Abstract:
A method and apparatus for multiplexing high bandwidth data signals uses a bank of sampling device, such as flip-flops, on the transmission side to essentially synchronize the data prior to multiplexing the data. The method and apparatus operate at the expense of increased jitter in the received signal, which is compensated for by proper choice of the clock and data recovery circuit in the receiver. By matching the increased jitter at the transmission side with a clock recovery circuit that can process this increased jitter, a truly simply and economical asynchronous multiplexing technique and apparatus can be constructed. By using a sampling device, such as a flip-flop, and several dividers, frame synchronization bits can be added to one channel to enable proper channel alignment at the receiving end.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed generally to methods and apparatuses for processing digital signals, and more particularly to a method and apparatus for processing digital signals in which multiple digital signals are multiplexed into a single data stream for transmission over a communications link and then demultliplexed for routing to desired locations.  
         BACKGROUND  
         [0002]    Many digital signals are processed and multiplexed in a synchronous manner. However, synchronous processing and multiplexing requires careful management of clock sources. Consequently, synchronous processing can be significantly expensive. Therefore, many digital transmission systems operate asynchronously.  
           [0003]    While more inexpensive than synchronous multiplexers, existing synchronous multiplexers for high bandwidth data are still too expensive for certain applications. Examples of such applications include video on-demand, networked video games and other services that require delivery of large amounts of content for entertainment purposes only. For video on-demand services to even compete with movie rental stores and the like, these services must reduce costs. Therefore, either synchronous multiplexing must be performed in a significantly less expensive manner or asynchronous high-bandwidth processing multiplexing must be accomplished in a cost-effective manner.  
           [0004]    The present invention is therefore directed to the problem of developing a method and apparatus for multiplexing high bandwidth data in an economical manner, which operates with sufficient quality for video on-demand and similar services.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention solves these and other problems by providing a method and apparatus for multiplexing high bandwidth data signals using a bank of sampling devices, such as flip-flops, on the transmission side to essentially synchronize the data prior to multiplexing the data in combination with a simple bit framing technique also employing an inexpensive sampling device, e.g., a flip-flop. This method and apparatus operate at the expense of increased jitter in the received signal, which is compensated for by proper choice of the clock and data recovery circuit in the receiver. By matching the increased jitter at the transmission side with a clock recovery circuit that can process this increased jitter, a truly simply and economical asynchronous multiplexing technique and apparatus can be constructed.  
           [0006]    According to another aspect of the present invention, exemplary embodiments of a transmitter and receiver are also disclosed.  
           [0007]    According to yet another aspect of the present invention, an exemplary embodiment of a method for synchronizing multiple asynchronous signals prior to transmission is disclosed. According to this embodiment, each of the asynchronous signals is first sampled with a sampling device, such as a flip-flop. Furthermore, each of the sampling devices or flip-flops is clocked with a clock having a clock rate in excess of almost twice (e.g., about 1.7 times) the data rate of each of the asynchronous signals. In addition, a simple bit framing insertion technique is employed using one of the sampling devices or flip-flops on one channel to permit proper channel alignment.  
           [0008]    According to still another aspect of the present invention, an exemplary embodiment for coupling multiple asynchronous signals to a communications link is disclosed. According to this embodiment, each of the asynchronous signals is first coupled to a sampling device, such as a flip-flop. The output of each of the sampling devices or flip-flops is then coupled to a multiplexer. Frame alignment bits are inserted into one channel of the input using a sampling device, such as a flip-flop, and a bit toggle technique. Each of the outputs of the sampling devices or flip-flops is multiplexed into a combined signal. The combined signal is then coupled to the communications link.  
           [0009]    Further aspects of the present invention will be apparent upon review of description herein in light of the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 depicts a block diagram of an exemplary embodiment of an apparatus for transmitting multiple data streams over a communication link according to one aspect of the present invention.  
         [0011]    [0011]FIG. 2 depicts a flow chart of an exemplary embodiment of a method for synchronizing multiple asynchronous signals prior to transmission according to another aspect of the present invention.  
