SUP vs PGS vs SRT: What Is the Difference?

SUP, PGS, and SRT are related to subtitles, but the names do not describe three equivalent subtitle formats.

PGS is an image-based subtitle format. SUP is commonly the file used to store raw PGS subtitle data. SRT is a text-based subtitle format that stores subtitle text with timing information.

The biggest difference is that PGS/SUP subtitles contain graphical subtitle data, while SRT contains actual text. This is why converting PGS or SUP subtitles to SRT normally needs OCR to recognize the words in the subtitle images.

SUP vs PGS vs SRT at a Glance

The important point is that SUP and PGS are closely connected. They should not be treated as two unrelated subtitle systems.

What Is PGS?

PGS stands for Presentation Graphic Stream. It is a bitmap subtitle format associated with Blu-ray and HDMV video.

Instead of storing a subtitle sentence as normal text characters, PGS stores graphical subtitle data. The decoder uses subtitle objects, palette information, timing, and presentation data to display the subtitle on screen.

This is why you cannot treat a PGS subtitle like an SRT text file.

For a deeper explanation, read What Are PGS Subtitles?.

What Is a SUP File?

SUP is commonly used as the file extension for raw HDMV PGS subtitle data. A file may look like this:

movie.sup

The file contains the graphical subtitle information used by PGS. In practical subtitle workflows, a PGS subtitle stream may be extracted into a .sup file.

So when someone says SUP subtitles, they are often talking about PGS subtitle data stored in a SUP file.

What Is SRT?

SRT stands for SubRip Subtitle format. It is a text-based subtitle format that normally uses the .srt file extension.

An SRT file stores subtitle text with start and end times. A basic subtitle block looks like this:

1
00:00:02,000 --> 00:00:05,000
This is an SRT subtitle.

Because the words are stored as text, an SRT file can be opened and edited with a text editor. SRT has a simple structure and only limited styling support compared with more advanced text subtitle formats.

For the full format explanation, read What Is an SRT File?.

PGS vs SRT Subtitles

The main difference between PGS and SRT is image versus text.

PGS stores subtitles as graphical data. SRT stores the words themselves as text with timing information.

That difference affects what you can do with the subtitle. SRT text can be searched, copied, and edited directly. PGS subtitle words first need to be recognized from the subtitle images if you want them as editable text.

If your goal is editable subtitle text rather than the original graphical subtitle presentation, SRT is usually the more practical result.

Why PGS or SUP to SRT Needs OCR

Changing a filename from .sup to .srt does not convert the subtitle.

The PGS data inside the SUP file contains subtitle graphics. An SRT file needs actual text. Software therefore has to recognize the characters shown in those graphics.

That process is optical character recognition, or OCR.

The basic conversion is:

PGS subtitle data in SUP
↓
Subtitle images
↓
OCR
↓
Recognized text + timing
↓
SRT

If you need to perform this conversion, use the site’s SUP to SRT converter.

PGS vs VobSub

PGS and VobSub are both bitmap-based subtitle systems, but they are not the same format.

VobSub commonly uses an .idx file together with a .sub file. The IDX file contains information used with the VobSub subtitle data, while the SUB file contains the subtitle bitmaps.

PGS is associated with HDMV/Blu-ray subtitles and is commonly stored as SUP data.

This means PGS vs VobSub is a comparison between two image-based subtitle systems. It is different from PGS vs SRT, where one side is image-based and the other is text-based.

SRT vs ASS Subtitles

SRT and ASS are both text subtitle formats, but ASS supports much richer subtitle presentation.

ASS stands for Advanced SubStation Alpha. ASS files can define styles and control details such as font, size, colors, alignment, margins, outlines, shadows, and positioning. SRT uses a much simpler timed-text structure and has limited formatting support.

So .ass vs .srt is not an image-versus-text comparison. Both contain text. The main difference is how much formatting and presentation control the format can carry.

Is SMI the Same as SUP?

SMI, also known as SAMI in subtitle software, is not the same as SUP. The names look similar, but they refer to different subtitle formats: SMI/SAMI is a text-based subtitle format, while SUP is commonly the file used to store raw, image-based PGS subtitle data. FFmpeg lists SAMI and SubRip (SRT) as separate supported subtitle formats, which confirms SMI is its own distinct format rather than a variant of SUP or PGS.

Which Format Should You Use?

If you already have PGS subtitles and want to keep their graphical presentation, keeping the original PGS/SUP subtitle data avoids turning the subtitle images into recognized text.

If you need editable subtitle text, SRT is the more useful format. Converting from PGS/SUP to SRT requires OCR because the source subtitle words are graphical.

ASS is useful when you need text subtitles with more detailed styling and positioning than SRT provides.

The right choice therefore depends on what you need to preserve: the original subtitle graphics, simple editable text, or detailed text styling.

SUP, PGS, and SRT FAQ

Sources

• FFmpeg subtitle format support
• FFmpeg PGS subtitle decoder
• FFmpeg SUP muxer
• Library of Congress SRT format description
• Library of Congress accessibility format list, including VobSub SUB and IDX
• libass project and ASS format references