Abstract:
Apparatus, and an accompanying method, for transmitting a frame synchronization signal and a data signal simultaneously through a serial transmission medium ( 170 ). Specifically within a data transmitter ( 105 ), a frame synchronization signal, a clock signal and a data signal, are encoded to form a single bi-phase mark signal having the frame synchronization signal incorporated into the bi-phase mark signal as a phase-shift. The bi-phase mark signal is then transmitted through a suitable serial transmission medium. A receiver ( 175 ), connected to the transmission medium, receives and amplifies an incoming bi-phase mark signal appearing on the medium, and, in turn, synthesizes the clock, frame synchronization, and data signals from this bi-phase mark signal.

Description:
BACKGROUND OF THE DISCLOSURE  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to data communication systems, and, more particularly, to methods and apparatus for synchronizing transmissions of serial data signals transmitted via such systems.  
           [0003]    2. Description of the Prior Art  
           [0004]    Currently, high speed serial line communications is experiencing increasing use across many application areas, not the least of which being telecommunications. Generally, serial line communication systems transmit digital data, whether digitized voice in a telephone system or data from a computer, as a serial bit stream over various transmission media, such as wire cables, radio waves, fiber optic cables and the like. Typically, to efficiently transfer large quantities of data, individual bit streams from multiple data sources are multiplexed to form a single, serial bit stream. After transmission along a single transmission medium, the multiplexed bit stream is demultiplexed to reproduce the individual bit streams.  
           [0005]    To facilitate proper routing of the individual bit streams to their respective destinations, the multiplexed serial bit stream must be organized so that a demultiplexer can identify and separate the bits, within the multiplexed bit stream, that are associated with each individual bit stream. Proper routing is accomplished by organizing the multiplexed bit stream into sequential frames each containing a sequence of time-slots. In practice, at a transmitter end of a serial line communication system, a multiplexer inserts bits from each individual bit stream into a corresponding time-slot within each frame. The locations of the bits within a frame comprising any one individual bit stream are known by counting bit positions, or time-slots, relative to the beginning, or end, of each frame. For example, in a 30 time-slot frame carrying bits from three data sources, bits from the first, second and third sources may be separately carried in the first, second and third successive ten time-slots relative to the beginning of the frame. Thus, specific time-slots, illustratively ten such slots, in each frame correspond to each data source. In this manner, each individual bit stream can be extracted from the frames and reassembled into individual bit streams associated with each particular data source.  
           [0006]    Consequently, through proliferation of such serial line communications systems utilizing frame formatted data, a need has existed in the art to accurately determine a beginning, or an end, of each frame. Typically, a specific sequence of bits known as a “framing sequence” marks the beginning (or end) of each frame. To maintain frame synchronization, a data receiver, which may be a component of a demultiplexer, monitors a received bit stream for the framing sequence. Subsequently, the data receiver produces a frame synchronization signal upon each occurrence of the framing sequence. For example, in a 30-channel pulse code modulation (PCM) system commonly used for telephone transmissions in Europe and Asia, a frame comprises 32 time-slots with each slot containing 8 bits of data. Of these, 31 time-slots contain information bits from individual bit streams with the remaining time-slot, specifically the first in the frame, containing an 8-bit framing sequence. The eight bits in the framing sequence are typically:  
           [0007]    X0011011, for even frames, and  
           [0008]    X1AYYYYYY, for odd frames,  
           [0009]    where the bits labeled X and Y are usually set to 1. The bit labeled A can be used as an alarm bit that is set to 1 whenever frame synchronization is lost.  
           [0010]    Detrimentally, due to errors which typically arise from noise in the transmission medium, the information bits within a frame can be corrupted such that these bits identically resemble the framing sequence. In this instance, the data receiver would erroneously synchronize to the information bits rather than the framing sequence thereby causing a frame synchronization error and a resulting loss of data. To avoid such errors, oftentimes a second signal, a frame synchronization signal, is transmitted via a separate and independent transmission medium to the data receiver. The frame synchronization signal, in combination with the framing sequence, indicates the beginning (or end) of the frame. This arrangement avoids any ambiguities that may arise between information bits and the framing sequence. However, due to the additional transmission medium and associated receiver circuitry, such a dual transmission arrangement is costly and complex.  
