Abstract:
This device ( 10 ) includes elements ( 12 ) for receiving an analogical signal including a data signal in a frequency channel, elements ( 18 ) for amplifying the received signal, elements ( 20 ) for filtering the amplified signal so as to cut out frequencies outside the frequency channel of the data signal, members ( 24 ) for converting the filtered analogical signal into a digital signal, elements ( 32 ) for measuring the power (P whole ) of the whole received signal after amplification and before filtering, and members ( 26 ) for determining an amplification control signal for the received signal amplification as a function of the received signal measured power (P whole ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of digitizing an analogical signal such as a digital television signal, and a corresponding device. 
     2. Description of the Related Art 
     Digital television data are generally transmitted to a receiver through an analogical data signal, using a predetermined frequency channel. 
     The receiver comprises means for processing the received analogical signal before it is converted into a digital data signal by means of an analogical digital converter. 
     The processing means usually comprises at least a filter, which cuts out the frequencies outside the data channel, frequency translation means, and other processing modules. 
     It is known to distribute amplifiers within the processing means so as to allow optimal use of the converter, and particularly it is known to place an amplifier before the filter and another amplifier before the converter. 
     The gains of the amplifiers are automatically controlled by Automatic Gain Control (AGC) means as a function of parameters of the converted digital data signal, usually the power of the digital data signal compared to a fixed reference. 
     Such receiver is described in FR-A1-2 826 525. 
     However, in harsh environment, the analogical data signal is mixed up with noise signals. 
     A noise signal can be either out of the data frequency range, i.e. out of the data channel, and it is referred to as adjacent channel noise, or inside the data frequency range and it is referred to as co-channel noise. 
     In the prior art receiver, these noise signals are amplified along with the data signal, and therefore lead to saturation of the amplifiers or of the converter. 
     Therefore it is desirable to develop a new method and a corresponding device that overcome this drawback. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a method of digitizing an analogical signal as described below. Other features of the method are also outlined below. 
     The invention further provides a device of digitizing an analogical signal as described below. Other features of the device are also outlined below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       These and other aspects of the invention will be apparent from the following description and drawings upon which: 
         FIG. 1  is a schematic diagram of a receiver according to the present invention; 
         FIG. 2  is a flow chart of a method of digitizing an analogical signal achieved by the receiver of  FIG. 1 ; 
         FIGS. 3 and 4  represent functional diagrams for determining control signals according to a first embodiment, and 
         FIG. 5  is a graph for determining control signals according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a receiver  10  intended to receive digital television data is illustrated. 
     This receiver  10  first comprises an antenna  12  for receiving a analogical signal S comprising digital television data coded in a predetermined frequency channel. It further comprises analogical means  14  for processing the received signal S, and a chip  16  connected to the output of the processing means  14 . 
     The analogical processing means  14  comprise a first amplifier  18  receiving the analogical signal S from the antenna  12  and connected to a filter  20  that cuts out the frequencies outside the data frequency channel. 
     The analogical processing means  14  further comprise, at their output, a second amplifier  22 . 
     The chip  16  comprises a first analogical digital converter  24  connected to the output of the second amplifier  22 . The first converter  24  outputs a digital signal that constitutes the output of the receiver  10 . 
     The chip  16  also comprises automatic gain control (AGC) means  26  delivering two control signals C 18  and C 22  to respectively the first and second amplifier  18  and  22 . 
     The AGC means  26  are able to determine the control signals from the following parameters: the power P partial  of the converted digital signal, an estimated optimal power value P 0  for P partial  (P 0  is the value which P partial  is expected to tend towards), and the power of the analogical signal S before filtering P whole . 
     In order to determine these parameters, the chip  16  first comprises means  28  for measuring the power P partial  of the converted digital signal, and means  30  for computing the estimated optimal power P 0  as a function of the amplitude distribution type, defined by a parameter of the amplitude distribution, both connected to the AGC and to the output of converter  24 . 
     In the described embodiment the parameter is a generalized moment, and preferably the generalized first moment. The estimated optimal power computing means thus  30  comprise a first element  30 A for computing the generalized first moment of the digital signal, representative of the spread of the signal, i.e. its amplitude distribution also called probability density. 
     The computation is achieved over a predetermined period of time by use of conventional formulas, for example the square value of the mean of the absolute terms of the signal divided by the power. Advantageously this computation is achieved on a time sliding window. 
     The estimated optimal power computing means  30  further comprise a second element  30 B for estimating the optimal power from the generalized first moment. In other embodiments, higher order of generalized moment could be used. 
     In the described embodiment, predetermined values of optimal power are stored with their corresponding generalized first moment. The nearest predetermined minimum and maximum values of the generalized moment computed by the first element  30 A are detected, and by interpolation, such as a linear interpolation or the like, the corresponding optimal power P 0  is computed. 
     Moreover, the receiver  10  comprises means  32  for measuring in an analogical way the power P whole  of the analogical signal S before filtering in filter  20 , at the output of the first amplifier  18 . They are connected to the AGC means through a second analogical digital converter  34  of the chip  16 . 
