HDMI cables do the same job for High Definition digital TV signals as SCART cables do for analogue TV. Not only do they convey the picture and sound signals but also some control signals. The original HDMI cable spec, Category 1, allowed for data rates of up to 75 MHZ, which is adequate for a 1080i HDTV signal.
The latest TVs and Blu-Ray DVD players both support 1080p video, which has twice as many lines a second and therefore a higher data rate. Consequently a category 1 cable just doesn’t cut the mustard. Although it might appear to work, it is quite likely to create intermittent problems.
The specification of Category 2 cables requires that they support signals of up to 340 MHz – over four times higher than Category 1. This allows them to support 1080p signals, even allowing for future increases in sample resolution to over 8 bits per colour and frame (refresh) rates of up to 120 frames per second.
So in other words when you hook your £700 TV to your £200 Blu-Ray player, don’t use a £5 cable!