Closed captions (line 21)

NTSC DVDs can carry EIA-608 closed captions hidden inside the MPEG-2 video stream. These are not the same thing as subtitles, and a disc can have either, both or neither.

The core extracts them and re-modulates them onto line 21 of the analog output, exactly as a real DVD player does — so your television's own caption decoder renders them.

Captions are not subtitles

	Subtitles	Closed captions
Stored as	Subpicture streams — bitmap overlays	Data bytes inside the MPEG-2 video
Rendered by	The core, drawn onto the picture	Your television
Output	HDMI and analog	Analog only
Selected with	B8, or the disc's menu	Your TV's caption setting
Typical content	Dialogue, often multiple languages	Dialogue plus sound descriptions, [DOOR SLAMS]

If you want text on HDMI, you want subtitles. There is no on-screen caption renderer in the core.

What you need

1. An NTSC disc that actually has captions. Only about 1 disc in 6 does. Every PAL disc scanned has none — line 21 is an NTSC construct, and PAL discs use subtitles instead.
2. The analog output engaged — Video Output = Interlaced, or Auto with the analog ini bits set; see Analog and CRT output.
3. A connection that carries line 21. Composite and S-video always do. Component does on many sets. Consumer sets generally do not slice captions from RGB, so an RGB SCART path often will not work even though the waveform is present on all three channels.
4. Line-21 CC on — Debug page, default On.
5. Your television set to C1. Not the DVD's own subtitle menu — the TV's caption setting. C1 is the correct choice: CC1 and CC2 both ride field 1, and field 2 is empty on every disc surveyed, so C2, T1 and T2 will show nothing on any real disc.

Every US television 13 inches and larger sold since 1993 has a caption decoder.

Is it working? The test line

Captions normally travel in the vertical blanking interval, where they are invisible unless something decodes them. That makes failure ambiguous: you cannot tell "my TV is not decoding" from "no caption data is reaching the output".

CC Test Line (Debug page, default Off) resolves it. Turn it on and the same waveform is painted on a visible line near the top of the picture:

• A band of dashes that changes as dialogue changes — everything on the core's side works. Extraction, pacing, waveform and the analog chain are all fine, and only the TV-side setup is left. Turn the test line back off and work on the television.
• Nothing at all — either the analog raster is not engaged, or this disc has no captions. Try a disc known to carry them.

This diagnostic is what separates a caption problem into "core" and "television" halves in one glance, which is why it exists as a user-facing setting rather than a developer tool.

Turning it off

Line-21 CC defaults to On because correct behaviour is simply on — an idle line 21 costs nothing. The Off position is an escape hatch for capture devices and upscalers that display VBI lines, where the caption waveform would otherwise show up as a flickering dashed line at the top of the captured image.

Limitations

• Analog output only. Nothing appears on HDMI.
• NTSC only.
• CC1 only in practice. Field 2 is empty on every disc surveyed, so there is no CC3/CC4 second-language stream and no XDS program metadata to display.
• A disc can emit well-formed but empty caption blocks — one concert disc in the survey sends a caption block on every GOP in which every byte pair is null padding. A disc like that will correctly show nothing.