Why do we need a center channel in a home theater system?

[JoachimStrobel]

I wonder if someone has actually made a test video for this. Someone should. There are plenty to test individual speakers but not the transitions that I know of.

I don't have a specific scene in mind and can't recall exactly what I may have used in the past.

Best bet would be using an animated movie with surround sound from the recording (not synthesized locally from stereo). These typically have quite exaggerated sounds corresponding to movements, are simpler in isolated sounds (like foot steps) and typically have plenty of such movements. Any of the recent Pixar or Disney animations should work. Just may have to go through the film and identify where the screen exits and screen positions are in relation to the audio stage.

A Mch music recording with an active centre channel might be good too. I start searching....

 

[Vasr]

A Mch music recording with an active centre channel might be good too. I start searching....

What you can align with just sound is seamless transition between speakers. You can do this with just a single tone and a set up to shift the balance between any two channels when playing it.

Or have a single tone recording that does this slowly and progressively between channels rather than jump between positions as in the video above. I have not seen such a recording for testing yet. I don't think this would be difficult to generate with the right software.

This, however, does not help with the problem of aligning the sound stage location to the visual stage location, the problem I was pointing to in home theater systems.

With just audio, there is no absolute reference point as to where the particular sound should be coming from only relative position between them and relative to the position of the speakers.

 

[JoachimStrobel]

What you can align with just sound is seamless transition between speakers. You can do this with just a single tone and a set up to shift the balance between any two channels when playing it.

Or have a single tone recording that does this slowly and progressively between channels rather than jump between positions as in the video above. I have not seen such a recording for testing yet. I don't think this would be difficult to generate with the right software.

This, however, does not help with the problem of aligning the sound stage location to the visual stage location, the problem I was pointing to in home theater systems.

With just audio, there is no absolute reference point as to where the particular sound should be coming from only relative position between them and relative to the position of the speakers.

Well, and then there is the issue of the head related transfer function
https://en.wikipedia.org/wiki/Head-related_transfer_function
Sound coming from the center aimed at your head will and should and can not sound identical to the left and right speakers as the angle how it hits the ear is different. That is the real reason one needs a centre speaker for serious mch music, a centre signal originating from two side speakers is very different from one originating from the centre speaker. One would hope that the recording engineer thought about that.
Hence, even in a perfect 5 speaker system with identical speakers surrounding you in a perfect setup, a moving signal will not sound the same. You would cross your finger that the surround engineer who generated a soundtrack or recording has the HRTF dialed in somehow. One of my pending projects is, to use ear mounted microphones with REW to quantify this effect for my personal geometry as a single Umik pointing upwards will not do that...

 

[Vasr]

Sound coming from the center aimed at your head will and should and can not sound identical to the left and right speakers as the angle how it hits the ear is different. That is the real reason one needs a centre speaker for serious mch music, a centre signal originating from two side speakers is very different from one originating from the centre speaker.

Personally, I think this is rather academic than an issue in practice like the bit rate/sample rate discussions. The variance from the different kind of speakers and their placement is so different from set up to set up with far more impact than HRTF, that there is no one set up that is for "serious mch music" whatever that means. And then there is the matter of hearing and ear lobe differences as well!

One would hope that the recording engineer thought about that.

No, since the recording engineer has no idea what kind of a system, layout the content would be played in, there is no one single adjustment that would be correct to do even if possible. If anything, this is best relegated to the system itself when possible and necessary. "Perfect reproduction" is an invention of the audio geek not the recording engineer who is just looking at making it sound balanced and good on a reference system which may be very different from what one hears at home and very different from what was on stage if it was live. Most of the mixing adjustments are gross adjustments such as volume levels, channel balances and tonal balance not some psycho-acoustic factors.

From all the multi-channel music I have heard, there isn't a well-recognized "norm" or "standard" or even an agreement of how a multi-channel sound needs to be in a recording. All of them are done by audio professionals who are more like Chefs with their own idiosyncrasies than some scientific approach. This is true even in stereo recording from the same master tapes.

People need to get away from the idea there is this one magical sound captured in a recording of what it should be like that can be perfectly reproduced elsewhere.

In my opinion, it is more practical to make sure that tonal balances and volume levels and phase differences between speakers are well adjusted and room modes eliminated and not worry about other nitpicking geeky stuff. What you hear is what you get and two people can appreciate the same music equally well even if they are hearing differently even by objective measures. It is lot more productive to spend that time on selecting the right content and learning to listen critically after some minimum thresholds and basic calibration and set up.

