GB2196510A

GB2196510A - A system for extracting frame sync signals from packet syn signals

Info

Publication number: GB2196510A
Application number: GB08724420A
Authority: GB
Inventors: Christain Guinchard
Original assignee: Telediffusion de France ets Public de Diffusion
Current assignee: Telediffusion de France ets Public de Diffusion
Priority date: 1986-10-17
Filing date: 1987-10-19
Publication date: 1988-04-27
Anticipated expiration: 2007-10-19
Also published as: FR2605479B1; FR2605479A1; GB8724420D0; DE3735367A1; DE3735367C2; GB2196510B

Description

1 GB2196510A 1

SPECIFICATION

A system for extracting frame sync signals from packet sync signals The invention relates to extraction of frame sync signals from a signal made up of successive 5 packets each comprising a heading of identifiable configuration and grouped in frames of con stant duration or comprising a constant number of packets.

More particularly the invention relates to a method and device of use when all the headings comprise one out of n binary words WI to Wn which are complementary to one another and are successively used in the sequence of packets, a change of frame being indicated by repetition of 10 the same word in two successive packets or by any other particular predetermined configuration, which is different from any heading configuration which can occur inside the frame.

The invention has a particularly important application to the extraction of frame or field sync signals from a television signal in accordance with one of the X- MAC/Packets standards (X denoting one of the stipulated modes of modulation, denoted by C, D and D2) where each line 15 contains analog chrominance and luminance signals and a digital part containing a line sync word. In this case, the contents of a line will be considered as a packet as per the definition given hereinbefore. The television signal also cmprises a frame sync word having a greater length than the line sync words, but this is not relevant to the invention.

A number of methods have already been proposed for extracting sync signals from an 20 aforementioned signal. One method is to recover the frame synchronization words and to derive the line sync signals- therefrom by counting. Another method is to detect a break in the alternation of words W1 and W2, which are replaced by the presence of the same word in the same position in two successive lines (or to find any other predetermined configuration other than alternation). Document WO 84/02242 gives a sample embodiment of this method. In such 25 cases, recovery of frame synchronization necessitates a preliminary phase of extracting the line sync signals (or, more generally, the heading signals of packets in the sense given hereinbefore, by detecting configurations identical with line sync words and by checking that the alternation recurs at regular intervals. This operation can be carried out in simple manner by identifying the line sync words, using shift registers and comparators and using a window generator to find 30 whether the alternation occurs at regular intervals. In that case a change of frame is character ised by the presence of a particular heading configuration over a given number of consecutive lines, i.e. consecutive packets.

There are two problems, apart from the problem of initial synchronization, in identifying the configuration characteristic of a change of frame. These problems are: the presence in the signal 35 of configurations which are similar to without being signals corresponding to a change of frame, and inevitable errors in detection of one of the words to be recognized. To solve the first problem, a search for configurations characteristic of a change of frame is made only during the windows at regular intervals. To solve the second problem, the conventional method is to provide a -flywheel- component circuit which, in the absence of recognition, preserves the 40 rhythm of the frame sync signals during a given number of frames.

The -flywheel- is necessary because the probability of recognising a change of frame by the conventional method is considerably lower than unity. If t is the proportion of errors at each binary element, and x is the number of binary elements in a line sync word, the probability of recognizing a line heading when the heading has really been transmitted is: 45 P= (1 -t)x The probability P of detecting a change of frame by detecting the characteristic configurations of recognition of a line heading over u successive lines, when a frame change has really 50 occurred, is then:

P=(1 _U The invention aims to provide a method and device which considerably increase the probability 55 of detecting a change of frame, to such an extent that a flywheel is often unnecessary.

