The Research And Realization Of AC-3 Audio Encoding Algorithms

AC-3 is a digital audio coding technology developed by Dolby Lab in the early 1990s. By adopting this technology, 5 full frequency band channels and a low frequency effect channel can be transferred or stored, while the space required is less than one channel linear PCM data in CD. So AC-3 technology has been widely adopted for the audio codec in DVD films, HDTV, digital movies, etc. Currently, the study on the realization of decoder is very mature, but concerning to the specific study on the encoding technology of AC-3, the achievement is rather limited. Therefore, this paper mainly disusses on several relative issues of AC-3 encoding.This thesis firstly describes the core encoding technology of the AC-3 standard, and realizes the encoder and decoder by C language, transplants C codes to the DSP developing platform TMS320DM642EVM and TMS320C6713DSK. The audio information is collected and coded in DM642. Then coded audio stream is transferred to C6713DSK through the serial port, and decoded in real-time. So AC-3 audio codec system is established finally.This paper mainly focuses on the analysis of the encoding algorithm of AC-3. Firstly, the framework of the encoder is accomplished. Secondly, the core arithmetic modules in AC-3 encoder are discussed in detail, including input PCM, analysis filter band, coupling technology, rematrix, encode exponent, bit allocation, quantize mantissas and pack AC-3 frame. Thirdly, the key modules are optimized in order to improve coding quality and increasing coding speed: the N point MDCT fast algorithm by N/4 point FFT is utilized, and realized by the assembly language. A new method based on the quantitative analysis is proposed so that the bit allocation can be accomplished more quickly under certain accuracy and a great amount of time can be saved. With the coupling module, the encoder can process the multi-channel audio in a low bit stream rate.The encoder algorithm is transplanted and optimized based on DM642 hardware platform. Now the encoder is passed through the subjective test which includes 12 test sequences, with 128kbps bit stream rate the average MOS point is 3.88. For the moment the encoder can run real-time on DM642 platform at 112 MHz for two-channel audio coding.