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atrac3: reverse the whole of an odd band's spectrum

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The spectra of the odd QMF bands are stored reversed. 8722c6f2bb
moved that reversal out of IMLT() and into the decoding of the
coefficients, but did it within each scale factor band, one place
off, and not at all for tonal components. The odd bands, which is
5.5 to 11 kHz and above 16.5 kHz at 44.1 kHz, have been decoded
wrongly since: against ffmpeg's decode that range was uncorrelated.

Reverse the whole band in IMLT() again, as ffmpeg does. The old
commit measured the saving it is giving up as 0.11 MHz on PP5024.

Checked with perfsim (Sansa e200v1 build) against ffmpeg's decode
of an OMA file and six RM files: the coherence of the 5.7 to 10.7
kHz range goes from under 0.01 to 0.99, the same as other bands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Michael Giacomelli 2026-10-06 01:15:08 -04:00
parent f1d7ae3bbf
commit 1bae7e0e0a
1 changed files with 21 additions and 27 deletions
Download patch file Download diff file Expand all files Collapse all files

48
lib/rbcodec/codecs/libatrac/atrac3.c
View file

@ -297,8 +297,17 @@ static void iqmf (int32_t *inlo, int32_t *inhi, unsigned int nIn, int32_t *pOut,
* @param odd_band 1 if the band is an odd band
*/
static void IMLT(int32_t *pInput, int32_t *pOutput)
static void IMLT(int32_t *pInput, int32_t *pOutput, int odd_band)
{
if (odd_band) {
/* Reverse the odd bands before the IMDCT; this is an effect of the
* QMF transform. The whole band is reversed, tonal components
* included, so it cannot be done as the coefficients are decoded. */
int i;
for (i = 0; i < 128; i++)
FFSWAP(int32_t, pInput[i], pInput[255-i]);
}
/* Apply the imdct. */
ff_imdct_calc(9, pOutput, pInput);
@ -438,31 +447,16 @@ static void inverseQuantizeSpectrum(int *mantissas, int32_t *pOut,
int *pIn = mantissas;
/* Inverse quantize the coefficients. */
if((first/256) &1) {
/* Odd band - Reverse coefficients */
do {
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
pOut[last--] = fixmul16(*pIn++, SF);
} while (last>first);
} else {
/* Even band - Do not reverse coefficients */
do {
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
} while (first<last);
}
do {
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
pOut[first++] = fixmul16(*pIn++, SF);
} while (first<last);
}
@ -998,7 +992,7 @@ static int decodeChannelSoundUnit (GetBitContext *gb, channel_unit *pSnd, int32_
for (band=0; band<4; band++) {
/* Perform the IMDCT step without overlapping. */
if (band <= numBands) {
IMLT(&(pSnd->spectrum[band*256]), pSnd->IMDCT_buf);
IMLT(&(pSnd->spectrum[band*256]), pSnd->IMDCT_buf, band & 1);
} else {
memset(pSnd->IMDCT_buf, 0, 512 * sizeof(int32_t));
}