Usually, line-21 caption data are encoded only on field 1. However, in mid-1993, the FCC issued a rulemaking which allows data on line 21, field 2, as well. This enables a second stream of captions to be encoded. The newly encoded video, with caption data now on line 21, emerges from the encoder and is routed to a record machine. This new tape of the program is sometimes referred to as the closed-captioned submaster, or CC master.
Encoding is always a dubbing process.
The closed-captioned master is always a dub or clone of the original tape. Therefore, it is important to make sure that the closed-captioned submaster is clearly labeled as such and is always used for air or duplication. Closed captions may be seen only with the use of a decoding device, now built in to most televisions as a result of the Television Decoder Circuitry Act, or with a set-top device. The resulting dub will be open captioned. Whenever it is played back or duplicated, the captions will always be visible, so a decoder is not necessary.