T.-H. Tsai and Y.-C. Yang

1. Low power and cost effective VLSI design for an MP3 audio decoder using an optimized synthesis-subband approach T.-H. Tsai and Y.-C. Yang Department of Electrical Engineering and National Central University, Taiwan ROC IEE Proceedings on Computers and Digital Techniques

2. Abstract • An optimized approach to MPEG layer-3(MP3) audio decoding is presented, with the main theme focused on the synthesis subband. Since the synthesis subband is the most power-consuming component in decoding, a cost-effective architecture is proposed based on a system-design consideration. By means of an algorithm and architecture, the synthesis subband archives a high throughput with reduced memory requirements and hardware complexity. With a two-stage pipeline architecture, it allows 100% hardware utilization and is suitable for low-power implementation. In addition, the chip design in a 0.35um process is also accomplished. It occupies a die area of about 2.7 × 3.2 mm2 with a transistor count of 157,469 and a low-power dissipation of only 2.92mW A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

3. What’s the problem • MPEG layer-3(MP3) coding has been widely applied to current digital audio broadcasting and multimedia application • A cost-effective and low-power implementation will largely reduce the hardware and computation complexity • From the MP3 decoder point of view, the computational load depends on the realization of a synthesis subband A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

4. Outline • Introduction of synthesis subband • Implementation considerations and analysis • Proposed method and architecture • Results and comparison • Conclusion A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

5. Introduction(1) • Elementary concept of MP3 • Multirate subband-based coding techniques • In the encoder, it performs analysis subband filtering with 32 equally spaced filterbanks based on a psychoacoustical model • In the decoder, it performs synthesis subband filtering • Most fast algorithms’ techniques interpret synthesis subband filtering as a modified discrete cosine transform (MDCT) with some additional windowing operations A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

6. Introduction(2) • One of the popular method • Translate DCT into a FFT kernel • Advantage：Because of FFT equations’ specific symmetric and recursive property, we can reduce the number of multiplications and additions • Disadvantage：these methods have complex control and irregular data flow which will introduce a high hardware cost • The proposed design • reduced memory requirements and hardware complexity • High efficiency with 100% hardware utilization using a two-stage pipeline architecture A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

7. Introduction(3) • MP3 decoding flow • Hybrid filter bank divided into inverse modified discrete cosine transfer with dynamic windowing and overlap (DWIMDCT), and the synthesis subband filterbank A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

8. Introduction(4) • Synthesis-subband decoding flow A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

9. Implementation analysis • Design target • Delivering the required high performance at the minimum cost and the smallest silicon area • The performance is determined by real-time constraints A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

10. Implementation analysis (cont.) • MOPS = Fs × ΣC × N • Fs：Sample frequency • ΣC：Total number of numerical calculations per sample • N：number of audio channel A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

11. Implementation consideration • In synthesis subband, IMDCT can be broken into an FFT, a data shift, preprocessing and post-processing • Three considerations • The initial transformer, the real-number computation is also translated into the complex number computation • Data shift, preprocessing and post-processing still contain complex multiplications • FFT algorithms always need many multipliers, and the butterfly recursive process leads to some complex interconnection and routing A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

12. Proposed method • Normal IMDCT： • Proposed IMDCT： • Require about ¼ amount of multiplier-accumulate computations • Required size for the ram buffer can be reduced to only 512 words per channel(½ amount of original) A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

13. Architecture IPQMF IMDCT A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

14. Architecture (cont.) • Pipeline architecture A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

15. Memory configuration (1) A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

16. Memory configuration (2) • Data conflicts in IMDCT and IPQMF A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

17. Memory configuration (3) • Memory data access with pipeline operation A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

18. Results and comparison (1) A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

19. Results and comparison (2) A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

20. Results and comparison (3) A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

21. Conclusion • By means of novel algorithm and architecture, the synthesis subband has a better performance • It also archives a high throughput, with a low-cost memory requirement and hardware complexity A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY

22. A FIXED-POINT MPEG AUDIO PROCESSOR OPERATING AT LOW FREQUENCY