@svta/cml-608

CTA-608/CEA-608 closed captioning functionality.

Installation

npm i @svta/cml-608
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Usage

import { CaptionScreen, Row } from "@svta/cml-608";
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Extracting captions from a media sample

CTA-608 captions are embedded in the video elementary stream as an ATSC A/53 cc_data() payload. Only the envelope around the payload differs by codec. The envelope is either a SEI message in a NAL unit, or an OBU. Choose the extractor that matches the sample entry type:

Both extractors return the same [field1, field2] byte arrays. Cta608Parser renders them as caption screens:

import { Cta608Parser, extractCta608DataFromAv1Sample } from "@svta/cml-608";
import type { CaptionScreen } from "@svta/cml-608";

const parser = new Cta608Parser(1, {
	newCue(startTime: number, endTime: number, screen: CaptionScreen) {
		console.log(startTime, endTime, screen.getDisplayText());
	},
}, null);

// Call once per AV1 sample. The sample starts at samplePos in view.
function onSample(view: DataView, samplePos: number, sampleSize: number, sampleTimeMs: number): void {
	const [field1] = extractCta608DataFromAv1Sample(view, samplePos, sampleSize);
	if (field1.length) {
		parser.addData(sampleTimeMs, field1);
	}
}
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The two extractors are separate because the two envelopes are framed differently, and a sample does not announce its framing. A NAL unit sample puts a fixed-width big-endian length before each unit. Only lengthSizeMinusOne in the avcC or hvcC config gives that width. An AV1 sample uses the low-overhead format. Each OBU has its own obu_size after the header, and the last OBU may omit it. In both cases the sample entry gives the framing.