           [0011]    Thus, a need currently exists in the art for a technique, specifically apparatus and an accompanying method, for providing accurate frame synchronization in a serial line communication system. Advantageously, this technique should be immune to frame synchronization errors caused by certain data sequences in a transmitted frame. Furthermore, this technique should carry both a frame synchronization signal and a data bit stream, i.e., the information and framing sequence bits over a single serial transmission medium.  
         SUMMARY OF THE INVENTION  
         [0012]    Accordingly, an object of the present invention is to provide a technique that can receive frame synchronization signals and data over a single serial transmission medium.  
           [0013]    A specific object is to provide such a technique that is substantially immune to frame synchronization errors caused by information bit sequences which are identical to the framing sequences.  
           [0014]    These and other objects are advantageously achieved through my inventive serial line synchronization technique. Specifically, in accordance with my inventive teachings, a frame synchronization signal, a clock signal and a data signal are encoded to form a single bi-phase mark signal wherein the frame synchronization signal is incorporated into the bi-phase mark signal as a phase-shift. The bi-phase mark signal is then transmitted through a transmission medium. A receiver, connected to the transmission medium, receives and amplifies the bi-phase mark signal. Subsequently, the receiver decodes the amplified bi-phase mark signal and reproduces the clock, frame synchronization and data signals.  
           [0015]    By advantageously incorporating a frame synchronization signal into the bi-phase mark signal, my inventive serial line synchronization technique does not require a second transmission medium and associated receiver circuitry to transmit a frame synchronization signal separate from the data signal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The teachings of the present invention can be readily understood in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is an amplified high level block diagram of data transmission system  100  which incorporates my inventive serial line synchronization technique;  
         [0018]    [0018]FIG. 2 is a schematic diagram of my inventive encoder  150 ;  
         [0019]    [0019]FIG. 3 is a timing diagram for the encoder depicted in FIG. 2;  
         [0020]    [0020]FIG. 4 shows the proper alignment of FIGS. 4A and 4B; and  
         [0021]    [0021]FIGS. 4A and 4B collectively show a schematic diagram of my inventive decoder  225 . 
     
    
       [0022]    To facilitate understanding, identical reference numerals have been used, where possible, to designate elements that are common to the figures.  
       DETAILED DESCRIPTION  
       [0023]    Though the ensuing disclosure of my inventive technique illustratively discusses my invention in the context of a telecommunications system, those skilled in the art will recognize that the invention is useful in any serial line communication system that uses frame formatted data. Such systems include integrated services digital networks (ISDN), local area networks (LAN), packet radio systems and the like.  
         [0024]    [0024]FIG. 1 depicts a simplified, high level block diagram of data transmission system  100  incorporating my inventive serial line synchronization technique. System  100  comprises serial line transmitter  105  connected, via optical fiber  170 , to serial line receiver  175 . Line  110  carries previously multiplexed and framed data (DATA) to an input of transmitter  105 . Additionally, line  115  carries a frame synchronization signal (SYNC) and line  120  carries a clock signal (CLOCK), both to respective inputs of transmitter  105 . Transmitter  105  merges the DATA, SYNC and CLOCK signals into a single serial bit stream using my inventive bi-phase mark modulation protocol. Subsequently, the serial bit stream is transmitted, via optical fiber  170 , to receiver  175 . A fiber optic transmission medium is, of course, illustrative. In practice, the transmission medium can also be, e.g., a twisted wire pair, a coaxial cable, a microwave transmission channel and the like. Upon reception of the signal propagating on the transmission medium, receiver  175  separates the CLOCK, SYNC and DATA signals and produces each of these signals on lines  230 ,  235  and  240 , respectively. Ultimately, a demultiplexer (not shown) utilizes the individual signals to demultiplex the various individual bit streams contained in the DATA signal appearing on lead  235 .  
         [0025]    Generally speaking, the CLOCK, SYNC and DATA signals are related to one another. In this regard, the CLOCK signal is phase synchronous with bits comprising the DATA signal; the SYNC signal indicates a reference point within a frame of data bits. Illustratively, the reference point is an end of a last time-slot in the frame. My invention uses a modulation technique known as a bi-phase mark protocol to combine the CLOCK and DATA signals into a serial bit stream. Simultaneously, the invention encodes the SYNC signal into this serial bit stream as a violation of the bi-phase mark protocol.  
