“SDH” is an American term the DVD industry introduced. It is an acronym for “Subtitles for the deaf and hard-of-hearing”, and refers to regular subtitles in the original language where important non-dialog audio has been added, as well as speaker identification, useful when the viewer cannot otherwise visually tell who is saying what.
The only significant difference for the user between “SDH” subtitles and “closed captions” is their appearance: SDH subtitles usually are displayed with the same proportional font used for the translation subtitles on the DVD; however, closed captions are displayed as white text on a black band, which blocks a large portion of the view. Closed captioning is falling out of favor as many users have no difficulty reading SDH subtitles, which are text with contrast outline. In addition, DVD subtitles can specify many colors, on the same character: primary, outline, shadow, and background. This allows subtitlers to display subtitles on a usually translucent band for easier reading, however, this is rare, since most subtitles use an outline and shadow instead, in order to block a smaller portion of the picture. Closed captions may still supersede DVD subtitles, since many SDH subtitles present all of the text centered, while closed captions usually specify position on the screen: centered, left align, right align, top, etc. This is very helpful for speaker identification and overlapping conversation. Some SDH subtitles do have positioning, but it is not as common.
DVDs for the US market now sometimes have three forms of English subtitles: SDH subtitles, English subtitles, helpful for viewers who are Hearing and whose first language may not be English (although they are usually an exact transcript and not edited into Simple English), and closed caption data that is decoded by the end-user’s closed caption decoder.
High definition disc media (HD DVD, Blu-ray disc) uses SDH subtitles as the sole method because technical specifications do not require HD to support line 21 closed captions. Some blu-ray discs, however, are said to carry a closed caption stream that only displays through standard definition connections. Many HDTVs allow the end–user to customize the captions, including the ability to remove the black band.
Video: Bleeding Love – Subtitled for Deaf and Hard of Hearing