Design of MPEG-4 AAC Audio Decoder
|Course||Signal and Information Processing|
|Keywords||MPEG-4 AAC IMDCT Joint coding LTP|
The MPEG AAC(Advanced Audio Coding)family is the consequent continuation of the truly successful audio codec MPEG Layer-3, widely known as MP3. With bit rates ranging from 24 kbit/s to 256 kbit/s, MPEG AAC combines excellent coding efficiency with highest audio quality, and is fully multi-channel capable. MPEG AAC has the characters of high reconstruction audio quality, modeling of coding and decoding process, flexible audio channel configuration, and there is different algorithm complexity at different profile.The development of MPEG AAC started at 1994. only three years later the new format became part 7 of the MPEG standard as MPEG-2 AAC, which is individual audio processing part to MPEG-2 part 3 Audio. Also because of the outstanding coding efficiency and audio quality, MPEG AAC was later brought into the MPEG-4 standard as the part 3 of MPEG-4. MPEG-4 AAC has multiple improvement and development based on the MPEG-2 AAC. MPEG-4 AAC now is widely applied for the next generation mobile communication, digital audio storing, Internet multimedia transmitting, digital audio broadcasting (DAB), etc.This paper focuses on analyzing the content of standard and completing the software realization and optimization of MPEG-4 AAC decoder.To summary, the contents of this paper include following aspects:Firstly, the audio signal knowledge, audio data compressing theory, the development history of MPEG AAC and other audio processing standard, and the future development direction is introduced. Because the MPEG AAC was firstly brought into the MPEG-2 system, the comparation between MPEG-2 AAC and MPEG-4 AAC is finished in this part, focusing on the technology character of MPEG-4 AAC.Secondly, this paper finishes the parsing design of the payload bitstream. The MPEG-4 AAC bitstream structure complies with the ISO new multimedia format--ISO Base Media File Format( the part 12 of MPEG-4) and MP4 file format(the part 14 Of MPEG-4). At the same time ,to be compatible with MPEG-2 bitstream,this paper also focuses on the parsing of MPEG-2 bitstream format—ADTS and ADIF.Then, the design scheme of the MPEG-4 decoding function module is proposed in this paper, including Huffman decoding, scalefactor decoding、IMDCT、 PNS、 joint coding、 LTP、TNS、gain control,etc. Such function modules are independent from each other. But there are some decoding order or between some specific modules.At last an integrated program structure introduction is proposed, focusing on some important function body and structure body, the conclusion of this paper and the future research direction is pointed out.