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Multiplying the first part of the analysis filter constant tables
by some coefficients and dividing the second part by the same
coefficients is a transformation which should produce the same
results if rounding errors are not taken into account. These
additional C0/C1/... coefficients can be varied in a certain
range (the requirement is that we still do not get overflows).
The 'magic' values for these coefficients are selected in such
a way that the rounding errors are minimized (rounding errors
are unavoidable when putting all the floating constants into
16-bit tables and losing some of the fractional part).
Also non-SIMD variant of the analysis filter is dropped because
keeping it would require applying a similar change to its tables,
which is a bit tricky and just increases maintenance overhead.
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Most SIMD instruction sets benefit from data being naturally aligned.
And even if it is not strictly required, performance is usually better
with the aligned data. ARM NEON and SSE2 have different instruction
variants for aligned/unaligned memory accesses.
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Added SIMD-friendly C implementation of SBC analysis filter (the
structure of code had to be changed a bit and constants in the
tables reordered). This code can be used as a reference for
developing platform specific SIMD optimizations. These functions
are put into a new file 'sbc_primitives.c', which is going to
contain all the basic stuff for SBC codec.
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This code is heavily based on the patch submitted by Jaska Uimonen.
Additional changes include preserving extra bits in the output of
filter function for better precision, support for both 16-bit and
32-bit fixed point implementation. Sign of some table values was
changed in order to preserve a regular code structure and have
multiply-accumulate oparations only. No additional optimizations
were applied as this code is intended to be some kind of "reference"
implementation. Platform specific optimizations may require
different tricks and can be branched off from this implementation.
Some extra information about this code can be found in linux-bluetooth
mailing list archive for December 2008.
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simplicity
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I also tagged the sbc project with "copied-to-bluez-utils" at the same time.
We will do sbc work under bluez and sync it with the old sbc project if necessary.
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