To this end, the invention inter alia provides a method of frame synchronization extraction of a signal made up of successive packets each having a heading comprising one out of n binary words W 1, W2,... Wn. The packets are grouped in frames of constant duration or comprising a constant number of packets, a change of frame being indicated by a particular configuration of 60 the sequence of binary words which is distinct from any heading sequence configuration which can occur inside the frame, during a given number u of successive packets. Changes of frame are identified by detecting a given characteristic configuration of absences of recognition of binary words complementary to those indicating the change of frame. In the particular case where each packet has a heading comprising one out of 2 binary words W1 and W2 used 65 2 GB2196510A 2 alternately in the sequence of packets, which are grouped in frames of constant duration or comprising a constant number of packets, a change of frame being indicated by a particular configuration of the series of binary words, different from alternation and over a given number u of successive packets, changes of frame are identified by detecting a given characteristic confi guration of absences of recognition of the binary words complementary to the words indicating 5 the change of frame.

In the particular case of frame sync extraction of an X-MAC/Packet television signal, a change of frame is identified by detecting the configuration/RW2,/RW2,/RW1,/RW1, (or /RW, /RW1,/RW2,/RW2) where /R1 indicates the absence of recognition of the word W.

The invention makes use of the discovery that, when a word W is inserted into the heading, 10 the probability of recognizing this word (i.e. of obtaining RW) is much more affected by the presence of errors than the probability of not recognizing /W (i.e. of not obtaining /RW) when W comprises a number of bits, all the more so when the word is longer. This discovery, however, is of no help in ifself, since /RW1 may indicate either recognition of W2 or absence of recognition of W1. A change of frame can be identified only through the fact that each 15 characteristic configuration of successive W1 and W2 corresponds to one (and only one) configuration of /RW1 and /RW2. The resulting gain in the probability of detecting the change of frame will be immediately evident. The probability q of not recognizing W2 (or W1) when W1 (or W2) has been transmitted is:

20 q= 1 -tx The probability Q of detecting any change of frame by observing the characteristic sequences of absence of recognition of packet heading during u packets and when a change of frame actually occurs, will then be: 25 Q= (1 -t,),.

In practice, the resulting values of Q are so close to unity that a change of frame is reliably detected, thus eliminating the need for a -flywheel-. The remaining problem then changes in 30 nature, it is no longer connected with the risk of not recognizing a change of frame, but with the risk of identifying configurations as a change of frame when they are not really so.

This second problem is easily solved by using time windows for searching for a new frame change configuration after a first detection, and a process of confirmation in which a configura tion is not recognized as indicating a change of frame until recognition has been carried out 35 during a number of successive time windows, at intervals equal to the frame repeition period.

The invention will be more clearly understood from the following description of an embodi- ment, given by way of non-limitative example, and consisting of recognition of changes of frame in a television signal as per Standard D2-MAC/Packet. The description refers to the accompany ing drawings, in which: 40 Figure 1 is a diagram showing the make-up of a television frame according to Standard D2/MAC/Packet; Figure 2 is a chronogram showing the configurations for identifying the transition from an even frame to an odd frame, by the conventional method and according to the invention, and Figure 3 is a block diagram of a synchronization extraction device constituting an embodiment 45 of the invention.

Fig. 1 is a very simplified diagram showing the distribution of synchronization words in an even television frame as per Standard D2/MAC-Packet. A complete description can be found in the -sp6cification du syst6me D12-MAC/Paquet-, September 1985, T616Diffusion de France page 11, 12 and 24. Note however that the word---packet-is used here to denote the digital 50 part (sound and data) occupying part 10 of the time for broadcasting a line, whereas in the Standard it means a block of digital information which can extend over a number of lines.

Each line 1 to 622 of the broadcast frame comprises a line sync word W 'I or W2 and six binary elements (bits), a digital part 10, and a vision part 12. Lines 623 to 625 also comprise a line sync word W1 or W2 and line 625 contains a 64-bit frame sync word FM The line sync 55 W1=001011 and W2==1 10100 60 alternate in lines 1 to 621. On the other hand the following configurations occur in lines 622 to 625:

W2 W2 W1 W1 in the even frames and W1 W1 W2 W2 in the odd frames. 65 3 GB2196510A 3 A conventional method of recognizing the end of an even frame is to look for this configura- tion, i.e.

