IIR filter and phase shift I heard that an IIR filter has delay and phase shift effect, but not clearly what are they. For example, 1-pole Lowpass has 90 degree phase shift, 2-pole Lowpass has 180 degree? So does it mean two 2-pole Lowpass filters has 360 degree phase shift? Beside the HighPass, LowPass, AllPass filter, other IIR filter like LowShelf, HighShelf, Peak... has phase shift too?
When I apply an IIR filter using biquad transfer function, does it has any phase shift diffirent if I use direct form I (4 delay registers) vs direct form II (2 delay registers)?
If IIR filter has phase shift, the phase shift occurred at the filter's target frequency, or all the frequencies? How can I phase shift (all frequencies) 90 degree clockwise &amp; counterclockwise just using AllPass filter?
I know some of my questions are noob, but I'm just study dsp myself, and there're somethings that I really confused about. Sorry for my bad English too :)
I believe you have received no answers to your questions because your first three questions are so strange, so puzzling, that no one where to start to even begin to compose answers.  See:

http://www.bing.com/videos/search?q=digital%20filter%20phase%20response&amp;qs=n&amp;form=QBVR&amp;pq=digital%20filter%20phase%20response&amp;sc=0-0&amp;sp=-1&amp;sk=#view=detail&amp;mid=3DC56A834B9C701B5FE93DC56A834B9C701B5FE9

and

http://www.bing.com/videos/search?q=digital%20filter%20phase%20response&amp;qs=n&amp;form=QBVR&amp;pq=digital%20filter%20phase%20response&amp;sc=0-0&amp;sp=-1&amp;sk=#view=detail&amp;mid=85921946745DEDD05D7385921946745DEDD05D73

Good Luck.