Phantom center with constant direcitivty speakers

I've done a number of installs where i choose not to use a center and have gotten great results.... Pesky neighbors visiting dont quite get it, but the sound was great..

I have been running a pair of 4Pi's for half a year and also have no desire to use a middle channel. Speaker setup is so much easier not having to worry about that pesky center channel.

Assuredly. I have a pair of B-52 LX-1515 as my mains with a phantom center, it is the absolute clearest sound. I will never use a center channel again. For the longest time I was happy with two channels handling the whole show, but now I run a 4.1 system. Every attempt to go back to 'conventional' speakers results in unhappiness. I'm sold on phantom center with waveguide compression drivers.

Edited by imagic - 7/19/12 at 5:52am

Same here, works fine.

It's great to get the millstone of a center channel off the neck, what with its too-low position, early reflections from floor and screen, and need for more amp channels if using external amps or biamping.

I have 18Sound 12ND710 + BMS 4552ND/XT1086 with lots of toe-in, will be replacing with JBL 2206 + SEOS12/360

Ditto. I've done it with dipoles, econowave variants and Synergys. Haven't noticedany need (but then, didn't try to compare with a real center)

removing the center will compress the signal down obviously, to send the info to the L/R. I noticed a difference in sound at times without the center but with a pretty broad listening area, I had trouble pinning down the center image across them. It was great at the MLP, but couldnt get it close anywhere else. The big question is, does this compression of the signal to downmix the center's info cause any issues?

Sure a center channel is likely a good idea if you are showing to a crowd, but for 2-3 people phantom center seems to image well enough. Then again the specific horns in use probably affect this, with more directional horns projecting a more accurate image within a smaller listening area and vice-versa with wide-dispersion tweeters. I'm curious what speakers you are using. Here's a picture of mine, B-52 LX-1515...



Edited by imagic - 7/19/12 at 12:26pm

There is no data or dynamic range compression when moving the center to the L/R so I'm not quite sure what you are talking about. If there is an issue, we wouldn't be able to use bass management and combine 8 signals together. This is no different than combining the multiple channels in a digital audio workstation when mixing music or movie audio.

There is a reduction of 3 dB to the center channel before it is combined with the L/R, but that won't make any noticeable difference in quality.

I wouldn't call it compression; downmixing at most.

I don't think mixing the center into L/R is in any danger of harming the signal quality.

Is this with constant direcitivty speakers?

If so, have you experimented with toe-in?

hey omega, lots of folks are in your camp. it works, so just enjoy.

Not sure how it works on today's modern AVR's but back not long ago it was known that Dolby decoders would apply dynamic compression when or if the center channel and/or subwoofer channel was not used.

I use an HTPC and forgot about that. I never could figure out if it was tied to the decoders or just to the receiver offering Dolby Dynamic Range Compression. I think for some receivers it still happened even when decoding DTS-HD.

Signal compression, that is what I am getting at. As Scott suggested above, this WAS the case, if it still is, I dont know. DD you are correct about bass management, but Dolby has taken this into account while at the same time assuming one would use a center channel as well. When removing that, the decoder has to RE-compensate for a channel not being there and that is when artifacts and issues can come about.

Noah, this was with JTR t-12's, so not horns, and could have been part of the problem.

I will say after stating all the above that I preferred a phantom center setup myself before I got my A-T screen and the triple 8 for the center. When before I had major issues with any center keeping up (obviously) and placement as it was basically on the ceiling above the screen.

Yikes

Ah, AT screen is another story.

I'm just not willing to give up the gain and other advantages of my Hipower.

I wouldnt either if I had the ability to use one

I've used phantom centre for years with horn/CD based speakers and direct radiators (KEF 104/2). For a brief time I used a KEF coax centre with the 104/2, but found the difference trivial.

Re DD signal compression with no centre: I've read this is so, but seen no actual evidence so far beyond assertion that this is the case.

As I don't use the internal amplifiers it's moot for me anyway, as I simply mix the C into L&R at line level post AVR.

Well that would be why.

I did it as a 'just in case' rather than compelled by evidence. Considering what I was building anyway, it added 2 resistors and a connector, so less than $1 and 10 minutes work. Better than finding out the compression is actually correct and having to add it later.

Pretty smart.

Hi A9X-308, Can you explain how you did it? I did it with 4 op-amps and some resistors, because I was afraid of causing cross-talk between left and right. It was a pretty brute-force approach, and I thought that there had to be a better way.

Standard inverting audio mixer using an opamp (in my case a hybrid forced class A LM4562). Feed C into summing node of L and R opamps via selected resistor to get it to -3dB.
The Land R opamps were already in place to feed signal to the modified DCX, so all I needed to add were the additional resistors, a short length of cable and an RCA.

Thanks, A9X-308, now I see. Having some of the opamps in place makes it a little simpler.

What I thought was awkward with my circuit was that I used two opamps to buffer the center channel, one to drive left and the other for right. I was trying to avoid any resistor path between the left and the right where crosstalk might occur. Each of those two opamps scaled by one half, while the left and right buffers were 1:1. I then summed with resistors.

But back on topic:
Can someone explain what is meant by "constant directivity speakers"? This thread makes it sounds like it is important to getting a good phantom center, but I have no idea why.

Bill Waslo wrote this very good article on them:
Setup of Controlled-Directivity Waveguide Speakers

You could do it with 4 resistors and no opamps. I needed the opamps to get the correct gain structure and drive for the low Zin of the ADC

Crosstalk is not really an issue, as you can get very good imaging from an LP with frequency dependent CT usually no better than 30dB down.

It all depends on the surround processing being used, but in a nutshell Dolby EX is much more flexible and capable than Dolby Pro Logic. Since it is a newer technology, once upon a time it was only available of high-end receivers.

I am heavily considering a phantom build for my new living room. (SEOS12 w/deltalites)

I cant seem to find any REAL data on the compression aspect for all of the newer codecs. Unfortunately it seems like Dolby no longer offers the same kinds of detailed engineering write-ups like they used to. Their website is just filled with marketing nonsense now.

why doesn't somebody ask dolby