The accuracy requirements specified in the main body of ISO/IEC 23002-1:2006 are essentially the same as those previously specified in IEEE 1180-1990 (which has since been withdrawn), in Annex A of ITU-T Rec. H.261, and in Annex A of ITU-T Rec. H.263. These requirements have been specified herein to resolve normative references to IEEE 1180-1990 in MPEG standards after its withdrawal and to provide improved clarity for the specification of IDCT accuracy requirements.
An additional requirement on encoded-bitstream intra refresh frequency was also previously specified in IEEE 1180-1990, establishing a requirement of bitstream conformance that each macroblock be intra-coded at least once within each series of 132 times that it is coded in a predicted picture without an intervening intra picture. That additional requirement is not specified in ISO/IEC 23002-1:2006 , in order to confine its scope to the domain of decoder conformance specification.
Some allowances for modification of the specified accuracy requirements are made within ISO/IEC 23002-1:2006. Additional accuracy requirements that may be invoked by a referencing specification are specified in Annexes A and B.