System and multiplexer/ demultiplexer for the transmission/ reception of digital television information

Abstract: It is described a system and related multiplexers/demultiplexers for the transmission/reception of digital television information (video + audio), included the high definition information (HDTV), in which it is used the transmission in packets and various work speeds are foreseen, among which a special transmission that uses two data flows transmitted contemporaneously for getting a sole information. The multiplexer is associated to external buffers that receive the data at different speeds, form the packet and generate requests of transmission to the multiplexer that receives them according to preestablished priorities.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method and a system for the transmission/reception of digital television information (video + audio), in particular for the high definition television information (HDTV). The multiplexing/demultiplexing systems usually order the data in transmission at a fixed distance in the frame and separate the data in reception according to this preestablished sequence. In the case where the data must be transmitted at different speeds, the operations become more complex and the classic system cannot be used. So, there is a problem, due to the necessity of transmitting data series with different speeds. Moreover until to by, for increasing the data speed to be transmitted by a system, a higher data hierarchy was used with the following necessity of using transmission means having a higher frequency. From Signal Processing of HDTV, II; Proc. of the 3rd International Workshop on HDTV, Turin, 30-09-1989; pages 587-594; M.Barbero et al.: "A flexible architecture for HDTV codec based on DCT", a method including the features of the first part of claim 1 and a system including the features of the first part of claim 7 are known. Further, Globecom '85 - IEEE Global Telecommunications Conference, New Orleans, 2-5 Dec. 1985; vol. 1, pages 1251-1255; L.Chiariglione et al.: "Multimedia Communication at the basic ISDN access"; discloses a realignment of data packets belonging to the same signal on the reception side by using a frame synchronisation signal. A realignment of data packets on the reception side is also described in Globecom '85; 2-12-1985; vol. 3/3, pages 1340-1344; L. Chiariglione et al.: "A family of frame structures for local video distribution", and IEEE Journal on Selected Areas of Telecommunications, vol. 7 n.5, June 1989; pages 739-751; G.Karlsson et al.: "Packet video and its integration into the network architecture". SUMMARY OF THE INVENTION The object of the present invention is to provide a simple and efficient method for high speed transmission of signals with different speeds and a system suitable to implement such a method. This object is met by a method according to claim 1 and a system according to claim 7. Preferred embodiments are disclosed in the depending claims. The system according to the invention allows to use a well established hierarchy for transmitting an information with double speed using two independent flows for the transmission of a sole information. In this manner it can be advantageously used the same type of transmission means without the necessity of having an access to other higher means. According to the invention the data at different speeds are no more transmitted unchanged directly to the multiplexing stage, but they are collected externally to the multiplexer in packet forming means of preestablished structure; each time said means have formed a packet they send a request to the multiplexing stage. The different sources are supplied with different priorities, therefore, in the case of contemporaneous transmission requests, the request coming from the packet source that is associated to higher priority is satisfied first. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the transmission circuitry of the present invention; Fig. 2 is a block diagram of the reception circuitry of the present invention; Fig. 3 shows the transmission of words having a length of N bits divided into two paths with words having a length of N/2 bits; Fig. 4 shows the elastic store that carries out the new alignment at bit level and the synchronisation at clock level; Fig. 5 shows the alignment generator; Fig. 6 shows the delay compensation; Fig. 7 shows a conventional variable delay line for providing the delay compensation of Fig. 6; Fig. 8 shows a delay line formed of a series of multiplexers preceded by shift registers for providing the delay compensation of Fig. 6; and Fig. 9 shows an example of the contents of two packets. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figures 1 and 2 show the transmission and reception schemes of the method according to the invention. The data at different speeds V1, V2, ...Vn arrive separately to the packet collectors-formers RFP1, RFP2....RFPn, in this case formed by buffer stores 1, 2, 3.... n, that are joined by means of the lines L1...Ln and the line in common with the multiplexer MUX. According to the invention, when one of the buffers (e.g. 1) reveals that a data number suitable to be transmitted exists, that is a number equal to the packet length, the request RQ 1 to the MUX is generated for informing MUX that the packet is available and for having the consent for the transmission thereof. The MUX processes the request coming from RQ 1 together with the possible other contemporaneous requests coming from RQ 2 to RQn and, if these have not higher priorities than the priority assigned to RQ 1 , it allows the output of the data packet accumulated with speed V1 into the buffer 1. The higher priority corresponds in the present embodiment to the data concerning the audio, followed by the fixed data of the video part. After these data are transmitted, the codes having