Rational Acoustics



smaudio
August 5th, 2011, 04:28 AM
Could you explain about using phase shifter and delay time on Klark Teknik DN 9848?
on practical site.
why a lot DSP don't have phase shifter like KT. they only have time delay.
:confused:

Dr. J
August 9th, 2011, 10:21 AM
Could you explain about using phase shifter and delay time on Klark Teknik DN 9848?
on practical site.
why a lot DSP don't have phase shifter like KT. they only have time delay.
:confused:

The delay function will be used to align the speakers for a phased aligned crossover and the phase shifter is a MUCH finer adjustment tool for possibly positioning a driver that is only 90 degrees away from another driver to be closer in phase by "Shifting" it.

The KT has or at least it appears to have actual "all pass filters" which is probably way better than a phase shifter.

The reason a lot of DSP devices don't have phase shifters is most likely because they are cheaper units that just don't have them OR they are called All Pass Filters instead of phase shifters.

I can't imagine any $5,000 DSP device wouldn't have all pass filters. Lower level DBX or Peavey controllers with no phase shifter......well that can be expected.

smaudio
August 10th, 2011, 03:07 AM
Thank you for your explanation. So when I'll alignment Sub and main, the first step is time alignment than phase shifter for fine tuning? Is it right?

Dr. J
August 10th, 2011, 12:51 PM
Yep. Keep in mind though that the phase shifter -- or in your case with the KT -- it will be the ALL PASS Filter you use and that may be advantageous for you since you will be able to select the frequencies where you want to use it at.

Do it very last after everything else has been done to the best of your ability. Once engaged and changes are made -- you may see your magnitude trace change as well (at the crossover area). In that case if it does -- you will have to make wise decisions on whether or not it is beneficial.

This is for the advanced tech & honestly on this forum -- I am probably the last guy you should talk to about it. :D:D

Won't hurt to experiment and learn tho.... Good luck!

lundbergsound
August 12th, 2011, 04:25 AM
Firstly, I am not familiar with this particular DSP, but I am happy to lend my 2 cents on using all-pass filters.

All-pass filters are handy in this situation because they can alter the phase response without altering the magnitude response. As such, we can use all-pass filters to make the slopes of the main and sub phase responses match over a greater frequency range, e.g., if the slope of the subs is even with the mains over the crossover range but falls below that of the mains above the crossover range, a second order APF in or below the crossover range will (albeit making you update your time delay) compensate for the concave down phase trace of the subs by sending a concave up component in the signal.

In many applications, this probably doesn't matter; the differences in the phase responses around the crossover range still create summation, and by the time they're out of sync the magnitude of one driver trumps the other - the isolation zone, if you will. For aligning them at the crossover, time delay will do just fine.

This does matter when using aux-fed subs: because the amount of signal in the subs relative to the mains (magnitude) changes, the crossover frequency (by definition the point where the magnitudes of each are equal) changes as well. Having the two phase aligned over a larger area will better accomodate this change in magnitude.

It also matters when doing an overlapping crossover, i.e., having a frequency range where both the subs and the mains are unattenuated. This a cool approach in line arrays that are flown below the subs because the subs essentially extend the length of the array and further control the directivity for this frequency range. Using subs and mains together, however, introduces the same issues encountered when designing the crossovers between high and low drivers in a single box, and all-pass filters can help there, too; check out http://www.excelsior-audio.com/Publications/Using_All_Pass_Filters_to_Improve_Directivity_Resp onse.pdf

Hope that helps. As with just about anything else that we do, best to walk before run with this: make sure your subs and mains are properly aligned at the crossover point using plain time delay, and only experiment with all-pass filters once you're confident in the system as a whole and have some spare time. Good luck!

Best,
Daniel

PS - I've seen some information saying that some FIR filters are capable of having flat magnitude responses and a variety of phase responses, but I don't know much about those yet. Can anyone suggest some good reading material on the matter?

Ferrit37
August 12th, 2011, 10:09 AM
Hey Daniel,
Here's two papers that will get you started on FIR filters
enjoy ;)

Ferrit37
August 12th, 2011, 10:53 AM
OOPS, graphics missing from Doc :o