         [0012]    [0012]FIG. 3 depicts a flow chart of an exemplary embodiment of a method for coupling multiple asynchronous signals to a communications link according to still another aspect of the present invention.  
         [0013]    [0013]FIG. 4 depicts a block diagram of an exemplary embodiment of an apparatus for inserting frame synchronization bits in one channel of the input data according to yet another aspect of the present invention.  
         [0014]    [0014]FIG. 5 depicts a block diagram of an exemplary embodiment of an apparatus for detecting the frame synchronization bits according to still another aspect of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]    It is worthy to note that any reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.  
         [0016]    Referring to FIG. 1, shown therein is an exemplary embodiment  10  of an apparatus for transmitting multiple signals over a single communications link as a single high bandwidth signal. In this embodiment  10 , there are four signals multiplexed into the single high bandwidth signal, however, the techniques herein can easily be extended to larger numbers of signals, such as eight, sixteen, thirty-two, etc. Moreover, these techniques can even be applied to one, two or three signals as well. At a high level of functionality, the embodiment  10  includes a transmitter  7 , a communications link  3  and a receiver  8 .  
         [0017]    Within the transmitter  7 , a bank of sampling devices, e.g., flip-flops,  1   b - d  and one frame synchronization device  40  acts to synchronize the incoming data signals (D 1 -D 4 ). The functionality of the frame synchronization device  40  will be explained vis-à-vis FIG. 4 below. Essentially, however, frame synchronization device  40  includes a sampling device, such as a flip-flop, at its center that operates similarly to sampling devices or flip-flops  1   b - 1   d  with regard to the input data signal.  
         [0018]    These flip-flops  1   b - d  can be D flip-flops or other types. An exemplary embodiment of these flip-flops includes a flip-flop available from ON Semiconductor, part number MC10EL31. One flip-flop is employed for each signal being multiplexed, so for example, in a sixteen signal multiplexer there would be a bank of sixteen flip-flops (however, at least one or more flip-flops may be replaced with the frame synchronization device  40 ).  
         [0019]    The flip-flops  1   b - d  and frame synchronization device  40  sample the incoming signal at three possible locations on the data pulse—the “one” position, the “zero” position or in the transition region (i.e., either transitioning from zero to one or from one to zero). Thus, by selecting the clock properly one can ensure sufficient samples are being obtained. Hence, the output of each flip-flop will be a 1, a 0 or a random value.  
         [0020]    Thus, the embodiment shown herein will synchronize the incoming signals to be multiplexed in a very simple and economical manner, but at the expense of increased jitter in the received signal. The increased jitter results from the additional random values output when the flip-flops sample the incoming signals during a transition region.  
         [0021]    The outputs of the flip-flops  1   b - d  and frame synchronization device  40  are coupled to a four-to-one (4:1) multiplexer  2   a,  which creates a single combined signal from the four inputs. Any standard multiplexer can be employed, as the signals are now synchronous. An example of a suitable multiplexer includes the Intel multiplexer GD16553.  
         [0022]    The combined signal is then transmitted over a communications link  3 , such as a fiber optic cable, a coaxial cable, or some hybrid fiber coaxial cable. Other activities that can be modeled as a communications link are also possible, such as data writing and reading to a memory device, such as a hard drive, thereby making the embodiments herein applicable to these types of applications as well.  
         [0023]    On the receiver side, at the end of the communications link  3  is a one-to-four (1:4) demultiplexer  2   b,  which converts the incoming signal to its constituent elements. The demultiplexer  2   b  is matched to the multiplexer  2   a.  So, if N signals are being multiplexed in the transmitter, then N signals are demultiplexed by a N:1 demultiplexer in the receiver. Standard demultiplexers can be used, such as the Intel GD16553.  
