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Fix faulty MP3 bitrate and frame size calculation (fixes #134)

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Vanilagy
2025-09-19 13:20:36 +02:00
parent e88bc15b0e
commit eb1ce86187
10 changed files with 151 additions and 129 deletions
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shared/mp3-misc.ts
+43 -43
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@@ -7,32 +7,20 @@
*/
export const FRAME_HEADER_SIZE = 4;
export const SAMPLING_RATES = [44100, 48000, 32000];
export const KILOBIT_RATES = [
// lowSamplingFrequency === 0
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, // layer = 0
-1, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, -1, // layer 1
-1, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, -1, // layer = 2
-1, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, -1, // layer = 3
// These are in kbps:
export const MPEG_V1_BITRATES: Record<number, number[]> = {
// Layer 3
1: [-1, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, -1],
// Layer 2
2: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, -1],
// Layer 1
3: [-1, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, -1],
};
export const MPEG_V2_BITRATES: Record<number, number[]> = {
// Layer 3
1: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, -1],
// Layer 2
2: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
// Layer 1
3: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
};
export const SAMPLING_RATES: Record<number, number[]> = {
// MPEG Version 2.5
0: [11025, 12000, 8000, -1],
// MPEG Version 2 (ISO/IEC 13818-3)
2: [22050, 24000, 16000, -1],
// MPEG Version 1 (ISO/IEC 11172-3)
3: [44100, 48000, 32000, -1],
};
// lowSamplingFrequency === 1
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, // layer = 0
-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1, // layer = 1
-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1, // layer = 2
-1, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, -1, // layer = 3
];
/** 'Xing' */
export const XING = 0x58696e67;
@@ -54,12 +42,21 @@ export type FrameHeader = {
audioSamplesInFrame: number;
};
export const computeMp3FrameSize = (layer: number, bitrate: number, sampleRate: number, padding: number) => {
if (layer === 3) {
// Layer 1
return Math.floor((12 * bitrate / sampleRate + padding) * 4);
} else {
return Math.floor((144 * bitrate / sampleRate) + padding);
export const computeMp3FrameSize = (
lowSamplingFrequency: number,
layer: number,
bitrate: number,
sampleRate: number,
padding: number,
) => {
if (layer === 0) {
return 0; // Not expected that this is hit
} else if (layer === 1) {
return Math.round(144 * bitrate / (sampleRate << lowSamplingFrequency)) + padding;
} else if (layer === 2) {
return Math.round(144 * bitrate / sampleRate) + padding;
} else { // layer === 3
return (Math.round(12 * bitrate / sampleRate) + padding) * 4;
}
};
@@ -93,11 +90,21 @@ export const readFrameHeader = (word: number, remainingBytes: number | null): {
return { header: null, bytesAdvanced: 1 };
}
let lowSamplingFrequency = 0;
let mpeg25 = 0;
if (secondByte & (1 << 4)) {
lowSamplingFrequency = (secondByte & (1 << 3)) ? 0 : 1;
} else {
lowSamplingFrequency = 1;
mpeg25 = 1;
}
const mpegVersionId = (secondByte >> 3) & 0x3;
const layer = (secondByte >> 1) & 0x3;
const bitrateIndex = (thirdByte >> 4) & 0xf;
const frequencyIndex = (thirdByte >> 2) & 0x3;
const frequencyIndex = ((thirdByte >> 2) & 0x3) % 3;
const padding = (thirdByte >> 1) & 0x1;
const channel = (fourthByte >> 6) & 0x3;
@@ -106,21 +113,14 @@ export const readFrameHeader = (word: number, remainingBytes: number | null): {
const original = (fourthByte >> 2) & 0x1;
const emphasis = fourthByte & 0x3;
const kilobitRate = mpegVersionId === 3
? MPEG_V1_BITRATES[layer]?.[bitrateIndex]
: MPEG_V2_BITRATES[layer]?.[bitrateIndex];
if (!kilobitRate || kilobitRate === -1) {
const kilobitRate = KILOBIT_RATES[lowSamplingFrequency * 16 * 4 + layer * 16 + bitrateIndex]!;
if (kilobitRate === -1) {
return { header: null, bytesAdvanced: 1 };
}
const bitrate = kilobitRate * 1000;
const sampleRate = SAMPLING_RATES[mpegVersionId]?.[frequencyIndex];
if (!sampleRate || sampleRate === -1) {
return { header: null, bytesAdvanced: 1 };
}
const frameLength = computeMp3FrameSize(layer, bitrate, sampleRate, padding);
const sampleRate = SAMPLING_RATES[frequencyIndex]! >> (lowSamplingFrequency + mpeg25);
const frameLength = computeMp3FrameSize(lowSamplingFrequency, layer, bitrate, sampleRate, padding);
if (remainingBytes !== null && remainingBytes < frameLength) {
// The frame doesn't fit into the rest of the file