I'd be glad to give you a quick tutorial on the filters.
Look at filters as a troubleshooting aid and not something that has to be applied to every object.
Keep in mind what works for me won't work for you so when you hear these blanket statements that "no filter" is best or this one is best over another, that's for where they sit and depends on their RF environment. Not yours.
People will say to use Sentinel to apply filters and I disagree, do it on the radio because you're looking for results from real-time reception looking at RSSI and noise levels.
Also avoid using Auto filters, they sample every filter and slows scanning way down and you never know what filter did the trick anyway.
Global filter is set to normal for a reason it's a good starting point. Global filter affects every object in the radio unless you go into the sites of a system and change them or the department settings of a conventional object and change them. Global is not where you want to set your filters and leave it or you will find that other objects will suffer.
You want to set filters on individual sites of a system or on conventional frequencies... you want to use Department options for the Department that the channel you want to apply the Filter to is controlled. No you can't just apply a filter to a channel it has to be to the department options that channel is under which means it affects all of the channels that are under that department... so you can get around that by the way you program your grouping of channels. Aviation frequencies are going to respond to the same filter, so Will Railroad so group them accordingly.
So any object you don't individually change to another filter will remain on normal filter if you leave Global on normal.
Global filter can be used to give you a good idea on how a filter will affect a system or conventional Channel. On the radio while listening to the system or Channel you're having problems with apply the different Global filters or no filter at all and look for the best RSSI and noise level. When you find the most optimal results put Global filter back on normal then...
... go into the system options and go to the sites that you have activated, the less sites the better of course and apply that filter that you found most optimal RSSI and noise levels to that site or sites. Now you're left with just that system having that filter and Global being normal for all the other objects that you have not changed.
Same goes for conventional. Sit on the channel, this could be tedious because sometimes they're not busy but try different Global filters other than Auto and see what gives you the best RSSI and noise level. Repeat steps above. Put Global filter back on normal and apply the appropriate filter to Department options that channel is under.
You can also toggle function 7 on conventional channels to see if that helps also but remember that is a universal application and will change that frequency everywhere in the radio it exists to also have ifx applied. For example in my location all Aviation frequencies have to have ifx applied or they will be deaf.
When you're done make sure to hook up to Sentinel and transfer all the information from your card into Sentinel so as to save your filter changes.
Hope that helps, any questions, just let me know... Bob.
PS... I forgot to mention to make sure to put the filter indicator on your display so as to be able to keep track of every filter that's on every object.