wjm@whuxj.UUCP (MITCHELL) (04/13/84)
Yes, it is possible for a filter to have a phase shift of more than 360 degrees especially some of the high (11+) order filters used in CD players. However, the key factor (as mentioned in an earlier article) is not the value of the phase shift but rather how this value varies with frequency. Ideally, one would like the phase shift to vary with frequency in such a way that the DELAY is constant for all frequencies - the so called linear phase character- istic. While a 90 degree phase shift (1/4 wavelength) represents a delay of 2.5 msec at 100 Hz (one complete cycle or wave = 360 degrees takes 10 msec at 100 Hz), the same 90 degree phase shift only represents .05 msec at 5kHz. The output of a filter with a linear phase characteristic will preserve the phase relationship between the different frequencies in the input signal in its output - any other phase characteristic will alter this relationship which will result in distortion. Incidentally, the May 84 "Stereo Review" has an interesting article about the pros and cons of digital audio (as currently available on 1984 CD's). Bill Mitchell Bell Communications Research, Inc. Whippany, NJ (whuxj!wjm)