Introduction:
This is fairly basic stuff and pretty obvious to most, but since the topic came up a couple of times in the IRC channel, i’ve made this basic tutorial with pictures, to visualize the filtering process.
I’ve used the free effects from GVST, you can get them here: http://www.gvst.co.uk
Of course the explained things apply to the internal filters of Renoise too.
First let me start with a short overview of the most useful filters, after that i’ll provide you with some tips on how to use them. All filters have in common, that they don’t introduce a hard cut. That means, when i use a low pass filter at 2k, there is still some activity above that frequency, since filters have a smooth transition also called slope. If the technical stuff is interesting for you, you might check the following links.
http://en.wikipedia.org/wiki/Lowpass_filter
http://en.wikipedia.org/wiki/High-pass_filter
http://en.wikipedia.org/wiki/Band-pass_filter
Have a look at the following pictures, the red parts in the spectrum view are the ones, which get affected by the filter. Moving the knob in the VST will relocate the red parts accordingly.
The Low Pass Filter:
Just as the name says, a low pass filter will remove high frequencies.
The High Pass Filter:
A high pass filter is the opposite of the low pass filter and removes low frequencies.
[b] 
The Band Pass Filter:[/b]
A band pass filter removes low and high frequencies. A band pass filter can also be simulated with chained low and high pass filters.
Tips:
If you have a song with lot’s of instruments playing, the probabilty that it sounds all overloaded is very high. Many instruments will overlap each others frequencys, so filters are really useful in such situations, to make room in the song and get a cleaner output.
A pad for examples usually doesn’t need much low content, so you can use a high pass filter on it. When soloing a pad with a high pass filter it may sound thin, but played together with the rest of your song it’ll probably sit well in the mix.
Kicks and basslines are perfect examples for low pass filters. Often the bass content of a song sounds muddy, so you may decide, if the kick or the bassline should dominate and filter accordingly. Let’s say you want the kick to dominate, use a low pass on the kick and a band pass filter on the bassline, adjust the low cut of the band pass to make room for the kick.
A lot of people are after some nice kicks for their songs. One method is layering a couple of them and then use filters on the layers, to extract the interesting parts for your new kick.
Maybe you’ve found some useful informations here. The filter settings are of course depending on your taste and on the sounds you use, so experiment a bit. Feel free to add your own tips.
