 14. Control signal S is supplied to data modulator 24 which generates a radio frequency signal RF which is modulated with the control signal and fed to the output terminal 14 in order to be transmitted to the receiving unit together with signals RF ' and RF ' . As diagrammatically shown in Figure 3, control signal S_n consists of a continuous data stream, composed of subsequent frames F of mutually equal length. Under the control from counter 16, data generator 23 is caused to generate frames F having a length corresponding to the cycle length of said counter, i.e. 20 ms, and each of which contains a frame synchronization word FW, con¬ sisting of an easily identifiable bit pattern having a specific character deviating from the character of the information contained within the remaining sections of the frame and which for instance may be Manchester- coded. The frame synchronization word FW defines a cyclically recurring reference time T _f (Figure 4) for the transmitting unit to which the time t, In or t,ID, respectively, for the occurrance of the vertical sync pulse as well as the time t„_A or t» , respectively, for an alteration of the operating mode of decoder 21, which is displaceable in relation to the firstmentioend time, may be related for each one of the two channels A and B.

Through a continuous supply of the output signal from each of the two registers 20 to data generator

23, control signal S is also caused, within predeter¬ mined sections of the total space within each frame F or within at least some frames F, to contain informa¬ tion I or I , respectively, about the count at the moment stored in register 20 of each channel A and B.

This count consists of the count of counter 17 which was written into register 20 in connection with the occurrance of the next preceding vertical sync pulse in video signal V_A or V_β, respectively, and defines the time t,IA.- or t,I_nB for the occurrance of the vertical sync pulse, related to reference time T _f. The reason why information I and I does not necessarily have to be transmitted to the receiving units within each frame of control signals S is that times t, and t, will vary in relation to reference time T _f at a very slow rate if, as assumed above, the time interval between the vertical sync pulses of video signals V and V is at least approximately equal to the cycle length of counters 16, 17 and 18.

Apart from serving to transmit frame synchroniza- tion word FW and information I and I , control signal S also serves to transmit the required enabling signals from microprocessor 22 to the authorized receiving units and to provide these units with information, obtained from the microprocessor, about the algorithms according to which the operating mode of the decoders contained in the receiving units has to be altered in order to correspond to the alteration in the operating mode of encoders 13.

The receiving unit shown in Figure 2 and generally designated 25 has an input terminal 26 which may be connected to the output terminal 14 of transmitting unit 10 for instance through a cable network and over which the receiving unit may be supplied with the radio frequency signals RF A. ' , RFts' and

delivered by the transmitting unit. To input terminal 26, there are con¬ nected, on the one hand, a data demodulator 27, serving to demodulate signal RF_n and to re-create the control signal S generated by data generator 23 in transmitting unit 10, and on the other hand, two decoders 28 which serve to decode respecitve ones of the two coded signals RF ' and RF ' and to generate a corresponding uncoded radio frequency signal RF " and RF " which is modulated with a TV program and which may be fed to a TV and/or video set 38, connected to the two decoders. In the illustrated embodiment, it has been assumed that data demodulator 27 and decoders 28 are provided with built- in band pass filters for filtering out those of the three signals received over input terminal 26 which are not intended to be utilized in the respective unit. Above it has been assumed that receiving unit 25 is provided with two decoders 28 in order hereby to be in a position to decode the two coded signals RF ' and RF ' simultaneously. However, if desired, the receiving unit may be provided with only one decoder, in which case it may decode only one incoming coded signal at a time. Conversely, it may also be provided with more than two decoders in order hereby to be able to decode more than two incoming coded signals simul¬ taneously.

In order to facilitate, under the control of control signal S_n, an alteration of the operating mode of each decoder 28 which corresponds to the successive alteration of the operating mode of encoders 13 and which is timely coordinated therewith, the receiving unit 25 is provided with a special control equipment for the decoders. This control equipment comprises a parity check logic 29, a frame synchronization logic 30, a frequency recovery logic 31, two registers 32, namely one for each decoder 28, and a microprocessor 33. These components of the control equipment are all connected to the output of data demodulator 27 in order hereby to receive the control data signal S re-created by the data demodulator. Furthermore, the control equip¬ ment also comprises a comparator 34 for each decoder 28 and a counter 35, a clock pulse generator 36 and a phase locking logic 37 for controlling the latter.

