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

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This commit is contained in:
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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src/mp3/mp3-demuxer.ts
+9 -2
View File
@@ -117,9 +117,16 @@ export class Mp3Demuxer extends Demuxer {
this.firstFrameHeader = header;
}
const sampleDuration = header.audioSamplesInFrame / header.sampleRate;
if (header.sampleRate !== this.firstFrameHeader.sampleRate) {
console.warn(
`MP3 changed sample rate mid-file: ${this.firstFrameHeader.sampleRate} Hz to ${header.sampleRate} Hz.`
+ ` Might be a bug, so please report this file.`,
);
}
const sampleDuration = header.audioSamplesInFrame / this.firstFrameHeader.sampleRate;
const sample: Sample = {
timestamp: this.nextTimestampInSamples / header.sampleRate,
timestamp: this.nextTimestampInSamples / this.firstFrameHeader.sampleRate,
duration: sampleDuration,
dataStart: result.startPos,
dataSize: header.totalSize,
src/mp3/mp3-muxer.ts
+1
View File
@@ -84,6 +84,7 @@ export class Mp3Muxer extends Muxer {
mpegVersionId: header.mpegVersionId,
layer: header.layer,
frequencyIndex: header.frequencyIndex,
sampleRate: header.sampleRate,
channel: header.channel,
modeExtension: header.modeExtension,
copyright: header.copyright,
src/mp3/mp3-writer.ts
+74 -71
View File
@@ -12,9 +12,7 @@ import {
computeMp3FrameSize,
encodeSynchsafe,
getXingOffset,
MPEG_V1_BITRATES,
MPEG_V2_BITRATES,
SAMPLING_RATES,
KILOBIT_RATES,
XING,
} from '../../shared/mp3-misc';
import { Id3V2TextEncoding } from './mp3-reader';
@@ -23,6 +21,7 @@ export type XingFrameData = {
mpegVersionId: number;
layer: number;
frequencyIndex: number;
sampleRate: number;
channel: number;
modeExtension: number;
copyright: number;
@@ -62,6 +61,78 @@ export class Mp3Writer {
this.writer.write(this.helper.subarray(0, text.length));
}
writeXingFrame(data: XingFrameData) {
const startPos = this.writer.getPos();
const firstByte = 0xff;
const secondByte = 0xe0 | (data.mpegVersionId << 3) | (data.layer << 1);
const lowSamplingFrequency = data.mpegVersionId & 1;
const padding = 0;
const neededBytes = 155;
let bitrateIndex = -1;
const bitrateOffset = lowSamplingFrequency * 16 * 4 + data.layer * 16;
// Let's find the lowest bitrate for which the frame size is sufficiently large to fit all the data
for (let i = 0; i < 16; i++) {
const kbr = KILOBIT_RATES[bitrateOffset + i]!;
const size = computeMp3FrameSize(lowSamplingFrequency, data.layer, 1000 * kbr, data.sampleRate, padding);
if (size >= neededBytes) {
bitrateIndex = i;
break;
}
}
if (bitrateIndex === -1) {
throw new Error('No suitable bitrate found.');
}
const thirdByte = (bitrateIndex << 4) | (data.frequencyIndex << 2) | padding << 1;
const fourthByte = (data.channel << 6)
| (data.modeExtension << 4)
| (data.copyright << 3)
| (data.original << 2)
| data.emphasis;
this.helper[0] = firstByte;
this.helper[1] = secondByte;
this.helper[2] = thirdByte;
this.helper[3] = fourthByte;
this.writer.write(this.helper.subarray(0, 4));
const xingOffset = getXingOffset(data.mpegVersionId, data.channel);
this.writer.seek(startPos + xingOffset);
this.writeU32(XING);
let flags = 0;
if (data.frameCount !== null) {
flags |= 1;
}
if (data.fileSize !== null) {
flags |= 2;
}
if (data.toc !== null) {
flags |= 4;
}
this.writeU32(flags);
this.writeU32(data.frameCount ?? 0);
this.writeU32(data.fileSize ?? 0);
this.writer.write(data.toc ?? new Uint8Array(100));
const kilobitRate = KILOBIT_RATES[bitrateOffset + bitrateIndex]!;
const frameSize = computeMp3FrameSize(
lowSamplingFrequency, data.layer, 1000 * kilobitRate, data.sampleRate, padding,
);
this.writer.seek(startPos + frameSize);
}
writeSynchsafeU32(value: number) {
this.writeU32(encodeSynchsafe(value));
}
@@ -170,72 +241,4 @@ export class Mp3Writer {
this.writer.write(imageData);
}
writeXingFrame(data: XingFrameData) {
const startPos = this.writer.getPos();
const firstByte = 0xff;
const secondByte = 0xe0 | (data.mpegVersionId << 3) | (data.layer << 1);
const bitrateGroup = data.mpegVersionId === 3 ? MPEG_V1_BITRATES : MPEG_V2_BITRATES;
const bitrates = bitrateGroup?.[data.layer];
if (!bitrates) {
throw new Error('Invalid MPEG version and layer combination.');
}
const sampleRate = SAMPLING_RATES[data.mpegVersionId]?.[data.frequencyIndex];
if (!sampleRate || sampleRate === -1) {
throw new Error('Invalid MPEG version and frequency index combination.');
}
const padding = 0;
const neededBytes = 155;
// Let's find the lowest bitrate for which the frame size is sufficiently large to fit all the data
const bitrateIndex = bitrates.findIndex((kbr) => {
return computeMp3FrameSize(data.layer, 1000 * kbr, sampleRate, padding) >= neededBytes;
});
if (bitrateIndex === -1) {
throw new Error('No suitable bitrate found.');
}
const thirdByte = (bitrateIndex << 4) | (data.frequencyIndex << 2) | padding << 1;
const fourthByte = (data.channel << 6)
| (data.modeExtension << 4)
| (data.copyright << 3)
| (data.original << 2)
| data.emphasis;
this.helper[0] = firstByte;
this.helper[1] = secondByte;
this.helper[2] = thirdByte;
this.helper[3] = fourthByte;
this.writer.write(this.helper.subarray(0, 4));
const xingOffset = getXingOffset(data.mpegVersionId, data.channel);
this.writer.seek(startPos + xingOffset);
this.writeU32(XING);
let flags = 0;
if (data.frameCount !== null) {
flags |= 1;
}
if (data.fileSize !== null) {
flags |= 2;
}
if (data.toc !== null) {
flags |= 4;
}
this.writeU32(flags);
this.writeU32(data.frameCount ?? 0);
this.writeU32(data.fileSize ?? 0);
this.writer.write(data.toc ?? new Uint8Array(100));
const frameSize = computeMp3FrameSize(data.layer, 1000 * bitrates[bitrateIndex]!, sampleRate, padding);
this.writer.seek(startPos + frameSize);
}
}