[FFmpeg-devel] [PR] avfilter/af_loudnorm: fix slow gain ramp after initial silence (PR #21449)

[realies via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21449 opened by realies
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21449
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21449.patch

## Summary
- Fixes fade-in artifact when audio begins with silence (3+ seconds)
- When first 3s of audio is below `measured_thresh`, the filter set `above_threshold=0`
- This caused gain to ramp at only 0.5 dB/second instead of applying correct gain immediately
- Audio took 20+ seconds to reach steady state after silence ended

## Root Cause
In INNER_FRAME processing with `above_threshold=0`:
- Gain was increased via `prev_delta *= 1.0058` (~0.05 dB per 100ms)
- `above_threshold` only changed to 1 when OUTPUT reached target
- Created a catch-22: output couldn't reach target because gain was too low

## Fix
When input short-term loudness exceeds `measured_thresh`, immediately transition to normal gain mode by:
1. Setting `above_threshold = 1`
2. Reinitializing all `delta[]` values with the correct gain

This ensures proper gain is applied immediately when audio starts after silence.

## Test plan
- [x] Tested with 11 different silence durations (0s to 10s) followed by constant tone
- [x] All tests pass with fix (previously 5 failed with silence >= 3s)
- [x] LUFS compliance tests pass
- [x] True peak compliance tests pass
- [x] Limiter and gating tests pass

Fixes: https://github.com/slhck/ffmpeg-normalize/issues/146


>From 5f0547f2e61cd472fc12715d301b2d608d11a945 Mon Sep 17 00:00:00 2001
From: realies <ffmpeg@reali.es>
Date: Tue, 13 Jan 2026 16:48:14 +0200
Subject: [PATCH] avfilter/af_loudnorm: fix slow gain ramp after initial
 silence

When audio begins with silence (below measured_thresh), the filter
would set above_threshold=0 and then slowly ramp up gain at only
0.5 dB/second when audio content started. This caused audible
fade-in artifacts taking 20+ seconds to reach steady state.

The bug was in the INNER_FRAME processing: when above_threshold=0,
gain was increased via prev_delta *= 1.0058 (~0.05 dB per 100ms),
and above_threshold only changed to 1 when OUTPUT reached target.
This created a catch-22: output couldn't reach target because gain
was too low.

Fix: When input short-term loudness exceeds measured_thresh,
immediately transition to normal gain mode by:
1. Setting above_threshold = 1
2. Reinitializing all delta[] values with the correct gain

This ensures proper gain is applied immediately when audio starts
after silence, rather than slowly ramping via Gaussian smoothing
of stale values.

Fixes fade-in artifacts reported in:
- https://github.com/slhck/ffmpeg-normalize/issues/146
---
 libavfilter/af_loudnorm.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/libavfilter/af_loudnorm.c b/libavfilter/af_loudnorm.c
index 432b9710a5..5c047cb57d 100644
--- a/libavfilter/af_loudnorm.c
+++ b/libavfilter/af_loudnorm.c
@@ -543,14 +543,19 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         ff_ebur128_relative_threshold(s->r128_in, &relative_threshold);
 
         if (s->above_threshold == 0) {
-            double shortterm_out;
-
-            if (shortterm > s->measured_thresh)
-                s->prev_delta *= 1.0058;
-
-            ff_ebur128_loudness_shortterm(s->r128_out, &shortterm_out);
-            if (shortterm_out >= s->target_i)
+            if (shortterm > s->measured_thresh) {
+                /* Input has exceeded threshold, transition to normal gain mode.
+                 * Reinitialize all delta values with the correct gain to ensure
+                 * immediate proper gain application when audio starts after
+                 * silence, rather than slowly ramping up via Gaussian smoothing
+                 * of stale values. */
+                double env_st = s->target_i - shortterm;
+                double new_delta = pow(10., env_st / 20.);
+                for (int i = 0; i < 30; i++)
+                    s->delta[i] = new_delta;
+                s->prev_delta = new_delta;
                 s->above_threshold = 1;
+            }
         }
 
         if (shortterm < relative_threshold || shortterm <= -70. || s->above_threshold == 0) {
-- 
2.49.1

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