Anthem ARC Room EQ Interview with Dr. Peter Schuck and Nick Platsis

When Anthem launched their proprietary Anthem Room Correction solution, commonly known as ARC, in 2007, it received widespread praise from many corners of the industry for a single reason: it worked and worked well.  Fast forward seven years later and most A/V enthusiasts are probably aware of many different room correction systems, including Anthem’s ARC. 

While room correction is becoming more wide-spread, what's less clear is how these systems actually work and the differences between each system.  Even the more knowledgeable audio buffs out there would be hard pressed to tell you the finer details of these systems i.e. what kind of filters they utilize, operational bandwidth, etc.  So what did we do? We started peppering the purveyors of Room EQ products with questions in hopes of getting a clearer picture of what  they offer.

As part of our ongoing series on room correction solutions, Audioholics talked with Anthem's Dr. Peter Schuck and Nick Platsis about the various aspects of ARC. 

Dr. Peter Schuck's background includes research with the Audio Research Group at the University of Waterloo, and at the Acoustics and Signal Processing Group of the National Research Council where, after research in active noise control, he assisted Dr. Floyd Toole in the formation of the Athena Project and became its Project Leader. In 1993 he joined Paradigm Electronics Inc. where his activities included the design and implementation of loudspeaker measurement for Paradigm’s anechoic chamber, design and implementation of digital signal processing including ARC (Anthem Room Correction) and PBK (Paradigm Perfect Bass Kit), with continuing focus on overall system design and software for Anthem surround sound processors and receivers and Paradigm active subwoofers, speakers and soundbars.

With a background including power distribution, professional audio and music, Nick Platsis joined Anthem Electronics in 1997 where his activities included circuit design, production testing, and technical support. His current focus is in product management.

ARC is available exclusively in Anthem’s MRX-line of receivers and their AVM50v and D2v preamp-processors.

Audioholics: Could you give us a brief history of ARC?

Dr. Peter Schuck: ARC grew out of studies conducted at Canada's National Research Council, especially the work of the Athena Project in 1990-1993 where the scale of the loudspeaker/room interaction problem and its effect on loudspeaker evaluations was established.  These conclusions were verified and strengthened through subsequent studies.  The foundations of the smoothing and positional averaging used to process in-room measurements were formulated and experimentally justified.  Later research at Paradigm led to the determination of target response.  All the work on using IIR equalization and designing the IIR filter coefficients was done here.

Audioholics: How long has ARC now been available?

Nick Platsis: Since 2007.

Audioholics: What kind of filters does ARC use, and at what resolution (i.e. 1/3 octave, 1/12 octave, etc)?

Dr. Peter Schuck: ARC uses IIR filters that can in principle approach arbitrary resolution, however, since the measurements that ARC uses are smoother in practice, the filters tend to be about 1/12 octave resolution at maximum depending on how smooth the in-room response is.

Audioholics: What is the maximum boost / cut ARC will apply? Is it possible for users to set a limit in this respect (i.e. no more than 3dB of boost)?

Nick Platsis: Boost at any one frequency is up to 6 dB to avoid overloading the amp and speaker.  This cannot be adjusted by the user.  Nulls are also much less audible than peaks.  Cut is any amount needed.

Audioholics: ARC comes with a calibrated, serialized, USB microphone.  Are there any limitations with the microphone's measurements or does it measure accurately from 20Hz - 20kHz?

Nick Platsis: It is reliable up to 5 kHz.  After that it starts becoming too directional with increasing frequency.  Maximum correction range for MRX receivers is 5 kHz.  This is the normally recommended setting, and the default, in all versions of ARC and is user adjustable down to 200 Hz.  In AVM/D it can be set as high as 20 kHz, but effectiveness of correction at these upper frequencies often depends on whether the tweeters (and mic) are at ear level.  A listening test is the best way to find out.

Anthem's ARC kit comes with a calibrated USB microphone, stand, and USB cable.

