Sd = Standard Definition
Remember there only-2-3 HD channels in India. Most of the other channels are broadcast in standard definition.
Some SD channels can appear heavily pixellated in large screen tv sets especially the new LCD panels.
You should lookout for the clarity of these channels before finalizing an DTH.
What I did was instead of visiting demo centers of DTH providers (where they had only small screen tvs to display) I visited TV showrooms like Sony World. This way you can personally see the clarity of big screen lcd panels before finalizing.
Tata Sky is providing two HD channels at present
You would also get to see a few sports events in HD on Tata Sky.
Discovery Turbo is available on Tata Sky
Airtel's claim of providing 7.1 audio is a joke in the industry. There is hardly any programming available in the world providing 7.1 audio and Airtel can be sued for misleading advertisements, but, alas, we don't have the necessary rules in place in the media industry.
I found every DTH provider's package so confusing that i went with their full package. Now that I know what channels i like to watch - will finalize a package based on my needs.
Hope this helps!