The manner of operation of the control equipment for the decoders above described is as follows. Similar to each counter 16, 17 and 18 in transmitting unit 10, counter 35 is designed as a cyclically operating counter which counts from 0 to 19999. It receives clock pulses having a frequency of 1 MHz from clock pulse generator 36 which is controlled from phase locking logic 37 which, in its turn, is supplied with, on the one hand a signal derived from control data signal S by means of frequency recovery logic 31 and having a frequency of for instance 62,5 kHz, corresponding to the bit rate of signal S , and on the other hand, a reference signal of the same frequency, obtainable from counter 35. Hereby, the cycle length of counter 35 will be the same as the cycle length of counter 16, 17 and 18, i.e. 20 ms. By means of frame synchronization logic 30, a check is made to ascertain that control data signal S from data demodulator 27 contains faultless frame synchronization words FW and that these words occur every 20 ms. When frame synchronization logic 30 detects a first correct frame synchronization word FW, it de¬ livers a signal to counter 35 to reset said counter to zero. Through this resetting of counter 35, a cyclical¬ ly recurring reference time T _f ' (Figure 4) for the receiving unit is determined which corresponds to the reference time T ,- for the transmitting unit and to which the time t_ ' or t- ' , respectively, for an altera¬ tion of the operating mode may be related for each decoder 28.

When detecting the frame synchronization word following next upon the first frame synchronization word, the frame synchronization logic 30 will deliver an enabling signal to the two registers 32, provided that the frame synchronization logic simultaneously receives a cyclically generated flag, delivered from counter 35. This enabling signal will then remain as long as faultless frame synchronization words and flags from counter 35 are detected every 20 ms by the frame synchronization logic.

In the receiving unit 25, the information I and I_D-----, respectively, contained in control data signal S_^ D and relating to the count, at a given instant stored in register 20 of each channel A and B of the transmit¬ ting unit, is utilized to write the same count into the corresponding register 32. However, in order to make it possible to write new counts into registers

32, these registers have to be supplied with the above- mentioned enabling signal from frame synchronization logic 30 as well as with an enabling signal from parity check logic 29.

The count stored in each register 32 is continously fed to one input of the corresponding comparator 34, which receives the output signal from counter 35 at its other input. Hereby, comparator .34 will deliver an output signal each time the count at the output of counter 35 is equal to the count stored in the corres- ponding register 32. For comparator 34 of channel A, this occurs at the time t» ' , while in respect of compa¬ rator 34 of channel B, it occurs at the time t ' . The output signal from the comparator is fed to the corres¬ ponding decoder 28 where it serves as a load pulse which may cause the decoder to load into a control circuit thereof an instruction relating to a new operating mode of the decoder, previously received from microprocessor 33. This instruction should be such as to adapt the alteration in the operating mode of decoder 28 to a simultaneously caused alteration in the operating mode of the corresponding encoder 13 in transmitting unit 10.

In Figure 3, the subsequent frames F, which the control data signal S_n is composed of, have been shown considerably shortened. However, it may be seen that frame synchronization word FW and the information I and I_D occur at predetermined allotted locations wihtin each frame F. In the illustrated example, it has been assumed that frame synchronization word FW occurs at the end of each frame, while information I, and I occurs at the beginning of each frame.