         [0026]    More specifically, transmitter  105  comprises delay line  125 , encoder  150  and light emitting diode (LED) driver  165 . Delay line  125  receives, on line  120 , the CLOCK signal and produces four delayed clock signals (CLK 1 , CLK 2 , CLK 3 , and CLK 4 ) each having a different delay relative to the CLOCK signal. For instance, the delayed clock signal on line  130  is delayed by approximately ¼ of a data bit relative to the input CLOCK signal. Similarly, the delayed clock signals on lines  135 ,  140  and  145  are delayed ½ bit, ¾ bit and 1 bit, respectively. Typically, a data bit has a duration (or interval) equivalent to one clock cycle. For example, in a system having a bit duration of approximately 40 nS, i.e., a bit rate of 24.576 Mb/s, the delay line provides delays in 10 nS increments.  
         [0027]    Encoder  150  has DATA, SYNC and the four delayed clock signals appearing on lines  130 ,  135 ,  140 , and  145  as inputs. Generally, the encoder merges the CLOCK, DATA and SYNC signals into a single serial bit stream. This is accomplished by phase synchronizing the CLOCK and DATA signals and then merging these two signals into a bit stream using a bi-phase mark protocol (as described in detail below). Suffice it to say, the bi-phase mark protocol permits the frame synchronization signal to be inserted into the bit stream as a phase-shift.  
         [0028]    Encoder  150  produces, on line  160 , a bi-phase mark signal which serves as an input to LED driver  165 . The LED driver converts an electrical signal representing the bi-phase mark signal into an optical signal. LED drivers are well-known in the art, and therefore, a detailed description of such a driver is unnecessary to understand my invention. The optical signal produced by LED driver  165  is then propagated over optical fiber  170  to receiver  175 .  
         [0029]    Receiver  175 , in general, reverses the encoding process of the transmitter and produces, on lines  230 ,  235 , and  240 , the CLOCK, DATA, and SYNC signals, respectively. Specifically, receiver  175  comprises electro-optic (E-O) device  180 , amplifier  190 , delay line  200  and decoder  225 . E-O devices are well-known in the art, and therefore, a detailed description of such a device is also unnecessary to understand my invention. E-O device  180  accepts, as an input, the optical signal emanating from optical fiber  170 . As is well-known in the art, an E-O device converts the optical signal into a representative electrical signal. Typically, the representative electrical signal, on line  185 , has relatively low signal strength and thus must be amplified. Amplifier  190  amplifies and “squares up” the electrical signal and produces a bi-phase mark signal that is substantially identical to the bi-phase mark signal that was produced, on line  160 , by encoder  150 . Circuitry for accomplishing the amplification and squaring functions is well-known in the art and thus need not be described in any further detail.  
         [0030]    Delay line  200  receives the bi-phase mark signal, appearing on line  195 , as an input and, as with the CLOCK signal above, generates four delayed output signals (D 1 , D 2 , D 3 , and D 4 ). Each delayed signal on lines  205 ,  210 ,  215 , and  220  is respectively delayed by a ¼, ½, ¾, and 1 bit interval, with respect to the bi-phase mark signal on line  195 . Using the four delayed bi-phase mark signals, decoder  225  separates the CLOCK, SYNC and DATA signals and produces each signal on lines  230 ,  235  and  240 , respectively.  
         [0031]    Next, a detailed discussion of the bi-phase mark protocol is presented. This discussion is followed by a detailed description of encoder  150  and decoder  175  that are used to implement my inventive synchronization technique.  
         [0032]    Generally, encoder  150  combines the DATA, SYNC and CLOCK signals into a serial bit stream. My inventive technique merges the data and clock signals into a serial bit stream using a bi-phase mark protocol. The frame synchronization signal is introduced into the bit stream as a violation of that protocol. Generally, the bi-phase mark protocol encodes a digital stream of ones and zeros as a series of transitions during each bit interval, i.e., a clock cycle. Specifically, each zero is encoded as a transition at the beginning of a bit interval and each one is encoded as a transition at the beginning and at the middle of each bit interval. In this manner, the clock signal is inherently encoded into the bi-phase mark bit stream, i.e., as a transition at the beginning of each bit interval.  