         [0024]    The outputs of the demultiplexer are then coupled to four Clock and Data Recovery (CDR) circuits  6   a - d,  which recreate the original data signals (D 1 -D 4 ). One output is first passed through a frame synchronization device  50  before being coupled to its respective CDR, which synchronization device detects the frame synchronization bits that are used to align the various channels. Once the synchronization bits are detected, the alignment of the channels is performed in the standard manner.  
         [0025]    Any increased jitter can be accommodated by proper selection of the clock and data recovery circuit in the receiver. Simply put, one must employ a clock and data recovery circuit that can handle more jitter than a typical synchronous communications system. Such circuits are available, for example, an adequate clock and data recovery circuit is the SY87701 CDR by Micrel.  
         [0026]    At the center of the embodiment  10  is a clock source  4   a,  which in this embodiment is a 2.5 GHz clock, which clocks the multiplexer  2   a.  A divider  5  converts the clock signal to a 622 MHz clock, which clocks the D flip-flips  1   a - d  and is input to a latch input of multiplexer  2   a.  The data rate of the incoming signals is about 270 Mb/s (shown symbolically as clocked by a clock  4   b  on the input side and clock  4   d  on the output side). The data rate of the signals being output by the demultiplexer  2   b  is about 622 Mb/s (shown symbolically as clocked by a clock  4   c ).  
         [0027]    Thus, the exemplary embodiment  10  operates as follows. A 2.5 GHz clock drives the 4:1 multiplexer, which receives four data streams of 622 Mb/s. These data streams are generated by sampling four 270 Mb/s data streams at 622 MHz. The 622 Mb/s signal is jittered with respect to 270 MHz, but not with respect to 622 MHz. If the sampling clock is twice the data rate of the signal being sampled, then there is a jitter width of 50%. A clock and data recovery (CDR) circuit in the receiver removes this jitter. With twice the data rate there is a jitter of 50% of the clock cycle. The eye closes completely with the sampling frequency being equal to the data rate of the signal to be sampled (which is the equivalent of the Nyquist limit called the sampling theorem). Therefore, sampling rates, which are lower than exactly twice the data rate are possible. A good practical number is 1.7 times the data rate (or ca. twice the data rate).  
         [0028]    Turning to FIG. 2, shown therein is an exemplary embodiment  20  of a method for synchronizing multiple asynchronous signals prior to transmission. According to this embodiment  20 , each of the asynchronous signals is sampled with a sampling device, such as a flip-flop (element  21 ). Furthermore, each of the sampling devices or flip-flops is clocked with a clock having a clock rate in excess of ca. twice (e.g., about 1.7 times) the data rate of each of the asynchronous signals (element  22 ). Frame synchronization buts are inserted into one of the asynchronous signals (element  23 ). An output of each of the sampling devices or flip-flops is coupled to a multiplexer converting the outputs from the plurality of sampling devices or flip-flops to a single signal (element  24 ). The single signal is then transmitted over a communications link (element  25 ). A clock and data recovery circuit is then used at a receiving end of the communications link, which clock and data recovery circuit is capable of handling jitter with a jitter width of at least 50% or more (element  26 ).  
         [0029]    Turning to FIG. 3, shown therein is an exemplary embodiment  30  of a method for coupling multiple asynchronous signals to a communications link. According to this embodiment, each of the asynchronous signals is coupled to a sampling device, such as a flip-flop (element  31 ). Frame synchronization bits are inserted into one of the asynchronous signals (element  32 ). The output of each of the sampling devices or flip-flops is then coupled to a multiplexer (element  33 ). Each of the outputs of the sampling devices or flip-flops is multiplexed into a combined signal (element  34 ). The combined signal is then coupled to the communications link (element  35 ). Each of the sampling devices or flip-flops is clocked with a clock having a clock rate in excess of ca. twice a data rate of each of the asynchronous signals (element  36 ). A clock and data recovery circuit is used at a receiving end of the communications link, which clock and data recovery circuit is capable of handling jitter with a jitter width of at least 50% or more (element  37 ).  
