Abstract:
An apparatus for eliminating noise is disclosed. The present invention includes a counter, which counts in a first direction when an input signal is active, and in a second direction otherwise. A determining device is used to determine a predetermined first threshold value, and assert an output signal while such value is reached. The present invention also includes a limiting device, which prevents the counter from counting beyond or below a predetermined limit value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for eliminating noise, and more particularly to an apparatus and method for cumulatively eliminating the superimposition of extra-signal fluctuations that corrupt an information signal. Still more particularly, the present invention relates to an apparatus and method for recovering an information signal corrupted by noise. 
     2. Description of the Prior Art 
     Noise commonly exists in a communication system to adversely affect the communication of a clean information signal. While the information signal is corrupted to some extent by the superimposition of the noise during the transmission, the voltage potential of the received information signal becomes unpredictable. 
     For instance, most of the modern telephone companies provide a call-waiting service through which a third party can be selectively connected to a called party. This is usually accomplished by firstly sending a composite signal (for example, a combined 2130 Hz and 2750 Hz signal) from a central exchange office to the called party. Subsequently, a detecting circuit located near the called party is used to detect this composite signal, and then respond to the central exchange office. Moreover, the central exchange office may further provide pertinent information of the third party to the called party. 
     Unfortunately, during the active period (for example, about 80 ms) of the composite signal, the detecting circuit may miss the composite signal due to the fact that the composite signal is probably corrupted by the surrounding sound signal. Moreover, the detecting circuit may mis-detect a normal sound signal as the composite signal, thereby false triggering a composite signal. 
     An apparatus in the art as shown in FIG. 1 was disclosed to overcome the aforementioned problem. A counter  11  is used to detect an incoming signal IDET by continuously counting under a system clock CK. The counter  11  is reset by a reset circuit  12  whenever absence of the input signal IDET exists, for example, at time t1, t2 or t4 as shown in FIG.  2 A. Further, the count of the counter  11  is fed to a determining circuit  13 , which outputs an active signal DET 1  whenever the count reaches a threshold value TH, for example, at time t3. 
     Unfortunately, the conventional apparatus of FIG. 1 can not be used to detect a severely corrupted signal such as that shown in FIG.  2 B. As demonstrated in FIG. 2B, the count of the counter  11  is repeatedly reset whenever encountering the noise (which has low voltage potential in this example), thereby missing the input signal IDET. The disadvantage of the conventional apparatus of FIG. 1 can not be improved even by using a lower threshold value TH, which will undesirably result in false triggers. 
     For the foregoing reasons, there is a need for an apparatus and method for eliminating the superimposition of extra-signal fluctuations that corrupt an information signal, and overcoming the disadvantages of the prior apparatus or method that usually misses detecting the input signal or generates false triggering in detecting a severely corrupted signal. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus and method is provided for cumulatively eliminating the superimposition of extra-signal fluctuations that corrupt an information signal. The present invention eliminates the disadvantages of the prior apparatus or method that usually misses detecting the input signal or generates false triggering in detecting a severely corrupted signal. 