Audioholics: Does ARC apply room correction below the audible spectrum, 20Hz (assuming the user has speakers or subs that will play below that frequency)?  Thus, am I correct in stating that "Anthem receivers can EQ from 20Hz-5kHz and pre-pros can eq from 20Hz to 20kHz with the default, user-selectable limit being 5kHz?”

Nick Platsis: Although not as much would happen at the limits, correction could go as low as 15 Hz because the frequency is not a hard cutoff but a blend into the next territory.  (The rest is correct.)

ARC measures, calculates, and corrects for sound anomalies caused by room boundaries and reflective surfaces

Audioholics: How does ARC help address the typical issues a room causes (resonances, modal peaks/nulls, etc)? Does room correction largely negate the need for room treatments for consumers, or would you suggest using both room correction and physical treatments?

 Dr. Peter Schuck: ARC removes resonances and modal peaks that are common at the measurement positions.  Traditional treatments for reverberant rooms, such as carpeting, retain their sonic as well as aesthetic benefits, while treatments such as large membrane absorbers are often difficult to implement effectively at low frequencies where resonances are most prevalent.  The easiest "treatment" is to find speaker locations that avoid the most prevalent resonances.  The fewer the major problems in terms of room interactions that ARC has to deal with, the better overall job it will do.

Detail view of ARC-1M Curve Viewer for the MRX-series receivers. This equalization curve is shown above, and is an indication that ARC generates correction that would be difficult to set manually using traditional graphic or parametric equalizers.

Audioholics: Does ARC utilize multi-point measurements? Why or why not? If so, how many measurement points are available?

Nick Platsis: The default is five measurement positions though for large listening areas up to ten may be used.  Response changes considerably even when changing position by as little as the distance when moving one's head side to side, therefore multiple mic positions are always needed.  At bass frequencies, multiple positions also prevent peaks and nulls at any one position from becoming the basis for all correction in the range or for determining crossover frequency.

Audioholics: Does ARC correct subwoofer response? How do your products calibrate and correct the response of multiple subwoofers? Please describe the benefits of ARC in this regard. 

Nick Platsis: ARC starts at 5 Hz and uses just as much resource in the sub channel as for any other channel.  When using multiple subs, run Quick Measure while adjusting each sub's level, and phase control if available, until response is flattest.  If the subs have Paradigm PBK, run that first.  The final step is the full ARC measurement.  ARC treats the subs as a mono group because once walls, a floor, and a ceiling are involved, there's no such thing as stereo bass, at least at subwoofer frequencies.

Audioholics: How can users best use ARC’s "quick measure" feature to determine the best placement for speakers.

Nick Platsis: This mostly applies to the subwoofer since it's the only speaker without a prescribed location.  Select a channel and run the sweep tone.  It sweeps continuously and after a few sweeps the graph becomes a live update.  If response is fairly smooth when the mic is in the listening area, there's probably no need to reposition the speaker.  This can be confirmed after running the full measurement.  If response during Quick Measure shows severe peaks and valleys, temporarily place the sub in the listening area if possible and measure from possible subwoofer positions.  Ones that don't show severe peaks and valleys are usually good candidates for subwoofer positioning in terms of flatter response.  Other considerations of course include aesthetics and, for the other speakers, imaging.  Check response again with the mic in the listening area.

Audioholics: Does ARC show users "before", "target", and "after" response curves? Is it possible for end users to adjust the final response curve such that they can flavor the sound to taste?

Nick Platsis: ARC shows all these curves.  The user can adjust correction range and amount of room gain, but nothing else to re-shape the curve.  Room gain is an increase in bass that is caused by the room.  ARC does not try to flatten this because doing so would make the sound unnatural.  It would be like taking how a speaker sounds outdoors and making it sound like that indoors.  Our ear/brain mechanism has issues with that when it knows it's in a room.

Before/after in-room response for two of the channels. The purple curve shows the result, before EQ.  

The bass rolloff is actually coming from the bass management crossover which is standard in all prepros and receivers.

