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Topic: What is a reasonable number of channels (Read 7541 times) previous topic - next topic
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What is a reasonable number of channels

Through the years, I've seen many discussions on the optimal bitrate and samplerate of for example CD-audio, but I can't remember much discussion about the third 'number' of the CD specs: the channel count. 1-channel, 2-channel, 5.1 and 7.1 have been in use, and today the first Dolby Atmos Blu-Ray disc was announced. I was wondering: is this going to continue to increase, like samplerate? (Just look at DSD512 or DXD to see what I mean)

I've seen various arguments over the number of front speakers. Apparently, the use of only two channels in stereo sound was a trade-off, where three would have been more efficient. However, mixes for systems with three front speakers (like most surround systems) do not always use the center channel for the soundtrack but only for speech. For example, the popular IRT cross recording arrangement does not use the center channel. I found one source, [1], that mentions the limited separation of available microphones to be the main reason for this.

So, techniques currently used for 2-channel sound are probably not adaptable to use with surround using the three front channels. However, things like wave field synthesis ask for a much larger number of channels. It looks like you either use a lot of speakers or just a few? Maybe because of this, all systems using more than 2 channels seem to rely on three front speakers and add rear and height channels, like 9.1, 11.1 and Dolby Atmos.

I found another source, [2] that argues that 4 channels are enough to create a very detailed surround image. It looks to me that that is at the cost of having a very narrow sweet spot, however. It also requires a specific recording technique.

With so many opposing views on the number of channels, is there any sane conclusion to draw? I'd say 3 for the front channels is about as good as it gets, but you can add a lot of surround speakers and height speakers if you want to. You can add wide fronts, height channels, more rear speakers ad nauseam.

TL;DR We can argue that 44.1kHz @ 16-bit is enough for human ears, can we say something similar about the number of channels?

[1] Slotte, B., Sharpening the image in 5.1 surround recording, Tonmeister Symposium 2005, available here
[2] Miller, Robbert, Contrasting ITU 5.1 and Panor-ambiophonic 4.1 Surround Sound Recording Using OCT and Sphere Microphones, AES 112th convention paper, 2002
Music: sounds arranged such that they construct feelings.

What is a reasonable number of channels

Reply #1 –
With both object based audio, and ambisonics (not to mention Dolby Pro Logic), the number of transmission channels is different from the number of speakers. We improve digital audio reproduction by oversampling the recorded signal temporally, so why not spatially? Then there are two separate questions to answer: number of transmission channels, and number of speakers needed, the two not being equal.

If you don't move your head, and match the HRTFs in the recording to the listener, 2-channel binaural and transaural recordings can already be "perfect" (i.e. indistinguishable from the source) in terms of spatial reproduction.

I don't know what is needed to deliver "perfect" spatial reproduction once you can move your head, and/or if you don't/can't have HRTFs in the recording (which IMO you can't if you are aiming for perfection, because everyone's HRTFs are different, and you can't make separate recordings for each listener).

Cheers,
David.

What is a reasonable number of channels

Reply #2 –
For movies/effects or for music?

I think you can simulate a concert hall with 5.1 or 7.1 since the reflected sound doesn't have a point-source.   

For movies/effects and the possibility of sound coming from 360 degrees in 3 dimensions, I imagine you could take advantage of many-many more channels.    I'm sure studies have been done related to how precisely a listener can pinpoint a source, and how precisely this can be "faked" by panning between two speakers.    For example, I don't perceive the phantom center channel as precisely as I perceive the location of the left & right tweeters.  (At least that's my un-scientific, non-blind impression.  ) 

Quote
I found one source, [1], that mentions the limited separation of available microphones to be the main reason for this.
With multitrack recording you can get 100% isolation...  Put the piano 100% on the right, the guitar 100% on the left, and the singer 100% in the center, and the drums 100% in the rear...  The "microphone separation" or directionality have nothing to do with it. 

If you are recording acoustic music "live", you could argue that 2 or 3 microphones are enough.  But in the real world, when this basic approach is taken (even for a stereo mix), there are often added "spot mics" for solo instruments/vocals and "room mics" to pick-up the reverb.

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So, techniques currently used for 2-channel sound are probably not adaptable to use with surround using the three front channels.
What?

Quote
I found another source, [2] that argues that 4 channels are enough to create a very detailed surround image. It looks to me that that is at the cost of having a very narrow sweet spot, however. It also requires a specific recording technique.
It seems to me that more channels reduces the sweet spot...  With mono, it's not too important where you sit.  With stereo, you'd like to sit in the, and sitting near a side speaker would also change the experience (more than sitting on the side with 3 or 3 channel stereo).

P.S.
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What is a reasonable number of channels...?
Hmmm..... "reasonable"... 