         [0033]    The frame synchronization signal is inserted by violating the requirement for a transition at the beginning of each bit interval. As discussed above, the data signal, after being formatted into frames, arrives at the transmitter. Thus, the framing sequence is already present in a time-slot within each frame when the frame arrives at the transmitter. The frame synchronization signal occurs, i.e., is high, during the occurrence of the framing bits in the data stream. For example, an individual frame may contain 32 time-slots each containing 8 bits of data. Illustratively, time-slots  0  through  30  carry information bits from a number of individual data sources and time-slot  31  carries the framing sequence to indicate the end of a frame. Hence, during all or a portion of time-slot  31 , the SYNC signal is high. Specifically, the frame synchronization signal should be high for at least one clock cycle in a bit position just prior to the position in the framing sequence where the protocol violation is to be inserted. Thus, continuing with the example, if the framing sequence in time-slot  31  is “00001011” and the protocol violation is to be positioned between the two consecutive ones, the frame synchronization signal should occur simultaneous with the second one, i.e., the second least significant bit. Accordingly, encoder  150  inserts a protocol violation between the last two bits, i.e., the framing sequence becomes “0000101v1”, wherein the violation (v) is a 180° phase-shift which removes a transition from the beginning of the last bit interval in time-slot  31 .  
         [0034]    Encoder  150  will now be discussed with simultaneous reference to FIGS. 2 and 3. Therefore, the reader should refer to both of these figures throughout the ensuing discussion. FIG. 2 depicts the circuit details of encoder  150 , while FIG. 3 depicts a timing diagram showing the interrelation of important signals used and produced by encoder  150 .  
         [0035]    As shown in FIG. 2, encoder  150  comprises timing generator  250  and bi-phase mark modulator  251 . Using the four delayed clock signals (CLK 1 , CLK 2 , CLK 3 , and CLK 4 ) on lines  130 ,  135 ,  140  and  145 , respectively, encoder  150  combines the CLOCK, DATA and SYNC signals and produces, on line  295 , a bi-phase mark signal (BIMDAT). Specifically, timing generator  250  processes the four delayed clock signals to generate a clock signal (CLK) on line  255  and a signal (2XCLK), on line  260 , the latter signal having double the frequency of the CLK signal and delayed by ⅛ of a bit interval relative to the CLK signal. The DATA signal, on line  110 , and the SYNC signal, on line  115 , in addition to the CLK and 2XCLK signals, form input signals to bi-phase mark modulator  251 .  
         [0036]    [0036]FIG. 3 shows a timing diagram indicating the relative temporal positions of DATA  305 , SYNC  310 , CLK  315 , 2XCLK  320 , and BIMDAT  325  signals that form the input and output signals of the bi-phase mark modulator. CLOCK signal  300  is shown for reference purposes. CLOCK signal  300 , DATA signal  305  and SYNC signal  310  are phase coherent, while CLK signal  315  is delayed by ¾ of a bit interval with respect to CLOCK signal  300 . The signals shown correspond to the illustrative example, wherein time-slot  31  contains the framing sequence “00001011”. For the sake of simplification, the timing diagram depicts only a portion of the framing sequence near where the protocol violation is inserted. Specifically, the sequence “01011” is shown.  
         [0037]    Bi-phase mark modulator  251 , shown in FIG. 2, contains logical AND gates  275  and  280 , NOR gate  285 , inverters  265  and  270 , and J-K flip-flop  290 . In effect, the modulator enables the CLK and inverted CLK signals to sample, through AND gates  275  and  280 , both an inverted DATA signal and the SYNC signal. To facilitate this sampling, inverters  265  and  270  invert DATA and CLK signals  305  and  315 , respectively. Subsequently, the resulting inverted signals are applied to one input of each AND gate  275  and  280 . Line  255  which carries CLK signal  315  connects to the second input of AND gate  275 . Line  115  which carries SYNC signal  310  connects to the second input of AND gate  280 . The outputs of AND gates  275  and  280 , i.e., a sampled inverted DATA signal and a sampled SYNC signal, serve as inputs to NOR gate  285 . The sampled inverted DATA signal and SYNC signal are combined in NOR gate  285  to produce a single bit stream. The output of NOR gate  285  connects to both J and K inputs of J-K flip-flop  290 . Additionally, line  260  connects the 2XCLK signal to the clock (CK) input of flip-flop  290 .  
