Changes between Initial Version and Version 1 of SV7FrameDecoding


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Timestamp:
05/25/07 17:25:10 (17 years ago)
Author:
r2d
Comment:

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  • SV7FrameDecoding

    v1 v1  
     1= Decoding a sv7 frame =
     2
     3For more details on the entropy decoding of musepack, see mpc_decoder_read_bitstream_sv7() in [source:libmpc/trunk/libmpcdec/mpc_decoder.c mpc_decoder.c]
     4
     5=== datas from stream header : ===
     6
     7max_band : max number of bands used in the stream[[BR]]
     8ms : 1 if mid/side stereo is used, 0 else[[BR]]
     9
     10
     11=== used datas : ===
     12
     13L/R means that there is a data for the right and the left channel[[BR]]
     14
     15max_used_band : number of bands used in this frame[[BR]]
     16res[L/R]![32] : Quantization level of each band, range from -1 to 17[[BR]]
     17ms_flag![32] : 1 if mid/side stereo is used for this band, 0 else[[BR]]
     18SCFI[L/R]![32] : Scalefactor index, range from 0 to 3[[BR]]
     19SCF[L/R]![32]![3] : Scalefactor, range from -6 to 121. There is 3 scalefactors per band (so 1 scalefactor for 12 samples)[[BR]]
     20Q[L/R]![32]![36] : Quantized samples values of the bands, ranges from -2^15^-1 to 2^15^-1[[BR]]
     21
     22
     23== 1) Decoding the quantization levels ==
     24
     25=== 1.1) first band ===
     26{{{
     27res[L][0] = read 4 bits from stream as unsigned integer
     28res[R][0] = read 4 bits from stream as unsigned integer
     29
     30if (res[L][0] != 0 && res[R][0] != 0) {
     31        max_used_band = 1;
     32        if (ms == 1)
     33                ms_flag[0] = read 1 bit from stream
     34}
     35}}}
     36
     37=== 1.2) other bands ===
     38{{{
     39for each band n from 1 until max_band do :
     40
     41        index = decode from stream as huffman code using table mpc_HuffHdr
     42        if (index != 4)
     43                res[L][n] = res[L][n - 1] + index
     44        else
     45                res[L][n] = read 4 bits from stream as insigned integer
     46               
     47        if (index != 4)
     48                res[R][n] = res[R][n - 1] + index
     49        else
     50                res[R][n] = read 4 bits from stream as insigned integer
     51       
     52        if (res[L][n] != 0 && res[R][n] != 0) {
     53                max_used_band = n + 1;
     54                if (ms == 1)
     55                        ms_flag[n] = read 1 bit from stream
     56        }
     57}}}
     58
     59== 2) Scalefactors index ==
     60{{{
     61for each band n from 0 to max_used_band - 1 do :
     62
     63        if (res[L][n] != 0)
     64                SCFI[L][n] = decode from stream as huffman code using table mpc_HuffSCFI
     65        if (res[R][n] != 0)
     66                SCFI[R][n] = decode from stream as huffman code using table mpc_HuffSCFI
     67}}}
     68
     69== 3) Scalefactors ==
     70
     71=== 3.1) Definition ===
     72
     73define decode_scalefactor(ref) as :
     74{{{
     75        index = decode from stream as huffman code using table mpc_HuffDSCF
     76        if (index != 8)
     77                return (ref + index)
     78        else
     79                return (read 6 bits from stream as insigned integer)
     80}}}
     81
     82=== 3.2) Decoding ===
     83{{{
     84for each band n from 0 to max_used_band - 1 do :
     85
     86        if (res[L][n] != 0) {
     87                SCF[L][n][0] = decode_scalefactor(SCF[L][n][2])
     88                if (SCFI[L][n] & 2)
     89                        SCF[L][n][1] = SCF[L][n][0]
     90                else
     91                        SCF[L][n][1] = decode_scalefactor(SCF[L][n][0])
     92               
     93                if (SCFI[L][n] & 1)
     94                        SCF[L][n][2] = SCF[L][n][1]
     95                else
     96                        SCF[L][n][2] = decode_scalefactor(SCF[L][n][1])
     97        }
     98       
     99        do the same as before replacing L channel by R
     100}}}
     101
     102,,note : SCF[L][n][2] when used as reference to decode SCF[L][n][0] is the last decoded scalefactor (in the last frame). So here there is a frame dependency,,
     103
     104== 4) Samples ==
     105{{{
     106for each band n from 0 to max_used_band - 1 do :
     107
     108        switch (res[L][n]) {
     109                case -1
     110                        set each sample in Q[L][n] to a random number whose range is -510 to 510 // distribution needs to be precised
     111                case 1
     112                        index = read 1 bit from stream as insigned integer
     113                        huff_table = mpc_HuffQ[0][index]
     114                        for each 3 samples k in Q[L][n] do :
     115                                index = decode from stream as huffman code using table huff_table
     116                                Q[L][n][k] = idx30[index]
     117                                Q[L][n][k+1] = idx31[index]
     118                                Q[L][n][k+2] = idx32[index]
     119                                with :
     120                                idx30[] = { -1, 0, 1,-1, 0, 1,-1, 0, 1,-1, 0, 1,-1, 0, 1,-1, 0, 1,-1, 0, 1,-1, 0, 1,-1, 0, 1};
     121                                idx31[] = { -1,-1,-1, 0, 0, 0, 1, 1, 1,-1,-1,-1, 0, 0, 0, 1, 1, 1,-1,-1,-1, 0, 0, 0, 1, 1, 1};
     122                                idx32[] = { -1,-1,-1,-1,-1,-1,-1,-1,-1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1};
     123                case 2
     124                        index = read 1 bit from stream as insigned integer
     125                        huff_table = mpc_HuffQ[1][index]
     126                        for each 2 samples k in Q[L][n] do :
     127                                index = decode from stream as huffman code using table huff_table
     128                                Q[L][n][k] = idx50[index]
     129                                Q[L][n][k+1] = idx51[index]
     130                                with :
     131                                idx50[] = { -2,-1, 0, 1, 2,-2,-1, 0, 1, 2,-2,-1, 0, 1, 2,-2,-1, 0, 1, 2,-2,-1, 0, 1, 2}
     132                                idx51[] = { -2,-2,-2,-2,-2,-1,-1,-1,-1,-1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2}
     133                case 3, 4, 5, 6, 7
     134                        index = read 1 bit from stream as insigned integer
     135                        huff_table = mpc_HuffQ[res[L][n] - 1][index]
     136                        for each sample k in Q[L][n] do :
     137                                Q[L][n][k] = decode from stream as huffman code using table huff_table
     138                other case
     139                        for each sample k in Q[L][n] do :
     140                                Q[L][n][k] = (read res_bits[res[L][n]] bits from stream as insigned integer) - dc[res[L][n]]
     141                                with :
     142                                res[] = { 0,  0,  0,  0,  0,  0,  0,  0,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16}
     143                                dc[] = { 0, 1, 2, 3, 4, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767}
     144        }
     145       
     146        do the same as before replacing L channel by R
     147}}}
     148
     149== 5) Dequantization and inverse filtering ==
     150
     151You will need SCF and Q for this step.[[br]]
     152I think that those steps are the same as mpeg layer 2 dequantization and inverse filtering. Is this true ?