-To generate and store a signal RW in the case where the word W is recognized and -To look for the following configuration on four successive lines: 5 RW2-RW2-RW1-RW1 In Fig. 2, this configuration appears in the form of shaded areas on lines 622-625. A single error on a single bit of any of the line sync words 622 to 625 is sufficient to.prevent detection, 10 since RW1 (or RW2) changes to /RW1 (or /RW2).

When the invention is applied to this particular case, changes of frame can be identified by the presence of configurations on the following four lines:

/RW1, /RW1, /RW2, /RW2 for even frames (i.e. a sequence of two nonrecognitions of W1 15 followed by two non-recognitions of W2), and /RW2, /RW2, /RW1, /RW1 for odd frames.

The proportion of incorrect recognitions can be reduced by adopting longer configurations, e.g. 20 the characteristic configuration, over eight lines, of the transition from an even frame to an uneven frame can be:

/RW1, /RW2, /RW1, /RW3, /RW1,-/RW1, /RW2, /RW2 25 A-simple calculation shows that, in the event of a search for the characteristic configuration over n=4 lines, the probabilities of recognition are:

for t= 1 01 (S/N ratio- 17 dB), Q= 1-4.10-6 and for t= 10-4 (S/N ratio-26 dB), Q= 1-8.10-24. 30.

It can be seen immediately that, even for t 10- 1, the recognition probability Q is sufficiently near unity for a flywheel to be unnecessary.

This conclusion remains valid even if the configuration is searched for over u=8 lines. We have e.g. 35 for t=10-1, Q X 1-8. 10-6 The advantage of the invention over the natural method of detecting a change of frame is even more evident if the average times T1 and T2 between absences of detection by the 40 conventional method (recognition of configuration W2 W2 W1 W1) are compared with the method according to the invention, with a frame duration of 40 ms:

for tX 10-2 and u=4 - T1=0.187s and T2=317 years u=8 T1=0.104s and T2=158 years 45 t= 1 O4 and u=4 T1=1.67s and T2=3.17x 1014 years u=8 T1=0.85s and T2=1.58X 1014 years.

According to the invention, an increase in the number of lines during which a search is made for the configuration has a negligible adverse effect on the rate of detection of changes of 50 framp. It is therefore often advantageous to go beyond four lines in order to reduce the number of faulty identifications during the synchronization phase and to reduce the duration thereof.

The method according to the invention can be worked by using the device shown in Fig. 3.

The device illustrated comprises:

-A circuit 18 for detecting configurations revealing a transition from an even frame to an odd 55 - frame and vice versa, and -A circuit 20 for eliminating incorrect detections due to detection of interfering configurations smilar to those indicating a chage of frame.

The device also comprises means (not shown) for identifying the alternation of line sync words and for supplying a signal RW1 or RW2, as the case may be, when the word W1 or W2 60 is recognized. These means can comprise a shift register in which successively received bits circulate, two devices for comparing the sequence in the register with two stored sequences, and two generators which supply the signal RW1 or RW2 during the entire line in the event of a coincidence.

Circuit 18 comprises two shift registers 22, 24 which receive signals RW1 and RW2 on their 65 4 GB2196510A 4 respective series inputs. The clock input H of registers 22 and 24 receives the line sync pulses, which are assumed to be available. The line sync pulses can be obtained by any of the well known methods, e.g. by the method described in the previously-mentioned document WO 84/02242.

It will be assumed hereinafter that recognition of configurations showing a change of frame is 5 carried out over only four successive lines. In that case, four successive parallel outputs of registers 22 and 24, denoted by BO, B1, B2 and B3, are used to search for characteristic configurations of absences of recognition. A first OR gate 26, has inputs receiving the outiputs BO and B l of register 22 and the outputs B2 and B3 of register 24. The gate is used for recognizing ends of even frames. A similar OR gate 262 is for recognizing odd frames. A NAND 10 gate 28 receives the outputs of OR gates 26, and 262 and outputs a signal SC indicating that one of the characteristic configurations of a change of frame has been recognized.