If the arrangement is intended to be utilized for coordinately coding and decoding further channels in addition to the illustrated channels A and B, informa¬ tion relating to those further channels and corresponding to I A. and I_B, may suitably occur at fixed locations for said information following immediately after the illustra- ted locations for I and I . Signal S may also be utilized to transmit all other required information from transmitting unit 10 to receiving unit 25. Thus, inter alia, it may be utilized for the transmission from microprocessor 22 of the transmitting unit to micro¬ processor 33 of the receiving unit of information about various algorithms for the alteration of the operating mode of each decoder 28 contained in receiving unit 25. Moreover, signal S_n also contains information about the location within said signal where the information I and I , respectively, occurs. Said information is transmitted from microprocessor 33 to registers 32 which also receive a continuous time information from counter 30. Hereby, it is ensured that the count written in each register 32 will correspond to the count just then stored in the corresponding register 20 in the transmitting unit. Thus, by being programmed from microprocessor 33, registers 32 may be caused to receive I, or I„. ^ ^J A B

In the upper portion of Figure 4, there is shown a time diagram illustrating the mutual relationship between the times for events occuring in transmitting unit 10. As previously mentioned, T _f designates a cyclically recurring reference time. This reference time occurs each time counter 16 assumes a zero value and it may for instance be brought to coincide with the end of the frame synchronization word FW.

The time for the occurrance of the vertical sync pulse in each of video signals V. and V_R is designated t₁ 1_Λ and t₁1_D-----, respectively. This time is defined in an unambiguous manner in relation to reference time T __.

^ ref by the count from counter 17 written in register 20 of channel A or B, respectively, and the programmed displacement in time between the operating cycles of counter 17 and counter 16. The time for an alteration of the operating mode of encoder 13 in channel A or B, respectively, is designa- ted t~_Δ and t--, respectively. This time occurs when the count of counter 18 is equal to the count from counter

17 stored in register 20 of the channel in question.

In the lower portion of Figure 4, there is shown a time diagram illustrating times for events occurring in receiving unit 25. This time diagram starts from the same starting time as the diagram shown in the upper portion of Figure 4 which means that it also illustrates the mutual relationship between times having reference to the transmitting unit and times having reference to the receiving unit.

As already mentioned, T _f ' designates a cyclically recurring reference time for the receiving unit. This time is displaced in relation to reference time T _£ ^ ref for the transmitting unit by a time interval corresponding to the delay of control data signal S_n occurring, on the one hand, in connection with the output of said signal from data generator 23 in the transmitting unit and, on the other hand, in connection with the input of said signal into receiving unit 25. In reality, a slig^htly ^J increased disp ^lacement between Tre _cf ' and Tref, may occur as a consequence of the delay of signal RF caused by the transmission of said signal from the trans¬ mitting unit to the receiving unit. However, this delay does not have any significance, as signals RF Pi' and

RF ' , which are to be decoded in the receiving unit, B are subjected to the same delay in connection with their transmission.

The individual time for an alteration of the ope- rating mode of each decoder 28, serving to decode a respective one of the two signals RF.' and RF ' , is designated t~ " and t~ ' , respectively. As mentioned above, for each decoder 28, this time occurs when the count at the output of counter 35 is equal to the count stored in the corresponding register 32. In the described embodiment of the transmitting unit and the receiving unit, counter 35 will operate in full synchronizm with counter 18 and start a new counting cycle simultaneously with the latter. For this reason, it is possible by programming a suitable displacement of the counting cycle of counter 18 in relation to the counting cycle of counter 16 to cause each one of times t~ ' and t„ * to occur at the same instant as the corresponding time t~ and t_2R, respectively. However, if desired, said displacement of the counting cycle of counter 18 may be chosen so as to cause times t_? ' and t~ ' to occur a selected time interval before or after the corresponding times t„_A and t„ , respectively.

The above described embodiment of transmitting unit 10 and receiving unit 25 ensures that the receiving unit may accomplish a decoding operation on the trans¬ mitted coded TV signals which is timely coordinated with, or synchronous with, the coding operation effected in the transmitting unit without making it necessary at the receiving end to effect any separation of the vertical sync pulses from said signals in order to facili¬ tate such a coordination.

The invention is not restricted to the embodiment above described and shown in the drawings. Instead, many other embodiments are feasible within the scope of the invention. Especially, it is emphasized that the invention may be utilized not only for coordinately coding and decoding one or more TV signals but also for coding and decoding other kinds of message signals.