     With reference to  FIGS. 1 and 2 , the steps of a method achieved by the receiver  10  is described. 
     An analogical signal S comprising OFDM or COFDM data signal D and a noise signal N is received by the antenna  12  in a step  40 . 
     In the example, the analogical data signal D carries digital television data. The amplitude distribution type of this data signal D is predetermined and is essentially Gaussian, according to used norms. 
     The noise signal N comprises a co-channel noise signal N 1  and an adjacent channel noise signal N 2 . 
     The method further comprises a step  42  of amplification of the received analogical signal S achieved by amplifier  18 . 
     Step  42  is followed by a step  44  of measurement of a first parameter of the received analogical signal S. More precisely, this parameter is the power P whole  of the whole received analogical signal S, including the adjacent channel noise signal N 2 , over a predetermined period of time. 
     Thereafter, the method continues in a step  46  by filtering by the filter  20  the analogical signal S which removes the adjacent channel noise signal N 2 . 
     The method further comprises a step  47  of amplification of the filtered analogical signal D+N 1  achieved by amplifier  22 . 
     This is followed in a step  48  by the conversion of the received, amplified and filtered signal by converter  24  into a digital signal D+N 1  that comprises the data signal D and the co-channel noise signal N 1 . 
     The method continues with computing an estimated optimal power P 0 , achieved at steps  50  and  52 , by corresponding measuring means  30 . 
     In step  50 , the spread of the digital signal D+N 1  is measured by computing the first generalized moment. 
     In step  52 , the optimal power P 0  is estimated by using the measurement of the spread of the digital signal realised at step  50 . 
     The method continues in a step  54  by computing the power P partial  of the digital signal D+N 1  after conversion by converter  24 . 
     Finally the method comprises a step  56  of determination of the control signals of the amplifiers  18 ,  22  by way of AGC means  26  as a function of:
         the converted digital signal power P partial , as measured in step  54 ,   the estimated optimal power P 0  determined in step  50  and  52 , and   the whole analogical signal power P whole  measured in step  42 .       

     Turning to  FIGS. 3 and 4 , in a first embodiment, the control signal C 18  of the first amplifier  18  is determined as a function of P whole , and the control signal C 22  of the second amplifier is determined as a function of P partial  and P 0 . 
     More precisely, with reference to  FIG. 3 , the AGC means  26  comprises a first error detector between P whole  and a predetermined reference P ref . The resulting error ε is then accumulated in a first module IC in order to obtain the control signal C 18  of the first amplifier  18 . 
     So as to obtain the control signal C 22  of the second amplifier  22 , with reference to  FIG. 4 , the AGC means  26  comprises a second error dector between P partial  and P 0 . Again, the resulting error ε′ is accumulated in a second module IC′ that gives C 22 . 
     In a second embodiment, a global control signal C global  is determined, by accumulating error ε′ between P partial  and P 0 , in the same way as C 22  is determined in the previous embodiment (cf.  FIG. 4 ). The control signals C 18  and C 22  are derived from the global control signal C global , by using the two graphs illustrated on  FIG. 5 . 
     A first graph represents C 18  while a second graph represents C 22  both as a function of C global . The graphs are traced empirically. 
     On a first portion of C global , C 18  increases while C 22  remains constant at a low level. On a second portion of C global , C 18  remains constant at a high level, while C 22  increases from the low level. 
     Hence, as P partial  decreases, the amplification of the received signal S is first achieved by the first amplifier  18 , then by the second amplifier  22 . 
     C 22  is directly determined from the second graph by using the actual value of C global . 
     C 18  however is determined from the first graph by using the actual value of C global  minus a quantity X being a function of P whole . Subtracting the quantity X avoids that C 18  reaches a high level, which could lead to saturation of the filter  20 . 
     The function between X and P whole  is determined by accumulating the error E between P whole  and P ref , in the same way as C 18  is determined in the previous embodiment (cf.  FIG. 3 ), such that C 18  makes amplifier  18  outputting a signal with a power P whole  corresponding to a predetermined reference P ref . 
     The described method and device clearly provide several advantages. 
     First, by using the estimated optimal power of P 0  as a reference, the amplitude distribution of the co-channel noise signal N 1  is taken into account in the AGC means  26 . More precisely, the estimated optimal power P 0  will generally be somewhere between the optimal power P 1  corresponding to the amplitude distribution of the data signal D alone, and the optimal power P 2  corresponding to the amplitude distribution of the co-channel noise signal N 1  alone. Using the estimated optimal power P 0  leads to optimal conversion of the digital signal D+N 1 , which can then be processed in digital circuits (not shown) following the chip  16  so as to retrieve D, and then the digital video data. 
     Moreover, the use of the mean power P whole  of the analogical signal S before filtering avoids saturation of the amplifier  18  that could result if the adjacent channel noise signal N 2  were not considered. 
     The device achieving the method of the invention can be a dedicated device or can be integrated in another general device such as a digital television decoder or a digital television set.