I have 5.1 and currently I have no desire for more channels.    When listening to 2-channel stereo on my home theater system I use a Pro Logic II soundfield for some rear-channel reverb.

If I was building a home theater from scratch, and if I had an unlimited budget, I'd go with 7.1 because there are Blu-Ray discs with 7.1.... and why not?

What is a reasonable number of channels

Reply #3 –
Quote from: 2Bdecided on
and/or if you don't/can't have HRTFs in the recording (which IMO you can't if you are aiming for perfection, because everyone's HRTFs are different, and you can't make separate recordings for each listener)

Yeah, I've given up the hope of ever hearing a binaural recording that sounds like it should for me.

Quote from: DVDdoug on
With multitrack recording you can get 100% isolation...  Put the piano 100% on the right, the guitar 100% on the left, and the singer 100% in the center, and the drums 100% in the rear...  The "microphone separation" or directionality have nothing to do with it. 

[...]

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So, techniques currently used for 2-channel sound are probably not adaptable to use with surround using the three front channels.
What?

I'm sorry, I was talking about overhead microphone arrangements. Sure, if you record every instrument separately it can be done, but when using an overhead microphone with spotmikes it won't work, because the overhead microphones don't have enough separation.

You're right though, I was only thinking of classical music. Of course it is not true for most other kinds of music.

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If I was building a home theater from scratch, and if I had an unlimited budget, I'd go with 7.1 because there are Blu-Ray discs with 7.1.... and why not?

I started this thread mostly because Dolby Atmos is coming to home theaters this year. They are selling 7.1 + 4 height channels for example. I can think of situations in which this would help... but I can think of cases where speakers on the floor might help too. Or anywhere else for that matter. I was wondering: is it sane?
Music: sounds arranged such that they construct feelings.

What is a reasonable number of channels

Reply #4 –
I think practicality plays an important role, and I think this is where the word "reasonable" comes in.

While you could convince a larger group of people that a certain bit-rate and sampling rate is ideal, It's not so easy with channels. It's obvious that more channels are more precise and reduce the sweet spot. But then there's a matter of costs and space requirements.

My personal preference for movies is 5.1, I just don't want to deal with positioning more speakers. With 7.1, unless the room where you watch movies is a perfect square, with no other obstacles in the way, there's always going to be one speaker that is gonna have issues.

When we move our heads, we can be pretty good at identifying that one sound is actually coming from more than one source. eg: Reproducing mono sound with 2 speakers. You're always going to be trying to recreate a virtual speaker, but if the person focuses, he should be able to tell where the sounds are really coming from.

A lot of research has been put into HRTFs lately and we have plenty of commercial products that take advantage of it. But it will always be an approximation, because like 2Bdecided said everyone's HRTF is different. The traditional virtual speaker feels a lot more accurate.

7.1 setups never got my attention, because it wasn't even where I expected the industry to go. The 2 side channels didn't mean much to me, and were even harder to place accurately in the room. I always found them to be "In the way"

I had 5.1 setups for my computer before I had them for my TVs, so most of my 5.1 content were 3D games instead of movies. Ever since I had 5.1 setups, I noticed the lack of vertical coverage. For me, the natural evolution would have been an 8.1 solution (or 9.1 with center speaker) where you'd have 8 speakers positioned to form a Cube. eg: A bottom 4 speaker surround, and a top 5 speaker surround.

That would be my ideal setup, and because each speaker is in a corner, and not the middle of the room, for me they are easier to place.

What is a reasonable number of channels

Reply #5 –
I've heard Dolby Atmos with, I think, 34 speakers. The sound did seem to be able to come, accurately, from pretty much anywhere. IIRC Atmos supports up to 128 audio objects rendered to up to 64 speakers (in a cinema).

I'm not sure how much relevance that has to capturing a real sound field though.

There's also Higher Order Ambisonics, NHK's 22.2 system, ...

Is it sane? At the end of the day, it's there to make money. Will enough people buy into one of these things for it to make money? Probably. That's sane.

Will I get it at home? No.

Will someone use it to deliver better (=more accurate) audio than we have now? Maybe. If it's just more pan-potted mono, I don't think I'll like it. I've only once heard something beyond stereo that I really did like.

Movie sound tracks are usually an artificial creation. They aim to create a sound field that never existed in real life. Audiophiles think they want accurate reproduction of a real sound field that did once exist. It's almost the opposite requirement.

People with a bit of discernment will note that we can't even reproduce the sound of a solo violin convincingly. It throws dramatically different sound waves out in different directions (especially height-wise), and a couple of microphones can't capture them properly. 5.1 (without height) doesn't help with the violin sound at all. It helps with the sound of the hall, but for me that's missing the point.

Cheers,
David.

