Do I have to upload an MP3?

No. A text-only list produces JSON, the RSS tag, Vorbis, mp4chaps, YouTube timestamps, SRT, and the support matrix. Upload an MP3 only if you want ID3v2.4 chapters burned in.

Which players read which format?

Apple reads MP3 ID3 since iOS 12 and RSS JSON since 2025. Pocket Casts and Overcast are ID3-only. Spotify ignores both, so include the description timestamp block for that platform.

How is the Podcasting 2.0 JSON structured?

It follows spec v1.2.0: version plus chapters with startTime in seconds. endTime is omitted and implied by the next chapter start. The RSS tag carries only url and type.

What happens to existing MP3 tags?

They are preserved. v2.3 sizes are re-encoded as v2.4 syncsafe. Tags marked unsync, compressed, or encrypted are rejected so the file is not corrupted.

Are there YouTube-specific rules?

Yes. The first timestamp must be 0:00, you need at least three chapters, and each chapter should last 10 seconds or more for the description block to work as expected.

Elysia Tools

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Podcast Chapter Marker Builder

Build every podcast chapter format from one timecoded list: Podcasting 2.0 JSON + RSS tag, ID3v2.4 CHAP+CTOC burned into an MP3, Vorbis comments, mp4chaps, YouTube timestamps and SRT, with a per-player support matrix.

Details

What this tool helps you do

Binary layout verified against the ID3v2 Chapters Addendum plus mutagen/ffmpeg: CHAP times are plain big-endian uint32 milliseconds (not syncsafe), offsets 0xFFFFFFFF when unused, a TIT2 sub-frame (UTF-8) carries each title, CTOC flags 0x03 set the two lowest bits (top-level + ordered), frame and tag sizes are syncsafe, and frames order as metadata → CTOC → CHAP in time order. Existing MP3 tags are preserved (v2.3 sizes re-encoded; unsync/compressed/encrypted tags are rejected instead of corrupted). The JSON follows spec v1.2.0 (startTime seconds, endTime omitted — implicit next start) and the RSS tag carries only url + type. Apple reads MP3 ID3 since iOS 12 and RSS JSON since 2025; Pocket Casts/Overcast are ID3-only; Spotify ignores both, so the description block matters there.

Execution

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Prepared example runs

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2 examples

Text-only build: six chapters across every format

A six-chapter list starting at 0:00 produces the Podcasting 2.0 JSON, the RSS tag, Vorbis pairs, mp4chaps text, the YouTube block and SRT — no MP3 needed, nothing burned.

<section class="pcm">6 chapters · JSON v1.2.0 · RSS tag · CHAPTER001 pairs · mp4chaps · YouTube block · SRT · player matrix</section>

Burn ID3v2.4 chapters into the sample rain MP3

Four chapters (all inside the 7:48 sample) are burned as CTOC + CHAP frames with TIT2 titles and millisecond BE times; chapter ends come from ffprobe's duration and the burned file is linked for download.

<section class="pcm">4 chapters burned as ID3v2.4 CHAP+CTOC into rain.mp3 · duration 7:48 · sidecars + player matrix</section>

Inputs

Set the required fields, then run the tool.

6 options

FilesUpload source files for this workflow.1

MP3 file to burn tags into (optional)fileOptional

Single file max size: 60 MBSupported types: audio/mpeg, audio/mp3

ContentPaste or type the main input values.5

Chapter list (one per line: timestamp Title)textareaRequiredPodcast name (JSON metadata)textOptionalEpisode title (JSON metadata)textOptionalChapters JSON URL (for the RSS tag)textOptionalDefault chapter image URL (JSON img)textOptional

Result

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Samples

Podcast Chapter Marker Builder

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Key facts

Overview

When to use

How it works

Use cases

Examples

1. Text-only six-chapter pack

2. Burn ID3 chapters into the episode MP3

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