If you want to stick mics in both ears and do REW measurements, knock yourself out. It is a hobby after all.

YMMV.

 

[Alexium]

Could you guys comment on my finding that removing (turning off) the center channel improves imaging/soundstage for upmixed stereo music due to the fact that all near-center sounds are hard-anchored to the center speaker even if they should come from slightly to the left, to the right, or even above the geometric center point?
Is this known, expected? Or a bug in the upmixing matrix of my particular AVR (Denon X2500H)? I've described it in a bit more detail here.

 

[Sancus]

Could you guys comment on my finding that removing (turning off) the center channel improves imaging/soundstage for upmixed stereo music due to the fact that all near-center sounds are hard-anchored to the center speaker even if they should come from slightly to the left, to the right, or even above the geometric center point?
Is this known, expected? Or a bug in the upmixing matrix of my particular AVR (Denon X2500H)? I've described it in a bit more detail here.

Turn center spread on in the surround parameter settings for Dolby Surround. Otherwise yes it does that. I stopped using DTS upmixing a while ago because Dolby w/center spread just seems better. That said, Auro2D/3D is the best at keeping spatial qualities and soundstage natural, but the 2500H doesn't have it. Auro2D also adds a bass boost, which is sometimes problematic. Auro3D is the best and most configurable but it requires 4 height channels.

 

[JSmith]

I prefer a phantom centre regardless of whether using surrounds etc. or not.

Some run 2 centres... or even use decent bookshelves as centres, actually running the centre channel of a multichannel mix, not a downmix.



JSmith

 

[Sal1950]

If your going to be playing modern 5.1 or better recordings I would just say yes, get a center channel. There are ways around it using various hacks but if you want things to sound the way the producer intended it's much easier and the results will be better just doing it the intended way.
YMMV

 

[tuga]

Perhaps but it cannot change the position of the speakers and their relationship to the HRTF.

Are we locating the speakers or the phantom images?

 

[Alexium]

Turn center spread on

Wow, live and learn! Just as I thought I've discovered and understood all the important settings of my AVR.

Thank you very much, center spread certainly does help, although stereo upmixes still sound worse with a center than without it, but much less worse now. I suspect it's not helping that my center channel is located slightly below the tweeters (and even the midwoofers) of the L/R speakers, and neither is the fact that my CC speaker is not matching the L and R (uses different drivers).

 

[Sancus]

Wow, live and learn! Just as I thought I've discovered and understood all the important settings of my AVR.

Thank you very much, center spread certainly does help, although stereo upmixes still sound worse with a center than without it, but much less worse now. I suspect it's not helping that my center channel is located slightly below the tweeters (and even the midwoofers) of the L/R speakers, and neither is the fact that my CC speaker is not matching the L and R (uses different drivers).

Yes, it's important that your center is of good quality and matches as closely as possible. Otherwise it will draw attention to itself. Assuming you're using a standard center, and don't have space for a speaker in regular orientation, it should be a 3-way(not MTM) or coaxial that is identical to or at least tuned to sound the same as your L/R. In terms of height, you want it less than 10 degrees off from your L/R at the listening position, and even less is better.

Honestly, upmixing is usually fine on my system with center spread off, but mine is also identical to L/R. But it is content dependent, some content just doesn't work quite right, and center spread usually helps, but even that doesn't always make things perfect. Auro3D is the only upmixing I've used where I think I could just leave it on for everything and never think about it.

 

[Alexium]

you want it less than 10 degrees off from your L/R

Thanks again for all the useful info and helpful answers!

Might I ask for a quick explanation why a horizontal MTM center speaker is inferior? Is it because of its narrow horizontal dispersion, or because it can't be tonally matched well enough with non-MTM main L/R speakers even if they all use the exact same drivers (e. g. frequency response or phase response differences due to different crossovers and physical layout)? Or is it something else?

 

[Thomas_A]

One question to you that have stereo speakers only. If you sit in front of the left speaker and localize the point where the center is for a dialogue in a film, where is this point? I can’t get any other point than the center of the TV screen, even if there are no visual cues present.

 

[Vasr]

One question to you that have stereo speakers only. If you sit in front of the left speaker and localize the point where the center is for a dialogue in a film, where is this point? I can’t get any other point than the center of the TV screen, even if there are no visual cues present.

