1. An image processing system, comprising: a content detection module having an input to receive a sequence of input pixels and configured to generate an adjustable parameter alpha (α) based on differences among every four consecutive input pixels; and a digital filter having an input for the sequence of input pixels and a control input coupled to an output of the content detection module, the digital filter adjusting filtering coefficients according to the parameter to generate one output pixel for every four consecutive input pixels, wherein each output pixel is generated by multiplying four coefficients C −1 , C 0 , C 1 and C 2 to the sequence of four consecutive input pixels x(−1), x(0), x(1) and x(2) respectively and adding the multiplication results as represented by an equation of 
 y ( k )= C 2 x (2)+ C 1 x (1)+ C 0 x (0)+ C −1 x (−1), wherein x(−1) is the first input pixel in the four consecutive input pixels, x(0) is the second, x(1) is the third and x(2) is the last, the generated output pixel is between x(0) and x(1), the four coefficients are determined from the adjustable parameter α by the following equations: 
 C −1 =αμ k 2 −αμ k , 
 C 0 =−αμ k 2 +(α−1)μ k +1, 
 C 1 =−αμ k 2 +(α−1)μ k , 
 C 2 =αμ k 2 −αμ k , and 0≦μ k <1.