     In one embodiment, an up-down counter is used to count up when an incoming signal is active, and count down when the signal is not active. A determining circuit, which asserts an output signal when the counter reaches a predetermined up-threshold value, is also included. The output signal remains asserted until the counter reaches a predetermined down-threshold value, wherein the up-threshold value is generally greater than the down-threshold value. Finally, a limit controller is used to prevent the counter from counting beyond a predetermined first limit value, and from counting below a predetermined second limit value, wherein the second limit value is generally equal to or less than the down-threshold value. In the embodiment, a resetting circuit is further included to respond to the output signal, so that the counter is reset when the counter counts down reaching the down-threshold value; and a setting circuit is further included for setting the counter to the first limit value when the output signal is asserted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a schematic diagram illustrating a traditional apparatus for detecting a signal corrupted by noise; 
     FIG. 2A shows a timing diagram pertinent to FIG. 1; 
     FIG. 2B shows another timing diagram pertinent to FIG. 1; 
     FIG. 3 shows a system block diagram of the apparatus according to one embodiment of the present invention; 
     FIGS. 4A and 4B illustrate the flow charts related to the apparatus of FIG. 3; 
     FIG. 5 shows an associated diagram of the count and the output signal DET according to one embodiment of the present invention; 
     FIG. 6 shows a pertinent timing diagram related to FIG. 5; 
     FIG. 7 shows a schematic diagram of an apparatus for recovering an information signal corrupted by noise according to the present invention; and 
     FIG. 8 shows a schematic diagram of another apparatus for recovering an information signal corrupted by noise according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows a system block diagram of an apparatus for cumulatively eliminating noise superimposed on an information signal according to one embodiment of the present invention. The accompanying flow of the operation of the apparatus is further shown in FIGS. 4A and 4B. A counter  30  (FIG.  3 ), which can controllably count up or down, is used to receive an incoming signal IDET. In this embodiment, a traditional up-down counter is used as the counter  30 . It is appreciated that other conventional counters that perform the same function as described hereinafter can be equivalently utilized. The counter  30  counts upward or downward under a system clock CK. When the presence of an active signal IDET (for example, IDET=1) is detected by the counter  30  (step  42 , FIG.  4 A), the counter  30  continuously count up toward an up-threshold value in step  43 . Subsequently, in step  45 , a determining subsystem or circuit  32  determines whether the count of the counter  30  has reached the up-threshold value in response to the count from the counter  30 . It is noted that the up-threshold value can be a fixed and predetermined value that is pre-stored in the determining subsystem  32 , or can be a variable that is dynamically written into the determining subsystem  32  via programmable data input terminals as depicted in FIG.  3 . 
     If the up-threshold value is reached, the YES branch of the block  45  leads to step  47 , in which the output signal DET of the determining subsystem  32  (FIG. 3) is asserted active, indicating that the presence of the active signal IDET is detected. On the other hand, if the up-threshold value is not reached yet, the NO branch of the block  45  leads back to the step  42  for further detecting the incoming signal IDET. 
     Referring back to the block  42 , when the presence of an active signal IDET is not detected, the NO branch of the block  42  leads to step  44 , in which a limit controller  34  (FIG. 3) decides whether the count of the counter  30  reaches a down-limit value. It is noted that the down-limit value can be a fixed and predetermined value that is pre-stored in the limit controller  34 , or can be a variable that is dynamically written into the limit controller  34  via programmable data input terminals as depicted in FIG.  3 . 
     If the count reaches the down-limit value, then the flow of the apparatus returns to the step  42  and nothing needs to be done; otherwise, the counter  30  continuously counts down. 
     The foregoing description concerning the flow of FIG. 4A relates to the detection of the presence of the incoming signal IDET. According to the same principle, the detection of the absence of the incoming signal IDET can similarly illustrated in the flow of FIG. 4B, which is self-explanatory. Specifically, the branch directions are reversed, and the up-threshold, down-limit, and down-threshold are replaced by down-threshold, up-limit, and up-threshold, respectively. 
     For thoroughly understanding the operation of the apparatus and method of the present invention, a more specific example according to the embodiment of the present invention is demonstrated in FIG. 5, an associated diagram of the count and the output signal DET, and in FIG. 6, a pertinent timing diagram. In the FIG. 5, the vertical axis represents the voltage potential of the output signal DET, while the horizontal axis represents the count of the counter  30 . At the beginning, the output signal DET remains at low level labeled L. In this embodiment, the counter  30  starts from an initial value NDT, which is also defined as the down-limit value in the embodiment. When the presence of the incoming signal IDET is detected, the count goes toward the up-threshold value NU; otherwise, the count goes toward the down-limit value NDT (for example, at the time labeled as  61 ) but never below that value NDT. The movement of the count before the assertion of the active output signal DET is shown as the arrows  504 . 
     Still referring to FIGS. 5 and 6, when the count reaches the up-threshold value NU, the output signal DET is asserted active (high in this embodiment), for example, at the time labeled as  62 . It is observed that, in this embodiment, the count is optionally set to the up-limit value NUT as shown in  510 . The set described above can be achieved by a set/reset circuit  36  of the present apparatus (FIG.  3 ). Afterwards, the count goes toward the down-threshold value ND. More specifically, when the presence of the incoming signal IDET is not detected, the count goes toward the down-threshold value ND; otherwise, the count goes toward the up-limit value NUT but never above that value NUT (for example, at the time labeled as  63 ). The movement of the count before the de-assertion of the output signal DET is shown as the arrows  507 . 