To a first approximation (neglecting reflections and other room modes), the distance of the perceived point away from any speaker is the weighted ratio of the corresponding volumes for the same sounds from each speaker reaching the ear. This is how balance (and individual channel balance settings in contemporary equipment) works for imaging. But it also depends on the frequency of the sound.

Lower frequencies are much less localized to the ear so other cues "trick" the mind into thinking it is clumped with the related sounds. This is how subs are incorporated into the system. Play the sub on its own, and you can fairly reliably locate the sub which can be totally off-center. Play it as part of the content and it will look like the bass parts are coming from wherever the related higher frequency sounds are coming from. So psycho-acoustics are also involved here even with only audio cues.

If you play a pure tone and move around, you will much more easily notice the change in center depending on your relative position to the speaker. Play it with mixed content and your mind plays a part in that perception, including an expectation that the dialog is in the center even without visual cues.

And if you have actual stereo content in which the far speaker may be carrying different content with stronger audio cues (frequencies for localization), your perception may still be swayed in that direction even if you are sitting closer to the other speaker.

 

[Sancus]

Might I ask for a quick explanation why a horizontal MTM center speaker is inferior? Is it because of its narrow horizontal dispersion, or because it can't be tonally matched well enough with non-MTM main L/R speakers even if they all use the exact same drivers (e. g. frequency response or phase response differences due to different crossovers and physical layout)? Or is it something else?

It's the horizontal dispersion, yeah. It's not just narrow, it also tends to be very messy with null patterns as close as 10 or 20 degrees off axis. Here is one that Amir reviewed. Completely aside from the fact that the on axis sound will be bad in some seats, that will also make a total mess of reflections. Compare that to a decent, but hardly expensive($400) Monoprice 3-way center. Or the Infinity RC263 which is practically free on sale.

I suppose if you have a heavily treated HT you can argue that the reflections "won't be that bad" but I think they should just be avoided. Unfortunately, there are some good low end speakers(Elac DBR62) that have an MTM matching center and that is... very unfortunate.

Fortunately, there are still good options like the Monoprice set, Kef, Infinity and many others.

 

[JoachimStrobel]

Personally, I think this is rather academic than an issue in practice like the bit rate/sample rate discussions. The variance from the different kind of speakers and their placement is so different from set up to set up with far more impact than HRTF, that there is no one set up that is for "serious mch music" whatever that means. And then there is the matter of hearing and ear lobe differences as well!

No, since the recording engineer has no idea what kind of a system, layout the content would be played in, there is no one single adjustment that would be correct to do even if possible. If anything, this is best relegated to the system itself when possible and necessary. "Perfect reproduction" is an invention of the audio geek not the recording engineer who is just looking at making it sound balanced and good on a reference system which may be very different from what one hears at home and very different from what was on stage if it was live. Most of the mixing adjustments are gross adjustments such as volume levels, channel balances and tonal balance not some psycho-acoustic factors.

From all the multi-channel music I have heard, there isn't a well-recognized "norm" or "standard" or even an agreement of how a multi-channel sound needs to be in a recording. All of them are done by audio professionals who are more like Chefs with their own idiosyncrasies than some scientific approach. This is true even in stereo recording from the same master tapes.

People need to get away from the idea there is this one magical sound captured in a recording of what it should be like that can be perfectly reproduced elsewhere.

In my opinion, it is more practical to make sure that tonal balances and volume levels and phase differences between speakers are well adjusted and room modes eliminated and not worry about other nitpicking geeky stuff. What you hear is what you get and two people can appreciate the same music equally well even if they are hearing differently even by objective measures. It is lot more productive to spend that time on selecting the right content and learning to listen critically after some minimum thresholds and basic calibration and set up.

If you want to stick mics in both ears and do REW measurements, knock yourself out. It is a hobby after all.

YMMV.

I was about to let it go ... but then, no. This centre sound issue is of prime importance, it is mentioned in Toole‘s book too.
Before Corona I often went to my Jazz club. Very often they had a jazz trio on stage and did not mic it up - so great. Piano left, saxophone centre, bass right. And that was it, the true sound of a saxophone being played in the middle between my eyes and ears. Every now and then they used microphones and fed them to a two speaker system. I should have left. Now, when the saxophone played, my brain saw a saxophone in the middle while the sound hit my ears sidewise, creating the illusion that it is in the middle, but soundwise it was not. When playing loud and/or moving around some of the saxophone‘s direct sound came through and that sounded good.
Now, if that were a recording, the same problem would happen. The saxophone would sound as being in in middle, only it is not. That is the reason why a centre speaker beats an upgrade of a DAC by a lot. And why Stereo should have had three loudspeakers as originally planned. I hear that some engineers have a way to massage a centrally placed instrument so that some of the true centre illusion returns - in that case a centre channel extraction is counter productive. One would hope that in a true Mch music recording is immune to that problem.

