| MIX MAGAZINE AUGUST 1994 REVIEW
by Mark Frink
Sometime around Woodstock, a sound engineer thought it would be a great idea to point a speaker at the musicians onstage so they could hear. In doing so, the speaker also ended up pointing at the microphone. Since then, feedback has become as inevitable in live sound as donuts and chicken backstage. Even before that, a group of audio pioneers demonstrated that by evenly spacing equalization filters three to an octave across the audible sound spectrum, all necessary adjustments to frequency response could be achieved. Standard ISO frequency centers were established for those filters, and the ubiquitous 1/3-octave equalizer became the tool for sound contouring and feedback control.
In 1990, Sabine introduced the TEC Award-winning FBX, a revolutionary product that automatically detected the onset of feedback and assigned a 1/5-octave notch filter, matching the problem frequency and eliminating the offending squeal. The following year, Sabine unveiled the second-generation FBX-900, a digital product with nine narrower 1/10- octave filters, and they followed this with a prototype called the FBX-1200 "Signature Series," a single-channel, DSP-based FBX device with an LCD screen displaying the frequency, depth and width of its 12 filters. Applying suggestions from users, Sabine refined this design, and the 2-channel ADF-2400 Workstation debuted at the Fall 1993 AES, along with the single-channel ADF-1200.
The precise digital parametric equalization of the Adaptive Digital Filters can be used for much more than feedback control. I don't think many recording studios are prepared to deal with the extra problems associated with artists who suddenly decide they need wedges instead of headphones. This product would certainly be useful on those amusing occasions. It might also make a fine insert for those special podium mic situations where you have five minutes to ring them out before they open the doors for the awards presentation.
The ADF-2400 has a single bypass button, an LCD screen and a 20-button keypad for control and data entry, making it perhaps a bit intimidating at first. Fear not! This well thought-out, fourth-generation product is easy-to-use, giving the greenest rookie some of the EQ chops of a veteran monitor engineer, while offering sophisticated users a wide variety of options and features.
As with the FBX-900, the ADF-2400 is a processor-controlled digital filter set that can automatically sense feedback, determine its exact frequency and place a narrow digital notch filter to cancel the ring. This is done by employing 1/10-octave filters instead of the much wider filters associated with typical 1/3-octave equalizers. The result is less gain reduction of musical material and less muting of the sound at frequencies adjacent to those feeding back. The unit's reaction time is typically under half a second, light-years faster than the average sound person could set up a parametric equalizer.
An in-line device with electronically balanced XLR connections (pin 2 hot) and plenty of headroom (+26 dBV), the ADF is intended to go just before the power amp or electronic crossover in the audio chain. Sabine has thoughtfully included audio ground lift switches on the back, and Jensen transformers are an option. Its two channels each include 12 digital equalization filters and adjustable highpass and lowpass filters. Its 18-bit sigma-delta A/D conversion has a sampling rate of 48 kHz. Like all digital devices, its "digital pipeline" has a small delay - about 56 samples long in this case (less than 2 ms -- you do the math).
The front panel has two eight-segment LED meters and a "clip level adjust" knob, which is used to match the headroom of the ADF with the clip level of the amplifier (overdriving the ADF's input definitely sounds bad). Other features include a delay and noise gate on each channel, a real-time analyzer, password protection and eight user memories. It has a separate switch and LED for the hard-wire bypass, which automatically engages when turned off.
The user interacts with the ADF using a keypad and a 64x240 dot matrix, backlit LCD screen. On power-up, a "main menu" appears, from which eight submenus are selected. The "filter control menu" shows the parameters of the ADF's equalization filters. This is the window into the operating room where all the brain surgery takes place. At a glance, you can see the parameters of the 12 filters plus the highpass and lowpass filters for one channel.
The type of filter is displayed in the first column, and the three types are designated P, F or D, for Parametric, Fixed or Dynamic. The next three columns are Frequency, Depth and Width. An unassigned filter has a center frequency of zero, a width of 1/10-octave and a depth of 0dB.
