Abstract:
A receiver for receiving digital data after transmission through a channel which produces inter-symbol interference or other distortion. In one embodiment, a received signal is differentiated before being digitized to form an output digital bit stream, to reduce the effects of inter-symbol interference and other distortion in the channel. The differentiated signal is compared to two threshold values, a first threshold value, and a second threshold value, the first threshold value being greater than the second threshold value. When the differentiated signal exceeds the first threshold, the output bit is 1, when the differentiated signal is less than the second threshold value, the output bit is 0, and when the differentiated signal is between the first threshold value and the second threshold value, the output bit is the same as the previous output bit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to and the benefit of Provisional Application No. 61/807,667, filed Apr. 2, 2013, entitled “SLOPE DETECTING RECEIVER”, the entire content of which is incorporated herein by reference. 
    
    
     FIELD 
     One or more aspects of embodiments according to the present invention relate to digital data transmission and more particularly to a system and method for improved data transmission through an imperfect channel, such as a channel producing inter-symbol interference. 
     BACKGROUND 
     Digital data transmission across an analog channel is used in displays, servers, data storage, and wireless internet, and may be performed using various modulation schemes, in which an analog signal is modulated according to the data content, i.e., bits of a digital data stream, to be transmitted across the channel. For example, the modulation may involve non-return-to-zero (NRZ) coding. 
     Referring to  FIG. 1 , in a conventional receiver, the input c(t) from the channel is, at each unit interval (UI), compared, in an act  110 , to a reference signal voltage or threshold value value V th . If the input exceeds the threshold value V th , then the data value d(n) at the current unit interval is set, in an act  115 , to 1; otherwise it is set, in an act  120 , to 0. Referring to  FIG. 2 , a well-formed data waveform  210  modulated using NRZ coding with the binary pattern ‘1100’, results, in the sampled waveform  215 , in two samples P 1 , P 2  exceeding the threshold value and two samples P 3 , P 4  below the threshold value, which results, at the output of the comparator  220 , in a digital output stream  225  having two ones followed by two zeros. 
     Referring to  FIG. 3 , after an NRZ waveform representing the bit stream ‘0001011111101000’, preceded and followed by some number of zeroes, is transmitted through a channel producing inter-symbol interference or otherwise distorting the transmitted waveform, the received waveform  310  may appear as illustrated and may lack sharp transitions between levels corresponding to 0 and 1, as, for example, illustrated in  FIG. 2  for the well-formed waveform  210 . Referring to  FIG. 4 , this may result in errors at, for example, the points in the received waveform circled with dashed lines. Thus, there is a need for a system and method for receiving, e.g., converting to a digital bit stream, a signal received at the output of an imperfect channel. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     According to aspects of embodiments of the present invention, a received signal is differentiated before being digitized to form an output digital bit stream, to reduce the effects of inter-symbol interference and other distortion in the channel. The differentiated signal is compared to two threshold values, a first threshold value, and a second threshold value, the first threshold value being greater than the second threshold value. When the differentiated signal exceeds the first threshold, the output bit is 1, when the differentiated signal is less than the second threshold value, the output bit is 0, and when the differentiated signal is between the first threshold value and the second threshold value, the output bit is the same as the previous output bit. 
     According to an embodiment of the present invention there is provided a receiver for generating a binary data stream from a modulated analog signal, the receiver including: a differentiator; a first comparator; a second comparator; and a digital circuit, an input of the differentiator configured to receive the analog signal, an output of the differentiator connected to a non-inverting input of the first comparator, an inverting input of the first comparator configured to receive a first threshold value, the output of the differentiator connected to an inverting input of the second comparator, a non-inverting input of the second comparator configured to receive a second threshold value, an output of each of the first and second comparators connected to an input of the digital circuit, and the digital circuit configured to generate the binary data stream. 
     In one embodiment, the first threshold value is greater than zero; and the second threshold value is less than zero. 
     In one embodiment, the digital circuit is configured to output: a value of 1 when the output of the first comparator is 1; a value of 0 when the value of the second comparator is 1; and an unchanging value otherwise. 
     In one embodiment, the receiver includes a first input, a second input, a third input, and an output; a two-input OR gate, including a first input, a second input, and an output; a delay block including an input and an output, configured to introduce a signal delay of one unit interval; and an inverter including an input and an output; the output of the first comparator being connected to the first input of the three-input NOR gate and to the first input of the two-input OR gate, the output of the second comparator being connected to the second input of the three-input NOR gate, the output of the three-input NOR gate being connected to the second input of the two-input OR gate, the output of the two-input OR gate being connected to the input of the delay block, the output of the delay block being connected to the input of the inverter, the output of the inverter being connected to the third input of the three-input NOR gate, and the output of the two-input OR gate being connected to the output of the digital circuit. 
