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Addressing AD issues
Including a description of the PVQ encoder and decoder
This commit is contained in:
Timothy B. Terriberry
Jean-Marc Valin
parent
eb8b3c2b07
commit
e4689464eb
1 changed files with 67 additions and 8 deletions
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@ -943,7 +943,8 @@ A receiver MUST NOT process packets which violate any of the rules above as
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They are reserved for future applications, such as in-band headers (containing
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metadata, etc.).
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Packets which violate these constraints may cause implementations of
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<em>this</em> specification to treat them as malformed, and discard them.
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<spanx style="emph">this</spanx> specification to treat them as malformed, and
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discard them.
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</t>
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<t>
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These constraints are summarized here for reference:
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@ -1983,6 +1984,7 @@ w0_Q13 = w_Q13[wi0]
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]]></artwork>
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</figure>
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N.b., w1_Q13 is computed first here, because w0_Q13 depends on it.
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The constant 6554 is approximately 0.1 in Q16.
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</t>
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<texttable anchor="silk_stereo_weights_table"
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@ -2105,7 +2107,8 @@ If the frame is an LBRR frame or a regular SILK frame whose VAD flag was set
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A separate quantization gain is coded for each 5 ms subframe.
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These gains control the step size between quantization levels of the excitation
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signal and, therefore, the quality of the reconstruction.
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They are independent of the pitch gains coded for voiced frames.
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They are independent of and unrelated to the pitch contours coded for voiced
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frames.
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The quantization gains are themselves uniformly quantized to 6 bits on a
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log scale, giving them a resolution of approximately 1.369 dB and a range
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of approximately 1.94 dB to 88.21 dB.
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@ -2762,6 +2765,7 @@ y = ((i&1) ? 32768 : 46214) >> ((32-i)>>1)
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w_Q9[k] = y + ((213*f*y)>>16)
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]]></artwork>
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</figure>
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The constant 46214 here is approximately the square root of 2 in Q15.
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The cb1_Q8[] vector completely determines these weights, and they may be
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tabulated and stored as 13-bit unsigned values (with a range of 1819 to 5227,
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inclusive) to avoid computing them when decoding.
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@ -3453,6 +3457,7 @@ a32_Q24[d_LPC-1][n] = a32_Q12[n] << 12 .
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Then for each k from d_LPC-1 down to 0, if
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abs(a32_Q24[k][k]) > 16773022, the filter is unstable and the
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recurrence stops.
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The constant 16773022 here is approximately 0.99975 in Q24.
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Otherwise, row k-1 of a32_Q24 is computed from row k as
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<figure align="center">
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<artwork align="center"><![CDATA[
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@ -4552,7 +4557,7 @@ Then for i such that j <= i < (j + n),
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4
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e_Q23[i] __ b_Q7[k]
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res[i] = --------- + \ res[i - pitch_lags[s] + 2 - k] * ------- .
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8388608.0 /_ 128.0
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2.0**23 /_ 128.0
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k=0
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]]></artwork>
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</figure>
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@ -4566,7 +4571,7 @@ For unvoiced frames, the LPC residual for
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<artwork align="center"><![CDATA[
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e_Q23[i]
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res[i] = ---------
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8388608.0
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2.0**23
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]]></artwork>
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</figure>
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</t>
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@ -5060,14 +5065,14 @@ total_bits, and set dynalloc_loop_log to 1. When the while loop finishes
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boost contains the boost for this band. If boost is non-zero and dynalloc_logp
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is greater than 2, decrease dynalloc_logp. Once this process has been
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executed on all bands, the band boosts have been decoded. This procedure
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is implemented around line 2352 of celt.c.</t>
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is implemented around line 2469 of celt.c.</t>
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<t>At very low rates it is possible that there won't be enough available
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space to execute the inner loop even once. In these cases band boost
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is not possible but its overhead is completely eliminated. Because of the
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high cost of band boost when activated, a reasonable encoder should not be
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using it at very low rates. The reference implements its dynalloc decision
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logic around line 1269 of celt.c.</t>
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logic around line 1299 of celt.c.</t>
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<t>The allocation trim is a integer value from 0-10. The default value of
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5 indicates no trim. The trim parameter is entropy coded in order to
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@ -5079,7 +5084,12 @@ available in the bitstream. To decode the trim, first set
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the trim value to 5, then iff the count of decoded 8th bits so far (ec_tell_frac)
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plus 48 (6 bits) is less than or equal to the total frame size in 8th
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bits minus total_boost (a product of the above band boost procedure),
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decode the trim value using the inverse CDF {127, 126, 124, 119, 109, 87, 41, 19, 9, 4, 2, 0}.</t>
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decode the trim value using the PDF in <xref target="celt_trim_pdf"/>.</t>
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<texttable anchor="celt_trim_pdf" title="PDF for the Trim">
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<ttcol>PDF</ttcol>
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<c>{1, 1, 2, 5, 10, 22, 46, 22, 10, 5, 2, 2}/128</c>
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</texttable>
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<t>For 10 ms and 20 ms frames using short blocks and that have at least LM+2 bits left prior to
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the allocation process, then one anti-collapse bit is reserved in the allocation process so it can
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@ -5188,7 +5198,30 @@ they are equivalent to the mathematical definition.
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</t>
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<t>
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The decoded vector is normalized such that its
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The decoded vector X is recovered as follows.
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Let i be the index decoded with the procedure in <xref target="ec_dec_uint"/>
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with ft = V(N,K), so that 0 <= i < V(N,K).
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Let k = K.
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Then for j = 0 to (N - 1), inclusive, do:
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<list style="numbers">
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<t>Let p = (V(N-j-1,k) + V(N-j,k))/2.</t>
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<t>
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If i < p, then let sgn = 1, else let sgn = -1
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and set i = i - p.
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</t>
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<t>Let k0 = k and set p = p - V(N-j-1,k).</t>
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<t>
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While p > i, set k = k - 1 and
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p = p - V(N-j-1,k).
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</t>
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<t>
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Set X[j] = sgn*(k0 - k) and i = i - p.
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</t>
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</list>
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</t>
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<t>
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The decoded vector X is then normalized such that its
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L2-norm equals one.
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</t>
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</section>
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@ -7204,6 +7237,32 @@ codebook and the implementers MAY use any other search methods. See alg_quant()
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</t>
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</section>
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<section anchor="cwrs-encoder" title="PVQ Encoding">
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<t>
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The vector to encode, X, is converted into an index i such that
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0 <= i < V(N,K) as follows.
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Let i = 0 and k = 0.
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Then for j = (N - 1) down to 0, inclusive, do:
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<list style="numbers">
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<t>
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If k > 0, set
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i = i + (V(N-j-1,k-1) + V(N-j,k-1))/2.
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</t>
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<t>Set k = k + abs(X[j]).</t>
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<t>
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If X[j] < 0, set
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i = i + (V(N-j-1,k) + V(N-j,k))/2.
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</t>
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</list>
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</t>
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<t>
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The index i is then encoded using the procedure in
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<xref target="encoding-ints"/> with ft = V(N,K).
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</t>
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</section>
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</section>
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