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driverack/FBX with "musical" feedback?


Hello All,
Noob to the forum here...hoping for some useful input on some pre-purchase questions.
Here's the background. Small-club/private party cover band, usually trio that occasionally expands to a four or five-piece as needs dictate. Drummer/soundman (mixer behind band). Different room each night, with no real time to "ring out" system in front of customers. Combination of in-ear and wedge monitoring. Powered mains, with powered subs about to be purchased.
I'm planning on a DriveRack PX for fast balancing and crossover management. What concerns me is the auto feedback system on the mains. I love the idea of it in principle given the unknown nature of each room we'll play and the tendency for vocalists to "wander." However, we also use "musical" feedback in many tunes, such as the guitar feedback swell in "The End" by The Doors...and we do mic the guitar cab for the mains.
I'm concerned that AFS will try to "knock down" intended feedback such as this guitar swell. Are there particular settings on the PX that will discern the difference between "controlled feedback" like this and unwanted, uncontrolled feedback on a vocal mic? If not, how do I defeat AFS on the PX?
February 8, 2012 @11:59am

Don't have a PX, but on the PA...
Each of the 12 filters can be set as fixed or live.
Fixed means you "ring out" ahead of time while the DriveRack is in learn mode, deliberately generating microphone feedback by walking around with a mic right in front of your wide open mouth, cupping it and dipping into the wedge and doing all the things that your singer might do. Try to set a few filters for each of these situations... weighted toward what your singer is most guilty of doing. When a pure tone (frequency is perfectly constant) above the threshold is detected, the DriveRack notches the frequency and moves on to the next filter, so have someone at the mixer pushing your fader as each of the notches are set. You'd be surprised at the difference between where that fader was before and after ringing out.
Live means the 'learning mode' mentioned earlier is ongoing throughout the whole show. Filters are lifted (disengaged, waiting to be set again) after a certain period of time that you set. This sounds wonderful in theory, but there is a huge flaw. The 'learning' isn't smart enough to differentiate between unwanted feedback, and any other perfectly constant pure tone, whether it be guitar feedback, certain synth patches, even a non-vibrato singer in some cases! So, I can't recommend using live filters.
I know you said you don't have the opportunity to ring out fixed filters, but it actually is possible. I mentioned "threshold" earlier... I don't know exactly what it is, but let's say it's 0dB (about 20dB below the DriveRack's clip point). You generate feedback on your mic, and the system engages the notch when that tone hits 0dB. Now, in terms of how loud this is to your audience is COMPLETELY dependent on how much amplification is going on post-DriveRack (and in many systems, the SPL is around 85 for a 0dB input). So here's the trick. Turn your amps down 10dB, turn your mixer up 10dB, and now the DriveRack will engage those filters that much sooner! Now your audience hears the ringing get killed at an SPL of 75 instead of 85. If it's still too annoying, go even deeper than my suggested 10dB. If they still give you funny looks, maybe you just need to approach several of the patrons closest to the speakers and explain that you're about to make some feedback so they won't hear any during the set.
When you're finished, don't forget to put your gain structure back to the original settings.
February 17, 2012 @06:21pm
Dave Burris

I was under the impression that the Driverack feedback suppression was a bit smarter than that. Are you sure it works the way you describe. It should not be that difficult in DSP world to detect that a frequency is rising faster at the output than it is at the input and base the filter decision on that.
I was also under the impression that the width of the filters are variable based on the detected output.
February 18, 2012 @03:47am

I would love to be schooled by a dbx engineer on how my description is completely wrong, but years of using the product taught me what I've just passed along.
"Rising faster at the output than it is at the input" isn't really the case... it may appear like that in a graphical explanation of the feedback phenomenon, but I'm not sure it's measurable. One thing that is measurable is the fact that the tone is increasing in volume exponentially, but regardless the other examples (guitar, synth, etc) do still manifest as false positives in my actual experience. It's not always noticeable to the ear, because when you remove the fundamental the remaining harmonics actually trick the ear into hearing the fundamental (this is how Waves MaxxBass works) so you might not even know unless you watch the DriveRack filters while the false positive is happening.
The width of the filters is a global parameter. To quote the literature:
Speech is a wide filter, Music Low is a narrow filter, Music Medium is a Very Narrow filter, and Music High is an Ultra Narrow filter
The most narrow is something on the order of 1/80 octave IIRC.
February 18, 2012 @06:22pm

Also in my first post I mentioned using the wedge to excite feedback... I take that back. Assuming the DriveRack is on the main bus and not the monitor sends, only the main stacks should be included in that ringout process. The wedges should have their own filters (commonly a 31-band EQ).
February 18, 2012 @07:14pm
Dave Burris

It would certainly be useful to have an exact description of how it works. I was under the impression it works as I described but I haven't tried to measure it. It certainly changes my impression of the usefulness of the live feature. If it works as you suspect, almost any live material could create the false positives you mentioned.
I'll have to give some thought as to how I would go about verifying the operation on a test bench.
February 18, 2012 @08:03pm
Dave Burris

Also in my first post I mentioned using the wedge to excite feedback... I take that back. Assuming the DriveRack is on the main bus and not the monitor sends, only the main stacks should be included in that ringout process. The wedges should have their own filters (commonly a 31-band EQ).

Agreed. I have used them in both situations, but not recently. I have to reevaluate how I use them in the future.
February 18, 2012 @08:05pm

My experience with the Sabine units is that they are very slow to lock on to feedback even if the feedback is the only signal going through the system. It's faster - and less annoying - for me to let my iDevice app tell me the frequency and for me to dial the EQ in manually. They will also falsely set filters over the course of an evening, turning things to mush.
When I was still schlepping my rig around, I rang it out in the most troublesome room I worked, and never touched those EQ bands again.
February 20, 2012 @07:33am
Dave Burris

I had a similar experience with the Sabine units.
February 20, 2012 @11:49am