I just bought an HDMI repeater and had connectivity issues until I realized the repeater had to know the cable length. It has (1,2,4,8) dip switches that you have to use (binary coded) to set the cable length.
What I'm wondering is: How is the cable length important? If it was just a question of power/loss over the cable, wouldn't the stronger setting always be the best? I assume that since the length is required it either has to do with capacity or signal echo bouncing back the cable end.
Technical details would be enlighting.