Broadcast subtitle and caption formats explained

Q: Is the subtitle .stl the same as a 3D printer .stl?

No. They share an extension and nothing else. The 3D .stl is a stereolithography mesh read by printers and CAD software. The subtitle .stl is EBU Tech 3264, a broadcast caption file. There is also a separate Spruce STL subtitle format, which is plain text and not the same as EBU STL. Opening the file settles it quickly: an EBU STL is binary, while a Spruce STL reads as normal text.

If someone hands you a file ending in .scc or .stl, it did not come from the usual subtitle world. These are broadcast caption formats, built for television delivery rather than for a video player on your laptop. They carry things a plain SubRip file never touches, like a timecode tied to the program clock and a character set fixed by the standard, and they pack the data at the byte level with rules a text format never needs. This page covers the two the site converts, SCC and EBU STL, so you can tell what you are looking at and turn it into something you can edit.

SCC and CEA-608

An SCC file (Scenarist Closed Caption) holds CEA-608 captions, the closed captioning that rode along with analog US television on what engineers called line 21. Open one in a text editor and you see a timecode followed by rows of four-character hex codes, something like 9420 94ae 947a. Each group of two bytes is either a command or a pair of characters, and every byte carries an odd-parity bit the decoder checks. The timecode is 29.97 drop-frame, the counting scheme US broadcast uses to keep clock time honest at that frame rate.

CEA-608 is the older of the two US caption standards. Digital broadcast moved to CEA-708, which supports much more, including proportional fonts and on-screen window positioning that 608 cannot express, and a 708 stream usually carries a 608 track inside it for backward compatibility. An SCC file describes 608 specifically, which is why it remains the format for legacy closed captioning and for the many playout and QC systems that still expect it.

To move a caption script into or out of SCC, use SRT to SCC or SCC to SRT. The converter reads and writes the standard pop-on caption subset of the format.

EBU STL

EBU STL is the European broadcast subtitle format, defined by EBU Tech 3264. Unlike SCC it is binary, so opening it in a text editor shows mostly unreadable bytes. The file starts with a 1024-byte GSI block, a header that records the frame rate, the character table, the subtitle count and similar metadata. After the header comes a run of 128-byte TTI blocks, one or more per subtitle, each holding an in and out timecode and the subtitle text. It has been the standard subtitle exchange format between subtitling houses and broadcasters across Europe for decades.

The .stl name collision

The extension .stl is badly overloaded, and it trips people up constantly. A broadcast subtitle .stl has nothing to do with the .stl used for 3D models, the stereolithography mesh format that 3D printers and CAD tools read. They only happen to share three letters. There is also a second, unrelated subtitle format called Spruce STL, a plain-text format from Spruce Technologies DVD authoring software, which is not the same thing as EBU STL even though both describe subtitles. This tool reads the binary EBU Tech 3264 file. If you open your .stl and it looks like readable lines of text with timecodes, you have the Spruce format rather than EBU, and the converter says so instead of guessing.

To convert, use SRT to EBU STL or EBU STL to SRT. As with SCC, the converter covers the common interoperable subset of the format.

How these differ from SRT and VTT

A SubRip (.srt) or WebVTT file is plain text with relative timings and almost no rules beyond that. Broadcast formats are stricter because they were built to be played out to air. Their timecode is pinned to a program clock, often starting at 10:00:00:00 rather than zero. Their character set is fixed by the format itself instead of depending on the file's text encoding. And they reserve room for styling and on-screen position that the playout chain knows how to render.

When you convert one of these to SRT you keep what matters for most work, the wording and the timing. What drops away is the broadcast layer: colour, boxing, precise positioning and the roll-up or paint-on behaviour do not survive the trip, because SRT has nowhere to put them. That is usually the point. You are pulling an editable script out of a delivery file, not trying to recreate the broadcast frame for frame.

Which one do I have, and how do I open it?

Start with the extension. A .scc is SCC and holds CEA-608. A .stl is most likely EBU STL if it reached you from a broadcaster or a subtitling house in Europe. When you are not sure, open the file. An SCC file is readable text, with Scenarist_SCC at the top and lines of hex below it. An EBU STL file is binary and looks like noise in a text editor, though you can usually spot a readable STL marker near the very start of the header.

You do not need special software to look inside either one. To get something you can actually edit, drop the file into the matching converter and download an SRT: SCC to SRT for a .scc, or EBU STL to SRT for a .stl. Going the other way, SRT to SCC and SRT to EBU STL build a broadcast file from an ordinary subtitle.

FAQ

What is an SCC file?

An SCC file is a Scenarist Closed Caption file. It stores CEA-608 captions, the closed captioning used on analog US television, as pairs of hexadecimal bytes with odd parity and a 29.97 drop-frame timecode. It is still used across US broadcast and by many systems that ingest legacy closed captioning. To read one as ordinary text, convert it with SCC to SRT.

What is an EBU STL file?

An EBU STL file is a broadcast subtitle file defined by EBU Tech 3264. It is binary, with a 1024-byte GSI header followed by 128-byte TTI blocks that hold each subtitle's in and out timecodes and its text. It has long been the standard subtitle exchange format for European broadcast. You can convert it with EBU STL to SRT.

Is the subtitle .stl the same as a 3D printer .stl?

No. They share an extension and nothing else. The 3D .stl is a stereolithography mesh read by printers and CAD software. The subtitle .stl is EBU Tech 3264, a broadcast caption file. There is also a separate Spruce STL subtitle format, which is plain text and not the same as EBU STL. Opening the file settles it quickly: an EBU STL is binary, while a Spruce STL reads as normal text.

Can I convert SCC or EBU STL to SRT?

Yes. Both convert to SRT in the browser here, with the text and timing preserved. The converters handle the common documented subset of each format. Broadcast-only details such as colour, on-screen positioning and roll-up styling are not carried into SRT, because SRT has nowhere to store them. See SCC to SRT and EBU STL to SRT.