when I connect my laptop or PC to video screen (LED Screen) when I make Master output 0% screen not getting 100% black, it having slight grey output.May I know the reson for this and how to fix it.
As Angelo stated, 95% of the time it's your GPU outputting a Limited RGB range (16-235) vs a Full RGB range (0-255). This is used mainly for TV's as they don't typically need or process 0-15 in the dark range, for LED's though this is much more sensitive and is needed. Check your GPU settings for this.
The other 5% is when you're using junk signal flavor converters like BMD micro converters, that force a limited range when using them. The soluton for this is to use better hardware like Decimator or AJA.