AC3/EAC3 Decoder Spectral Banding Fix

Please refer to:

https://www.rationalqm.us/board/viewtopic.php?p=24199#p24199

This fixes the issue in the latest ffmpeg stable.

$ diff -u libavcodec/ac3dec.c.bak libavcodec/ac3dec.c
--- libavcodec/ac3dec.c.bak     2026-06-19 16:11:14.198323700 -0500
+++ libavcodec/ac3dec.c 2026-06-19 18:47:26.631695100 -0500
@@ -362,8 +362,20 @@
              if (s->channel_in_cpl[ch]) {
                  int cpl_coord = s->cpl_coords[ch][band] << 5;
                  for (bin = band_start; bin < band_end; bin++) {
-                    s->fixed_coeffs[ch][bin] =
-                        MULH(s->fixed_coeffs[CPL_CH][bin] * (1 << 4), cpl_coord);
+                    if (!s->bap[CPL_CH][bin]) {
+                        /* A/52 s7.3.4: dither is applied after channel extraction
+                           so each coupled channel gets independent dither */
+                        if (s->dither_flag[ch]) {
+                            int mantissa = (((av_lfg_get(&s->dith_state)>>8)*181)>>8) - 5931008;
+                            mantissa >>= s->dexps[CPL_CH][bin];
+                            s->fixed_coeffs[ch][bin] = MULH(mantissa * (1 << 4), cpl_coord);
+                        } else {
+                            s->fixed_coeffs[ch][bin] = 0;
+                        }
+                    } else {
+                        s->fixed_coeffs[ch][bin] =
+                            MULH(s->fixed_coeffs[CPL_CH][bin] * (1 << 4), cpl_coord);
+                    }
                  }
                  if (ch == 2 && s->phase_flags[band]) {
                      for (bin = band_start; bin < band_end; bin++)

_______________________________________________
ffmpeg-devel mailing list -- [email protected]
To unsubscribe send an email to [email protected]