Screen Subtitling

Imitext and SCTE-27 are bitmap subtitles standards that pre-date the DVB standard.

Imitext is a bit-map subtitle transmission technique proprietary to Screen Subtitling Systems. It was a forerunner to the DVB bit-map subtitling technique and allows a single transmission feed to carry multiple languages of subtitles along side a clean video signal. Imitext subtitles are rendered as bit-maps and then encoded into the VBI of the video signal using a technique similar to the transmission of Teletext subtitles but using the Packet 31 data format. At the receiving end of the transmission chain and Imitext decoder or Imitext capable IRD is used to rebuild the bit-map subtitles and burn them into the video signal prior to re-distribution across a cable or other DTH network.

SCTE-27 formally known as Digicipher II (DCII) is based on the Imitext system, used initially by General Instruments and now in Motorola broadcast systems. SCTE-27 is now part of the US cable networks SCTE standards for digital cable systems.

One of the great advantages of the both Imitext and SCTE-27 system is its ability to have several subtitle languages available in a single video feed. For example a number of head-ends located in different geographical regions all receive the same broadcast signal with a clean video and multiple closed (hidden) subtitle languages. Each headend then selects the local language from the subtitles in the signal and these are burnt-in to the video by an Imitext Decoder or Imitext capable IRD for distribution to local viewers. The viewers then see their language and as it's part of the video picture they do not need a special decoder.

Imitext & SCTE-27 subtitles can also be delivered direct to the viewer where Imitext or SCTE-27 capable set top boxes are available.