see Captioning Laws
47 C.F.R. 76.606
"The requirements for closed captioning are as follows:
As of June 30, 1992, the operator of each cable television system shall not take any action to remove or alter closed captioning data contained on line 21 of the vertical blanking interval: and
As of July 1, 1993, the operator of each cable television system shall deliver intact closed captioning data contained on line 21 of the vertical blanking interval, as it arrives at the headend or from another origination source, to subscriber terminals and (when so delivered to the cable system) in a format that can be recovered and displayed by decoders meeting 47 C.F.R. 15.119 of the Rules"
Section added by order in FCC Docket Nos. 91-169 and 85-38, effective June 30, 1992, 57 FR 11000. For report, see 70 RR 2d 679.
also more at Captioning Laws