Ogg Vorbis DecoderImplementationin BluespecMarek DoniecMIT CSAIL 2008Image source: http://www.vorbis.com2Ogg What!?Ogg Vorbis audio codec: General purpose, perceptual.Decoder computationally simple.Open source! License & royalty free!3Outline1.) Internal decoder structure.2.) Current hardware implementation.3.) Decoder performance and future work.4Ogg Vorbis Decoderstream decoderesidue decoder*01000101 residue vectorfloor vectorfloor decode IMDCTframe spectrum input bit streamframe overlapPCM output audio frame5Ogg Container for Vorbis DataOgg Vorbis streamheadersegment size tablesegments=Vorbis packetsheader...Ogg packetmore packetsinitialization headercomment headersetup headeraudio packet 0...Vorbis packets6Initialization and Comment Headers●Initialization header:–Vorbis version–Number of channels, sample rate–Small and large frame size (block sizes)●Comment header:–Song title, artist, weblink, ...–Not important for decoder!7Setup Header●Contains probability model:–Huffman codebooks–Vector quantization (VQ) tables–Floors (type, size, resolution, cb index)–Residues (type, size, resolution, passes, cb index)–Modes and Mappings●Needs to be stored for further decode:–Large memory consumption: header > 2kb–When unpacked > 256kb!8Audio Packets●Each packet contains one frame for all channels.●First Floors are stored (coarse spectrum)●Residues follow (spectral details)●Decoder reconstructs spectrum and feeds it to IMDCT. The result is overlapped with previous frames to generate output.9Bluespec ImplementationRAM withOgg Vorbis fileOggReaderRAM → bit streamrule basedhigh level controlinit FSMcomment FSMsetup FSMcb gen FSMaudio FSMreq respstartResidueGenFloorGenCodebooksVQ tablesMemoryCouplingDotIMDCTOverlapOutputPCMreset10Audio Packet Setupm Modesblocksizemappingmode[0]...mode[m-1]mode[1]mode[2]packetmodenumbern Mappingsc Couplingsmagnitude[0]angle[0]mapping[0]...mapping[n-1]mapping[1]mapping[2]currentchannelnumber2 Blocksizesblocksize[0]blocksize[1]...magnitude[c-1]angle[c-1]mux[0]...mux[a-1]s Submapssubmap floor[0]submap residue[0]...submap floor[s-1]submap residue[s-1]f Floorsfloor[0]...floor[f-1]floor[1]floor[2]r Residuesresidue[0]...residue[r-1]residue[1]residue[2]a Muxescouplingstepnumber11Huffman Codebooks●Need low memory usage + fast decode!●Binary tree with one 16 bit entry per node–MSB: data?–15LSB: data/ right child offset–Left child is in next entry.●Current usage: ~16kbHuffman Tree RAM+1001+30+1011213111......01 23000111bit streamfrom packetdecodedentry number......12Codebooks and VQ TablesHuffman Tree RAM+1multiplicand[0]n Codebookscodebook[0]codebook[1]codebook[2]...codebook[n-1]d dimensionse entrieslookup typecase (lookup type)1: l = log (e) lookup values2: l = e / d lookup valueshuffman ram offsetd...multiplicand[l-1]Optional: if lookup type > 0001+30+1011213111......01 23000111bit streamfrom packetdecodedentry numberVQ RAM...value[0,d-1]...value[0,0]...value[e-1,d-1]value[e-1,0].........codeword length[0]...codeword length[e-1]VQ ram offset+·dreturnfixed point vectorVQ context?yesnoreturn integercodebooknumberdelta valuemin value...13Performance and Future Work●Tested with 16 bit mono, 22050 Hz audio files:–Lowest quality < 1 MHz / sec–Highest quality < 2 MHz / sec–Verilog multiplications and Divisions are used!●To be done next:–Place queues and pipeline!–Optimize VQ tables (clock cycles vs RAM)–Test multichannel
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