Audioholics: As a follow-up to the "room gain" question: at what frequency does room gain begin? To my eye, it looks like it's somewhere between 200-400Hz with a gradual slope.

Nick Platsis: This depends on the in-room measurement, and it's not unusual to have no room gain at all especially in open concept areas or where bass traps are used.

Audioholics: What do you feel are the important differentiators between ARC and competitors?

Dr. Peter Schuck: It works!  Listeners prefer ARC on vs ARC off.  We don’t hide behind pseudo-scientific explanations, and apparently other companies are slowly gravitating towards our approach as they can test our system and see for themselves that it works.

Audioholics: Are there any plans to expand ARC beyond PC support (Mac or Linux?).

Nick Platsis: Not at this time.

Audioholics: Are there any plans to offer ARC as a stand-alone product/solution or only bundled as part of an AVR/pre-pro?  I know you have the perfect bass kit as an option but that's bundled only with subs.

Nick Platsis: Bundled only.  A standalone unit would in essence be a prepro without digital inputs, video capabilities, and sales volume.  The ADCs (at least eight channels' worth in this case), DSP, DACs, level control, power supply, chassis, electrical certifications, production, marketing, etc. would still be required and regardless of the bill for all this, the thought of inserting additional A/D and D/A conversion stages between preamp and power amp doesn't sit well among purists.

Audioholics: Do users have the ability to set multiple profiles—for example, setting a separate "music" and "movie" room correction profile where one can be tailored for a different frequency response range, bass response, different crossovers, etc?

Nick Platsis: Yes - all our AVRs and pre-pros have two profile memories.

Through ARC's Target Customization Window  you can optionally set separate movie and music configurations for ARC. 

By default, both the music and movie configurations use the same response cutoffs.

Audioholics: Are there any forthcoming upgrades or improvements to ARC that users can look forward to?  Because ARC's processing is performed externally on a PC, will current ARC users be able to take advantage of those upgrades? 

Nick Platsis: Although it doesn't happen as much as it used to, every now and then an unusual measurement with an unusual problem is sent to tech support, and sometimes the solution is to tweak the algorithm for the next ARC build.  Anyone can download the software.  Most users would probably not encounter a situation where the latest tweaks would come into effect.  The most significant change lately is the faster connection type and enhanced user interface used by the current generation of MRX receivers but the algorithms are pretty much the same as in the legacy ARC version.  The rest comes down to how much filtering the hardware allows.

Audioholics: ARC's responses in the application window are not crossovers.  Can you explain for the average end user what they're seeing?

Nick Platsis: It's the correction range limit on the crossed-over side of the graph.  In all cases except for the subwoofer it matches the crossover frequency set in the prepro/AVR.  For the subwoofer, the crossover frequency isn't necessarily reflected in the graph because the sub channel plays two things - the LFE and the bass that's redirected from other channels.  The crossover frequencies are shown in the prepro/AVR setup menu.  On top of this, the sub correction limit normally extends beyond the crossover for a couple of reasons.  It's usually easier to attain flat EQ by correcting to the higher frequency, and any peaks beyond the crossover frequency but still high enough in amplitude to provide directional cues are also removed.

ARC's PC-based software shows the measured response of the speaker, the target response, and the calculated response post-correction. 

Audioholics: Are there any specific instances/situations where a user would measure separately for "movie" and "music" modes? Or, is the best practice to simply do one set of measurements?

Nick Platsis: The main reason for taking separate measurements is when the acoustics change, for example screen up vs screen down or door open vs door closed.  Music, a movie, or music in the movie soundtrack are all the same as far as the playback equipment is concerned but if someone wants different profiles regardless, for example with sub vs without, they can be set as desired.

Audioholics: Following up on the previous question, do you have a best-practice suggestion if users have true full-range speakers (going into the low 20Hz range) and are setting up the full range speakers for two-channel listening in addition to home theater?