     According to an embodiment of the present invention there is provided a method of generating, in a receiver, a binary data stream from a modulated analog signal, the method including: differentiating the analog signal to form a differentiated signal; comparing the differentiated signal to a first threshold value; comparing the differentiated signal to a second threshold value; forming a binary output of 1 at an output of the receiver when the differentiated signal exceeds the first threshold value; forming a binary output of 0 at the output of the receiver when the differentiated signal is less than the second threshold value; and leaving the binary output unchanged when the differentiated signal is less than the first threshold value and greater than the second threshold value. 
     In one embodiment, the acts of: forming a binary output of 1 at an output of the receiver when the differentiated signal exceeds the first threshold value; forming a binary output of 0 at the output of the receiver when the differentiated signal is less than the second threshold value; and leaving the binary output unchanged when the differentiated signal is less than the first threshold value and greater than the second threshold value include: comparing the differentiated signal to the first threshold value in a first comparator, to form a first binary result which takes the value 1 when the differentiated signal exceeds the first threshold value and which takes the value 0 when the differentiated signal is less than the first threshold value; comparing the differentiated signal to the second threshold value in a second comparator, to form a second binary result which takes the value 1 when the differentiated signal is less than the second threshold value and which takes the value 0 when the differentiated signal exceeds the second threshold value; combining the first binary result, the second binary result, and a third binary result in a three-input NOR gate to form a fourth binary result; combining the fourth binary result and the first binary result in a two-input OR gate to form a fifth binary result; delaying the fifth binary result by a unit interval and inverting it to form the third binary result; and providing the fifth binary result at the output of the receiver. 
     According to an embodiment of the present invention there is provided a system for generating, in a receiver, a binary data stream from a modulated analog signal, the system including: means for differentiating the analog signal to form a differentiated signal; means for comparing the differentiated signal to a first threshold value; means for comparing the differentiated signal to a second threshold value; means for forming a binary output of 1 at an output of the receiver when the differentiated signal exceeds the first threshold value; means for forming a binary output of 0 at the output of the receiver when the differentiated signal is less than the second threshold value; and means for leaving the binary output unchanged when the differentiated signal is less than the first threshold value and greater than the second threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, aspects, and embodiments are described in conjunction with the attached drawings, in which: 
         FIG. 1  is a flow chart of a related-art method employed in a receiver; 
         FIG. 2  is a block diagram illustrating processing of a received signal in a related-art receiver; 
         FIG. 3  is a waveform received by a receiver constructed according to an embodiment of the present invention; 
         FIG. 4  is a plot of received signal against time illustrating errors in the operation of a related-art receiver; 
         FIG. 5  is a flow chart illustrating a method of operation in a receiver constructed according to an embodiment of the present invention; 
         FIG. 6  is a block diagram illustrating processing of a received signal in a receiver constructed according to an embodiment of the present invention; 
         FIG. 7  is a slope waveform formed in a receiver constructed according to an embodiment of the present invention; and 
         FIG. 8  is a block diagram showing functional elements and logic gates in a receiver constructed according to an embodiment of the present invention; and 
         FIG. 9  is a block diagram of a digital communication link configured to transmit data to a display according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a slope detecting receiver provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. Further, the use of “may” when describing embodiments of the present invention refers to “one or more embodiments of the present invention”. 
     Referring to  FIG. 5 , in a method according to an embodiment of the present invention, the bit value in the digital bit stream is digitized according to the signal&#39;s slope rather than the signal&#39;s amplitude. The derivative s(t) of the input c(t) from the channel may be formed by a differentiator  510 . Subsequently, in an act  515 , the derivative may be compared to a first threshold value S+, and if the derivative exceeds the first threshold value, the data value d(n) at the current unit interval may be set, in an act  520 , to 1; otherwise the derivative may be compared, in an act  525 , to a second threshold value, S−, less than the first threshold value S+. In one embodiment the first threshold value S+ is greater than zero and the second threshold value S− is less than zero. If the derivative is less than the second threshold value, the data value d(n) at the current unit interval may be set, in an act  530 , to 0; otherwise, it may be set, in an act  535 , to be equal to the data value at the previous unit interval. 
     The process of converting an input to a digital bit stream is further illustrated in  FIG. 6 , for a well-formed data waveform  210  modulated using NRZ coding with the binary pattern ‘1100’. Processing by the differentiator  510  results in the slope waveform  610 , in which the signal is positive at the point P 1 , near zero at the points P 2  and P 4 , and negative at the point P 3 . Sampling the slope waveform  610  results in the sampled slope waveform  615 , which again is positive at the point P 1 , near zero at the points P 2  and P 4 , and negative at the point P 3 . The output of a comparator/processor  620 , the input to which is the sampled slope waveform  615 , may be determined according to the method illustrated in  FIG. 5 , with the first bit being 1, as a result of P 1  exceeding the first threshold value, the second bit being 1 (the same as the preceding bit) as a result of P 2  falling between the first threshold value and the second threshold value, the third bit being 0 as a result of P 3  being less than the second threshold value, the fourth bit being 0 (the same as the preceding bit) as a result of P 4  falling between the first threshold value and the second threshold value. Thus, in this example, a system and method according to an embodiment of the present invention faithfully reproduces the bit pattern modulated onto the data waveform  210 . 