         [0038]    As is well-known in the art, a J-K flip-flop toggles at each rising edge of a signal applied to its CK input as long as high level signals are applied to both J and K inputs. Thus, in effect, the output of OR gate  285  controls when flip-flop  290  toggles. As shown, flip-flop  290  toggles upon each rising edge of 2XCLK signal  320  as long as the output of NOR gate  285  is high. Since a rising edge of 2XCLK signal  320  occurs at the beginning and a middle of a bit interval, the NOR gate output must blank, i.e., become low, during each zero data bit and during the protocol violation. As a result, the Q output of flip-flop  295  is a bi-phase mark signal, i.e., BIMDAT signal  325 .  
         [0039]    In operation, bi-phase mark modulator  251  transforms DATA signal  305  into a bi-phase mark signal wherein each zero in DATA signal  305  appears as a transition at the beginning of a bit interval and each one in DATA signal  305  appears as a transition at the beginning and at the middle of a bit interval. In addition, SYNC signal  310  is encoded into the bi-phase mark signal as a violation of the protocol mandating that a transition occur at the beginning of each bit interval. In essence, SYNC signal  310  is used to block or mask a transition within the framing sequence. Alternatively, SYNC signal  310  can be viewed as causing a 180° phase-shift at the bit interval transition. Illustratively, the protocol violation occurs between the last two bits of the framing sequence in BIMDAT signal  320 . A driver circuit coupled to a transmission medium transmits BIMDAT signal  320  to a receiver and ultimately, to a decoder to recover the individual DATA, CLOCK and SYNC signals.  
         [0040]    The encoder depicted in FIG. 2 is shown only as an illustration of one possible implementation using discrete components. Alternatively, for example, a circuit having a similar function can be implemented using programmable logic devices (PLD) such as the EP330-12 PLD manufactured by Altera Corporation of Irvine, Calif., or using an application specific integrated circuit (ASIC). Additionally, the position of the protocol violation discussed above is merely illustrative. The actual position of the violation will depend upon each individual application of the inventive technique. Also, the frame synchronization signal was illustratively described as occurring simultaneously with the position in the data where the protocol violation is to be placed in the framing sequence. However, those skilled in the art will realize that, broadly speaking, the protocol violation could be positioned anywhere within the entire frame.  
         [0041]    [0041]FIGS. 4A and 4B, in combination, collectively show a schematic diagram of decoder  225 . Decoder  225  comprises 2X clock recovery circuit  400 , transition detector  405 , bit clock generator  410 , mask generator  420 , dejitter filter  425 , data recovery circuit  430 , frame sync recovery circuit  435 , and bit resynchronizer  440 .  
         [0042]    In particular, from the four delayed BIMDAT signals, inputs (D 1 , D 2 , D 3  and D 4 ) on lines  205 ,  210 ,  215 , and  220 , respectively, decoder  225  recovers the CLOCK, DATA and SYNC signals. Generally, using the delayed BIMDAT signals, 2X clock recovery circuit  400  generates a 2XCLK signal which is two times the bit interval frequency. Simultaneously, transition detector  405  generates a pulse that represents each transition in the BIMDAT signal. Subsequently, an output of transition detector  405 , shown as the transition indicator signal (TRS), is used by bit clock generator  410  to generate a bit clock signal (BCLK). However, the missing transition at the protocol violation must be replaced to generate an accurate bit clock signal. Mask generator  415  produces a signal that inserts a transition into the BCLK signal at the protocol violation. Subsequently, the bit clock signal is filtered by dejitter filter  425  to produce, on line  230 , the CLOCK signal.  
         [0043]    Data recovery circuit  430  and frame sync recovery circuit  435  use BCLK, and its inversion, in combination with transition indicator signal TRS, and its inversion, to recover data and sync signals, RDATA and RSYNC, respectively. Subsequently, the recovered CLOCK signal is used by bit resynchronizer  440  to reclock the RDATA and RSYNC signals and produce, on lines  235  and  240 , the DATA and SYNC signals, respectively. The DATA, SYNC and CLOCK signals are then passed along to a demultiplexer (not shown) to recreate the individual bit streams that comprise the information carried by the frames.  
