Can somebody explain, how exactly the digital filters affect the sound: sharp roll-off, short delay sharp roll-off, slow roll off, short delay slow roll-off, super slow roll-off?
I don't hear any difference. Or are those not working for third party player apps (using Rockbox)?
Slower rolloffs take out some of the top end. The slower the rolloff the more so.
The short-delay filters put any ringing caused by antialiasing at the back end of the impulse, which theoretically masks the ringing better.
The rationale for using each filter may be as follows:
Sharp rolloff: your high frequency hearing doesn't go up to the Nyquist frequency (22050Hz in the case of 44.1kHz sample rate) so you choose to use a sharp rolloff filter. This way you get flat response up to the limits of your hearing and no audible ringing (only inaudible ringing at 20000-22050Hz)
Slow (or super slow) rolloff: your high frequency hearing does go up to and above 20000Hz so much that you do hear ringing at those frequencies; besides, modern pop music is mastered too brightly for your tastes. You kill two birds with one stone, the slow rolloff filter: it makes ringing less noticeable and the highest frequencies less prominent.
Linear phase filters (i.e. the ones not labelled "short delay"): you believe preserving the phase relationships of instrumental sounds up to the highest frequencies is beneficial to correct soundstage reconstruction.
Minimum phase filters (i.e. the ones labelled "short delay"): you hear ringing with the "not short delay" version of the same filter and want to make it less obvious without switching to a slower rolloff filter. Or, you really have use for a shorter delay (playing twitch games on your i5???)
That said, don't be surprised if you don't hear a difference; the differences are subtle at best.