There are significant differences between conventional analog filters and digital filters. The ADF's digital filters do not drift with temperature, and the device is capable of radical EQ correction without imposing the phase shift that accompanies analog filter sets. When using any digital equalizer to make EQ adjustments, users who are not familiar with this aspect of digital EQ may find it more difficult to hear changes, because they will not have the accompanying phase shift. With the absence of the phase shift, it may take more radical adjustments to convince the uninitiated listener that digital filters are "working." I encourage curious engineers to sit down and compare digital and analog equalization if they haven't already.
The ADF's other two filter types are FBX automatic feedback control filters. Like Sabine's other FBX devices, the ADF uses a patented algorithm to monitor the input signal and detect feedback. Once detected, it sets a filter, usually 1/10-octave wide, at the offending frequency. This filter is only set as deep as necessary to eliminate feedback, in 3dB increments. If feedback persists, the filter's depth is rapidly increased. Both the width and the maximum depth of all the ADF's FBX filters are global parameters, with the depth going up to -80dB and the width adjustable from 1/20 to 1 octave wide. Most causes of feedback can be cured with only a few dB of adjustment, and the maximum depth usually only needs to be -20.
Controls for "threshold" and "persistence" allow the user to fine-tune the FBX algorithm. Threshold adjusts how the algorithm looks at the harmonic content of a suspected feedback signal before it can be classified as feedback. Higher threshold settings indicate higher sensitivity to feedback and require a lower harmonic content to be perceived as feedback, to help distinguish between musical tones, which have higher harmonic content. Persistence determines the length of time that a pure tone must be present before it is classified as feedback and automatically suppressed. Higher values require the feedback to be present for a shorter time before acted upon. Conservative adjustment (lower threshold and persistence) increases the amount of time it takes the unit to react to feedback, with maximum reaction times of up to 5 seconds available. At the default settings, the unit reacted in under half a second.
"Fixed" filters are used to eliminate feedback from physical characteristics that are unlikely to change much, such as the combined response of the microphone and speakers. The ADF's ability to precisely set these filters provides the dramatic increase in gain before feedback. Once a fixed filter is set, its center frequency remains stationary, but it may be deepened automatically to control additional feedback at the same frequency. The fixed filters are programmed during the setup procedure, where the microphone levels are raised to the point of feedback so the unit can "learn" those frequencies and set the filter to the frequency and depth needed.
"Dynamic" filters are used to eliminate transient feedback that can come and go at different points of a live performance. If a new feedback frequency occurs during the show, an unused dynamic filter is automatically assigned. When all of the dynamic filters have been assigned, the filter that was set the earliest is reassigned, if needed, to handle subsequent feedback, and so on. If the feedback frequency detected is close to an existing filter, it is presumed that the new feedback is a result of a "drift" in the frequency of the original as a result of changes in air temperature or humidity. In this case, a nearby filter is moved to the new frequency.
Each time the sound system components are moved or changed, FBX devices should be "re-taught" where to place the fixed filters that are used to eliminate response peaks. Before setting up the fixed FBX filters, there are several basic adjustments that should be made. First the speakers and microphones must be placed where they'll be used during the performance.
I used Sennheiser MD 431s, Audix OM-5s and Beyer M-88 mics for my tests, going into an Allen & Heath GL-2 mixer. The speaker system I used was four JBL-loaded bi-amped floor monitors, in pairs, on two mixes. The wedges were loaded with JBL 2225 l5s and 2425 compression drivers on 2370 horns and powered with Carver amps, with a BSS 310 crossover at about 1,500 Hz.