Nick Platsis: You only need to measure once because the measurement can be copied to the second profile, where the sub can be removed easily.  The center and surrounds can also be removed but this is unnecessary for 2-channel listening since not selecting a surround mode during listening has the same result.  Make all the changes in the ARC software, not in the setup menu - this ensures that ARC will recalculate the curves accordingly.  Deleting the sub forces the fronts to full-range but the green ARC graph, the one that shows corrected response, may still give the impression that it's rolling off the extreme low end. 

You can override this too but keep in mind that when a speaker manufacturer says the speaker can play in the low 20 Hz range, the conditions to attain this aren't always mentioned.  ARC sets it parameters according to the in-room response it sees, and where there's a steady decline in measured bass response, ARC sets its target accordingly.  The red curve, the one that shows measured response, essentially says, "This is what you've been getting all along without EQ" and if you create settings such that the green curve ends up 6 dB higher than the red one at 20 Hz, it's not an improvement.  In such a case click on "Cancel" or "Autodetect" which restores the original targets.

ARC will save two profiles, labeled "movies" and "music".  You can have both profiles be the same or have one profile for movie listening and one for music.  Each can have different settings. 

For example,  you can set the ARC "Music Profile" to run your speakers full range without a sub and then engage the sub just for movies.  Paradigm Signature v3 speakers are pictured above. 

Audioholics: If using ARC, am I correct in understanding that if I theoretically feed it a 192kHz source, then the MRX units will downsample to 48kHz and the pre-pros will down sample to 96kHz?  In other words, none of the models will be able to apply ARC at the native 192kHz?

Nick Platsis: This is correct, and for the longest time AVM/D was the only unit on the market that could operate room eq at higher than 48 kHz.

Audioholics: If I'm using the built-in ADC (Analog-to-Digital Converter) on the receivers and pre-pros via one of the analog inputs (let's say I'm playing an LP) and I choose to apply ARC will the units likewise down sample the incoming analog signal to 48kHz for the receivers and 96kHz for the pre-pros?  Let's assume that there is no recording going on and I don't need to match a destination format.

Nick Platsis: A/D conversion in MRX is to 48 kHz.  In AVM/D, it is according to the ADC setup menu.  The selections range from 44.1 to 96 kHz and as a side note 176.4 and 192 weren't included because all they do is capture more noise.  There's no extra audio signal to be had up there from an analog input.

Audioholics: What are the settings for in the "Advanced Target Customization" window?  What are some real world examples where a user would benefit from using the "advanced Target Customization" window?

Nick Platsis: Very little reason remains.  We've been looking at the oddest user files sent to tech support since ARC came out and made necessary tweaks to auto detection so targets would no longer need to be manually overridden.  Lately there haven't been very many tweaks but if a reason to continue them presents itself, we would.  One reason that remains, and this may never change, is that if small speakers are to be played loudly, you may want to set a higher frequency to help prevent the woofer from reaching its excursion limit.  The ARC sweeps aren't very loud so they're unlikely to make a woofer bottom out.

ARC's Advanced Target Customization Window

Audioholics: Once a user configures ARC, what is the relationship between ARC and Anthem's speaker calibration setup menu?  Can a user change crossovers, speaker volumes, distances, etc. after running ARC?  If they do, do they need to re-run ARC or re-upload parameters?

Nick Platsis: ARC sets the crossovers and level calibration.  If changes to crossover frequency are needed, it's best done through the Targets panel (then click OK, Calculate, Upload).  If such changes are made in the menu alone then ARC may end up being be mis-calibrated in the crossover ranges.

Audioholics: If a user has "full range speakers" that are +/- 3db at 23 or 25Hz, what would "full range X-Over" and "Flat" do with ARC?

Nick Platsis: Full Range sets the DSP's crossover to "large" or "crossover off".  Flat turns off the bottom end rolloff but be careful when using this.  Even though the option is available on all channels it normally shouldn't be used with anything except subs that have plenty of output at frequencies below 20 Hz.

