feat: better audio bitrate handling

detect bitrate of audio and uses it, falls back to ffmpeg's default bitrate on error, and defaults bitrate to 320kbps for converting from lossless codecs to lossy

src/lib/converters/ffmpeg.svelte.ts

@@ -121,7 +121,8 @@ export class FFmpegConverter extends Converter {
 		);
 		ffmpeg.terminate();
 
-		return new VertFile(new File([output], input.name), to);
+		const outBuf = new Uint8Array(output).buffer.slice(0);
+		return new VertFile(new File([outBuf], outputFileName), to);
 	}
 
 	private async setupFFmpeg(input: VertFile): Promise<FFmpeg> {
@@ -153,6 +154,46 @@ export class FFmpegConverter extends Converter {
 		return ffmpeg;
 	}
 
+	private async detectAudioBitrate(ffmpeg: FFmpeg): Promise<number | null> {
+		const args = [
+			"-v",
+			"quiet",
+			"-select_streams",
+			"a:0",
+			"-show_entries",
+			"stream=bit_rate",
+			"-of",
+			"default=noprint_wrappers=1:nokey=1",
+			"input",
+		];
+
+		try {
+			let bitrate: number | null = null;
+
+			const bitrateListener = (event: { message: string }) => {
+				if (bitrate !== null) return;
+				const n = parseInt(event.message.trim(), 10);
+				if (!n) return null;
+				bitrate = Math.round(n / 1000);
+				log(
+					["converters", this.name],
+					`Detected stream audio bitrate: ${bitrate} kbps`,
+				);
+			};
+
+			ffmpeg.on("log", bitrateListener);
+
+			try {
+				await ffmpeg.ffprobe.call(ffmpeg, args);
+				return bitrate;
+			} finally {
+				ffmpeg.off("log", bitrateListener);
+			}
+		} catch {
+			return null;
+		}
+	}
+
 	private async buildConversionCommand(
 		ffmpeg: FFmpeg,
 		input: VertFile,
@@ -161,13 +202,32 @@ export class FFmpegConverter extends Converter {
 		const inputFormat = input.from.slice(1);
 		const outputFormat = to.slice(1);
 
+		const lossless = ["flac", "alac", "wav"];
+		let audioBitrateArgs: string[];
+		if (
+			lossless.includes(inputFormat) &&
+			!lossless.includes(outputFormat)
+		) {
+			audioBitrateArgs = ["-b:a", "320k"];
+		} else {
+			const inputBitrate = await this.detectAudioBitrate(ffmpeg);
+			audioBitrateArgs = inputBitrate ? ["-b:a", `${inputBitrate}k`] : [];
+		}
+
 		// video to audio
 		if (videoFormats.includes(inputFormat)) {
 			log(
 				["converters", this.name],
 				`Converting video ${input.from} to audio ${to}`,
 			);
-			return ["-i", "input", "-map", "0:a:0", "output" + to];
+			return [
+				"-i",
+				"input",
+				"-map",
+				"0:a:0",
+				...audioBitrateArgs,
+				"output" + to,
+			];
 		}
 
 		// audio to video
@@ -178,13 +238,13 @@ export class FFmpegConverter extends Converter {
 			);
 
 			const hasAlbumArt = await this.extractAlbumArt(ffmpeg);
+			const codecArgs = toArgs(to);
 
 			if (hasAlbumArt) {
 				log(
 					["converters", this.name],
 					"Using album art as video background",
 				);
-				const codecArgs = toArgs(to);
 				return [
 					"-loop",
 					"1",
@@ -200,11 +260,11 @@ export class FFmpegConverter extends Converter {
 					"-r",
 					"1",
 					...codecArgs,
+					...audioBitrateArgs,
 					"output" + to,
 				];
 			} else {
 				log(["converters", this.name], "Using solid color background");
-				const codecArgs = toArgs(to);
 				return [
 					"-f",
 					"lavfi",
@@ -218,14 +278,26 @@ export class FFmpegConverter extends Converter {
 					"-r",
 					"1",
 					...codecArgs,
+					...audioBitrateArgs,
 					"output" + to,
 				];
 			}
 		}
 
-		// fallback
-		log(["converters", this.name], `Converting ${input.from} to ${to}`);
-		return ["-i", "input", "output" + to];
+		// audio to audio
+		log(
+			["converters", this.name],
+			`Converting audio ${input.from} to audio ${to}`,
+		);
+		const { audio: audioCodec } = getCodecs(to);
+		return [
+			"-i",
+			"input",
+			"-c:a",
+			audioCodec,
+			...audioBitrateArgs,
+			"output" + to,
+		];
 	}
 
 	private async extractAlbumArt(ffmpeg: FFmpeg): Promise<boolean> {
@@ -318,26 +390,11 @@ const toArgs = (ext: string): string[] => {
 			break;
 		}
 
-		case "mpeg4": {
-			// for avi and divx
-			break;
-		}
-
 		case "mpeg2video": {
 			// for mpeg, mpg, vob, mxf
 			if (ext === ".mxf") args.push("-ar", "48000"); // force 48kHz sample rate
 			break;
 		}
-
-		case "libtheora": {
-			// for ogv
-			break;
-		}
-
-		case "wmv2": {
-			// for wmv
-			break;
-		}
 	}
 
 	args.push("-c:a", codecs.audio);
@@ -352,6 +409,7 @@ const toArgs = (ext: string): string[] => {
 
 const getCodecs = (ext: string): { video: string; audio: string } => {
 	switch (ext) {
+		// video <-> audio
 		case ".mp4":
 		case ".mkv":
 		case ".mov":
@@ -364,29 +422,45 @@ const getCodecs = (ext: string): { video: string; audio: string } => {
 		case ".3gp":
 		case ".3g2":
 			return { video: "libx264", audio: "aac" };
-
 		case ".wmv":
 			return { video: "wmv2", audio: "wmav2" };
-
 		case ".webm":
 		case ".ogv":
 			return {
 				video: ext === ".webm" ? "libvpx" : "libtheora",
 				audio: "libvorbis",
 			};
-
 		case ".avi":
 		case ".divx":
 			return { video: "mpeg4", audio: "libmp3lame" };
-
 		case ".mpg":
 		case ".mpeg":
 		case ".vob":
 			return { video: "mpeg2video", audio: "mp2" };
-
 		case ".mxf":
 			return { video: "mpeg2video", audio: "pcm_s16le" };
 
+		// audio
+		case ".mp3":
+			return { video: "libx264", audio: "libmp3lame" };
+		case ".flac":
+			return { video: "libx264", audio: "flac" };
+		case ".wav":
+			return { video: "libx264", audio: "pcm_s16le" };
+		case ".ogg":
+		case ".oga":
+			return { video: "libx264", audio: "libvorbis" };
+		case ".opus":
+			return { video: "libx264", audio: "libopus" };
+		case ".aac":
+			return { video: "libx264", audio: "aac" };
+		case ".m4a":
+			return { video: "libx264", audio: "aac" };
+		case ".alac":
+			return { video: "libx264", audio: "alac" };
+		case ".wma":
+			return { video: "libx264", audio: "wmav2" };
+
 		default:
 			return { video: "libx264", audio: "aac" };
 	}