Abstract:
Current generation digital media processors support multi-format video resolutions, SDTV, Progressive Scan and HDTV. Built-in video encoders directly support NTSC and progressive 480P video outputs. These two video formats have different image bandwidth and output gain requirements, but are normally filtered by fixed bandwidth filters. This invention provides adjustable filter bandwidth for improved video filtering and solves the dilemma on filter bandwidth design for multi-format video applications. The invention is applicable to video reconstruction filter applications requiring bandwidth adjustable filters.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The technical field of this invention is video filtering. 
   BACKGROUND OF THE INVENTION 
   New generation digital media processors support multi-format resolutions among which are standard density TV (SDVT), progressive scan and high density TV (HDTV). The built-in video encoder of these new media products directly supports national television standards committee specification (NTSC) and the progressive scan 480P video standard specification. These two video formats have different sampling rate requirements for different video signal bandwidth. NTSC video bandwidth is about 4.2 MHz to 5 MHz and requires video filter with cutoff frequency at about 6.25 MHz. Progressive scan 480P requires cutoff frequency at about 12.5 MHz for its wider video bandwidth filtering. 
   When the video encoder digital-to-analog (DAC) outputs NTSC signals a gain of 5.3 is needed in order to meet international television standards (ITU) video standard on level requirement. However, when the media processor outputs progressive scan 480P video in component video or RGB signals, the video standards require a gain of 4. Thus a design employing a fixed-bandwidth filter and gain can meet only limited video format requirements. Accordingly there is a need in the art for a video encoder and filter able to operate in plural video formats. 
   SUMMARY OF THE INVENTION 
   Current generation digital media processors support multi-format video resolutions, such as standard density TV (SDTV) and progressive scan and high density TV (HDTV). Built-in video encoders directly support NTSC and progressive 480P video outputs. These two video formats have different image bandwidth and output gain requirements, but are normally filtered by fixed bandwidth filters. This invention provides adjustable filter bandwidth for improved video filtering and solves the dilemma regarding filter bandwidth for multi-format video applications. The invention is applicable to video reconstruction filter applications requiring bandwidth adjustable filters. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of this invention are illustrated in the drawings, in which: 
       FIG. 1  illustrates the bandwidth requirements for digital media processors supporting multi-format video (Prior Art); 
       FIGS. 2A ,  2 B, and  2 C together illustrate the circuit configurations of second, fifth, and sixth order low pass filters (Prior Art); 
       FIG. 3  illustrates a multi-format video filter implementation of the invention; 
       FIG. 4  illustrates a multi-format video filter of the invention with switches selecting a third order Butterworth configuration; 
       FIG. 5  illustrates a multi-format video filter of the invention with switches selecting a sixth order Butterworth configuration; and 
       FIG. 6  illustrates the frequency response and delay characteristics of the sixth/third order video filter of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A filter design of fixed frequency response and gain meets only one video format requirement. Some designs have previously used simple filters with wider bandwidth to provide filtering for progressive video and passed SDTV with little or no filtering. The present invention is a filter providing adjustable filter bandwidth and gain control at nominal cost and power consumption. 
     FIG. 1  illustrates the bandwidth requirements for digital media processors supporting multi-format video. The video processor input channel  100  supplies luminance component  102  and chrominance components  103  and  104  to the three respective filter networks  105 ,  106  and  107 . When video processor input channel  100  outputs SDTV  108 , the filters  105 ,  106  and  107  use a cutoff frequency fc=6.25 MHz. When video processor input channel  100  outputs progressive scan 480P in RGB format, filters  105 ,  106  and  107  use a cutoff frequency fc=12.5 MHz. For the Y—Pr—Pb format, filter  105  for the Y component uses a cutoff frequency fc=12.5 MHz while filter  106  for the Pr component and filter  107  for Pb component use fc=6.25 MHz. 
   Low pass filter (LPF) designs normally have a topology with a serial inductor in the video signal path and a capacitor in shunt to ground. Changing the shunt capacitance by switching in another capacitance can easily change the filter bandwidth. This apparently simple low cost solution has significant mathematical complications. 
     FIG. 2A  illustrates a basic second order low pass filter  200 . Filter  200  illustrated in  FIG. 2  includes shunt resistance R S , shunt capacitance C 1 , serial inductor L 1  and shunt resistor R L . These drive an operational amplifier having a gain dependent upon the relative resistance values of resisters R 1  and R 2 . Filter  200  has a transfer function: 
                   H   ⁡     (   S   )       =         (       R   1     +     R   2       )     /     R   1             C   1     ⁢     L   1     ⁢     S   2       +       (         R   S     ⁢     C   1       +       L   1       R   L         )     ⁢   S     +       R   S       R   L       +   1               (   1   )               
The normalized transfer function for this filter with equal termination resistance values (Rs=R L ) is:
 
