SMPTE RP 202-2008

As bit-rate reduction has become pervasive in emission, contribution, and distribution of video content, multiple compression and decompression (codec) cycles are required. Concatenation of codecs may be needed for production, post-production, transcoding, or format conversion. Any time video transformations to or from the coefficient domain of macroblock-based algorithms are performed, care must be exercised in alignment of the video both horizontally and vertically as it is coded from the raster format or decoded and placed in the raster format.

Over multiple compression and decompression cycles, it is highly desirable to maintain the original positioning of the video signal within the raster. Less obvious, but just as important, is the need for macroblock alignment to reduce artifacts among encoders and decoders from various equipment vendors. If concatenated encoders do not share common macroblock boundaries, then additional quantization noise, motion-estimation errors, and poor mode decisions may result. Likewise, encoding decisions that may be carried through the production and post-production process with recoding data present will rely upon macroblock alignment. Conforming to this practice will minimize artifacts in multiple generations of compression (bit-rate reduction) encoding and decoding by optimizing macroblock alignment.