What is a reasonable number of channels

Reply #6 –
Quote
I was wondering: is it sane?
  I suppose it was insane when that Earthquake movie come out in the 1970s and they used the Cerwin Vega subwoofers to rumble the theater!  3D has always been a little insane!    Half the fun is insane effects! 

And if something "catches on", we seem to get used to it...  A movie in regular 2-channel stereo or a system without a subwoofer just doesn't cut it anymore...   

I doubt that 7.1 +4 will ever "catch on" with the average consumer at home...  "Regular" 7.1 isn't that popular in home theater setups (at least not yet).  But, all of these additional channels might make some movies more fun and interesting in the theater.

What is a reasonable number of channels

Reply #7 –
Quote from: 2Bdecided on
I don't know what is needed to deliver "perfect" spatial reproduction once you can move your head, and/or if you don't/can't have HRTFs in the recording (which IMO you can't if you are aiming for perfection, because everyone's HRTFs are different, and you can't make separate recordings for each listener).

Cheers,
David.



Probably some sort of device that stores your HRTF and tracks your head as it moves and adjusts DSP to suit?

What is a reasonable number of channels

Reply #8 –
Quote
Probably some sort of device that stores your HRTF and tracks your head as it moves and adjusts DSP to suit?
You can buy head tracking headphones that work 5.1 sources.  (There is no custom HRTF.)

What is a reasonable number of channels

Reply #9 –
As far as channels goes, Dolby Atmos looks interesting in that it seems to be channel-independent: while it can handle lots of speakers, it doesn't require lots of speakers.  There's apparently something in its processing that will map between the spatial info in the objects in the sound file and the actual number, location and type of speakers you really have to best map the intended sound to your system.  Or so says the marketing - I've never heard it, so YMMV.

As far as accommodating one's specific HRTF and dealing with head movements, this seems well-regarded, if not expensive, but I've never tried it.  Also no idea what happens if you try using Atmos with Smyth - perhaps be wary of  crossing the streams.

What is a reasonable number of channels

Reply #10 –
Quote from: 2Bdecided on
If it's just more pan-potted mono, I don't think I'll like it. I've only once heard something beyond stereo that I really did like.

Interesting. That's one possible addition that has been proposed to 5.1: you can add rear speakers, height speakers and 'wide front' speakers. It's not used often, but it is a possibility in for example Audyssey DSX and used in the 10.2 surround format. However, seeing that it is not going to be a format introduced for movies, it probably won't be introduced for music as a different format.
Music: sounds arranged such that they construct feelings.

What is a reasonable number of channels

Reply #11 –
Quote from: ktf on
However, mixes for systems with three front speakers (like most surround systems) do not always use the center channel for the soundtrack but only for speech.

Forgive me for going off on a tangent. I'm not sure exactly what you mean by your statement. It seems to me that you feel that using the front center channel only for speech is some kind of failing.

If that is what you're saying, I strongly disagree. The modern fashion in film and TV is to clutter the soundtrack up with all manner of background environmental noises and unnecessarily loud incidental music, which makes understanding the dialogue really difficult at times. This is why the front center channel should only be used for speech - so we can turn down all the incidental stuff in relation to the dialogue to give us a chance hearing what's being said.

What is a reasonable number of channels

Reply #12 –
Quote from: ktf on
It looks like you either use a lot of speakers or just a few? Maybe because of this, all systems using more than 2 channels seem to rely on three front speakers and add rear and height channels, like 9.1, 11.1 and Dolby Atmos.

For movies I would agree.
For music, if you are referring to "existing" stereo media, I find 4 sufficient. No center. But not just any fronts.
For proprietary constructs, it seems like 5 is preferred.

cheers,

AJ
Loudspeaker manufacturer

What is a reasonable number of channels

Reply #13 –
i've moved to 7.1 from 5.1 and i love it.  i don't have the scratch to be an early adopter of atmos, but i'm very much interested in many, many more channels.  i would easily be that guy with 64 speakers running Atmos, if i could afford to do so.  i find a properly set up 7.1 can be wildly immersing with stereo recordings as DSP's/convolutions are getting insanely good for up-mixing stereo.  even better if you can get a hold of a well done true multichannel mix!

my answer to the OP question:  IMO there is no limit to number of speakers that could be used.  unlike say high res audio files that have a provable audibility threashold, it can be easily proven that every single speaker added (or subtracted) will make some difference (sweet spot size/position, reverb, cancellations, etc.)...and much more difficult to prove audibility of those changes (what if the blind tester moves her head 1mm to the left...maybe she can hear it?).  until one creates a perfect sphere of carefully designed speakers with the listening point at exact center, i guess.  but hey, even that will be unpleasant sounding to someone.  all i can say for sure is that i will remain forever, and extremely stubbornly so, in the camp that wants more and more speakers.  most of us mortals can't afford this stuff anyways, so things like atmos (done to it's limits) will remain more of a dream anyways

wave field synthesis in real life    http://www.soundandvision.com/content/true-stereo

