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celt_sin() computes sin(pi^2/4 * x) instead of sin(x), so BBWENet's activation no longer matches the trained model

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
88/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c, python
Domain
audio-video-rtc

Research direction

Start in celt/mathops.h at celt_sin(), then compare its behavior with test_cos() in celt/tests/test_unit_mathops.c and the activation path in dnn/osce.c. Verify the sine approximation against the training activation in dnn/torch/osce/models/bbwe_net.py, add coverage in test_unit_mathops.c, and confirm the affected BWE behavior matches the trained model.

Written by the indexing model from the issue text.

Description

note: this was found by claude, not by me. it came up while the agent was porting bbwenet to rust for a hobby project. the agent then checked it against the current source, the git history and the existing issues. i'm posting it on its behalf. everything below (analysis, numbers, repro) is the agent's work. the repro is short, so it's quick to verify. i hope it's not bogus!

in float builds, celt_sin() scales its argument by π/2 where 2/π is needed:

/* celt/mathops.h:353 (main 503d81b, v1.6.1, v1.6) */
#define celt_sin(x) celt_cos_norm2((0.5f*PI) * (x) - 1.0f)

celt_cos_norm2(u) approximates cos(π/2·u) (its comment and test_cos() in celt/tests/test_unit_mathops.c), so

celt_sin(x) = cos(π/2·(π/2·x − 1)) = cos(π²/4·x − π/2) = sin(π²/4·x) ≈ sin(2.467·x)

for sin(x) the argument has to be (2/π)·x − 1.

where it is used

the only caller is apply_valin_activation() in dnn/osce.c (line 989), which bbwenet_process_frames() runs at lines 1229 and 1311:

y[i] = celt_log(y[i]);      /* natural log */
...
x[i] *= celt_sin(y[i]);

celt_sin was added in 940d4e5 ("extending mathops for use in osce"), which replaced

x[i] *= sin(log(fabs(x[i]) + 1e-6f));

bbwenet is trained with the same function in radians (dnn/torch/osce/models/bbwe_net.py:204, activation ImPowI):

self.nlfunc = lambda x : x * torch.sin(torch.log(torch.abs(x) + 1e-6))

so since 1.6 the c code applies x·sin(π²/4·ln|x|), while the model was trained with x·sin(ln|x|). the training code has not changed since 2025-09 (still torch.sin), and the model data was last updated the day before 940d4e5.

expected vs actual

output of the repro below on main (503d81b), gcc 16.2, x86-64:

y = -2.0000000   celt_sin(y) =  0.9753680   sin(y) = -0.9092974   sin(pi^2/4*y) =  0.9753680
y = -1.0000000   celt_sin(y) = -0.6242660   sin(y) = -0.8414710   sin(pi^2/4*y) = -0.6242660
y =  0.5000000   celt_sin(y) =  0.9437192   sin(y) =  0.4794255   sin(pi^2/4*y) =  0.9437192
y =  1.0000000   celt_sin(y) =  0.6242659   sin(y) =  0.8414710   sin(pi^2/4*y) =  0.6242660
y =  1.5707964   celt_sin(y) = -0.6699874   sin(y) =  1.0000000   sin(pi^2/4*y) = -0.6699876
y =  3.0000000   celt_sin(y) =  0.8996722   sin(y) =  0.1411200   sin(pi^2/4*y) =  0.8996722

over y in [−14, 14) (the caller's inputs start at ln(1e-6) ≈ −13.8): max |celt_sin(y) − sin(π²/4·y)| = 1.6e-6, max |celt_sin(y) − sin(y)| = 1.99.

impact

only float builds with osce compiled in (--enable-osce, OPUS_OSCE, -Dosce=enabled) and bwe enabled at run time (OPUS_SET_OSCE_BWE(1), 48 khz output, complexity ≥ 4, silk-only wideband frames or their concealment). default builds and default decoder settings are not affected.

with bwe on, the agent ran osce_bwe() on 8 librispeech dev-clean utterances (16 khz in, 48 khz out), once with the current macro and once with libm sin() (the pre-1.6 code and the training activation):

  • above 8 khz, the band bbwenet generates, the two outputs differ strongly: snr −1.6 to 8.7 db (median 2.3 db), and the current output is 0.2–2.8 db louder there (median 1.4 db);
  • full band: 18–44 db snr (median 33 db), since the band below 8 khz passes through;
  • with the fix below, the output matches the libm version to int16 rounding (median snr 87 db above 8 khz, 113 db full band).

predicted quality barely changes: on 32 clips, nisqa 4.066 vs 4.049, sigmos 3.187 vs 3.181, utmos 3.967 vs 3.968 (current vs correct sine). so this is mainly the c inference no longer matching the trained model, not an obvious audible defect.

minimal repro

run from the opus source root:

/* gcc -O2 -I celt -I include celt_sin_check.c -o celt_sin_check -lm */
#include <math.h>
#include <stdio.h>
#include "mathops.h"

int main(void)
{
   const float ys[] = {-2.f, -1.f, 0.5f, 1.f, 1.5707964f, 3.f};
   int i;
   for (i = 0; i < 6; i++)
      printf("y = %10.7f   celt_sin(y) = %10.7f   sin(y) = %10.7f   sin(pi^2/4*y) = %10.7f\n",
             ys[i], celt_sin(ys[i]), sin(ys[i]), sin(PI*PI/4*ys[i]));
   return 0;
}
suggested fix
-#define celt_sin(x) celt_cos_norm2((0.5f*PI) * (x) - 1.0f)
+#define celt_sin(x) celt_cos_norm2((2.f/PI) * (x) - 1.0f)

with it, max |celt_sin(y) − sin(y)| is 8.6e-7 over [−14, 14), and the bwe output returns to the pre-1.6 behaviour. a test_sin() next to test_cos() in celt/tests/test_unit_mathops.c would catch a regression. if the current output is preferred instead, the model would need retraining with that activation, and the macro renaming, since it does not compute sin().

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