a variable length coming from processors and as last data, the data having the lowest speed and importance as the telex transmissions, the ancillary data, etc. are transmitted. These priorities can be changed in any moment. Each time the transmission request RQi is recognized, MUX generates the corresponding timings and synchronisms Kl, and SIN:MUX communicates with the circuits forming the packets by means of the lines l 1 , l 2 , li. The data transmission is carried out after having made a redundancy insertion for a future error correction by means of a FEC (Forward Error Corrector). In reception (Figure 2) the circuit must recognize the buffer 1', 2'... n' to which each received packet is destined. For making this, the definition of the packet type hold in the initial part of the same packet is used as indicated in the Figure 9, that explains how the frame and packets must be formed assigning well-established packet types to each area. For allowing the changing and programming of this information according to the future requirements of the total system, the output decision is not taken by the demultiplexer DEMUX in a fixed manner, but it provides said information, after having extracted it from the frame, to an external circuit deciding the destination of the same information PAL, using the bus PT. PAL decides in relation to the assignment table supplied case-by-case (that can be changed easily as indicated for the priority RQi) and provides the destination signal of the packet by means of the bus OS. Depending on the information sent by OS, the demultiplexer DEMUX decides the type of synchronism and timings K'1, SIN' that it must generate for the data output; these signals are transmitted onto the lines l'1, l'2 ...l'n, while the information is transmitted onto the lines L'1, L'2...L'n. As data output speeds and according to the quantity of information to be transmitted, the following channel speeds are used: 1 channel with 34 Mbits/s 2 channels with 34 Mbits/s 1 channel with 140 Mbits/s The higher the speed, the better the signal quality that can be transmitted in the same transmission time interval. Considering the first two cases, all multiplexing/demultiplexing part is processed in the semicustom circuit, and considering the last case, due to the high speed of the final stages, the series-parallel or parallel-series conversions and the interfaces of the lines are done externally using ECL logics. The information distribution is based on frames (Figure 9) formed by two data packets having 238 words of 16 bits, and further a first head word that holds the above mentioned definition of the packet type in the first 8 bits, said information being protected by a Hamming code, error corrector, and further 16 redundancy words of 16 bits for the error correction. The frame is started with an alignment word of 24 bits, and further 8 bits that is formed by a status word provided by an external system. The continuation of the alignment word is formed by an information of 16 bits, that is the information allowing to recover the video synchronism in reception. All these data are transmitted in the form words of 8 bits even if they can be grouped in words having a greater or shorter length after the buffers. In the case where 34 Mbit/s speed is used, the running of mux/demux is similar to a normal system, excepting the matter that the data transmission is carried out in packets and not according to a fixed sequence. The demultiplexing is made revealing the alignment word and, in the moment of the alignment acquisition,the initial heads of the packets are revealed and the information related to the area that must receive the transmitted packet is extracted. In the case where 140 Mbit/s speed is used, the data are multiplexed in MUX, but the series and the line code conversions are applied externally to the circuit in the ECL logics for obvious reasons of work speed; in reception the data enter in DEMUX, already transformed by parallel conversion, directly to the input elastic store and they follow the above said normal process. The more interesting part is that related to 2*34 Mbits/s, in which two independent channels C1, C2 (respectively C'1, C'2) are used for transmitting the whole information. Until today, for increasing the data speed to be transmitted by a system, a higher data hierarchy was used with the following necessity of using a transmission means having a higher frequency. This fact requires to use, e.g. in the case of radiocommunications, higher and higher channels with consequent problems of the increasing frequency, included in the microwave field. In the present embodiment a speed of 2x34 Mbit/s with 8 bits words is advantageously used, but the following explanation refers to a generical system. The system according to the invention allows to use a well established hierarchy for transmitting an information with double speed using two independent flows for the transmission of a sole information. In this manner it can be advantageously used the same type of (doubled) transmission means without the necessity of having an access to other higher means. According to a feature of the invention, the information is divided in two flows separating a part from the other part and rejoining these parts in reception for obtaining the original information. For acting in this manner, it is necessary to synchronize the two independent flows and compensate the time differences between the flows for the different delays to which they are submitted due to the differences in the transmission means. The Figure 3 represents schematically the system according to the invention. In transmission the word of N bits, Fn , is divided in two word of N/2 bits, F1, F'1. Due to the differences in the transmission time TO and the delay changes, the two flows F1 and F'1 arrive with delays DELAY1 and DELAY2, in adddition to