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Author Topic: London Phase Filter  (Read 3461 times)

John Neil

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London Phase Filter
« on: February 08, 2012, 09:25:10 pm »

Attempting to use BSS London phase filter modules failed to have the expected impact during a recent alignment session.  I expected them to exhibit "all pass" behavior.  Today I snapped traces of a single filter.

The filter has two variables - frequency and phase (from 0 to 180 deg).  Shown is a 1k filter at 0, 5, 10, 30, 60, 90, 120, 150, 175, and 180deg.  180 appears as a polarity inversion at all frequencies.

Funny thing is that 10k 50deg filter and a 200 Hz 1deg filter both show up similar to the 1k 5deg filter.  A 200 Hz 15deg filter shows up roughly the same as a 1k 60deg.

It appears these aren't really conventional all-pass filters as there is no "Q" adjustment.  Every case (even if not within bandwidth of interest) adds 180deg of phase and both attributes only impact the position of the phase shift relative to frequency.

What am I missing?  This "phase filter" appears to be an animal of limited utility.  Literature searches elude.

« Last Edit: February 08, 2012, 09:27:50 pm by John Neil »
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Timo Beckman

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Re: London Phase Filter
« Reply #1 on: February 09, 2012, 03:03:25 am »

As far as i know it's a 1st order all-pass filter wich only gives you 90dg phase shift @ the desired frequency . I've asked the guy's from bss if there was going to be a 2nd order allpass in a future update but the awnser was no.
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Langston Holland

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Re: London Phase Filter
« Reply #2 on: February 09, 2012, 12:38:41 pm »

Timo is correct - those are 1st Order All-Pass filters. He's also correct in that their function will create a 90˚ phase shift at the selected frequency - that is unless it's one of these BSS units. In that case, the same thing is happening even though the designer chose to add a confusing degree selection field. As you've noticed, you are able to create identical phase responses with different frequency selections by varying the degree selection. For people that don't do FFT measurement and don't give a rip about what's happening other than at crossover, it may be helpful to know the phase offset at a given frequency. I could see that in the 70's with nothing but a RTA and a microphone. Just in case BSS would like to know, it's not the 70's anymore...

1st Order APF's for the most part are useless because they affect most of the passband when what we want to do is fix a small portion of same without making things worse everywhere else. Helping crossover summation with a 1st Order APF is like using dynamite to catch a trout.

I've recreated John's measurements using a 1st Order APF in a Lake processor. I simply looked at John's plot to see where the trace crossed the 90˚ line and selected that frequency in the Lake. Notice that APF's do not affect magnitude, only phase:



One thing that may be helpful though it still affects a large portion of the passband is to use two 1st Order APF's in series assuming the BSS processor can do that. In the following I've redrawn the 1kHz single APF but added measurements showing the combined effect of dual 1st Order APF's. The yellow-ish trace is a truly useful 2nd Order APF with a .33 octave bandwidth that cannot be made with dual 1st Order filters.

« Last Edit: July 19, 2012, 03:02:19 am by Langston Holland »
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God bless you and your precious family - Langston

Timo Beckman

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Re: London Phase Filter
« Reply #3 on: February 09, 2012, 01:00:29 pm »

One thing that may be helpful though it still affects a large portion of the passband, is to use two 1st Order APF's in series assuming the BSS processor can do that.

I have 2 BSS BLU80 wich is the follow-up on the green series . It's possible to program 2 1st order allpass filter within the LA software and to upload your design to the units . However you do not have the possibility of changing the bandwith .
To bad the awnser was no 2-3 years ago when i asked the people from BSS for a 2nd order allpass but as far as i've heard the new crown amps have these filters so being part of the Harman-Pro group would make my life a lot easier if BSS would use the knowledge from crown .

The new series from XTA/APEX/BIAMP and others have them available so i hope they get a wake-up call @ BSS .
In the mean time i'm seriously thinking about the APEX processor wich is a very nice machine with software that is easy to use . I know Lake has these filters but i do not have enough experience with the interface from lake so the APEX might be the thing for me .......
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John Neil

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Re: London Phase Filter
« Reply #4 on: February 10, 2012, 01:48:56 am »

Langston, thanks for the input.

One can insert as many of these in into various signal chains as CPU load will allow.  Considering my current venue setup...12 in, 12 zones out including the majority of speaker processing...only uses 46% even while supporting a 96kHz digital backbone, seems like there would be some extra power for a true all-pass module.  I inserted 15 of these filter modules and took a hit of less than 10% CPU load.

I can think of a few of my delay zone alignments where this would be useful to a small degree.
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