Audioholics: Is there a provision in ARC that allows redirection of LFE to the Main Front L/R channels as well as the dedicated subwoofer channel too?  (This is valuable for the few people who have true full range tower speakers that can exceed the Audioholics Extreme Bassaholic rating)

Nick Platsis: LFE only plays from the sub unless the sub channel is disabled, in which case the AVPs, and not the AVRs, redirect LFE to the L/R front speakers.

Anthem ARC Room EQ Interview with Dr. Peter Schuck and Nick Platsis Part 2

Anthem's D2v 3D processor comes standard with ARC

Audioholics: With ARC, once there is a dip above a few hundred Hz, how does ARC know whether it's the loudspeaker response (and therefore able to be equalized) vs. it being a characteristic of acoustical interference and therefore should be left alone?  If it's the latter, for example, wouldn't adding a boost to the dip  add a resonance to the loudspeaker and that resonance would then radiate everywhere?
 
Nick Platsis: ARC sets its curve according to interference common in all mic positions.  It wants to EQ as little as possible, the point being to address acoustical interference as opposed to EQ'ing the speaker.  It's not very often that a midrange frequency needs a 6 dB boost and ARC was never meant to serve as a crutch for inadequate speaker design, with the expectation being that the speaker has certain minimum performance including smooth anechoic response both on and off axis.

 
Audioholics: Are there any speaker technologies that ARC is not designed to correct.  For example, 360 degree speakers (like Mirage) electrostatic, magneplanar, or exotic speakers (like a plasma arc).  I assume that dipole and bipole are not a problem since that's a supported speaker type in the Anthem menu?
 
Nick Platsis: We experimented with various directivities and ARC continues to work well with all of them.  It may also bear mentioning that a few years ago Paradigm/Anthem and Martin-Logan became sister companies.  Note that with dipoles, including planars, the amount of room gain can be lower depending on whether the speaker also uses a regular woofer.  We haven't tried plasma arc due to ozone issues, but the expectation is that the room correction would work just the same.  The reason that the prepros have a dipole setting, and this was in place before EQ came about, is unrelated and meant to prevent over-delaying surround channels in terms of the time alignment menu, since most of the sound from a dipole or bipole surround speaker bounces off the front and back walls before reaching the listener.

Detail of the calibrated ARC USB Microphone

Audioholics: Is there any reason why ARC doesn't auto-level and phase match single or multiple subwoofers (assuming a maximum dual configuration where a sub would be one connected to each outputs with no Y connector)?  Would there be any advantage or disadvantage to having a future version of ARC—rather than the enthusiast user—auto-level and phase-match multiple subwoofers automatically?
 
Nick Platsis: There are many reasons.  We experimented with independent sub channels and connecting a mic into the unit instead of computer, but it kept coming back to leaving these things as they are.  It may not be apparent to the enthusiast community but the majority of our receivers and prepros are dealer-installed and "multiple subs" most often means using four of them.

 
Audioholics: If a user has multiple subs that are either different models or from different manufacturers, would ARC do an adequate job correcting the multiple subs and any acoustic differences they would present such a setup?
 
Nick Platsis: It shouldn't matter as long as system design principles that existed before EQ came along are still observed, which means using the more capable subs at the front of the room, and the ones at the back to fill in the valleys.  Initial setup using Quick Measure is the same - start with rear sub level turned down and increase it until response on the live graph is smoothest, then run full measurement.

Audioholics: To wrap things up, is there anything specific about ARC you would like to tell our readers?

Nick Platsis: Although graphs provide the results, listen with the ears not the eyes.  ARC was designed to benefit all systems and was blind-tested with good speakers in a good room, good speakers in a bad room, bad speakers in a good room and bad speakers in a bad room.  Listener preferences were consistent - sound was better with the correction turned on in all of the above.

Many thanks to the Nick and Peter for taking the time to answer our questions about their excellent ARC room correction system.  Fellow Anthem enthusiasts, please share your experiences with ARC in our forum.