The active part of a PAL or NTSC scanline is 52 - 53 usec long and to fill that part with 704 alternating white-black pixels equals a period time of 148.148 nsec which is the same as a 6.75 Mhz signal.

And regarding the bandwidth of LD it's important to remember that the 6.75 Mhz bandwidth of DVD is at full signal strength (well, perhaps with a little drop from imperfect reconstructions filters), but analog LD bandwidth figures usually is somewhat arbitrary measured with a signal drop of 6 dB or even 20 dB.

I've measured the multiburst VITS (Vertical Interval Test Signal) on a few of my PAL LDs and at best the 5 Mhz burst was at -4 dB and quite noisy, so the 5.5 Mhz bandwidth of PAL LD is in practice more like just below 5.0 Mhz. But it's still very player dependant and I'm very sure a X0 quality PAL player would give both higher bandwidth and cleaner signal.