Polyphase implementation of a video scalar

Image scaling is important in the conversion between different formats such as NTSC, PAL, HDTV and between the CCIR 601 video resolution to the various sizes included in MPEG coding. The change of the resolution proves specially beneficial for improving the coding efficiency. The scaling operation can be generalized by decimation by a factor of M followed by filtering and then interpolation by a factor of L where M and L are integers. The choice of the digital filter depends on the values of L and M. For certain resolution changes, M and L can be rather large integers. Conventional implementation of the filter may result in huge memory and computational requirements. To reduce this factor for practical applications, a polyphase implementation of the digital filter for the video scalar is presented. Some factors determining the choice of the digital filter are discussed. Finally, examples are shown for different resolution scaling of an image.