Patent Publication Number: US-5025460-A

Title: System and circuits for the processing and transmission of a clock signal, in particular of the video synchronization frequency

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the processing of a video synchronization signal and, more specifically, to systems for the processing and transmission of a clock signal having a period 1/Fsr and a subject to &#34;jitter&#34;, using a measure of the period 1/Fsr with the aid of a sample signal having a frequency Fc. 
     2. Description of the Related Art 
     Generally, to transmit the video frequency or the line frequency or the display frequency or a signal bound to these latter frequencies, a comparison is made between the magnitude of the signal frequency and a sample frequency at the receiver (typically the sample frequency is set at the frequency with which the data are transmitted). 
     Several well known methods for accomplishing this comparison are as follows: 
     direct sampling of the video frequency with the sampling frequency known by the receiver. The trouble is that, for the reconstructed signal to have low distortion, it is necessary to use a high bit rate. 
     phase comparison between the video frequency and the sampling frequency; the transmission occurs, e.g., by sending 0 (zero) with -n≦φ&lt;0 and 1 with 0≦φ&lt;π, where φ is the phase angle. The drawbacks of this method are the variable bit rate and the difficulty of compressing the comparison jitter. 
     discrete measure of the video frequency period by means of the sample frequency. The drawback of this method is that, in the presence of the jitter, the information to be transmitted is of the multilevel type, that is, it increases the bit rate. 
     SUMMARY OF THE INVENTION 
     The present invention is based upon an adaptation of the third method described above in a manner which allows the transmission of minimum information (one bit) for each single period measurement, regardless of the presence of jitter on the video frequency (and, in general, regardless of the clock frequency that is transmitted). 
     The invention involves taking a discrete binary measurement of the ratio Fc/Fsr; the result of this measurement is processed according to the algorithm described below in which the jitter of the signal is compressed and discharged by means of the integration of an error, whereby a sole binary digit is still sufficient for each period of the clock signal. 
     The algorithm used in the invention is as follows: the video synchronism frequency is measured by means of a sampling frequency Fc; the ratio Fc/Fsr is, in general, a decimal number n.m., that is a number between whole numbers n and n+1. Therefore, it is possible to convert the single measurement into a whole number equal to n or to n+1. 
     This result can be achieved, e.g. by means of a circuit that surveys the upward or downward slopes of Fsr and initializes a counter at O at a certain slope; this counter, that increments at frequency Fc, supplies the measurement, i.e., the value of the counting end, that shall be thus n or n+1. 
     The transmission occurs; a O is transmitted the case in which the counting ends at n, and a 1 is transmitted in the case when the counting ends at n+1. All these considerations are valid when Fsr is a nominal value. 
     However, in the presence of the jitter in Fsr, the measurement or the counting can have values different from n and n+1. 
     Instead of transmitting the actual value of where counting ends, (a number of bits&gt;1), the invention only transmits n or n+1, based upon whether counting ends at a value ≦n or ≧n+1, and it is calculated a final error as the actual counting -n, or n+1, according to what it is transmitted. 
     The error, a quantity with sign, is added to the value of the counting end of the following cycle; this latter magnitude determines whether it is suitable to transmit n or n+1. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above and other features of the present invention will be more readily understood when the following detailed description is considered in conjunction with the accompanying drawings wherein like characters represent like parts throughout the several views and in which: 
     FIG. 1 is a block diagram of the overall invention; 
     FIG. 2 is a block diagram of the invention with overflow and underflow detection; 
     FIG. 3 is a detailed schematic view of the circuitry of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The block scheme of the invention is illustrated in FIG. 1, where in 1 the actual counting is carried out; in 2, the addition between counting and error is carried out; in 3, the decision is made, in 4 the difference between addition and value transmitted in 6 is determined in 5, this difference is stored as the error 7, that is re-applied to the adder 2 by means of loop 8. 
     For example, having as cumulated a negative error (n being transmitted) it is necessary to bring the error back towards positive values, that the counting end at values ≧n+1, while n is still being transmitted. 
     Thus: 
     
