fix: preserve timeline clip gain during audio export

src/core/editor/render-project-timeline.ts

@@ -2,6 +2,7 @@ import { spawn } from 'node:child_process';
 import { mkdtemp, readFile, rm, writeFile } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
+import { buildTimelineAudioMixFilter } from './timeline-audio-filter';
 
 import { getResolvedCaptionStyle } from './captions';
 import {
@@ -363,8 +364,7 @@ async function buildFfmpegArgs({
   }
   if (audioLabels.length > 0) {
     filters.push(
-      `${audioLabels.join('')}amix=inputs=${audioLabels.length}:duration=longest:dropout_transition=0,` +
-        `atrim=duration=${time(document.duration)},apad=whole_dur=${time(document.duration)}[aout]`
+      `${audioLabels.join('')}${buildTimelineAudioMixFilter(audioLabels.length, document.duration)}[aout]`
     );
   }
 

src/core/editor/timeline-audio-filter.test.ts

@@ -0,0 +1,36 @@
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { test } from 'node:test';
+import { buildTimelineAudioMixFilter } from './timeline-audio-filter';
+
+const ffmpeg = process.env.BEATDESIGN_FFMPEG?.trim() || 'ffmpeg';
+const available = spawnSync(ffmpeg,['-version'],{stdio:'ignore'}).status === 0;
+function render(count:number, overlap=false) {
+  const args=['-hide_banner','-loglevel','error'];
+  for(let i=0;i<count;i++)args.push('-f','lavfi','-i','sine=frequency=440:sample_rate=48000:duration=1');
+  const filters=Array.from({length:count},(_,i)=>`[${i}:a]volume=${overlap?10:1},adelay=${overlap?0:i*1000}:all=1[a${i}]`);
+  filters.push(Array.from({length:count},(_,i)=>`[a${i}]`).join('')+buildTimelineAudioMixFilter(count,overlap?1:count)+'[out]');
+  args.push('-filter_complex',filters.join(';'),'-map','[out]','-ac','1','-c:a','pcm_f32le','-f','f32le','pipe:1');
+  const result=spawnSync(ffmpeg,args,{maxBuffer:4*1024*1024});
+  assert.equal(result.status,0,result.stderr?.toString());
+  const samples=new Float32Array(result.stdout.buffer.slice(result.stdout.byteOffset,result.stdout.byteOffset+result.stdout.byteLength));
+  return samples;
+}
+function rms(samples:Float32Array,start:number,end:number) {
+  let sum=0;for(let i=start;i<end;i++)sum+=samples[i]**2;
+  return Math.sqrt(sum/(end-start));
+}
+
+test('sequential delayed clips retain unity gain as later inputs are added',{skip:!available},()=>{
+  const one=render(1),three=render(3),expected=rms(one,4800,43200);
+  assert.ok(expected>0.08);
+  for(let i=0;i<3;i++)assert.ok(Math.abs(rms(three,48000*i+4800,48000*i+43200)-expected)<0.0001);
+  assert.equal(three.length,3*48000);
+});
+
+test('overlapping loud clips are limited without automatic makeup or timeline delay',{skip:!available},()=>{
+  const samples=render(2,true);
+  let peak=0;for(const sample of samples)peak=Math.max(peak,Math.abs(sample));
+  assert.ok(peak<=0.951 && peak>0.90,`Peak was ${peak}`);
+  assert.equal(samples.length,48000);
+});

src/core/editor/timeline-audio-filter.ts

@@ -0,0 +1,10 @@
+/** Preserve clip gains independently of silent/delayed timeline inputs. */
+export function buildTimelineAudioMixFilter(inputCount: number, duration: number) {
+  if (!Number.isSafeInteger(inputCount) || inputCount < 1 || !Number.isFinite(duration) || duration <= 0)
+    throw new Error('Audio mixing requires inputs and a positive timeline duration.');
+  const seconds = duration.toFixed(3);
+  return `amix=inputs=${inputCount}:duration=longest:dropout_transition=0:normalize=0,` +
+    // Disable the limiter's automatic makeup gain; compensate its lookahead.
+    'alimiter=limit=0.95:level=0:latency=1,' +
+    `atrim=duration=${seconds},apad=whole_dur=${seconds}`;
+}