When a hi def source outputs an HDMI signal it takes the source video and audio data and packages it up. It sends one copy along one wire in the cable and an inverse copy of the signal along the another wire in the same cable. In many cases especially with LONG cable runs some of the signal will be lost or corrupted along the way. This is where the inverse copy comes in. Your TV compares the two signals and uses the difference between them to recreate the missing data. When theres a big difference (as is more likely with a lower quality cable) the TV has to sketch in more of its own detail which appears as noise blocking and smearing and a loss of clarity in the audio.
