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fix(tts): write mp3 to temp file to ensure accurate VBR duration metadata

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Switch mp3 transcoding to output a seekable temp file instead of piping to stdout.
This allows ffmpeg to backpatch the Xing/Info header with correct total-duration
metadata, which is required for HTML5 audio elements to handle VBR mp3 playback
accurately. Prevents playback issues caused by missing or incorrect duration,
such as premature or stalled `ended` events.
This commit is contained in:
Richard R 2026-06-09 11:19:11 -06:00
parent 34b94534cf
commit 65c9f03976
1 changed files with 18 additions and 3 deletions
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21
src/lib/server/tts/audio-format.ts
View file

@ -1,5 +1,5 @@
import { spawn } from 'child_process';
import { mkdtemp, rm, writeFile } from 'fs/promises';
import { mkdtemp, readFile, rm, writeFile } from 'fs/promises';
import { tmpdir } from 'os';
import { join } from 'path';
import { serverLogger } from '@/lib/server/logger';
@ -133,27 +133,42 @@ function spawnFfmpegToBuffer(args: string[], signal?: AbortSignal): Promise<Buff
* what the user listens to live, and aggressive low-bitrate encoding of pristine
* high-fidelity TTS (e.g. 44.1 kHz wav) introduces high-frequency artifacts that
* cause listening fatigue. The audiobook export still re-encodes to 64k for size.
*
* The mp3 is written to a seekable temp file rather than piped to stdout: VBR mp3
* stores its total-duration metadata in a Xing/Info header at the *start* of the
* file, which ffmpeg can only emit by seeking back after encoding finishes. On a
* pipe that seek is impossible, so a piped VBR mp3 carries no valid duration. The
* HTML5 <audio> element then extrapolates duration from the first frame's bitrate
* (correct for CBR, wrong for VBR), which makes the `ended` event fire at the
* wrong time — or stall without firing — and stops segment-to-segment playback.
* Writing to a real file lets ffmpeg backpatch the Xing header so duration is
* accurate and `ended` fires reliably.
*/
export async function transcodeToMp3(buffer: Buffer, signal?: AbortSignal): Promise<Buffer> {
let workDir: string | null = null;
try {
workDir = await mkdtemp(join(tmpdir(), 'openreader-tts-transcode-'));
const inputPath = join(workDir, 'input');
const outputPath = join(workDir, 'output.mp3');
await writeFile(inputPath, buffer);
return await spawnFfmpegToBuffer(
await spawnFfmpegToBuffer(
[
'-nostdin',
'-loglevel', 'error',
'-y',
'-i', inputPath,
'-vn',
'-c:a', 'libmp3lame',
'-q:a', '2',
'-write_xing', '1',
'-f', 'mp3',
'pipe:1',
outputPath,
],
signal,
);
return await readFile(outputPath);
} finally {
if (workDir) {
await rm(workDir, { recursive: true, force: true }).catch(() => {});