the speed changes. In reception these changes must be compensated for turning to the original words of N bits. According to a feature of the invention, as an indication of the difference between the transmission times of the two flows, the information obtained from the time difference at the apparition of the alignment word is used. In transmission a channel C1 transmits the N/2 bits of greater weight of the word of N bits and the second channel C2 transmits the other N/2 bits of less weight (e.g. see Figure 1). In reception the following factors must be considered: each channel has different delays for being independent, therefore the time differences between the two information must be compensated, as stated above; as a further performance a revelation of the channel fixing each information can be added. This second possibility requires the introduction of an alignment word revelation different in each channel, generating a high or low channel indication. In this manner/the input channel changes are compensated automatically. For compensating the delay, a more complex process is applied that can be understood better with reference to the Figures 4, 5 and 6. Figure 4 shows the input flows F1, F1', that are now introduced into an elastic store ME with different clocks. The name of elastic store means that it is a store where the reading-writing distance is variable, the data being introduced by means of a clock and the data being read by means of a second clock. The aim of a memory of this type is to absorb the instantaneous changes of the writing clock and further, in the present embodiment, to synchronize the bits, as the writing uses a clock for each flow, while the reading uses a same master clock in the two memories. Obviously a PLL must be used for getting that the writing arid reading frequencies are equal, for avoiding fillings and emptyings of the elastic store ME, that, as stated above, must absorb the instantaneous changes. Therefore the output of the elastic store provides two synchronized information along the duration of bits, read in the two flows by the same clock, as it is shown in Figure 4. Therefore a synchronization at clock level and a new alignment at bit level is obtained. Each of information (packets PA11...PA21; respectively PA12..PA22) is transmitted to an alignment detector that provides the words correctly aligned and formed as they were originally, due to the detecting of the alignment word, and so the right bit for each word being started can be known. Therefore at the input of the alignment generator RI, as shown in Figure 5, two flows can be got (represented by the continuous line) that must be aligned correctly. The words are aligned correctly at output, but it exists a time difference between the apparition of the alignment words in each flow. This difference supplies the mean value of the delay between the two frames that is compensated. Until this moment the word synchronism is got. For understanding as the time difference is compensated, one flow must be defined as master flow and the other flow as slave flow. The master flow is the flow that was joined to the circuit clock frequency. The difference between the apparition of the alignment words in the secondary flow can be positive or negative with respect to the master flow, i.e. it can be in advance or delay. In other words, the time difference can be positive or negative. The time difference between the apparition of two alignment words is revealed. This difference must be compensated. To do this, the master flow is always delayed of a fixed value T, so that the alignment word appears always at output shifted by T words after the detecting thereof. The slave flow can be shifted in advance or in delay with respect to the other flow. For discovering this, the alignment detector starts a counter for each of the flows. At the moment in which the alignment word appears in the two channels, the time difference between the two channels is given by the difference between two counters CNT1-CNT2 (CNT1 - counter of the master channel, CNT2 - counter of the slave channel). This value can be positive or negative, according to whether the alignment word appeared in master channel or in slave channel for the first time. This value is named D, so that: CNT1-CNT2=D As stated above, the master flow was delayed of a fixed value T, therefore, at the instant t01, the alignment word appears in the master channel and in the instant t02, the same word appears in the slave channel. The difference between the two times shall be: t01 - t02 = D*tp ; where tp is the duration of a word. The alignment word after the delay appears at the time t0t1, given by the expression: t0t1 = t01 + T*tp ; The slave channel includes a delay TS that provides the output at the instant t0t2, given by: t0t2 = t02 + TS*tp ; The solution that it looked for is that t0t1 = t0t2, from which: replacing [1]: TS*tp = D*tp+T*tp ; TS = D + T Therefore a variable delay line is used, in which it is assumed that the minimum value of TS = 0, (to delay the signal of a negative number shall be equivalent to accelerate it), therefore: D + T = 0 ; - T < D < T from which: 0 < TS < 2T; In the present embodiment, the value of T was selected equal to 64 therefore, as words are treated, a delay of 64*4 bit = 256 bit is presented; TS being variable between 0 and 512 bit, that in the case of 2*34 Mbit/s provides time of bit equal to 1/34 µs and a delay compensation of 256*1/34,368 µs = 7.5 µs in advance or delay. Comparing this value of total frame time, that should have a length (always for our particular application): 2*(238*16+16+16*16)+2*16+16 = 8208 bits with a frame time for 2*34 Mbits/s equal to: 8208*1/(2*34.368) µs. = 119.413 µs Expressing the compensation in other manner, it can be seen as: 256/8208 = 3,118 % The embodiment of the variable delay line allows two variants: a