Circuit for detecting a synchronizing signal in frame synchronized data transmission

Abstract: A circuit for detecting a frame synchronizing signal and a circuit for detecting subframe synchronizing signals in the frame synchronization data transmission are presented. The former circuit generates a first signal which is at the logic 1 exclusively during the period that the frame synchronizing signal is received, and produces the logical product of the first signal and the data signal, the logical product corresponding to any error present in the frame synchronizing signal. In the latter circuit, a window pulse signal which corresponds to the bits of the subframes assigned to the subframe synchronizing signal is first produced, the bits of the data signal which are synchronous to the window pulse signal are next extracted, and an error bit is selected by excluding corrects bits from the extracted bits of the data signal. In this way an error bit in the synchronizing signal in the frame synchronization data transmission is detected.

Description: The present invention relates to a frame synchronization data transmission system. A circuit for detecting an error in a synchronizing signal in a frame synchronization data transmission system will be described below by way of example in illustration of the invention. In a particular frame synchronization data transmission system, the frame is made up of a plurality of subframes, and a frame synchronizing signal, which represents the start of the frame of the data signal, is assigned to the starting or first subframe with a code of a specified pattern. In order to punctuate, if required, the remaining part of the frame, a subframe synchronizing signal of a predetermined logic level is assigned to a specified bit, say the kth bit, of each subframe. The kth bit is normally the first bit. There will now be described previously proposed frame formats and circuits with reference to Figs. 1 to 4 of the accompanying drawings in which:- Fig. 1 shows a previously proposed frame format of a data signal, an enable signal and a clock pulse signal for use in a frame synchronization data transmission system, Fig. 2 shows another frame format, in which subframe synchronizing signals are assigned, Fig. 3 shows a data transmission system including a circuit for detecting a frame synchronizing signal, and Fig. 4 shows a block diagram of a typical previously proposed circuit for detecting a frame synchronizing signal. Referring to Fig. 1 there is shown the frame format of a data signal S3, an enable signal S1 and a clock pulse signal S2 for use in a frame synchronization data transmission system. In the figure, one frame is made up of a plurality of subframes, each subframe having 8 bits, each bit being transmitted in synchronization with a clock pulse signal S2. Hereafter, each subframe with 8 bits is referred to as an octet or an OCT. The enable signal S1 makes logic 1 only when a datum is present. A code made up of octadic Os is assigned to the starting octet as the frame synchronizing signal. There is no subframe or octet synchronizing signal in the frame format shown in Fig. 1. Fig. 2 shows another frame format, in which there is an octet synchronizing signal. The synchronizing signal is assigned to the first bit of each octet and designated with the logic 1. Hereafter, the frame synchronizing signal and the subframe synchronizing signal are referred to as the FRM SYN and the SBFRM SYN, respectively, and the two SYNs are referred to in general as the SYN. Fig. 3 shows a data transmission system, in which a circuit for detecting an FRM SYN is used. Hereafter the circuit for detecting an FRM SYN is referred to as the FRM SYN detecting circuit. At the transmitting end, a digital data signal sent from a transmitting terminal is converted to an analog data signal by means of digital-analog converter 91. The analog data signal is then transmitted through a transmitting antenna 92, received by a receiving antenna 93 and supplied to an analog-digital converter 94 to be converted to a digital data signal. An FRM SYN detecting circuit 95 detects an FRM SYN from the digital data signal in order to distinguish whether the digital data signal is a frame of the transmitted data signal, or a data signal which involves any other information, and then sends the digital data signal to a receiving terminal. In an actual case the FRM SYN detecting circuit is provided at the transmitting end as well, in order to enable a check to be made to see if there has been any possible change in the data signal caused by noise. Fig. 4 shows a block diagram of a typical previously proposed SYN detecting circuit. The circuit is composed of a serial-parallel converter 10, a memory 12 and a CPU 13. The serial-parallel converter 10, which is enabled by an enable signal S1, receives a serial data signal S3, and converts the serial data signal S3 to a parallel data signal S11 in synchronization with a clock pulse signal S2. A parallel data signal S11 is first stored in the memory 12. After all of the data signals transmitted by the data transmission system concerned have been stored, the CPU 13 detects the SYN by means of software. A problem encountered in the SYN detecting circuit described above is that, since all the serial-to-parallel-converted data have to be stored once in the memory in order to check the bits assigned to the SYN, it takes a long time to detect them. In the specification of German Offenlegungschrift No. 37 22 567 which was published on January 19 1989, there was proposed a frame synchronization circuit employing an adjustable frame pulse counter having a synchronization word recognition circuit and a counting system for evaluating synchronization and loss of synchronization criteria, in which the counting system was preceded by a logic circuit to which a frame pulse from the pulse frame counter and a synchronization pulse from the synchronization word recognition circuit were applied, and the logic circuit delivered one synchronization criterion if the two pulses agreed and another synchronization criterion if the two pulses did not agree, the counting system being a bi-directional counter which added the one criterion and subtracted the other respectively, and the frame pulse counter being synchronised at a maximum value on the basis of a received synchronization word. Features of embodiments to be described below by way of example in illustration of the present invention are that an FRM SYN detecting circuit is described which is capable of effecting detection in real time and which is capable of detecting any error in the frame format in which all bits of the synchronizing signal are designated with codes of an identical logic level. In a particular embodiment to be described an FRM SYN detecting circuit includes first means for providing a first signal which takes logic 1 exclusively during the period in which the frame synchronizing signal is received, and second means for extracting any error bit present in the frame synchronizing signal, the second means having first gate means which receives both the first signal and the data signal transmitted through the frame synchronization data transmission system and produces a second signal corresponding to the logic product of the two signals. The second signal is the error bit. The following description and Figs. 5 to 7 of the accompanying drawings disclose, by means of examples, the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby. In Figs. 5 to 7 of the drawings:- Fig. 5 shows a block diagram of a first embodiment, Fig. 6 represents a block diagram of error detecting circuit 4 shown in Fig. 5, and Fig. 7 represents a timing chart to illustrate the operations of parts of the circuits shown in Figs. 5 and 6. Fig. 5 shows a block diagram of a first embodiment illustrative of the present invention, and Fig. 6 represents a block diagram of error detecting circuit 4 shown in Fig. 5. These figures illustrate a circuit for detecting a frame synchronizing signal or an FRM SYN detecting circuit. Fig. 7 represents a timing chart to illustrate the operations of parts of the circuits shown in Figs. 5 and 6. In this embodiment, the format of the frame is identical with that illustrated in Fig. 1, the FRM SYN being assigned to the starting octet with each bit being designated by logic 0. In Fig. 7, an FRM SYN including an error of a logic 1 in the third bit is shown. The FRM SYN detecting circuit includes a first-signal generating circuit 1 for providing a first signal S5 and an error detecting circuit 4. The first-signal generating circuit 1 is made up of an 8 bit shift register circuit 2 and an AND gate 3. The shift register circuit 2 receives an enable signal S1, and shifts it by 8 bits in synchronization with a clock pulse signal S2, thereby delaying the enable signal S1 by 8 clock-pulse periods. The AND gate 3 produces a logical product of the enable signal S1 and an inverted delayed enable signal S4 and delivers the logical product as a first signal S5. Thus, the first signal S5 takes the value of a logic 1 exclusively while the FRM SYN is received (cf. signal S5 in Fig. 7). The error detecting circuit 4 is arranged to extract any error present in the FRM SYN. The error detecting circuit 4 has an AND gate 5 and a register circuit 6. The AND gate 5 receives both the first signal S5 and a data signal S3 and delivers the logical product as a second signal S6, thereby extracting the error bit in the FRM SYN (cf. signal S6 in Fig. 7). The register circuit 6 has AND gates 7, 8 and a D flip-flop 9. The AND gate 7 receives both the clock pulse signal S2 and the first signal S5 and delivers a clock pulse train S7, which is active only while the FRM SYN is received (cf. signal S7 in Fig. 7). The AND gate 8 receives both the second signal S6 and the clock pulse train S7 and delivers a clock pulse S8 which is synchronous with the two signals S6 and S7. The D flip-flop, which is enabled by the enable signal S1, receives the second signal S6 at its data input and the clock pulse S8 at its clock input, latches the second signal S6 at the trailing edge of the pulse S8 and provides an alarm signal S9. In this way the FRM SYN detecting circuit is able to detect any error present in the FRM SYN in real time. It will be understood that, although particular embodiments have been described by way of example in illustration of the invention, variations and modifications thereof, as well as other embodiments may be made within the scope of the protection sought by the appended claims.

