An estimation of mismatch error in IDCT decoders implemented with hybrid-LNS arithmetic

This paper investigates the magnitude of the mismatch error produced when an IDCT unit, built using the Hybrid-Logarithmic System (Hybrid-LNS), is used as part of an MPEG or videophone decoder. Mismatch errors occur when different IDCT algorithms are used in the encoding and decoding unit of an image compression system. As the IDCT forms part of the feedback loop in such a system, the errors can accumulate and affect the overall picture quality. To remove the errors it is necessary to periodically transmit intra-frames. However, if the intra-frames are transmitted too frequently the efficiency of the transmission is reduced (i.e. the required bandwidth. increases). This paper uses a well known standard, the IEEE 1180-1990, to assess the performance of a range of decoders built using Hybrid-LNS and compares them with a typical fixed-point equivalent decoder. The results show that to achieve the performance levels defined by 1180-1990 the major advantages of Hybrid-LNS: low-power and low complexity are somewhat compromised.