scope:

The purpose of this document is to be a technical guide for those who wish to provide encoding equipment and/or decoding equipment to produce material with encoded data embedded in line 21 of the vertical blanking interval of the NTSC video signal. It is also a usage guide for those who will produce material using such equipment.

Where recommendations are made for service providers, they apply to anyone who creates, transmits, or modifies data, ¡.e., someone other than an equipment manufacturer. For example, a 'kaption sewice provider" could be the agency which creates the captions for a program, the network which carries the captions on line 21, or the local afiliate which uses its own data encoder to insert Text Mode or Extended Data Services between the captions.

The document is divided into sections which concentrate on technical issues and those which deal with different aspects of usage.

It is recommended that regardless of the function to be performed, the reader should become familiar at least with all the introductions to sections of this document to avoid unintentionally degrading other services, and then concentrate on the sections which are appropriate to the actnnty being undertaken.

This document represents the best advice of the Electronic Industries Association, as compiled by its Television Data Systems Subcommittee, made up of representatives from the television receiver manufacturing and captioning industries. While there is no legal requirement to abide by the advice herein, it is strongly recommended that this advice be followed by line 21 data service providers and manufacturers of equipment used to transmit and receive these data services. Failure to follow these practices will result in a degraded and inferior service and a non-uniform, unpredictable display of captions, text, or operation of Extended Data Service features on the consumer's receiver.