Claims: A circuit for detecting a frame synchronizing signal in a frame synchronization data transmission system, characterised in that all bits of the frame synchronizing signal are designated with codes of an identical logic level, and in that the circuit includes first means (1) for providing a first signal (S5) which has the value of logic 1 exclusively during the period in which the frame synchronizing signal is received, and second means (4) for extracting any error bit present in the frame synchronizing signal, the second means (4) having first gate means (5) which receives both the first signal (S5) and the data signal (S3) transmitted through the frame synchronization data transmission system and produces a second signal (S6) corresponding to the logic product of the two signals, the second signal (S6) being the error bit. A circuit as claimed in claim 1, wherein the first means (1) includes delaying means (2) and second gate means (3), the delaying means (2) delaying an enable signal (S1) received through the frame synchronization data transmission system by a transmission period of the frame synchronizing signal and delivering the delayed enable signal (S4), and the second gate means (3) providing the logical product of the enable signal (S1) and an inverted signal of the delayed enable signal (S4) and delivering the logical product as the first signal (S5). A circuit as claimed in claim 2, wherein the delaying means (2) includes a shift register (2) with the same number of bits as that of the frame synchronizing signal, the shift register (2) receiving a clock pulse signal (S2) and the enable signal (S1) transmitted through the frame synchronization data transmission system, shifting the enable signal (S1) in synchronization with the clock pulse signal (S2) and delivering the serial output as the delayed enable signal (S4). A circuit as claimed in claim 2, wherein the second means (4) includes a register (6) for registering the second signal (S6) . A circuit as claimed in claim 4, wherein the register (6) has third gate means (7,8) and a D flip-flop (9), the third gate means (7,8) receiving both the second signal (S6) and the clock pulse signal (S2) transmitted through the frame synchronization data transmission system to select from the clock pulse signal (S2) a clock pulse (S7) which is synchronous with the second signal (S6), and the D flip-flop (9) latching the second signal (S6) in synchronization with the output of the third gate means (7,8).

Inventors: UOMOTO YASUTOMO, UOMOTO, YASUTOMO, Uomoto, Yasutomo, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H04J3
IPC codes: H04J3