Last year, I started a very complex project to expand my Dolby Atmos surround sound system to incorporate extra height speakers, more than most consumer A/V receivers support. One of the drawbacks I encountered in doing so was that Audyssey EQ room correction interfered with the process and had to be turned off. There is a solution for this problem, but it’s kind of a pain to make work.
It’s been a few months since I last wrote on this subject. I strongly recommend reading from the beginning in order to understand the theory behind using three A/V receivers in conjunction to derive a total of six height channels in an Atmos system.
In brief, the idea is to feed the Top Front and Top Rear signals from a 7.1.4 configuration into two extra AVRs, one for the left side of the room and one for the right. Each of these additional receivers will then use Dolby ProLogic II processing to extract a center (Top Middle) channel between them.
As I explained in Part 2, this center extraction process will only work if each downstream AVR can identify the mono sound information common to both the TF and TR channels. The only way that can happen is if the incoming signals to both of those channels are identical to one another. The primary Atmos AVR must be programmed with the exact same speaker distance, trim (channel level), and bass crossover settings. If any one of those parameters differs between the front and the rear channels, the secondary AVR interprets the signal as stereo and will not extract it to the center.
Unfortunately, if you turn on Audyssey room correction in the main AVR, it will automatically apply unique EQ filters to every channel. A difference in EQ between the TF and TR channels is enough to confuse the Dolby ProLogic II processor in the secondary AVR into seeing the signal as stereo, not mono. As a result, sounds that are supposed to collapse discretely into the Top Middle speaker will instead be spread across the entire side of the room from front to rear.
At least in the version of Audyssey MultEQ XT32 in my receiver (which is a few years old), there’s no way to adjust the EQ settings per channel, or even turn off EQ to select channels without turning it off to everything. Audyssey is an on/off, all-or-nothing feature. Newer receiver models allow integration with the new Audyssey app, which I understand has some feature for setting manual target curves. Whether this would solve the problem I describe or not, I’m not able to test.
After discovering this issue, the only way I could get the Top Middle extraction to work was to turn off Audyssey completely. That meant no room correction, and no Audyssey Dynamic EQ. Both were features that I like and had hoped to continue using. Without Audyssey, the sound quality in my home theater seemed a little flat. My receiver has a manual EQ feature that helped somewhat, but it’s very rudimentary and lacks Audyssey’s proprietary filters.
How to Get Audyssey Back
Thankfully, all hope for me was not lost. As it turns out, there is a way to use Audyssey room correction without disrupting the Dolby ProLogic II processing in the second and third AVRs. Getting it to work, however, is a challenge.
First, I want to acknowledge that I did not come up with this solution all on my own. Other contributors in the “Official” Audyssey Thread Part II and Beyond 7.1.4… Multi-AVR Set-up for Immersive Audio discussions at AVSForum helped me work though it. My intent here is not to claim credit, but just to describe the process.
Because the root of the problem is the way that Audyssey assigns separate EQ parameters to the Top Front and Top Rear speakers, the way to defeat that is to trick Audyssey into applying identical EQ to both. Under normal circumstances, that’s not possible. Even if you use the same model of speaker in both locations, your room acoustics will invariably cause one to sound different than the other. The whole point of EQ is to compensate for that.
As such, the only way to make Audyssey set identical EQ to both the front and rear is to make the test tones for both play through just one of those speakers. In other words, the test tone for the Top Front channel plays through the speaker in the Top Front location, and the test tone for the Top Rear channel also plays through the same speaker in the Top Front location.
Sadly, you can’t just wire the receiver terminals for both channels to the same speaker. That may cause a short circuit as the electrical signal leaves the receiver through one terminal and then travels through the other speaker wire back into the second receiver terminal. This is dangerous. Don’t try it.
As a reminder, the multi-AVR system is accomplished by connecting the Top Front and Top Rear pre-outs from the primary Atmos AVR into the stereo inputs on the second and third AVRs. Therefore, the way to combine the TF and TR signals into just one speaker is to connect both to the same TF input on the downstream AVR.
