Line 21

Line 21 was chosen as a location for data to carry television closed captions as specified by EIA 608. (The top of active video begins in line 22.) The data rate was set very low, 480 bits per second, to insure that the signal could still be decoded after passing through noisy transmission channels and low-grade consumer VCRs. The primary use is for closed captions on Caption Channel 1, now legally mandated for many programs. Text Channel 2 is used for ATVEF triggers to TV enhancements, as specified in EIA 746.(Related ETVCookBook page: ATVEF Transport A)