fig1 a shows a disc - shaped information carrier 1 having a track 9 intended for recording , which track is arranged in a helical pattern of windings 3 . the windings may also be arranged concentrically instead of helically . the track 9 on the information carrier is indicated by a servo pattern such as , for example , a pregroove 4 which enables a read / write head to follow the track 9 during scanning . a servo pattern may also be formed , for example , by regularly spread sub - patterns which , in the servo tracking system , periodically cause tracking signals to occur . fig1 b shows a cross - section along a line b -- b of the information carrier 1 , in which a transparent substrate 5 is covered by a recording layer 6 and a protective layer 7 . the pregroove 4 may also be arranged as a land or other material property that differs from its environment . furthermore , the servo pattern may also be formed by alternately raised and depressed windings , forming so - called land and groove patterns , with a transition from land to groove or vice versa in each winding . the recording layer 6 can be written optically , magneto - optically or magnetically by an apparatus for reading and / or writing information such as the known cd recordable or hard disk for computer use . the fig1 c and 1d show two examples of a periodic modulation ( wobble ) of the pregroove . this wobble causes an additional signal to arise in a servo tracking detector . the wobble is , for example , frequency modulated and in the modulation position - information is coded , such as an address or a time code . a description of an inscribable cd system comprising position - information obtained in such manner can be found in u . s . pat . no . 4 , 901 , 300 ( phn 12398 ) and u . s . pat . no . 5 , 187 , 699 ( phq 88002 ) both assigned to philips . another example of position - information modulation are header symbols included in the servo pattern , which header symbols are indicative of an address and indicate the beginning of the area for recording an information block . fig2 shows a land / groove pattern in which a wobble 21 is introduced on one side in which , for example , from the inside out ( from the bottom up in fig2 ) a land / groove ( l / g ) transition is straight and a groove / land ( g / l ) transition shows a wobble . the same wobble signal is then developed in land and groove , but since it is also known whether a land or a groove is scanned , the winding can be determined unambiguously . on the information carrier according to the invention , windings 3 in the area intended for recording contain winding information which indicates , for example , the radial position of the winding such as , for example , a winding number and / or sector addresses . a winding number , for example , counts up from 0 from the innermost winding outward . the winding information may also denote a distance to the center of rotation . for decoding the winding information , it is advantageous for the modulation of the pregroove to have a pattern such that a constant frequency evolves during reading , irrespective of the radial position . with a simple filter it is possible to separate well a modulated signal having a constant frequency from noise signals . for this purpose , the period of the space modulation preferably increases proportionally with the distance to the center of rotation 10 . as a result , a constant angular velocity of the information carrier produces to a constant modulation frequency . alternatively , another pattern of the space modulation period and a linked pattern of the angular velocity may be selected . for verifying the exact winding after a jump , it is advantageous for the winding information to be repeated several times per winding in winding information frames . such winding information frames are to be decodable independently . this also provides a good protection against local damage . it is advantageous to number the winding information frames sequentially with segment numbers , so that always an indication is available of the angular position relative to the reference line 8 , where the winding numbers increase for a helical track , or where the transitions take place at a land / groove . fig3 a - 3c diagrammatically show the modulation of a servo pattern according to the invention . this modulation may be realized as pregroove wobble on the information carrier as described in fig1 . the modulation 30 , 31 is shown in two adjacent windings n and n + 1 ( or winding pairs in the case of l / g shown in fig2 ). the modulation has a wobble 32 with a relatively long period and has synchronization elements 35 , 36 , 37 , 38 , 39 , 40 with a relatively short period . the amplitude of the synchronization elements is also larger than the amplitude of the wobble 32 . this is advantageous in that the synchronization elements are easy to detect . the synchronization elements may also be modulated in a different manner , such as , for example , a phase jump in the wobble . the synchronization elements may be used for synchronizing the writing and / or reading , such as controlling the rotation or scanning velocity of the information carrier , inserting the information blocks in the track at a precisely determined position on the information carrier , or reading the position - information from the wobble of the servo pattern . the combination of the wobble and the synchronization elements permits information to be recorded in blocks in a simple manner , even if information has already been recorded in track where the new information is to join . fig3 a shows an embodiment of the invention in which the synchronization elements in adjacent tracks always have an alternating polarity . the synchronization elements 35 in the winding n continue to have the same , for example , positive polarity , whereas the synchronization elements 36 in the adjacent winding n + 1 always have a different , for example , negative polarity . obviously , this pattern is repeated , so that track n - 1 is again equal to track n + 1 . the synchronization elements 37 in the winding n in fig3 b have an ever alternating polarity , whereas the corresponding synchronization elements 38 in the adjacent winding n + 1 always have the different , opposite polarity . in the embodiment shown in fig3 a and 3b , the synchronization elements in adjacent windings are always aligned . fig3 c shows a fixed shift between adjacent windings over half the distance between synchronization elements in lieu of aligned synchronization elements . in that case , the detection of the winding jump can be derived from the instant at which a synchronization element is read . this is advantageous in that an error is established more rapidly for an equal number of synchronization elements ( as a result of the lack of or early appearance of a synchronization element ). if , for example , one third of the distance is considered a shift , it is also possible to detect an undesired jump over two windings . obviously , further patterns of shifts are possible , possibly combined with the use of various types of synchronization elements . alternatively , a different number of synchronization elements ( and thus a different distance between them ) may be used per winding . in lieu of the different polarities , it is also possible to use various types of synchronization elements which comprise , for example , two short and two long pulses in different sequences . with the above different synchronization elements , distances and shifts it is possible to form a recurrent pattern of distinguishable differences over the number of tracks , by which also an undesired displacement over various tracks and / or the direction of the displacement can be detected . while