Subtitles for the Deaf and Hard of Hearing (SDH) combine the same guidelines for both closed captioning and subtitles into one final export since they often differ in their intended audience, use case, appearance, and another important area often overlooked: data encoding. SDH is an excellent option when your required export needs to be subtitle data, but your intended audience is the deaf and hard-of-hearing.
In short, SDH mimics the presentation of closed captions on media that does not support it.
Closed captioning is intended for the deaf and hard-of-hearing and includes speaker identification and non-dialogue sound effects. Subtitles assume the viewer can hear the video but just may not understand the language. SDH will assume the opposite: that the viewer is unable to hear the audio.
This style of subtitling is also purposefully set at a reduced reading rate of 300 words-per-minute for increased readability. In order to achieve this, the subtitles may omit some non-necessary words that otherwise may have appeared in closed captioning. The synchronicity may also be slowed down or sped up to accommodate the proper on-screen duration to achieve that reading rate. And for these reasons, this style will not satisfy the FFC’s closed captioning requirements for broadcast audiences.
When you think about closed captioning, you may envision white text on a black background with text that either rolls up or pops on the screen, moves around frequently to follow the speaker, and is often presented in all CAPS. Subtitles tend to have that more polished look – mixed-case text over a transparent background, centered in the bottom third of the screen. And in subtitles, unlike captioning, the font, color, and size can be manipulated to the client's liking.
Because of the limitations of video presentations from HDMI, closed captioning data is not supported, but subtitle data is – so, this is a way to trick it. It's frequently used on films and programming distributed on DVD or Blu-ray discs – media that is viewed on hardware connected via HDMI.