Isn't it annoying when so called experts throw around terminology in their marketing literature?
From this paper: http://www.ayre.com/pdf/Ayre_MP_White_Paper.pdf
"There is no phase shift, so it is called a "Linear Phase" filter.
What Linear Phase really means is that the phase shift is a straight line (linear) where all frequencies are delayed the same amount such that the waveshape or transient response is preserved. When all frequencies are delayed (phase shifted) an equal amount it is as if we heard the signal the delay time later. Pure time delay preserves waveshape. Good for those who believe that perfect transient response is important.
Continuing from this paper:
"However, a filter can only achieve a "Linear Phase" response by introducing pre-ringing. This means that before every single musical transient, these is a "pre-echo. In nature, there is no such thing as a "pre-echo". All events must be "causal" in the real world - the cause must precede the effect."
That is a good one! Linear phase filters are not non-causal, it is not possible to build non-causal filters as this paper suggests, since they require knowledge of the signal before it arrives!
While the phrase minimum phase sounds like it might be something good, it is actually a complex mathematical characteristic that is not necessarily good:
http://en.wikipedia.org/wiki/Minimum_phase
When the minimum phase constraint is put on a filter it excludes delay equalization circuits that can *improve* the phase linearity when properly designed. Delay equalizers are non-minimum phase filters!
It is difficult to read further in that paper ...
Isn't it annoying when so called experts throw around terminology in their marketing literature?
From this paper: http://www.ayre.com/pdf/Ayre_MP_White_Paper.pdf
"There is no phase shift, so it is called a "Linear Phase" filter.
What Linear Phase really means is that the phase shift is a straight line (linear) where all frequencies are delayed the same amount such that the waveshape or transient response is preserved. When all frequencies are delayed (phase shifted) an equal amount it is as if we heard the signal the delay time later. Pure time delay preserves waveshape. Good for those who believe that perfect transient response is important.
Continuing from this paper:
"However, a filter can only achieve a "Linear Phase" response by introducing pre-ringing. This means that before every single musical transient, these is a "pre-echo. In nature, there is no such thing as a "pre-echo". All events must be "causal" in the real world - the cause must precede the effect."
That is a good one! Linear phase filters are not non-causal, it is not possible to build non-causal filters as this paper suggests, since they require knowledge of the signal before it arrives!
While the phrase minimum phase sounds like it might be something good, it is actually a complex mathematical characteristic that is not necessarily good:
http://en.wikipedia.org/wiki/Minimum_phase
When the minimum phase constraint is put on a filter it excludes delay equalization circuits that can *improve* the phase linearity when properly designed. Delay equalizers are non-minimum phase filters!
It is difficult to read further in that paper ...