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Trasm. value                                                              
        . . .                                                             
           n n + 1                                                        
                 n n   n   n   n  n  n   n   n   n n + 1                  
actual counting                                                           
        . . .                                                             
           n n + 1                                                        
                 n n - 1                                                  
                       n - 1                                              
                           n - 2                                          
                               n  n  n + 1                                
                                         n + 2                            
                                             n + 1                        
                                                 n n + 1                  
error   . . .          -2  -4  -4 -4 -3  -1  0   0 0                      
__________________________________________________________________________
 
    
     Therefore, having a bit for each period of Fsr, the remote terminal can reconstruct, on the average, the video synchronization frequency. 
     Proper reconstruction is possible due to the integration of the error caused by the jitter and due to the compression of the jitter. 
     U.S. Pat. No. 4,894,710 discloses a system for the processing of the television signal based upon mono- or bidimensional transformation coding to obtain a high compression of the information to be transmitted. The following description refers to a circuit implementation for the multiplexer, e g. operating at 34368 Kb/s, used in the system of U.S. Pat. No. 4,898,710. 
     The counter for measuring the video synchronization frequency does not carry out a counting from 0 up to n or at n+1; it is preferred, to simplify the circuit, to use a five-bit counter that is saturated many times in a period of Fsr. 
     Further, to simplify the arithmetics, it is desired that the counting end nominally at 15 or 16: this requires that the counter be initialized at a particular value calculated according to Fsr and Fc. For example, if Fsr is 31250 Hz (2×15625) and Fc is 4.296 MHz, it is necessary to count 137 or 138 clock strokes at Fc frequency, then to initialize the counter at 7, so that the counting ends at 15 or 16. To take consideration of the jitter of Fsr and the value of Fc, counting must end at values included between 3 and 28; in the other cases, where counting is equal to 0, 1 and 2 or 29, 30 and 31, an alarm signals indicating an out of limit condition. 
     To always consider the allowed jitter, it is assumed that, in conditions of correct functioning, the accumulated error is included between -31 and 31; an overflow situation exists if the actual counting + error ≧48, or an underflow condition exists if the actual counting +error ≦-17. Both overflow and underflow situations generate an alarm signaled to the outside. 
     In the case of overflow and underflow, the result of the addition is controlled so that it occurs as follows: 
     
         -16≦controlled addition ≦47 
    
     The consequent block scheme is indicated in FIG. 2. Further, another counter is used (e.g. as identified by reference number 11 in the FIG. 3) that counts the number of transitions of the video synchronization frequency in a pre-determined interval, in particular in the network period of the multiplexer. 
     On the basis of the jitter of Fsr and of the frame frequency, that in this case is 4 KHz, the number of transitions can be 7, 8 or 9; in any other case, an alarm signals. 
     This last information that requires two bits to be transmitted, is protected by transmitting a suitable coding of six bits (e.g. using the circuit identified by reference numeral 13 in FIG. 3); in reception, it is recovered with a decision at majority two/three. 
     Finally, always nine information bits are transmitted, of which the bits 7, 8 or 9 will be significant on the basis of the indication of the six bits obtained by the transition counter, and one external alarm bit coming from the plate that supports the circuit. Therefore, 16 information bits flow on one frame at 4 KHz for a total flow at 64 KHz. 
     FIG. 3 illustrates in detail the circuit construction of this particular embodiment. 
     This figure shows the counter 1 with five bits C0, C1, C2, C3 and C4 for the measurement, the counter 11 with four bits CF0, CF1, CF2, and CF3 for counting the transitions of the video synchronization frequency FSINC in the frame period and the circuit 12 that surveys the transitions (the slope fronts) of FSINC. Central to the scheme is the circuit that carries out the real algorithm: the adder 2 that carries the algebraic addition (it uses a complementary two&#39;s representation of the negative numbers) of the counting 1 and of the error 7 ES, ER4, ER3, ER2, ER1 and ER0, followed by the limiter 10 controlled by the signals UD and OVN; the limiter is followed by the adder 4, of the controlled addition and the transmitted datum D (0), whose output forms the future error. 
     The logic circuit 16, necessary for the coding of the transition number of FSINC in the main frame period and for the preparation of 16 bits SYPA (15:0) to be sent into the frame, completes the present embodiment. 
     Obviously, the invention has been described for clarity&#39;s sake with reference to the drawings and examples, but it is not limited to these embodiments as it is susceptible of changes and modifications which, being within the reach of the person ordinarily skilled in the art, fall naturally within the spirit and scope of the following claims.