Circuit 20 comprises a NAND gate 30 having an input which receives the signal SC and 1 another input which receives an enabling signal which does not allow a frame sync signal STB to be supplied until the time elapsed since the preceding synchronization signal is greater than a 15 value near the nominal duration of a frame or until the number of lines scanned since the preceding signal STB is at least equal to the number of lines in a frame. In the embodiment shown in Fig. 3, the enabling signal is supplied by a circuit comprising a counter 32 and a comparator 34. The signal STB is applied to the zero-resetting input of counter 32, whose incrementation input receives one pulse per scanned line. Comparator 34 compares the contents 20 of the counter with a predetermined value (e.g. 624 in the case of a 625- line frame) and supplies an enabling signal when the contents of the counter exceeds this number.

Other constructions are of course possible for circuit 20 when used for recovering the frame synchronization of a television signal. More particularly, a measurement of time can be used instead of a measurement of the number of lines. The circuit can also be directly adapted to the 25 general case of frames made up of packets each having a heading, when enabling is carried out after a given number of packets after the previous transmission of signal STB.

The device shown in Fig. 3 can also comprise a circuit for facilitating synchronization during starting-up. Often this circuit will be unnecessary, since permanent operating conditions are rapidly established in the case covered by the invention. If necessary, however, the first signals 30 51 B, which do not coincide with true moments of transition from one frame to the next, can be eliminated by using the following criterion: under established operating conditions, at least a predetermined number R of zero resettings of counter 32 have occurred each time at the end of the frame period T or after N packets.

In order to use this criterion, the synchronization circuit can comprise a store for the zero 35 resetting state, e.g. an R-position shift register operating at the rhythm of the STB signals and a combinatory logic unit supplying an enabling STB signal when the criterion is satisfied.

Claims

1. A method of frame synchronization extraction of a signal made up of successive packets 40 each having a heading comprising one out of n binary words W 1, W2_.. Wn, the packets being grouped in frames of constant duration or comprising a constant number of packets and a change of frame being indicated by a particular series configuration of binary words different from any heading series configuration capable of occurring inside the frame and over a given number u of successive packetsm, characterised in that changes of frame are identified by 45 detecting a given characteristic configuration of absences of recognition of binary words comple mentary to those indicating the change of frame.

2. A method frame synchronization extraction of a signal made up of successive packets each having a heading comprising one out of 2 binary words W1 and W2 used alternately in the sequence of packets, which are grouped in frames of constant duration or comprising a 50 constant number of packets, a change of frame being indicated by a particular configuration of the series of binary words, different from alternation and over a given number u of successive packets, characterised in that changes of frame are identified by detecting a given characteristic configuration of absences of recognition of the binary words complementary to the words indicating the change of frame. 55

3. A method according to claim 1 of frame synchronization of an X- MAC/Packet television signal in which each packet corresponds to a television scanning line and the change of frame is indicated by the sequence W1, W1, W2, W2 (or W2, W2, W1, W1), W1 and W2 being complementary, characterised in that a change of frame is identified by detecting the configura tion/RW2JRW2JRW1JRW1, (or /RW1,/RW1,/RW2,/RW2) where /R1 indicates the absence 60 of recognition of the word W.

4. A method according to claim 1 or 2, characterised in that the change of frame is identified over a number of successive packets greater than the number of binary words in the minimum characteristic configuration.

5. A device for frame synchronization extraction of a signal made up of successive packets 65 GB2196510A 5 each having a heading comprising one out of n binary words W 1, W2,... Wn, the packets being grouped in frames of constant duration or comprising a constant number of packets and a change of frame being indicated by a particular series configuration of binary words different from any heading series configuration capable of occurring inside the frame and over a given number u of successive packets, characterised in that it comprises means, each of which is 5 adapted to identify a given characteristic configuration of absences of recognition of binary words complementary to the words indicating the change of frame, second means supplying an output signal of any of the given characteristic configurations, and third means which enable the signal supplied by the second means when the time which has elapsed since the preceding enabled synchronization signal is greater than a value near the nominal duration of a frame. 10

6. A device according to claim 4, characterised in that the third means comprise a counter incremented at the line rhythm and a comparator adapted to supply an enabling signal when the contents of the counter reaches a given value.

7. A method substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. 15

8. A device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC I R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.