Let’s stop for a second. You may be thinking that a simple Y-adaptor will do the trick. This is not recommended. It has the same risk of a short or, at the very least, feedback distortion as I described above. The purpose of a Y-adaptor is to split one signal into two, not to combine two signals into one.
Instead, what I did was connect the pre-outs for both Top Front and Top Rear into an old manual A/V selector switch that I happened to have lying around. (You can acquire these very inexpensively if you don’t already have one.) The Top Front channels went into Input 1, the Top Rear channels went into Input 2, and the sole output went to… you guessed it, the inputs designated for Top Front speakers on my second and third AVRs. Unlike a passive Y-adaptor, the selector switch isolates each signal individually, so there’s no risk of feedback or a short.
The annoying thing about this plan is that it requires a lot of manual interaction during what should be an automated calibration process. As the Audyssey test tones moved from speaker to speaker, I had to switch between Input 1 and Input 2 and then back again at the appropriate moments to make sure all the signals got to the right speakers. Doing so also meant that I had to lie down on the floor behind my Surround speakers so that my body wouldn’t physically block any sound waves or interfere with the calibration. I’d periodically hop up to move the calibration microphone to a new location, then get back down on the floor again… and up and down again and again for all the mic positions.
However, the end result of this is that Audyssey measured identical readings (or close enough) from both the Top Front and Top Rear channels, and assigned them the same EQ parameters. Therefore, after I wired everything back to the correct speakers (and adjusted the distance, trim and crossovers to the same values in the primary AVR), both my second and third AVRs received a clean mono signal that could be extracted between the left and right inputs. After that, I just had to do some channel level tweaking in AVRs 2 and 3 using Dolby Atmos test tones and a sound level meter.
Frustratingly, those Atmos test tones are still only available on the Dolby Atmos demo discs given out at CEDIA in 2015 and 2016, but cannot be found anywhere for retail sale.The Atmos test tones are now available for download from the Dolby web site.
Once I was done, further testing with the test tones confirmed that I now have fully discrete Top Front, Top Middle and Top Rear channels in my theater, even with Audyssey room correction engaged.
This bit of success does have some caveats. As you may have deduced on your own, fooling Audyssey into assigning the same EQ values to both the Top Front and Top Rear speakers means that only one of those sets of speakers will have correct EQ in the end. In my case, only the Top Front speakers have accurate EQ. The Top Rears and the new Top Middles mimic the EQ settings for TF, which are not the ideal values for those speaker locations.
Ultimately, I’ve decided that EQ to the Top Rear and Top Middle speakers is less critical, and I can live with some error there so long as my 7.1 ground level channels and Top Fronts are all correct and my Top Middle extraction process still works.
Disappointingly, I also found that, even though Audyssey room correction now works in my system, engaging Audyssey Dynamic EQ (which is designed to adjust EQ settings depending on your playback volume level) throws off the Dolby ProLogic II decoding in the downstream AVRs – meaning that the Top Middle extraction gets screwed up again. I had to turn Dynamic EQ off. Luckily, I’ve decided that I can live without that since I tend to watch movies and TV at a loud volume anyway. (The higher your playback volume, the less Dynamic EQ does.) For me, the room correction feature is more important than the Dynamic EQ.
Who Is This For, Anyway?
I’m glad to have Audyssey room correction back. I feel that my home theater sounds best with it on.
Admittedly, the problem I had will only affect the very small number of people like myself who have experimented with extending a Dolby Atmos home theater to more than four height channels – and who, at the same time, cannot afford ridiculously expensive boutique high-end products such as the $37,000 Trinnov Altitude 32 that are currently the only single-box solutions for getting those extra channels. None of what I’ve described will affect a typical home theater owner with 7.1.4 or fewer channels, and who connects all of his or her speakers to just one A/V receiver from a major-market brand name. Which is pretty much everyone, I’m sure.
I write all of this for the benefit of those few HT gearheads adventurous (and perhaps foolish) enough to jury-rig a D-I-Y solution to get more out of their audio systems. This project has consumed a great deal of my time and attention over the past year. While there have been obstacles to getting it done, I hope to show that workarounds for them are possible, though you may have to get a little creative.