Claims

C l a ims

1. A method for coordinately coding and decoding a TV signal or other message signal which is transmitted from a transmitting unit to at least one receiving unit in a state, coded by means of an encoder incorpo¬ rated in the transmitting unit, during simultaneous transmission of a separate control signal, generated by the transmitting unit and utilized in the receiving unit to cause a decoder, incorporated in the latter unit, to accomplish a decoding operation on the received message signal, timely coordinated with the coding opera- tion, characterized in that the transmitting unit is caused to generate a control signal, consisting of a continuous data stream, composed of subsequent frames of mutually equal length, each of which contains a prede¬ termined frame synchronization word which defines a cyclically recurring reference time for the transmitting unit and which is utilized in the receiving unit to determine a corresponding cyclically recurring reference time for the receiving unit, at least some of said frames also containing data which define the time for an altera- tion of the operating mode of the encoder, related to said reference time for the transmitting unit, and which are utilized in the receiving unit to determine a time for a corresponding alteration of the operating mode of the decoder, related to said reference time for the receiving unit.

2. A method according to claim 1, wherein two or more message signals are transmitted simultaneously from the transmitting unit to the receiving unit, indi¬ vidually coded by means of respective encoders incorpora- ted in the transmitting unit, and said signals are decoded in the receiving unit by means of respective decoders incorporated in this unit, characterized in that the transmitting unit is caused to generate a single control signal serving to control the decoding of all message signals.

3. A method according to claim 2, characterized in that at least some of said message signals are coded and decoded in synchronism with each other.

4. A method according to claim 2, characterized in that at least some of said message signals are coded and decoded in non-synchronism with each other.

5. A method according to claim 4, characterized in that the transmitting unit is caused to generate a control signal, consisting of frames, at least some of which contain data which define individual times for altering the operating mode of different encoders and which are utilized in the receiving unit to determine corresponding individual times for altering the operating mode of different decoders.

6. An arrangement for coordinately coding and decoding a TV signal or other message signal which is transmitted from a transmitting unit (10) to at least one receiving unit (25) in a state, coded by means of an encoder (13) incorporated in the transmitting unit (10), during simultaneous transmission of a control signal, generated by the transmitting unit (10) and utilized in the receiving unit (25) to cause a decoder (28), incorporated in the latter unit, to accomplish a decoding operation on the message signal received by the receiving unit, timely coordinated with the coding operation, characterized in that the transmitting unit (10) is arranged to generate a control signal (S_n), consisting of a continuous data stream, composed of subsequent frames (F) of mutually equal length, each of which contains a predetermined frame synchronization word which defines a cyclically recurring reference time (T ,) for the transmitting unit (10) and which is utilized in the receiving unit (25) to determine a corresponding cyclically recurring reference time (T _f') for the receiving unit (25), at least some of said frames also containing data which define the time (t_?A, t_?R) for an alteration of the operating mode of the encoder (13), related to said reference time (T _f) for the transmitting unit (10), and which are utilized in the receiving unit (25) to determine a time (t~ ', t~ ' ) for a corresponding alteration of the opera¬ ting mode of the decoder (28), related to said reference time (Tref __- ' ) for the receiving -a unit ( ^ι25) *^•.

7. An arrangement according to claim 6, wherein two or more message signals are transmitted simultaneously from the transmitting unit (10) to the receiving unit

(25), individually coded by means of respective encoders (13) incorporated in the transmitting unit (10), and said signals are decoded in the receiving unit (25) by means of respective decoders (28) incorporated in this unit, characterized in that the transmitting unit (10) is arranged to generate a single control signal (S_n) serving to control the decoding of all message signals.

8. An arrangement according to claim 7, characterized in that it is arranged to code and decode at least some of said message signals in synchronism with each other.

9. An arrangement according to claim 7, characterized in that it is arranged to code and decode at least some of said message signals in non-synchronism with each other.

10. An arrangement according to claim 9, characterized in that the transmitting unit (10) is arranged to generate a control signal (S_D), consisting of frames (F), at least some of which contain data which define individual times (t~_A, t~_R) for altering the operating mode of different encoders (13) and which are utilized in the receiving unit (25) to determine corresponding individual times (t~ ', t„ ') for altering the operating mode of different decoders (28).