Introductory article for beginners in this area
Subtitles are text descriptions that accompany the video, duplicating or supplementing the movie or the TV show sound. Subtitles mainly reflect the characters’ speech.
Basically, subtitles are arranged as text descriptions written with medium size font and are located at the bottom of the screen. Subtitles are useful to those people who badly hear the movie sound, or those studying a foreign language. However, subtitles are often used for watching movies (often anime) with the original audio channel containing no voice translation. (In some countries e.g., Israel, and Bulgaria, it is a common way to watch movies. Voice translation in these countries is used for PG movies and soap serials only.) Subtitles sometimes contain additional information about what is happening on the screen e.g., director’s comments to the movie and (or) explanation of some scenes that are difficult to understand.
Subtitles may vary in detalization. Usually they are used to show the movie characters’ speech. In case the subtitles are used for entire movie translation, they also describe all the inscriptions that appear in the frame. In case when the subtitles are used to help people with limited hearing, they often depict important movie sound events, e.g. in the moment when the hero of a horror movie hears the ominous grinding, you may see the "(grinding)" subtitle.
Subtitles can be applied to a video sequence, imposed over the image, or obtained from a separate text file. Typically, most DVD movies contain subtitles. Creation of homemade subtitles, the so-called fan subtitles (fansubs), is also popular. This kind of subtitles is distributed either together with the video file (hard subtitles), or as a separate file (external, energized, or sometimes soft subtitles).
Digital video (DVD-video, MPEG-2, MPEG-4) subtitle types:
— a kind of subtitles where the original video is layered with an image containing the subtitles text when creating (encoding) the final file. This kind of subtitles doesn’t impose any additional requirements on the movie players, but they it can’t be turned off, and it also makes the encoded video quality lower due to increase of small contrast details in the frame. The quality and complexity of hard subtitles may be maximal, since their creation can be much longer and scrupulous than their viewing.
— a set of ready-made images that are stored separately from the video file (although, perhaps, may be stored in a single file as well). This kind of subtitles requires additional plugins to be installed in the movie player and are usually of not very high quality (due to the fact that the raster formats with a small number of colors, which does not allow fonts anti-aliasing, are used to store them), but have no problems with displaying texts in different languages and cause no excessive load to the media player. This is the kind of subtitles used for DVD-video.
— a kind of subtitles, representing the text itself; often goes together with layout elements (size, color, typeface, position on the screen). This kind of subtitles imposes higher requirements for reproducing equipment (software), if compared to other subtitles types, due to the fact their fonts (usually vector fonts), and other possible effects need to be rendered in real time. Besides, some formats have problems with the representation of text in multiple languages due to using a single-byte encoding.
One of the main advantages of prerendered and software subtitles is the possibility of using multiple translations (e.g. translations to multiple languages) as well as the ability to disable them (which cannot be done to hardware subtitles).
Subtitles also divided into two categories according to the form they are stored in:
Embedded Subtitles —put together with the video file.
External Subtitles — can be represented by one or more separate files, with name usually fully or partially matching the name of a video file but having a different extension: SRT, SUB, TXT (see below).
Popular formats of External Subtitles
srt — text subtitle format of SubRip software. Initially it didn’t support the text elements layout change but was later updated to support text decoration (colors and styles (italic, bold).
SMI (smil) — text subtitle format, based on SGML. The format allows implementing virtually any text decoration, which is supported by HTML and CSS standards.
S2k — text subtitle format of Sasami software.
SSA — text subtitle format of Sub Station Alpha software. The format allows using the following text decoration: color, character position, font size, and typeface; allows using sophisticated video effects regarding the characters (shading, motion, rotation, etc.); can contain text in several different encodings.
ASS — text subtitle format of Advanced Sub Station Alpha software (was first implemented in the Medusa software), and is the further development of the SSA format.
sub/idx — binary format of rendered subtitles, which is used by the VobSub software. Sometimes referred to as "Vobsub Subtitle Format”, though it’s not true. The format is based on graphical subtitles extracted from DVD’s (e.g. using the DVDDecrypter software, etc.)