   
     
       
         
           
             
               
                 
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   The ladder filters illustrated in  FIG. 2  have conventionally been implemented in Butterworth or Bessel designs but the Butterworth filter has been found to have phase linearity characteristics preferable to the Bessel filter. The circuit of  FIG. 2A  is known as a second order Butterworth filter. 
   The transfer function of the second order Butterworth filter of the type illustrated in  FIG. 2A  is given by 
                   h   ⁡     (   s   )       =       1       s   2     +       2     ⁢   s     +   1       =     1         A   2     ⁢     s   2       +       A   1     ⁢   s     +     A   0                   (   4   )               
To implement second order Butterworth filter, first set:
 λc=aA 2  and λ+ c= 2 A   1   (5) 
Solving equations (5) simultaneously yields:
 λ= A   1 ±√{square root over ( A   1   2 −2 A   1 )}  (6) With A 1   2 =2A 2   (7) 
in the 2nd order Butterworth polynomial, we have:
 λ=A 1  and c=A 1   (8) 
   Therefore the result is the following two equations: 
                 C   =     2     2   ⁢   π   ⁢           ⁢     f   C     ⁢     R   L                 (   9   )               L   =         2     ×     R   L         2   ⁢   π   ⁢           ⁢     f   C                 (   10   )               
In equations (9) and (10) a change in f c  requires a change in both L and C. Thus it is impossible to change second order Butterworth filter bandwidth from one cutoff frequency to another frequency by adjusting only capacitance and retaining fixed inductance. Both L and C have to be adjusted when cutoff frequency f c  varies. This is true not only in second order filter, but can be shown mathematically as also true in higher order filters.
 
     FIGS. 2B and 2C  illustrate circuit configurations for fifth and sixth order Butterworth filters. These filters have been widely used in LPF design. These will be further described below 
   Table 1 lists the capacitances and inductances of other higher order Butterworth filters derived mathematically similar to equations (9) and (10). 
                                           TABLE 1               Order   C1   L1   C2   L2   C3   L3                   1           2     2   ⁢     πf   c     ⁢     R   L                           2             2       2   ⁢     πf   c     ⁢     R   L                           2     ⁢     R   L         2   ⁢     πf   c                           3           1     2   ⁢     πf   c     ⁢     R   L                         2   ⁢     R   L         2   ⁢     πf   c                       1     2   ⁢     πf   c     ⁢     R   L                           4           0.7654     2   ⁢     πf   c     ⁢     R   L                         1.8478   ⁢           ⁢     R   L         2   ⁢     πf   c                         1.8478   ⁢               2   ⁢     πf   c     ⁢     R   L                         0.7654   ⁢           ⁢     R   L         2   ⁢     πf   c                           5           0.618     2   ⁢     πf   c     ⁢     R   L                         1.618   ⁢           ⁢     R   L         2   ⁢     πf   c                       2     2   ⁢     πf   c     ⁢     R   L                         1.618   ⁢           ⁢     R   L         2   ⁢     πf   c                       0.618     2   ⁢     πf   c     ⁢     R   L                           6           0.5176     2   ⁢     πf   c     ⁢     R   L                           2     ⁢     R   L         2   ⁢     πf   c                       1.9319     2   ⁢     πf   c     ⁢     R   L                         1.9319   ⁢           ⁢     R   L         2   ⁢     πf   c                         2       2   ⁢     πf   c     ⁢     R   L                         0.5176   ⁢           ⁢     R   L         2   ⁢     πf   c                                
To implement a second order Butterworth filter set:
 
                     C   1     =       2       2   ⁢   π   ⁢           ⁢     f   C     ⁢     R   L           ⁢     
     ⁢       L   1     =         2     ×     R   L         2   ⁢   π   ⁢           ⁢     f   C                   (   11   )               
Extensive analysis has shown that it is not possible to meet both NTSC broadcast standards and 480P video standards using an adjustable filter of a given order by half frequency without adjusting both the inductances and capacitances. On the other hand transforming the filter from one order to another order allows adjustment of bandwidth to be achieved by adjusting only the capacitance values. Considering only third to sixth order Butterworth filters, there are 56 possible filter pairs.
 