         [0044]    I will now discuss each individual circuit mentioned above in FIGS. 4A and 4B in detail. 2X clock recovery circuit  400  comprises exclusive OR (XOR) gate  445 , AND gates  450  and  455 , OR gate  460  and buffer  465 . The AND gates, the XOR gate, and the OR gate logically combine the four delayed BIMDAT signals to produce a clock signal (2XCLK) having a cycle duration that is half the duration of a bit interval in the BIMDAT signal. Specifically, XOR gate  445  combines, using an exclusive OR function, the ½ bit delayed (D 2 ) and the ¾ bit delayed (D 3 ) BIMDAT signals, on lines  210  and  215 , respectively, to produce a pulsatile signal having a ¼ bit long pulse corresponding to each transition in the BIMDAT signal. Simultaneously, AND gate  450  inverts and then combines, using an AND function, all of the delayed BIMDAT signals on lines  205 ,  210 ,  215 , and  220 . Simultaneously, AND gate  455  combines, using an AND function, all of the delayed BIMDAT signals. OR gate  460  combines, using an OR function, the output signals from XOR gate  445  and AND gate  450  and  455 . Buffer  465  buffers a resultant output from OR gate  460 . The OR gate output exits 2X clock recovery circuit  400  as output 2XCLK. The 2XCLK has a frequency which is twice the bit interval.  
         [0045]    Transition detector  405  comprises XOR gate  470 , buffer  475  and D flip-flop  480 . Generally, XOR gate  470  produces pulses representing each transition in the BIMDAT signal; and flip-flop  480  lengthens the pulses representing each transition. Specifically, XOR gate  470  combines, using an exclusive OR function, the ¼ bit delayed (D 1 ) and the ½ bit delayed (D 2 ) BIMDAT signals, on lines  205  and  210 , respectively, to produce a pulse having a ¼ bit duration for each transition in the BIMDAT signal. Buffer  475  buffers each such pulse as it exits XOR gate  470 . D flip-flop  480  operates as a pulse stretcher to elongate each ¼ bit duration pulse to a ½ bit duration, i.e., equivalent to 1 cycle of the 2XCLK signal. The Q and {overscore (Q)} outputs from flip-flop  480  are respectively labeled TRS and {overscore (TRS)}. Importantly, the TRS signal is high for the first half of each zero bit and high for the entire bit interval for each one bit except at the protocol violation.  
         [0046]    Bit clock generator  410  comprising J-K flip-flip  490  produces the bit clock signal (BCLK) in response to the TRS signal and the 2XCLK signal. Flip-flop  490  produces BCLK by toggling its Q output at each rising edge of the 2XCLK signal. In this manner, the BCLK signal has a frequency that is double the 2XCLK signal frequency. In other words, the BCLK signal has a cycle duration equivalent to one-bit interval. However, the protocol violation will cause flip-flop  490  to produce an improper transition in the BCLK signal. Therefore, mask generator  420  produces a masking signal at the protocol violation. OR gate  485  inserts the masking signal into the TRS signal. The insertion of the masking signal causes flip-flop  490  to ignore the protocol violation while producing BCLK.  
         [0047]    Mask generator  420  comprises counter  415  and J-K flip-flop  510 . Counter  415 , itself containing AND gate  505  and flip-flops  495  and  500 , is a two-bit counter which is enabled by the {overscore (Q)} output of flip-flop  510 . In this manner, when a bit pattern occurs in the BIMDAT signal having the binary form “10” this counter is enabled and flip-flop  510  generates, at its Q and {overscore (Q)} outputs, a masking pulse having a two-bit duration. The masking pulse masks the occurrence of a protocol violation, if any, that follows the “10” bit sequence. During the occurrence of data bits with a binary “10” pattern that are not within the framing sequence, the flip-flop  510  generates a masking pulse, but the masking pulse has no effect upon the bit clock generator. However, when a “10” pattern appears in the framing sequence “0000101v1”, the “10” pattern initiates the masking pulse which replaces the improper TRS signal at the input of flip-flop  490  within bit clock generator  410 . As a result, the bit clock generator produces an accurate bit clock signal (BCLK). As those skilled in the art will readily recognize, the bit pattern that initiates the masking pulse could be any pattern. Also, circuitry could be implemented requiring recognition of a longer bit pattern than 2 bits prior to producing the masking pulse.  