memory in which the reading-writing distance fixes the delay thereof; a shift register line, forming the delays of 2^ N and selecting the delay to be applied by means of a multiplexer selection. As preferred embodiment the second option is used, whose implementation is easier .A representation of the embodiment forms of the two options is visible in Figures 7 and 8 respectively. At the output of the delay lines the frame synchronism is obtained, by which, always considering that the delay difference between the two frames is lower than the fixed value, the two channels can be completely synchronized. In the case in which it is not possible to compensate the delay, it is generated an external alarm. This alarm must indicate only that the absolute value of D is higher than the maximum foreseen value. As it can be noticed in the Figure 6, at output of the system SC of delay compensation, the two frames are completely synchronized and the sole action that must be done is to join the two words of N/2 bits in words of N bits for reconstructing the original information. Another performance carried out by the circuit is the possibility of working as a "repeater" introducing directly the data from another demultiplexer and using an external selection to replace the packets that are required in the point of the distribution network and inserting further the supplementary information in the packets that in transmission were let free just for realizing this possibility, as indicated above. This is obtained by means of the data introduction by another bus and, in the case of revelation of a "replaceable" packet with one of the packets fixed externally is replaced. For better explanation: Figure 3 shows the flow fu formed by words having a N length, that at the time TO is divided in two flows F1 and F1' having words of N/2 length. F1 arrives to the demultiplexer DEMUX at the moment TO + delay 1, F'1 arrives AT TO + delay 2; DEMUX joins again F1 and F'2 compensating the respective delays 1 and 2 and forming again the original flow F'n of words having a N length; Figure 4 represents the elastic store ME that carries out the new alignment at bit level between F1 and F2 and the synchronization at clock level; Figure 5 represents the tester RI of synchronzation and therefore at word level; Figure 6 shows the compensation system with new alignment at packet level; Figure 7 represents the variable delay line (for compensating the delay of Figure 6) made with classic system, that is a store that receives the data in input IN and sends them at output OUT with a delay that is equal to the distance between reading and writing; for this aim the writing is controlled by the counter WC, whose counting, added with the delay TS, controls the reading; Figure 8 shows the delay line that is advantageously formed by a series of MUX preceded by registers that supply shifts 2 ∧ N, 2 ∧ (N-1)...respectively 1. The first MUX receives the signal at input IN, the signal from the register with shift 2 ∧ N and the signal of a LATCH fed by the delay signal TS. Each of MUXs following the first MUX shall receive the signals from the preceding MUX, from the decreasing shift register 2 ∧ (N-1) etc. and from the LATCH; Figure 9 shows the formation scheme of two exemplifying packets (first, second packet PAC1, PAC2). In a particularly simple and advantageous embodiment the frame is really formed by two packets PAC1, PAC2 of 255 words formed each of 16 bits; the head of each packet uses two words of 16 bits reserved to the alignment words (24 bits) ALIN1, ALIN2, ALIN3, and to the status word (STATUS) (8 bits) and further two 16 bit words for the video synchronism (VIDEO FREC). Each packet is protected by an error correction code (FEC). The first 16 bits of each packet hold the information related to the source from which the same packet comes (PACKET TYPE). The transmissidn from the buffer store Bi to the multiplexer MUX and from the demultiplexer DEMUX to the buffer B'i occurs by means of the 8 bit words (P1, P2). It was stated above that the multiplexer can run with three different speeds: 1 flow with 34 Mb/s; 2 flows with 34 Mb/s; 1 flow with 140 Mb/s. In the first two cases all of operations of a multiplexer-demultiplexer are advantageously made internally to the semicustom circuit, while in the third case the series/parallel conversion and the interface of lines are carried out externally by ECL logics due to the very high speed. In the case 2 flows with 34 Mb/s are used, the advantage of working with a frequency double of 34 Mb/s is obtained, the transmission speed of 34 Mb/s is used on two independent flows. For getting the original information, the two flows are joined in reception in a correct manner, synchronizing the two input flows. The 8 bit words in transmission are divided in two 4 bit words (the most significant 4 sits and the less significant 4 bits), and transmitted independently. In reception it is possible to recognize the most significant bits and the less significant bits (as it is necessary to recognize different alignment words) being possible, in this manner, to compensate eventual channel exchanges. The two received flows are recorded in two elastic stores with different writing clocks extracted from flows and are read with the same clock, that is the one related to the master flow (Figure 4). Then an alignment generator RI (Figure 5) is presented in each path and at output from each alignment generator a correctly aligned flow is obtained; as a time difference between the position of the alignment word can be present in a flow with respect to the other flow, it is compensated from the variable delay line SC (Figure 6) so that, in the preferred embodiment of Figure 8, registers are inserted by means of MUX. As stated above, the system can run as repeater, in which case the free packets are replaced with the desired packets.

