Who knows. The cynic in me says it's most likely due to cost reasons. The wear and tear from switching on/off is 10x more with a PWM rate > 1000hz, which would require higher spec'd parts.jstabb and SGSeeker...any idea why Samsung would go with such a low PWM frequency? I can't imagine it costing more (or at least not much more) to make it > 1000 Hz, or even DC for flicker free backlighting.
Even if they did some research and found that most people aren't sensitive to PWM and wouldn't even notice a rate of 120 Hz, it doesn't make much sense to me why they wouldn't hedge on the safe side and go higher.
I'm using a backlight setting of 5...no inability to produce an erection so far, anyway.
But, the fact that it's 120hz PWM on a 120hz panel seems like more than a coincidence. I'm guessing the PWM pulse is time aligned with the panel refresh.
Which means:
1) You get a LightBoost like effect for reducing eye-tracking motion blur.
2) The "off" cycle is time aligned with the transition of the shutters in the 3D glasses. (This benefit is probably moot, because 3D mode should be running at max backlight to begin with.)
*shrug* Only a Samsung engineer could say for sure.
Note: The above applies to the 7 series, the 6 series with its supposed 60hz panel would not receive as much of a benefit in the motion blur reduction and obviously, has no 3D capabilities.
Also, I like your tolerance criteria. If it doesn't cause ED, it's good enough for me!