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The phrasing we (I) have been using is a bit misleading. You do not clamp TO back porch. You clamp the back porch to some level. In a PAL receiver, you clamp the back porch to the DC level of black in the video amp chain. In an NTSC reveiver, you clamp the back porch to a level below black (7.5 IRE units below) so that black is at the right level in the video amp chain.
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