Legacy SD video signals from home videotapes and copy-protected commercial tapes and DVDs introduce their own problems in multistandard HDTV receivers. One problem is simply that SD sync signals are unterminated, and the resulting reflections on the transmission line can distort transitions and lead to false triggering. VCR head switching and fast-forward/rewind modes introduce other challenges, as does Macrovision copy-protection.
The problem with head switching is that it can introduce timing discontinuities so that the next track appears slightly earlier or later than it should, confusing the AFE PLL. The problem is compounded in the fast-forward and rewind modes. Here, viewers want to be able to see something resembling the content of the actual video so that they can find a particular point in the content.
Macrovision technology inserts pseudo-horizontal-sync (HSYNC) pulses between the normal HSYNC pulses that follow the vertical blanking interval. They generally don't affect ordinary viewing on a conventional SD TV. However, that causes the automatic gain control in the VCR that is trying to record the signal ? to swing wildly, spoiling the attempted recording. An AFE that sees the pseudo-sync pulses could try to lock to them, forcing the AFE PLL to slew toward a pixel clock four times higher than it's normal level.