Contrast ratio is simply:
max brightness / minimum luminance
That's the formula period.
If you increase the numerator or you decrease the denominator, you get a higher contrast ratio.
No magic. No other things to think about.
(For what it's worth, most LCDs on the market do not match the CR of the best plasmas because you cannot realistically use a brightness over a certain level and their black floor, aka minimum luminance is just too high. The exceptions to this rule use LCD brightness -- which again is "capped" by the fact that over a certain level the brightness is beyond useless, it's both detrimental to the picture and literally blinding. The excitement around the F8500 is that with brightness anywhere near the "LCD cap" and a minimum level anywhere near the "best in breed" plasma level, you'd have a CR that's in league with best-of-breed displays like the Sharp Elite, but ideally without any of its weaknesses, including its high price and apparent discontinuedness).
There is no difference in HDMI cables. If you can see the picture without visible dropouts or sparklies, the cable is working at 100%. No other cable will display a better version of that picture. You're simply wrong if you think there is a better digital cable than one that is already working.