If you have a number of devices connected together to supply your TV then no doubt you have a lot of cables (mains, HDMI, maybe scart, cables to speakers etc).
These cables are often near each other and can cause interference.
The better cables usually have better insulation and can withstand this interference.
I had two cheap Scart cables connecting my Sky box to my DVD Recorder and then from my DVD recorder to my TV (all digital) and they were running near each other and near the mains cables.
I was getting interference on my TV so bought two decent Scart cables for about �15 each and got a much better picture.
I am not saying pay �75 for a cable but low end Scart and HDMI cables can make your TV signal worse.