 

[JoachimStrobel]

Could you guys comment on my finding that removing (turning off) the center channel improves imaging/soundstage for upmixed stereo music due to the fact that all near-center sounds are hard-anchored to the center speaker even if they should come from slightly to the left, to the right, or even above the geometric center point?
Is this known, expected? Or a bug in the upmixing matrix of my particular AVR (Denon X2500H)? I've described it in a bit more detail here.

For sure Dolby surround had the habit to catch slightly of centre sound and nail it to center. There was no way it would pan from left to right. The released Dolby surround music CD suffered from that, they tried to combat it with stuff like Circle surround and others names that would try and trick the Dolby decoder, but it did not and those CDs were discontinued.. luckily DVDAUDIO arrived then.
So, there might be a special surround mode that is gentle on the centre catching, but I guess one would have to invest into Lexington grade stuff for that. You could try it with vinyl, as that is more forgiving due to its already weak channel seperation.

 

[phoenixdogfan]

Pic from a cigarette ad?

Amazing he lived into his 80's, isn't it?

 

[Alexium]

So, there might be a special surround mode that is gentle on the centre catching

As Sancus pointed out above, there is a "Center spread" option that yields a tremendous improvement, although it is still difficult to achieve soundstage that's as good as just the two main speakers provide.
Based on my own experience so far - mind that my center speaker is not perfectly matching and is not located at the perfect ear-level height - I would argue that for one seat only, and even for two closely located seats (e. g. you and your significant other on a couch), with a less than perfect center channel speaker, one is better off without a dedicated center speaker. But I'm now curious about a perfect center channel. I also barely listened to any DVD-Audio mch music yet, which I'm going to listen to as time allows.

 

[Vasr]

I was about to let it go ... but then, no. This centre sound issue is of prime importance, it is mentioned in Toole‘s book too.
Before Corona I often went to my Jazz club. Very often they had a jazz trio on stage and did not mic it up - so great. Piano left, saxophone centre, bass right. And that was it, the true sound of a saxophone being played in the middle between my eyes and ears. Every now and then they used microphones and fed them to a two speaker system. I should have left. Now, when the saxophone played, my brain saw a saxophone in the middle while the sound hit my ears sidewise, creating the illusion that it is in the middle, but soundwise it was not. When playing loud and/or moving around some of the saxophone‘s direct sound came through and that sounded good.
Now, if that were a recording, the same problem would happen. The saxophone would sound as being in in middle, only it is not. That is the reason why a centre speaker beats an upgrade of a DAC by a lot. And why Stereo should have had three loudspeakers as originally planned. I hear that some engineers have a way to massage a centrally placed instrument so that some of the true centre illusion returns - in that case a centre channel extraction is counter productive. One would hope that in a true Mch music recording is immune to that problem.

Imagine the situation of listening to a recording where there is a trio playing - one in the center, one slightly to the left and one slightly to the right. With your "logic" we really should have two more speakers at the exact position of where the two side instruments should be because the waves coming from such a position if live would hit the ears differently (angle) than the proportionate mix of L and R to create that stereo image. Even if you had a center speaker where such an image would be created by a proportionate mix to L and C or R and C, it wouldn't be the same with the same logic because it would be still creating the "illusion" of a position in those two locations with two "artificial" sound sources.

The logical extension (to show the absurdity) of this is to have a zillion speaker array in front for every possible location of where the instrument might be on stage so that the wave coming from that instrument live would be exactly be the same as what is coming from a speaker corresponding to that position.

A moment of sanity would suggest that not only is this impractical but wholly unnecessary and that human hearing is perfectly capable of appreciating the content equally with far more imperfection than this theoretical artifact. Our entire visual depth perception is based on this "illusion" as well as making movies possible with small and finite frame rates.

Perhaps this is the difference between people who have an audio system primarily to appreciate the content that is being played vs people who want the system to "perfectly reproduce" something without which they are convinced they can not appreciate the content as well. Both have a point but only within limits, far from academic extremes.