         [0048]    Dejitter filter  425  comprises buffer  515 , filter  520  and schmitt trigger  525 . Dejitter filter  425  reduces variation, which can result during the transmission and receiving processes, in the positions of the rising edges of the BCLK signal. Specifically, buffer  515  buffers the BCLK signal. The buffered BCLK signal is then sent to filter  520 . Filter  520  has a low pass frequency response and a phase response such that the input signal, BCLK, and an output signal, CLK, maintain a phase relationship having a difference of less than 90°, with transitions of CLK always occurring later than corresponding transitions of BCLK. Subsequently, schmitt trigger  525  sharpens the edges of a filtered signal at the output of the filter and produces the CLK signal.  
         [0049]    As previously noted, the TRS signal is high for the first half of each zero bit and is high for the entire bit interval for each one bit. From the TRS signal, data recovery circuit  430  converts the TRS signal into a data signal having a one represented by a high level and zero represented by a low level. Data recovery circuit  430  comprises AND gates  530  and  535 , and flip-flops  540  and  545 . The data recovery circuit decodes the TRS signal using the BCLK and 2XCLK signals. The output of the data recovery circuit (RDATA) represents the data which was encoded by the transmitter. Importantly, the BCLK signal is high during the second half of each bit interval. The BCLK signal samples the TRS signal, and its inversion, through AND gates  530  and  535 . When the TRS signal indicates a logical one data bit, the TRS signal is high during the second half of each bit interval. When signals TRS and BCLK are both high, the output of AND gate  530  is high, as is the J input of flip-flop  540 . Consequently, output Q of J-K flip-flop  540  is high during an entire bit interval. Conversely, when TRS is low ({overscore (TRS)} is high), during the second half of the bit interval, the K input controls the output of flip-flop  540  and the Q output of this flip-flop is low for an entire bit duration. D flip-flop  545  reclocks the output of flip-flop  540  and generates, at its Q output, signal RDATA having a zero represented as a low signal for a full bit interval and one represented as a high signal for a full bit interval.  
         [0050]    Similarly, frame sync recovery circuit  435  recovers the frame synchronization signal in response to the BCLK and TRS signals. As with data recovery circuit  430 , the frame sync recovery circuit decodes the TRS signal using the BCLK and 2XCLK signals. The output of the frame sync recovery circuit represents the SYNC signal that was encoded by the transmitter. Importantly, the {overscore (BLCK)} signal is high during the first half of each bit interval. Signal BCLK samples the TRS signal, and its inversion, using AND gates  550  and  555 . To effectuate decoding the SYNC signal, the output of AND gate  550 , and consequently the J input of flip-flop  560 , both become high only when a protocol violation occurs, i.e., when {overscore (TRS)}, during the first half of a bit interval, is high. Usually, AND gate  555  maintains a high signal at the K input of flip-flop  560 , thus maintaining a low output at the Q output of flip-flop  560 . However, upon the occurrence of a protocol violation, the Q output of flip-flop  560  becomes high and produces RSYNC.  
         [0051]    Both the RDATA and RSYNC signals are reclocked by bit resynchronizer  440  using the CLK signal. The bit resynchronizer comprises two D flip-flops  570  and  575 . Specifically, flip-flop  570  reclocks RDATA with the CLK signal; similarly, flip-flop  575  reclocks RSYNC with the CLK signal. The outputs of decoder  225  are buffered by buffers  580 ,  585  and  590  to produce the CLOCK, DATA and SYNC signals, respectively.  
         [0052]    As with the encoder, the decoder depicted in FIGS. 4A and 4B is shown only as an illustration of one possible implementation. For example, a circuit having a similar function can be produced using a programmable logic device (PLD) or an application specific integrated circuit (ASIC).  
         [0053]    Additionally, the foregoing discussion only described a transmission in a single direction. Duplex transmission is easily implemented by installing a transmitter with a receiver at each end of the transmission medium. Those skilled in the art will recognize that some minor modification to the receiver and transmitter would be necessary to propagate full duplex signals on a single transmission medium.  
         [0054]    Although I have shown and described, in detail, a single embodiment of my invention, those skilled in the art can readily devise many other varied embodiments that still incorporate my inventive teachings.