   Table 1 shows that two pairs of Butterworth filters have a transform meeting the design requirements of both NTSC and 480P formats. These two pairs are: fifth order filter and third order filter; and sixth order filter and third order filter. 
     FIG. 2B  illustrates the change required to convert a fifth order filter into a third order filter. A third order filter is implemented using bypass connection  201 . A fifth order filter removes the bypass connection  201  and inserts the components of block  202  consisting of serial inductor L 2  and shunt capacitor C 3 . 
     FIG. 2C  illustrates the change required to convert a sixth order filter into a third order filter. A third order filter is implemented using bypass connection  203 . A sixth order removes the bypass connection  203  and inserts the components within the block  204  consisting of serial inductors L 2  and L 3  and shunt capacitor C 3 . 
   Holding inductance values constant for a given filter, a change from sixth order to third order requires that:
 
( L   1   +L   2 ) 6th =( L   1 ) 3rd   (12)
 
Table 1 gives the relationship between bandwidth and inductance values of a sixth order filter:
 
                     (       L   1     +     L   2       )       6   ⁢   th       =           2     ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   6         +       1.9319   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   6                   (   13   )               
Table 1 gives the relationship between bandwidth and inductance values of a third order filter:
 
                     (     L   1     )       3   ⁢           ⁢   r   ⁢           ⁢   d       =       2   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   3                 (   14   )               
Solving (12), (13), and (14) simultaneously yields:
 
                   (           2     ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   6         +       1.9319   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   6           )     =       2   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   3                 (   15   )               
And the ratio of f 3  to f 6  is given by:
 
                     f   3       f   6       =           2   ⁢     R   L         2   ⁢   π               2     ⁢     R   L         2   ⁢   π       +       1.9319   ⁢     R   L         2   ⁢   π           =     1   1.673               (   16   )               
Similarly, the ratio of f 3  to f 5  is given by:
 
                             (       L   1     +     L   2       )       5   ⁢   th       =     (         1.618   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   5         +       1.618   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   5           )                 =       (     L   1     )       3   ⁢   r   ⁢           ⁢   d                   =       2   ⁢     R   L         2   ⁢   π   ⁢           ⁢     f   3                 ⁢     
     ⁢   or           (   17   )                   f   3       f   5       =           2   ⁢     R   L         2   ⁢   π         2   ×       1.618   ⁢     R   L         2   ⁢   π           =     1   1.618               (   18   )               
These equations indicate that if a sixth order Butterworth filter with cutoff frequency at f 6 =12.5 MHz is designed for progressive 480P video, then a cutoff frequency at f 3 =7.47 MHz (=12.5/1.673) of 3rd order could be reached by changing only the capacitances.
 
   Alternatively, if a fifth order Butterworth filter with cutoff frequency at f s =12.5 MHz is designed for progressive 480P video, then a cutoff frequency at f 3 =7.725 MHz (=12.5/1.618) of third order could be reached by changing only the capacitances. 
     FIG. 3  illustrates the example of a sixth order Butterworth filter switchable to a third order Butterworth filter providing the solution for multi-format video filter implementation. This filter accomplishes the switching illustrated in  FIG. 2C  and has two −3 db cutoff frequencies: (a) 7.5 MHz for SDTV filtering; and (b) 12.5 MHz for 480P video filtering. 
   The filter bandwidth is selected by: 
                                       TABLE 2                               Butterworth   Cutoff           SW1   SW2   Order   Frequency                           high   low   Sixth   12.5 MHz           low   high   Third    7.5 MHz                        
Transistors  301 ,  302 ,  303 ,  304  and  305  are P-type metal oxide field effect transistors (MOSFET). These MOSFETs are conducting with a high voltage on the gate and non-conducting with a low voltage on the gate. High and low voltages are determined relative to the voltage threshold of the MOSFET.  FIG. 3  lists the preferred component values.
 
   With SW 1  low and SW 2  high transistors  301 ,  302  and  303  are on, transistors  304  and  305  are off, and the circuit of  FIG. 3  becomes the equivalent of the third order filter of  FIG. 4 . In this case: C 1  is CA in parallel with CD; and C 2  is CC in parallel with CE. CB and RL are floating and may be neglected. The contribution of LC is made negligible by isolation of RL. 
   With SW 1  high and SW 2  low transistors  301 ,  302  and  303  are off, transistors  304  and  305  are on, and the circuit of  FIG. 3  becomes the equivalent of the sixth order filter of  FIG. 5 . In this case: C 1  is CA; L 1  is LA; C 2  is CB; L 2  is LB; C 3  is CC; and L 3 =LC. CD, CE, R 0  and R 3  are floating and may be neglected. 
     FIG. 6  illustrates the frequency response characteristics of the sixth/third order video filter embodiment of this invention illustrated in  FIG. 3 . Curve  601  illustrates the condition when SW 1  is low and SW 2  is high having a cutoff frequency of 7.5 MHz. Curve  602  illustrates the condition when SW 1  is high and SW 2  is low having a cutoff frequency of 12.5 MHz.