Before making feedback adjustments, all equalization corrections for tonal balance should be made. The FBX cannot change the laws of physics, and excessive highs or lows will make it work harder to eliminate feedback, sucking up more of its fixed filters and making it necessary for it to cycle through its dynamic filters more quickly. Although basic sound contouring can be done from a separate graphic or parametric equalizer, using a few of the ADF's filters in parametric mode will be adequate with good speakers and mics.
Once you are satisfied with the sound contour of the speakers, you next "teach" the unit where to set the fixed FBX filters. I began setting the ADF's fixed filters in the usual way by slowly making the system feed back, raising the master volume on the console with the speakers and mics in place and everything turned on. As feedback occurs, there are a series of short howls or squeals that are quickly cut off. You can watch the filter's frequency and depth parameters being set on the filter control screen. Further feedback recurring at these fixed frequencies later on causes the appropriate filter to be deepened as necessary. Some well-behaved microphone and speaker combinations require only a few filters. I found six fixed filters to be more than enough, leaving the last three of the 12 filters to be used as dynamic FBX filters.
Without reading the manual, I just replaced my two 1/3-octave EQs with the ADF, and, presto, it started working, grabbing a half-dozen offending frequencies right away. It kicked in and set each filter in less than half a second, but even that brief amount of feedback can be irritating.
The dynamic FBX filters are for feedback associated with changes in the orientation of the microphone to the speaker during the performance. This may happen by the microphone actually being moved or by a change in some acoustically reflective surface causing sound energy to be bounced onto the microphone differently. Some performers may take their microphone off the stand and travel around the stage quite a bit. The extra dynamic filters available in this case are helpful, particularly if the performer gets "lazy arm" and points the mic into the monitor.
An overlooked source of monitor feedback is the narrow-band cancellation effects resulting from pathlength differences when using multiple wedges at individual microphone positions. The ADF's dynamic filters are useful when narrow dips in the combined response of multiple sound sources suddenly change. The ADF's unique ability to rapidly adapt to changing physical and environmental conditions make it an order of magnitude more powerful than a simple notch filter set.
The filter display screen shows the combined graphical representation of the filter settings. This display is shown in two forms. The first screen is a graphical representation of the location, width and depth of all the filters and their relative positions; a second mode shows the combined response of all filters with display scales of 60, 30 and 15 dB of vertical resolution available.
Like the filter display, the RTA screen has three scales with fast or slow decay. The manual has instructions for using a pink noise source, a mic preamp and a reference microphone to set up the equalization of a speaker system, but these items are not included with the ADF. Although this RTA feature is a nice extra, it will not take the place of a dedicated, full-featured RTA. The response of the RTA display is slow, as is the response of the filter display mentioned earlier, because the ADF's processor assigns the highest priority to monitoring and control of feedback. But it's useful for visual information about the frequency content of the signal.
Digital delay settings for either channel can be set in feet, meters or milliseconds, with up to 170 ms of delay available (340 ms on the single-channel ADF-1200). This can be a useful tool for delaying a monitor mix to the reference of the main house mix or the sidefills, which arrive at the stage at different times.
There are eight user presets that can be saved and named. Preset zero, the factory default configuration, provides a useful starting point and is where I jumped in headfirst. Each time the unit is turned on, it automatically returns to the same settings it had when the unit was last turned off.
All in all, I was very impressed with the ADF-2400 as both an equalizer and as a feedback exterminator. The added ability to program precise digital parametric filters and digital delay make it well worth its list price for two channels. While it may not be the right tool for some, I would bet my next monitor gig that this will outperform traditional 1/3-octave equalizers in many applications.
I used the test-rig as a rehearsal system and then for a live show. It was comforting knowing that, in addition to the increase in gain before feedback I experienced in the monitor rig, there was always the invisible hand of the processor, ready to step in and avoid a train wreck. I was able to let the band rehearse for hours on end, while I went home for a carefree meal. At the show, I was able to get the monitors ripping in no time at all without having to run back and forth to the graphic, and I was then able to go mix the house without having to worry about feedback onstage. This one's a winner.
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