Avoiding oddification to simplify MPEG-1 decoding with LNS

Abstract

Low-precision logarithmic number system (LNS) arithmetic can reduce the power consumption for MPEG decoding compared to conventional fixed-point techniques. Although this introduces small numeric errors, which violate the IEEE-1180 standard for the inverse discrete cosine transform (IDCT), the visual effects of such error may be tolerable for portable battery-powered devices, like videophones, that have limited-resolution displays. The MPEG standard achieves video compression by quantization of the data fed to the IDCT. The MPEG decoder must multiply this data by dequantization factors. Such multiplication, by itself, is trivial with LNS since adding logarithms is equivalent to multiplication. The IEEE-1180 standard suggests oddification, where fixed-point data is forced to become odd after dequantization to minimize IDCT mismatch between the encoder and the decoder. Oddification poses an implementation problem for data in LNS format. This paper suggests that the visual effect of LNS without oddification is nearly indistinguishable from LNS with oddification, meaning that the benefits of LNS in MPEG are even greater than previously expected.