Reply posted by Chip on September 19, 2001
Tom,

Two questions:

1) would you consider preparing a "idiots guide to LF phase"? I'm very interested in the phase artifacts caused by different types of boxes and venting / porting methods. I never realy considered this as such a huge factor in the differing performance of different types of boxes.

2) How would you best describe an all-pass filter, and it's effects?

David, et all, please respond as well. This type of shared information is why we are here.

Thanks,
Chip

Reply posted by Nathan Butler on September 18, 2001
Whenever I think of this, I like to reference some simple mathematics. Bear with me...

A pure tone can be described by:

1) cos(wt) where w = frequency, t = time

With a phase shift (1) becomes:

2) cos(wt + p) where p = phase in degrees

With a time delay, (1) becomes:

3) cos(w(t + td)) where td = time delay

As an example, let's say w = 100, p = 90, and td = 0.9

(2) becomes cos(100t + 90)

(3) also becomes cos(100t + 90)

Now let's make the frequency, w = 200

(2) becomes cos(200t + 90)

(3) becomes cos(200t + 180)

Essentially, a time delay yields similar results to a phase shift, except that a time delay increases the phase shift with frequency.

Hope this helps.
Nathan Butler

Reply posted by Patrik Arnekvist on September 18, 2001
I gotta go and try if changing phase angle on my omnidrive to, say 90 deg changes the delay, I guess it will. but how does one come to the conclusion that here I need to shift the phase angle 90 deg?

I can´t really inderstand why one would need to do that.

But changing the delay must be different, cause the time of 90 deg at 5kHz is quite different than 90deg at 100 Hz..right?

I´m getting a headache here, excuse me for thinking out loud.

Patrik

Reply posted by Chip on September 18, 2001
Patrik,

If I understand the electronics correctly, the phase adjustment in the DSP simply "rolls" the phase without adjusting time.

If I'm correct, changing the phase would not change the result of an impulse time measurement.

To the best I can understand it, at this point, the differentiating factor between the requirement for a phase adjustment VS. a time adjustment would be the phase angle relationship AT the crossover frequency. If the angles are not parallel, some time adjustment is required.

Once the phase angles are parallel, if necessary, you can adjust the phase to make the phase angles lay one on top of the other while remaining parallel. If you were to continue to adjust the time, you would loose the coherance of the phase angles and have a less phase coherant area above, and below, the crossover point.

Reply posted by David Gunness on September 18, 2001
Everything you've said is true - so I think you've got it. Let's see if I can clarify it.

If the arrival times are the same, the phase response curves will be parallel. If the two signal paths are "in phase" at crossover, the phase response curves will have the same value at the crossover frequency. The ideal situation is for both of these conditions to be true, but occasionally you can't achieve both by only adjusting delay.

David Gunness

Reply posted by Chip on September 18, 2001
David,

This is exactly what I was getting at. You were able to deliver it in a much more understandable way than I was.

Can you think of any other way to differentiate between the two?

Chip

Reply posted by David Gunness on September 18, 2001
Delay always produces phase shift, but the phase response can be shifted without producing delay.

Dave