     The aforementioned operation continues until when the count reaches the down-threshold value ND, and the output signal DET is then de-asserted (becomes low in this embodiment), for example, at the time labeled as  64 . It is observed that, in this embodiment, the count is optionally reset to the down-limit value NDT as shown in  509  (FIG.  5 ). The reset described above can be achieved by a set/reset circuit  36  of the present apparatus (FIG.  3 ). 
     FIG. 7 shows a schematic diagram of an apparatus for recovering an information signal corrupted by noise, wherein the same numerals are used as in the corresponding diagram of FIG.  3 . The counter  30  includes a conventionally three-stage up-down counter. It is appreciated that number of the stage is not limited to that in the present specific embodiment. At the beginning, the level of the output signal DET of the determining circuit  32  remains at low (i.e., DET=0) before the presence of the incoming signal IDET is detected. The counter  30  counts from Q[1:3]=‘000’, which is also used as the down-limit value. When the active signal IDET is present (i.e., IDET=1), the counter  30  continuously counts up toward the up-threshold value; otherwise, the counter  30  counts down toward the down-limit value. The limit controller  34  decides whether the count of the counter  30  reaches the down-limit value by using, for example, a decoder  340 , thereby preventing the counter  30  from further counting down below the down-limit value. The determining circuit  32  continuously checks the count Q[1:3] by, for example, a decoder  320  to determine whether the count reaches the up-threshold value. The determining circuit  32  outputs an active signal DET=1 when the count reaches the predetermined up-threshold value. It is noted that the up-threshold value is generally equal to or less than the up-limit value. 
     Afterwards, the count goes down whenever the absence of the signal IDET (i.e., IDET=0) exists. The output signal DET remains high (i.e., DET=1) until the counter  30  counts down to the predetermined down-threshold value. Further, the limit controller  34  continuously checks the count of the counter  30  so that the counter  30  never counts below the predetermined down-limit value. It is noted that the down-threshold value is generally equal to or greater than the down-limit value. 
     Moreover, a set/reset circuit  36 , such as a conventional one-shot generator, is used to reset the count Q[1:3] when the output signal DET changes state from active to inactive (i.e., DET=1−&gt;0 in this embodiment). The up-threshold value determined by the determining circuit  32  and the down-limit value checked by the limit controller  34  can, respectively, be a fixed data stored therein, or be a variable that is dynamically written thereunto for effectively eliminating the noise, or accommodating various specifications of devices and environment. 
     FIG. 8 shows a schematic diagram of another apparatus for recovering an information signal corrupted by noise. The circuit of FIG. 8 is structurally similar to that of FIG. 7, and therefore, only major difference therebetween is described below. First of all, the system clock CK in FIG. 7 is directly fed to the corresponding clock terminal of each stage of the counter  30 ; while the clock signal CK in FIG. 8 is derived from the system clocks CK 1  and CK 2 , which are gated by a NOR gate  81  corresponding to the present state (i.e., up or down) decided by the limit controller  34 . Secondly, the system clock CK 1  and CK 2  are used for counting up and down, respectively. Particularly, the frequency of the system clock CK 1  and CK 2  can be made different for optimally accommodating a particular situation. For example, in a telephone conversation, a sound signal may probably be mistaken as a call-waiting signal requiring an interrupting connection to a called party. By adjusting the frequency of the system clock CK 1 , for example, larger than that of the system clock CK 2 , the false triggered output signal DET can be easily distinguished from a normal active output signal DET due to its narrower active width than the normal one. 
     Accordingly, the present invention provides an apparatus and method for effectively eliminating noise, and more particularly, for cumulatively eliminating the superimposition of extra-signal fluctuations that corrupts an information signal. The present invention overcomes the disadvantages of the prior apparatus or method that usually misses detecting the input signal or generates false triggering in detecting a severely corrupted signal. 
     Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.