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atrac3: fix gain compensation at adjacent gain points

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applyVariableGain() applied the constant gain before a gain point
in a do-while loop, so at least eight samples of it, even when
there are none: when the point is at the start of the block or
follows straight after the previous one. Every later gain point in
the block was then eight samples late. This came in with the loop
unrolling of 51a8be1a0f.

Use a while loop, as ffmpeg does.

Checked with perfsim (Sansa e200v1 build) against ffmpeg's decode.
Over the whole of atrac3_lp2_132.oma the SNR goes from 45 to 51 dB
and the worst frame from 14 to 21 dB; the right channel of a joint
stereo RM file goes from 37 to 54 dB. What is left is a noise floor
near -72 dBFS in every frame, from the 2 fractional bits the
decoder works with.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Michael Giacomelli 2026-10-06 10:46:37 -04:00
parent b7170e03c4
commit 8667af5cd7
1 changed files with 5 additions and 3 deletions
Download patch file Download diff file Expand all files Collapse all files

8
lib/rbcodec/codecs/libatrac/atrac3.c
View file

@ -717,8 +717,10 @@ static int applyVariableGain (int32_t *pIn, int32_t *pPrev, int32_t *pOut,
{
int32_t i = start;
/* Apply fix gains until end index is reached */
do {
/* Apply fix gains until end index is reached. There is nothing to do
* here when the gain point is where the previous one ended, or at the
* start of the block. */
while (i < end) {
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;
@ -727,7 +729,7 @@ static int applyVariableGain (int32_t *pIn, int32_t *pPrev, int32_t *pOut,
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;
} while (i < end);
}
/* Interpolation is done over next eight samples */
pOut[i] = fixmul16((fixmul16(pIn[i], gain1) + pPrev[i]), gain2); i++;