All pass filters?

[Ethan Duni]

>> Consider this: theory says you'll never be able to phase shift a fourier
>> frequency more than 180 deg in either direction.
>
>This I don't understand. Can you be more specific ?
-i think what he means is that if you shift a given frequency is a fourier
series by more than |180| degrees (in respect to the other frequencies), it
looks just the same as having shifted it the other direction by a
complementary amount.  i don't think this would be true in joint
time-frequency analysis though.. 

Ethan