Reduced Power Consumption for MPEG Decoding with LNS

By reducing the accuracy of the Logarithmic Number System (LNS) it is possible to achieve lower power consumption for multimedia applications, such as MPEG, without sig-niicantly lowering the visual quality of the output. An LNS wordsize of 8 to 10 bits produces a comparable MPEG output as a xed-point wordsize of 14 to 16 bits. The switching activity of an LNS ALU that computes the Inverse Discrete Cosine Transform (IDCT) is one quarter that of xed point, implying lower power consumption. By skipping inputs that are zero (which MPEG can do naturally with its run-length coding and zigzag ordering) the switching activity of LNS MPEG becomes one-tenth that of xed point, in contrast to the minimal impact zero skipping has on xed-point power consumption. 1. LNS and power consumption Portable devices, like WAP phones and DVD players, often use the Motion Picture Expert Group (MPEG) compression standard 12] for video. It is likely that such battery-powered multimedia devices will beneet from Application Speciic Processors fabricated using Complementary Metal Oxide Semiconductor (CMOS) technology. The less power consumed by the CMOS circuit, the longer the batteries will last, which is a signiicant issue for a demanding application like MPEG. The power consumption, P , for a CMOS circuit composed of simple gates (such as ANDs/ORs) can be estimated 14, 7] as (1) where a i is the average switching activity (transitions per clock cycle) on the wire(s) that connects to gate i, C i is the capacitance associated with gate i, V dd is the supply voltage to every gate and f clk is the clock frequency. The major thrust in low-power design is to reduce the eeective capacitance, a i C i. This term is called the eeective capacitance because the activity, a i , mitigates how much of the physical capacitance, C i , aaects the circuit. The designer can reduce the eeective capacitance by cutting the switching activity, a i. One way to compare the merit of alternative designs is to measure the switching activity of each gate using a simulation.