Claims: A method for the transmission and reception of video, audio and data signals, said signals being digital signals coming from different sources and having speeds different with one another, wherein in transmission each of said signals is accumulated into one of a plurality of buffers (1, ..., n) so as to form respective packets, and when a packet is ready the respective buffer requires a multiplexer (MUX) to transmit said packet, said multiplexer fulfilling the requestsaccording to a predetermined priority order, and wherein in reception the packets are distributed by a demultiplexer (DEMUX) to one of a plurality of buffers (1', ..., n'), and then to the related receivers, characterised in that in transmission the information to be transmitted is supplemented with alignment words and is divided into two flows, wherein said two flows are transmitted on two independent channels (C'1, C'2), and in reception said two flows are received and joined together by the steps of writing said two received flows into two elastic stores (ME) with two different writing clocks extracted from said two flows, reading said two flows from said two elastic stores (ME) with the same reading clock, and aligning said two flows read out of said elastic stores on the basis of said alignment words. The method according to claim 1, wherein the difference between the transmission times of the two flows received on said two channels (C'1, C'2) is compensated on the basis of the time difference between the detection of said alignment words in each flow. The method according to claim 2, wherein said compensation is carried out by defining the flow received on one channel as master flow and the flow received on the other channel as slave flow, by delaying said master flow with a fixed value and by delaying said slave flow with a variable value so that the alignment words of said two flows appear at the same time. The method according to claim 2 or 3, wherein, when said compensation of the difference between the transmission times is not possible, and external alarm is generated. The method according to any of claims 1 to 4, wherein said two flows are transmitted at a rate of 34 Mbit/s each. The method according to any of claims 1 to 5, wherein some packets are replaced by packets from another demultiplexer. A system for the transmission and reception of video, audio and data signals, said signals being digital signals coming from different sources and having speeds different with one another, comprising in transmission means (V1, V2, ..., Vn) for feeding said signals to means (1, ..., n) for forming packets, means for assigning a predetermined priority to said means (1, ..., n) for forming packets, a multiplexer (MUX) for multiplexing and transmitting said packets, and means (RQ1, ..., Rqn) for requiring said multiplexer (MUX) to transmit a packet when the packet is ready, said multiplexer fulfilling the requests according to said predetermined priority order, and comprising in reception a demultiplexer (DEMUX) for distributing each packet to one of a plurality of buffers (1', ..., n'), and then to the related receivers, characterised by means for supplementing the information to be transmitted with alignment words, and for dividing said information to be transmitted into two flows, two independent channels (C'1, C'2) for transmitting said two flows from said multiplexer (MUX) to said demultiplexer (DEMUX), and means for receiving and joining together said two flows, said means including elastic store means (ME) for writing in said two received flows with two different writing clocks extracted from said two flows, and for reading out said two flows with the same reading clock, and alignment generating means (RI) for aligning said two flows read out of said elastic store means on the basis of said alignment words. The system according to claim 7, further comprising compensation means (SC) for compensating the difference between the transmission times of the two flows received on said two channels (C'1, C'2) on the basis of the time difference between the detection of said alignment words in each flow. The system according to claim 8, wherein said compensation means (SC) includes a variable delay line for each flow preferably formed by many registers and multiplexers and one latch. The system according to any of claims 7 to 9, wherein said means for forming packets is a series of buffers. The system according to any of claims 7 to 10, wherein said demultiplexer (DEMUX) is associated to a circuit (PAL) for packet decision and destination.

Inventors: CAMPOS ANTONIO, ROSSI ALESSANDRA, CAMPOS, ANTONIO, ROSSI, ALESSANDRA
Applicants: ALCATEL ITALIA, ALCATEL ITALIA S.P.A
Application year: 1990
Main classification: H04N7
IPC codes: H04N11, H04N7, H04N9, H04N5