Thai subtitles from word timestamps

This post shows how to build Thai cues from the word timestamps Paxa STT Lite returns. Each span is a whole Thai word with its start and end, which is exactly the set of places a cue may be cut. The rules for turning spans into cues fit on one page, and the result is a subtitle file any player reads.

Ask for timestamps set to "word" and the response carries a words array beside the transcript. Each entry is one word with its start and end in seconds. For Thai the words are segmented on the way out. A span covers a whole word and never a character. The spans join back into the transcript with nothing lost.

Walk the spans in order and collect them into a cue until one of three limits is reached. The cue then ends at the last span's end, and the next cue starts at the next span's start.

• Length. A Thai line reads comfortably at around thirty-five characters, and a cue holds at most two lines. Count the characters of the collected words and cut before the word that would push past the limit.
• Duration. A cue on screen for longer than about six seconds reads as stuck. Cut when the next word would push the cue past the limit, even when the line has room.
• Pause. A gap between one span's end and the next span's start longer than about half a second is a breath or a full stop. Cut there, and the cue ends when the speaker does.

A cue that ends at a pause reads better than one that ends at a count. A small overrun of the length limit to reach a pause is worth allowing. Where a cue has two lines, break the line at the widest gap between spans inside the cue, which is usually a phrase boundary.

Subtitles want the spoken convention, which is the default. The viewer hears สองพันสามร้อยห้าสิบบาท and reads the same words. A digit on screen while the speaker says five syllables reads as a mismatch, and a cue built from the written form also loses the span boundaries inside a number, since the digits arrive as one token.

Keep the verbatim style for the same reason. A viewer who hears a false start and reads a cleaned sentence notices. The clean style is for a transcript read on its own, away from the audio.

SRT numbers each cue, writes the start and end as hours, minutes, seconds, and milliseconds with a comma before the milliseconds, and follows with the text. WebVTT is the same with a period before the milliseconds and a header line. Both are plain text, and both carry Thai as UTF-8 with no further setting. Write the seconds from the spans with three decimals and the file is done.

For a translated track, send the cues' text to the translation API one cue per segment with the ids echoed back. The timings stay with the ids. A Thai video gets an English track with the same cut points, and the two tracks stay aligned.

• Read the transcription guide for the word timestamps beside the model's own spans on one recording.
• Read the timestamps page for the span shape shared with speech synthesis, where the same rules build read-along captions.
• Read the numbers post for why a subtitle keeps the spoken form.
• Read the captions use case for a caption pipeline end to end and what a season of video costs.