I'm thinking of buying a lt8 dva water softener, i want to know what is the softener effect on tds and ph in the water?

It's a sodium exchange unit, so it will reduce the calcium and won't react with the chloramine/chlorine if you're on city water so it won't lower (acidify) the water to the machine. Even if you did provide any details on your water, it would be nearly impossible to accurately foretell the result, that's why you would test the water in and the water out. A dealer in your area may have some stats recorded for your water.

my water tds is 170 but unfortunately i don't know the ph but my water has high calcium and magnesium that is way ivm buying a softener i could mesaure the output tds from the softener but not the ph. but there is one issue if this softener increases sodium it won't be good for coffee.

Agree with JRising, this is a conventional softener that exchanges sodium ion for hardness minerals, or potassium ion if you use potassium salts to recharge it. It does not change the ph, and does not lower the TDS (in fact it may increase slightly). It also does nothing for chlorine or chloramine so you typically use it in conjunction with a carbon block or an activated granular charcoal filter.

The amount of sodium (or potassium) ion that it adds is related to the amount of softening that it performs. For each reduction in GH of 1 °dH (or 17.8 mg/L as CaCO3) it will add 8 mg/L sodium or 14 mg/L potassium to the brew water. Probably not a concern for coffee taste - - espresso contains about 130 mg/L sodium and about 1200 mg/L potassium.

To measure the effectiveness of this type of softener you would want to measure total hardness (GH) with a drop titration test kit.

would it be better if i use an RO system with bleeding valve?

Possibly, but to advise about that we would need to know a lot more about your water. If it has high chloride or high silica then you may need RO to deal with that, and to advise about blending vs. remineralization we would want to know the chloride, hardness, and alkalinity numbers.