         [0030]    The present invention thus provides an extremely inexpensive yet effective technique for performing asynchronous communications. The D flip-flops set forth herein are very inexpensive parts, e.g., on the order of $2 per part. This avoids the costly and complex synchronization circuits.  
         [0031]    Adding Frame Synchronization  
         [0032]    Frame synchronization is needed for the recognition of the order of bits in the serial bit stream. In order to do that, every fourth bit of one 622 MB/s bit stream is a synchronization bit. One approach is to use a toggle bit as the synchronization bit. On the receive side, it is sufficient to recognize which bit toggles in order to identify the order of the payload bits. See the block diagram of an exemplary embodiment  40  of the frame synchronization apparatus shown in FIG. 4.  
         [0033]    The embodiment  40  shown in FIG. 4 is included in the transmitter  7  in lieu of one of the flip-flops  1   a - 1   d  as shown in FIG. 1. The output of embodiment  40  provides one of the inputs of the 4:1 multiplexer  2   a.  The input of embodiment  40  is one of the data inputs shown as input to one of the flip-flops  1   a - 1   d  in FIG. 1.  
         [0034]    The remaining three data inputs (270 Mb/s each) remain unchanged. Thus, one of the four data inputs to the bank of flip-flops  1   a - 1   d  includes a frame synchronization bit, the recognition of which will allow proper allocation of the data to the at the receive end.  
         [0035]    The 622 MHz clock (which is available in the transmitter  7  from clock  4   a  that has been divided by 4 by divider  5 , see FIG. 1) is further divided by 4 (in divider  45 ) in order to obtain a 155 MHz clock signal. The 622 MHz clock is then sent through an x¾ multiplier  41  as well to produce a 466.5 MHz clock signal. A D-Flip-Flop  42  samples the asynchronous 270 MB/s data at a clock speed of 466.5 MHz. A following shift register  43 , which is clocked at 466.5 MHz as well, loads three samples into cells  43   a - c,  respectively. The 155 MHz clock loads the first three cells ( 44   a - c ) of the second shift register  44  with the three data samples of the first shift register  43 . The 4th cell ( 44   d ) of the second shift register  44  is loaded with the toggle bit (e.g., the frame synchronization bit). The toggle bit is obtained by dividing the 622 MHz clock signal by two in divider  46 . The second shift register  44  is then clocked out at 622 MHz, thereby producing a serial bit stream of the original data signal that contains three bits of the original payload data and one bit of synchronization at the data rate of 622 MB/s. This serial bit stream fits into the 4×622 MB/s transport scheme discussed above, which runs at 2.48 GB/s.  
         [0036]    Frame Synchronization at the Receive Side  
         [0037]    Turning to FIG. 5, shown therein is an exemplary embodiment  50  of the frame synchronization device (FSD) according to one aspect of the present invention. The received 622 MHz clock is processed into a 155 MHz clock signal and a 466.5 MHz clock signal by divider  52  and multiplier  51 , respectively. A first shift register  53  is loaded with the serial 622 Mb/s data at a clock rate of 622 MHz. The 155 MHz clock loads the second shift register  54 , which is read out at a rate of 466.5 MHz. The third bit of the second shift register  54  represents the sampled version of the original 270 MB/s data stream. A low jitter Clock and Data Recovery circuit  6   a  removes the sample jitter.  
         [0038]    The fourth bit of the first shift register  53  is clocked into a D-Flip-Flop  55 . The present and previous value is compared in an X-OR gate  56 , where the situation is detected, when both values are always of opposite sign, as is the case in a toggle sequence. That information is used to synchronize the position of the four data signals of the 2.48 GB/s data stream in the normal manner, thereby resulting in the correct assignment of the channel numbers.  
       SUMMARY  
       [0039]    Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, certain types of flip-flops are discussed in the embodiments; however, other types may be sufficient to practice the inventions herein. Furthermore, these examples should not be interpreted to limit the modifications and variations of the invention covered by the claims but are merely illustrative of possible variations.