1. A method of decoding video data in uni-directional prediction by a decoding apparatus, the method comprising: deriving, by the decoding apparatus, a reference picture index and a motion vector of a current prediction unit; generating, by the decoding apparatus, a prediction block of the current prediction unit using the reference picture index and the motion vector; generating, by the decoding apparatus, a quantized block by inversely scanning quantized coefficient components; generating, by the decoding apparatus, a transformed block by inversely quantizing the quantized block using a quantization parameter; generating, by the decoding apparatus, a residual block by inversely transforming the transformed block; and generating, by the decoding apparatus, reconstructed pixels using the prediction block and the residual block, wherein prediction pixels of the prediction block are generated using an interpolation filter selected based on the motion vector, the interpolation filter being a 7-tap filter if the motion vector indicates a quarter pixel position, the interpolation filter being an 8-tap filter if the motion vector indicates a half pixel position, wherein the quantization parameter is by adding using a differential quantization parameter and a quantization parameter predictor, when only one of a left quantization parameter and an above quantization parameter is available, where the quantization parameter predictor is an average of a previous quantization parameter and the available one of the left quantization parameter and the above quantization parameter, wherein the differential quantization parameter is restored using a bin string indicating an absolute value of the differential quantization parameter and a bin indicating a sign of the differential quantization parameter, and wherein the quantization parameter is derived per quantization unit, and a size of the quantization unit is one of allowable sizes of a coding unit.