# 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. > Canonical page: https://elysiatools.com/en/tools/podcast-chapter-marker-builder - **Category:** Media - **Keywords:** podcast chapters, id3 chap frame, ctoc, podcasting 2.0 chapters, mp3 chapter tags, youtube chapters ## Overview 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. ## Inputs - **Chapter list (one per line: timestamp Title)** (textarea): 0:00 Intro 2:35 News roundup 18:40 Interview: scaling Kafka 41:05 Listener questions 52:30 Outro - **MP3 file to burn tags into (optional)** (file): Upload an .mp3 to embed ID3v2.4 CHAP+CTOC frames - **Podcast name (JSON metadata)** (text): The Daily Ship - **Episode title (JSON metadata)** (text): Ep. 204 — Scaling Kafka Consumers - **Chapters JSON URL (for the RSS tag)** (text): episodes/204/chapters.json - **Default chapter image URL (JSON img)** (text): optional per-chapter artwork ## When to use - You have a simple list of timestamps and titles and need Podcasting 2.0 JSON plus the matching RSS tag. - You want to burn verified ID3v2.4 CHAP+CTOC frames into an episode MP3 while keeping existing tags. - You also need YouTube timestamps, SRT, Vorbis comments, and mp4chaps from the same list. ## How it works - Enter chapters as one line each: timestamp then title (for example 0:00 Intro). - Optionally add podcast name, episode title, chapters JSON URL, and a default chapter image URL. - Optionally upload an MP3 so ID3v2.4 CHAP+CTOC frames (millisecond big-endian times, TIT2 titles, CTOC flags 0x03) can be written while existing tags are preserved. - Receive the JSON, RSS snippet, sidecars, player matrix, and a downloadable tagged MP3 when a file was supplied. ## Use cases - Publish Podcasting 2.0 chapters and the RSS podcast:chapters tag for a new episode. - Embed ID3v2.4 chapters in the episode MP3 for Apple, Pocket Casts, and Overcast. - Produce a YouTube description timestamp block and SRT file from the same chapter list. ## Frequently asked questions ### 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. ## Related tools - [Audio Melody Contour Extractor](https://elysiatools.com/en/tools/audio-melody-contour-extractor): Extract a dominant melody path from audio and export MIDI, note events, a pitch contour, SVG, and JSON in one ZIP. - [Batch Audio Converter](https://elysiatools.com/en/tools/audio-batch-converter): Convert multiple audio files between different formats like MP3, WAV, FLAC, AAC, OGG, Opus with quality settings and batch processing - [Audio Codec Swap](https://elysiatools.com/en/tools/audio-codec-swap): Change the codec of an audio file within a chosen output format - [Image Sequence to Video](https://elysiatools.com/en/tools/image-sequence-to-video): Convert uploaded image files or a ZIP archive of frames into a video, sorted naturally by filename - [Audio Chord Progression Detector](https://elysiatools.com/en/tools/audio-chord-progression-detector): Estimate an audio file's chord sequence locally and download CSV, JSON, SVG, and method notes in one ZIP. - [Audio to Text Transcriber (AI)](https://elysiatools.com/en/tools/audio-to-text-transcriber): Transcribe speech from audio (wav/mp3/m4a/flac/ogg/webm/aac) to text, SRT, VTT or JSON using the grok-stt AI model. 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