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ietf doc: An initial attempt at explaining the allocation machinery.

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Gregory Maxwell 2009-07-03 22:44:27 -04:00
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104
doc/ietf/draft-valin-celt-codec.xml
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@ -658,6 +658,108 @@ optimal bit allocation, it provides good results without requiring the
transmission of any allocation information.
</t>
<t>
The allocation process begins with a table of prototype allocations,
specified in band_allocation (<xref
target="modes.c">modes.c</xref>). Each row of the table is a single prototype allocation,
in bits per Bark band. These rows must be projected onto the actual band layout in use at the
current frame size and sample rate, using exact integer calculations. The reference
implementation
pre-computes these projections in compute_allocation_table() (<xref
target="modes.c">modes.c</xref>) but implementations are free to use any
approach which produces bit-identical allocation results.
</t>
<t>
Every entry in the allocation table is multiplied by the current number of channels and
the current frame size. Each prototype allocation is projected
independently using the following process: the upper band frequencies (in Hz) from the current Bark band and current CELT band are compared. (When the process begins, these will each be the first band, but will increment independently.) If the current Bark band's upper edge frequency
is less than the current CELT band's upper edge frequency, the entire value of the Bark band plus any carried remainder is assigned to the current CELT
band, and the process begins again with the next
Bark band in sequence and zero remainder. If the current Bark band's upper edge frequency is equal to or greater than that of
the current CELT band, the CELT band will receive only part of this Bark band's allocation.
This portion allocated to the CELT band is then calculated by multiplying the Bark band's allocation by the
difference in Hz between the Bark band's upper frequency and the current
CELT band's lower frequency, adding the width of the current Bark band
divided by two, and then dividing this total by the width of the current Bark
band in Hz. The partial value plus any carried remainder is added to the current
CELT band, and the difference between the partial value and the Bark target is
taken as the new carried remainder. The process begins then again starting at the
next CELT band and next Bark band. Once all bands in a prototype allocation have been considered, any
remainder is added to the last CELT band. All of the resulting values are
rescaled by adding 128 and dividing by 256.
</t>
<t>
For every encoded or decoded frame, a target allocation must be computed
using the projected allocation. In the reference implementation this is
performed by compute_allocation() (<xref target="rate.c">rate.c</xref>).
The target computation begins by calculating the available space as the
number of whole bits which can be fit in the frame after Q1 is stored according
to the range coder (ec_[enc/dec]_tell()), and iff the frame has pitch prediction,
subtracting the number of pitch bands and then multiplying by 16.
Then the two projected prototype allocations whose sums multiplied by 16 are nearest
to that value are determined. These two projected prototype allocations are then interpolated
by finding the highest integer interpolation coefficient in the range 0-16
such that the sum of the higher prototype times the coefficient, plus the
sum of the lower prototype multiplied by
the difference of 16 and the coefficient, is less than or equal to the
available sixteenth-bits.
The reference implementation performs this step using a binary search in
interp_bits2pulses() (<xref target="rate.c">rate.c</xref>). The target
allocation is the interpolation coefficient times the higher prototype, plus
the lower prototype multiplied by the difference of 16 and the coefficient,
for each of the CELT bands.
</t>
<t>
For every encoded or decoded frame, a target allocation must be computed
using the projected allocation. In the reference implementation this is
performed by compute_allocation() (<xref
target="rate.c">rate.c</xref>). The target computation begins by first
calculating the available space as the number of whole bits which can be fit in the
frame after Q1 is stored according to the range coder (ec_[enc/dec]_tell())
and iff the frame has pitch prediction subtracting the number of pitch bands then multiplying
by 16. Then the two projected prototype allocations whose sum times 16 is nearest
to that value are determined. These two projected prototype allocations are then interpolated
by finding the highest integer interpolation coefficient in the range 0-16 such
that the sum of the
higher prototype times the coefficient, plus the sum of the lower prototype times
16 minus the coefficient, is less than or equal to the remaining sixteenth-bits.
The reference implementation performs this step using a binary search in
interp_bits2pulses() (<xref target="rate.c">rate.c</xref>). The target
allocation is the interpolation coefficient times the higher prototype, plus 16
minus the coefficient times the lower prototype, for each of the CELT bands.
</t>
<t>
Because the computed target will sometimes be somewhat smaller than the
available space, the excess space is divided by the number of bands, and this amount
is added equally to each band. Any remaining space is added to the target one
sixteenth-bit at a time, starting from the first band. The new target now
matches the available space, in sixteenth-bits, exactly.
</t>
<t>
The allocation target is separated into a portion used for fine energy
and a portion used for the Spherical Vector Quantizer (PVQ). The fine energy
quantizer operates in whole-bit steps. For each band the number of bits per
channel used for fine energy is calculated by 50 minus the log2_frac(), with
1/16 bit precision, of the number of MDCT bins in the band. That result is multiplied
by the number of bins in the band and again by twice the number of
channels, and then the value is set to zero if it is less than zero. Added
to that result is 16 times the number of MDCT bins times the number of
channels, and it is finally divided by 32 times the number of MDCT bins times the
number of channels. If the result times the number of channels is greater than than the
target divided by 16, the result is set to the target divided by the number of
channels divided by 16. Then if the value is greater than 7 it is reset to 7 because a
larger amount of fine energy resolution was determined not to be make an improvement in
perceived quality. The resulting number of fine energy bits per channel is
then multiplied by the number of channels and then by 16, and subtracted
from the target allocation. This final target allocation is what is used for the
PVQ.
</t>
</section>
<section anchor="pitch-prediction" title="Pitch Prediction">
@ -725,7 +827,7 @@ both the encoder and the decoder.
<section anchor="bits-pulses" title="Bits to Pulses">
<t>
Although the allocation is performed in bits units, the quantization requires
Although the allocation is performed in 1/16 bit units, the quantization requires
an integer number of pulses K. To do this, the encoder searches for the value
of K that produces the number of bits that is the nearest to the allocated value
(rounding down if exactly half-way between two values), subject to not exceeding