Fix channels for mp3

Author: dy

.gitignore

@@ -1,2 +1,3 @@
 /.claude
 /node_modules
+.DS_Store

audio-encode.js

@@ -44,7 +44,7 @@ function fmt(init) {
 		if (!opts.sampleRate) throw Error('sampleRate is required')
 		let ch = channels(data)
 		if (!ch.length || !ch[0].length) return EMPTY
-		let enc = await init(opts)
+		let enc = await init({ channels: ch.length, ...opts })
 		try {
 			let result = await enc(ch)
 			let flushed = await enc()

packages/mp3-encode/mp3-encode.js

@@ -16,7 +16,8 @@ import { createMp3Encoder } from 'wasm-media-encoders'
  * free() → void
  */
 export default async function mp3(opts) {
-	let { sampleRate, bitrate = 128, quality, channels = 2 } = opts
+	let { sampleRate, bitrate = 128, quality, channels } = opts
+	if (!channels || channels < 1 || channels > 2) channels = 2
 
 	let encoder = await createMp3Encoder()
 

test.js

@@ -119,3 +119,22 @@ t('AudioBuffer input', async () => {
 	is(dec.sampleRate, 44100)
 	almost(rms(dec.channelData[0]), rms(ch), 0.001, 'rms matches')
 })
+
+t('mp3 mono — channels inferred from data', async () => {
+	let mono = sine(44100, 440, 0.5)  // 1 channel
+	let buf = await encode.mp3(mono, { sampleRate: 44100, bitrate: 128 })
+	ok(buf.length > 0, 'encoded without error')
+	// verify MP3 frame header says mono (channel mode = 3)
+	for (let i = 0; i < buf.length - 4; i++) {
+		if (buf[i] === 0xff && (buf[i + 1] & 0xe0) === 0xe0) {
+			is((buf[i + 3] >> 6) & 3, 3, 'MP3 frame is mono')
+			break
+		}
+	}
+})
+
+t('ogg mono — channels inferred from data', async () => {
+	let mono = sine(44100, 440, 0.5)
+	let buf = await encode.ogg(mono, { sampleRate: 44100 })
+	ok(buf.length > 0, 'encoded without error')
+})