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How to build a flat-phase DSP preset?
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<blockquote data-quote="Michael John" data-source="post: 57229" data-attributes="member: 830"><p>Re: How to build a flat-phase DSP preset?</p><p></p><p></p><p></p><p>Hi Silas,</p><p></p><p>"All Pass 1" sounds like a 1st order all-pass filter : 90 degs at the centre and 180 degrees up high. "All Pass 2" sounds like a 2nd order all-pass filter : 180 degrees at the centre and 360 degrees up high.</p><p></p><p>Since you mentioned that the phase of the tweeter was increasingly leading with increasing frequency, all that says is that the tweeter is physically forward a little of the woofer. (Looking at a product photo, it's quite possible.) And by adding all-pass filters, you're simply adding group delay to bring tweeter in line with the woofer. (The analog passive cross-over will get in the way a little.)</p><p></p><p>In a passive speaker with a 4th or lower order crossover, I'd hardly expect you to be able to hear any difference with the phase as-is or flattened with a few all-pass filters.</p><p></p><p>And the high frequency phase wrapping you're correcting is only at the mic position. Any minor change in listening position will bring back a phase variation. (This is obviously less of a problem with a coaxial loudspeaker.)</p><p></p><p>Note: If the tweeter lags the woofer, as is the case for most speakers with deep hf horns, then all-pass filters can't be used to "time-align" the drivers.</p><p></p><p>Another experiment you could try is to bi-amp the speaker, time align the drivers, use a linear-phase brick-wall crossover and see how this sounds. However.... flattening the drivers with parametric stages will add phase changes ... (Unless you can do arbitrary linear phase FIR filtering. And, linear phase FIR filters can have significant group delay, and can have audible pre-ringing if too long.)</p><p></p><p>Cheers,</p><p>Michael</p></blockquote><p></p>
[QUOTE="Michael John, post: 57229, member: 830"] Re: How to build a flat-phase DSP preset? Hi Silas, "All Pass 1" sounds like a 1st order all-pass filter : 90 degs at the centre and 180 degrees up high. "All Pass 2" sounds like a 2nd order all-pass filter : 180 degrees at the centre and 360 degrees up high. Since you mentioned that the phase of the tweeter was increasingly leading with increasing frequency, all that says is that the tweeter is physically forward a little of the woofer. (Looking at a product photo, it's quite possible.) And by adding all-pass filters, you're simply adding group delay to bring tweeter in line with the woofer. (The analog passive cross-over will get in the way a little.) In a passive speaker with a 4th or lower order crossover, I'd hardly expect you to be able to hear any difference with the phase as-is or flattened with a few all-pass filters. And the high frequency phase wrapping you're correcting is only at the mic position. Any minor change in listening position will bring back a phase variation. (This is obviously less of a problem with a coaxial loudspeaker.) Note: If the tweeter lags the woofer, as is the case for most speakers with deep hf horns, then all-pass filters can't be used to "time-align" the drivers. Another experiment you could try is to bi-amp the speaker, time align the drivers, use a linear-phase brick-wall crossover and see how this sounds. However.... flattening the drivers with parametric stages will add phase changes ... (Unless you can do arbitrary linear phase FIR filtering. And, linear phase FIR filters can have significant group delay, and can have audible pre-ringing if too long.) Cheers, Michael [/QUOTE]
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