     Referring to  FIG. 7 , a system and method according to an embodiment of the present invention also reproduces a modulated waveform when the input to the receiver has been transmitted through a channel producing inter-symbol interference or otherwise distorting the transmitted waveform. The slope waveform  710  illustrated in  FIG. 7  may result, for example, from taking the derivative of the received waveform  310  ( FIG. 3 ). The bit stream d(n) produced at the receiver output when the slope waveform  710  is the output of the differentiator  510  ( FIGS. 5 and 6 ) may be seen to be the following. The first three bits may be zero, each being the same as the preceding bit because the slope waveform  710  is initially between the first threshold value S+ and the second threshold value S− for a duration of several UI. At UI number  5 , the slope waveform  710  exceeds the first threshold value, resulting in an output of 1, at UI number  6 , the slope waveform  710  falls below the second threshold value, resulting in an output of 0, and at UI number  7 , the slope waveform  710  again exceeds the first threshold value, resulting in an output of 1. The following five output bits are all 1, because for each corresponding UI the slope waveform  710  is either greater than the first threshold value S+ (for UIs numbered  8  and  9 ) resulting in an output of 1, or between the two threshold values (for UIs numbered  10  through  12 ) resulting in the same output as the output at the preceding UI. At UI number  13 , the slope waveform  710  falls below the second threshold value, resulting in an output of 0, at UI number  14 , the slope waveform  710  exceeds the first threshold value, resulting in an output of 1, and over UIs numbered  15  through  17 , the slope waveform  710  falls below the second threshold value, resulting in an output of 0. Thus, the original bit stream ‘0001011111101000’ is reproduced by a portion of the slope waveform  710 . 
       FIG. 8  shows a differentiator connected to two comparators  810 ,  815  and a digital circuit (logic circuit)  817 , which together are used to implement a system and method according to one embodiment of the present invention. The output of the differentiator and a first threshold value, or reference voltage, S+ are connected to the non-inverting and inverting inputs respectively of the first comparator  810 . The output of the differentiator and a second threshold value, or reference voltage, S− are connected to the inverting and non-inverting inputs respectively of the second comparator  815 . The outputs of the two comparators  810 ,  815  are connected to two respective inputs of a 3-input NOR gate  820 , i.e., a gate the output of which is low if and only if one or more of its three inputs is high. The output of the first inverter  810  and the output of the three-input NOR gate  820  are connected to the two inputs  822 ,  823  of an OR gate  825 , the output of which is the digital bit stream which forms the output of the receiver. The output of the OR gate  825  is also connected to a one-UI delay  830 , the output of which is connected, through an inverter  835  to the third input of the three-input NOR gate  820 . 
     In the digital circuit  817  illustrated in  FIG. 8 , when the output of the differentiator is between the two threshold values S+ and S−, the outputs of both inverters are low; as a result the output of the three-input NOR gate  820  is the inverse of its third input, which is the inverse of the receiver output one UI previously. Thus in this case the output of the three-input NOR gate  820 , which is connected to the second input  823  of the OR gate  825 , is equal to the receiver output one UI previously. The output of the first comparator  810  being low, the output of the OR gate  825  is the same as the logic level at its second input  823 . In this situation the output of the receiver is thus equal to the receiver output one UI previously. 
     When the output of the differentiator exceeds the first threshold value S+, the output of the first comparator is high, causing the first input  822  of the OR gate  825  to be high, which causes the output of the circuit to be high. When the output of the differentiator is less than the second threshold value S−, the output of the second comparator is high; this causes the output of the three-input NOR gate  820  to be low, and, because when the output of the differentiator is less than the second threshold value S−, is it also less than the first threshold value S+, the output of the first comparator  810  is also low. Thus both inputs  822 ,  823  to the OR gate  825  are low, and the receiver output is low. 
     The receiver illustrated in  FIG. 8  thus exhibits the behavior illustrated in  FIG. 5 : when the output of the differentiator  510  is greater than the first threshold value S+, the receiver output is 1, when the output of the differentiator  510  is less than the second threshold value S−, the receiver output is 0, and when the output of the differentiator  510  is between the first threshold value S+ and the second threshold value S−, the receiver output is the same as it was one UI previously. 
       FIG. 9  shows an exemplary application of a receiver constructed according to an embodiment of the present invention. A source  910  of data for displaying on the display  915  is transmits digital data over the channel  920  to the receiver  925  inside the display  915 . The receiver  925  receives the data, and the display  915  generates a corresponding display of information for a user to view. 
     The digital circuit may be implemented with logic gates as illustrated in  FIG. 8 , or with any other embodiment of a processing unit. The term “processing unit” is used herein to include any combination of hardware, firmware, and software, employed to process data or digital signals. Processing unit hardware may include, for example, application specific integrated circuits (ASICs), general purpose or special purpose central processing units (CPUs), digital signal processors (DSPs), graphics processing units (GPUs), and programmable logic devices such as field programmable gate arrays (FPGAs). 
     Although limited embodiments of a slope detecting receiver have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that the slope detecting receiver employed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.