such an information carrier according to the invention is being written , it is easy to detect whether a read / write head is positioned over the correct track or over an adjacent track . since always the instant and the polarity of the next synchronization element is known , an undesired jump to an adjacent winding can be detected when an erroneous synchronization element is found . the writing operation can then be interrupted immediately so as to damage the least possible data in an adjacent winding by overwriting . a next synchronization element may be waited for , and the writing will not be interrupted until after a recurrent error . it is also possible to combine the detected information with information from the servo system in which system often track - loss information can also be derived . combining servo track - loss information with synchronization element detection information enhances the reliability of the detection of undesired displacements of the read / write head . the synchronization elements are desirably recorded in the servo pattern at regular intervals . this provides a relatively fast detection of an undesired displacement of the read / write head . preferably , the mutual distance of synchronization elements in adjacent tracks is smaller than the length of an information block , so that at most this distance can be overwritten erroneously . in a reading / writing system in which the information is protected by an error correction code , the maximum length of the track that can be written erroneously is desirably selected smaller than the maximum burst error to be corrected . the distance between the synchronization elements and the number of erroneous detections thereof necessary for interrupting the writing operation can be adjusted to this . it is observed that the pattern of synchronization elements per winding of a helical track may differ . at a certain angular position , there will thus be an exchange of patterns as denoted along the imaginary line 8 in fig1 a . fig4 shows a device for writing and reading information blocks on a disc - shaped information carrier which can be written magneto - optically or optically ( via phase change ). the device comprises drive means 45 for causing the information carrier to rotate and a read / write head 42 for scanning the track indicated on the information carrier by the servo pattern . the read / write head 42 is positioned on the track in radial direction by servo control unit 44 . the read / write head scans the track with known tracking and focusing methods and a modulation caused , for example , by the wobble in the pregroove is present in the servo signals . the servo signal is demodulated and the position - information coded therein is recovered in the servo control unit 44 and passed on to the system control 46 . the radial position of the read / write head may be checked via the recovered position - information . the information applied to the input of the coding unit 41 is divided into blocks , as required , and converted into a writing signal for the read / write head 42 . the coding unit 41 comprise , for example , an error coder and channel coder . during the reading operation , the signal from the read / write head 42 is reconverted into the information in the decoding unit 43 comprising , for example , a channel decoder and an error corrector . the device comprises synchronization a unit 47 for synchronizing the writing and reading operations . the timing of the writing is then customarily controlled in dependence on the recovered synchronization elements . the system control 46 controls the synchronization unit 47 and , in addition , the servo control 44 , the coding unit 41 , the decoding unit 43 and the drive means 45 and is arranged for executing the procedures to be described hereinafter . when an information carrier 1 has been inserted into the device and a writing instruction has been received , the system control 46 positions the read / write head 42 over the desired winding and the writing is started at the correct instant , for example , synchronized with a first synchronization element . the pattern of the synchronization elements in the track to be written is known , however . the system control is arranged according to the invention for detecting a second synchronization element of a certain type and / or a synchronization element at a certain moment , and subsequently , further synchronization elements . however , if a synchronization element or a wrong type of synchronization element is detected at another instant that means that the read / write head 42 has wandered off to an adjacent winding as a result of a shock or a defect , and the system control will interrupt the writing operation . an example of a pattern of aligned synchronization elements in adjacent windings is shown in fig3 b . the polarity of the successive synchronization elements in the track to be written then alternates . if the system control detects the same polarity twice in succession , that signifies that the read / write head has wandered off to an adjacent winding . fig3 c shows a pattern of shifted synchronization elements . if the system control 46 detects a synchronization element too early , or does not find a synchronization element at the expected instant , that means that the read / write head will have wandered off to an adjacent track . when , for example , 4 different types of synchronization elements are used in a pattern which recurs every 4 windings , the system control will detect the error even with an undesired displacement over a maximum of 3 windings , because it detects an unexpected type . in an embodiment of the system control the writing will not be stopped until the second or further unexpectedly detected synchronization elements has occurred . this is advantageous in that a single synchronization element erroneously detected as faulty does not immediately lead to an interruption of the writing process . a further embodiment of the system control also evaluates the servo signals and derives therefrom information about track loss in known manner . this information may be combined with the unexpectedly detected synchronization elements . a decision to interrupt may then be made both in the case of a track loss indication and an erroneous synchronization element . this is advantageous in that the reliability of the decision is enhanced , because the two indications are formed independently . fig5 shows a reading device according to the invention by which an information block can be similarly read . the reading device comprises drive means 45 for causing the information carrier 1 to rotate and a read head 52 for scanning the track on the information carrier . the read head 52 is positioned in radial direction on the track by servo control unit 44 . furthermore , the device comprises a system control 46 . during the reading operation , the signal from the read head 52 is converted into the information in the decoder unit 43 which comprises , for example , a channel decoder and an error corrector . in the embodiment according to the invention , the system control 46 detects the synchronization elements during the reading operation . in the event of a deviation , the reading may be interrupted . a control signal may be sent to the servo control unit 44 , as required , so that a jump is made back in the direction of the desired winding , provided that this direction can be derived from the detected synchronization element ( the pattern is different over more than 2 adjacent windings ) and / or from the variation of the servo signals . furthermore , the system control can , after an interruption , directly carry out a repetition and cause the desired track to be read out once again .