What is a reasonable number of channels

Reply #14 –
Quote from: ktf on
Through the years, I've seen many discussions on the optimal bitrate and samplerate of for example CD-audio, but I can't remember much discussion about the third 'number' of the CD specs: the channel count. 1-channel, 2-channel, 5.1 and 7.1 have been in use,


Interesting that the above post skips over 3.x and x.x (where x >1) which is probably the actual minimums for a reasonable audio system based on speakers.

What is a reasonable number of channels

Reply #15 –
Quote from: cliveb on
Forgive me for going off on a tangent. I'm not sure exactly what you mean by your statement. It seems to me that you feel that using the front center channel only for speech is some kind of failing.

Well, yes, if you have it, why not use it to solidify the phantom image created by the left and right speaker? You say that like speech is only ever panned to center. However, I've seen quite some movies where the speech moves to the left if the person is at the left side of the screen, so I'd say that is not really a reason not to pan anything else to center. If you're not able to hear the dialogue, something went wrong at mixing to surround I'd guess.

Quote from: ajinfla on
wave field synthesis in real life    http://www.soundandvision.com/content/true-stereo

Indeed looks great, but not really possible to record that in any practical way. Only synthesis seems to be doable.

Quote from: Arnold B. Krueger on
Interesting that the above post skips over 3.x and x.x (where x >1) which is probably the actual minimums for a reasonable audio system based on speakers.

I just mentioned the most common setups. For 3-channel, I know only of a few SACDs remastered from three track recordings.
Music: sounds arranged such that they construct feelings.

What is a reasonable number of channels

Reply #16 –
Quote from: ktf on
Well, yes, if you have it, why not use it to solidify the phantom image created by the left and right speaker? You say that like speech is only ever panned to center. However, I've seen quite some movies where the speech moves to the left if the person is at the left side of the screen, so I'd say that is not really a reason not to pan anything else to center. If you're not able to hear the dialogue, something went wrong at mixing to surround I'd guess.


It's also used to lock sounds to the screen. If you just have a phantom centre and your L&R speakers are not evenly placed or you're sitting off access, the sounds can wander across the front when they should be locked to the screen.

What is a reasonable number of channels

Reply #17 –
This video by Francis Rumsey I recently came across, has some interesting discussion about the number of channels for spatial audio, though it's lengthy....and I'm not quite sure what his area of expertise is. Lots of hypotheses.
Oddly, the same folks (NHK) proposing a 22.2 ch format, seemed to have found 4 channels sufficient for diffuse soundfields.
JJ used 5 decode from 7 encode for PSR.

So...maybe 4-5 for music, 5-22 for "Object-based" audio, where one might want to detect leading edges from various directions (not too many explosions from ceilings of concert halls)?

cheers,

AJ
Loudspeaker manufacturer

What is a reasonable number of channels

Reply #19 –
Personally I find 2 channels enough, especially if you sit in the sweet spot (like in-front of a computer screen) or wear headphones (if using HRTF or some other processing to tweak your "stage", I kind of like Dolby Headphones if I sparingly apply the effect.)

Although I have found 4 channels if placed optimally will handle (bold statement) 90% of music/audio out there.
If the player (software/hardware) supports mapping then you can map any number of source channels to those 4 speakers (phantom center and so on)

A circular placement around a sweet spot would be ideal if the player support it.
Though I do see value in 4 "corner" speakers and one speaker under and one speaker above you, if you manage to put them in a a sphere they should do a impressive job.

I guess you could add finer granularity by 4 floor, 4 middle and 4 roof speakers all in a ideal sphere arrangement, again the player would need to be able to map to it.
As to source material, discrete channels are obviously preferred, but 4 full range channels should be able to provide a rather complete sound stage.

The key is speaker placement/angle and if the player has been calibrated to take advantage (i.e. knows of the placement) of the speakers, if done properly you can map anything to anything.
I'm sure that binaural would sound awesome with 4 circular placed + 1 above + 1 below speakers, and ambisonic recordings would probably sound the best.

Then there are recordings which play with the channels, like somebody mentioned here a instrument or group of instruments may only be in one channel (speaker), and in those cases (if it's fully discrete and on that channel alone) you may want the player to map it directly to retain that direct effect.


Personally I want to see smarter players and speakers, by that I mean speakers that talk to the player so that the player knows how they are placed and maybe each speaker has a test mic built in too so that room characteristics can be easily tested.
That way you could just sit in the "sweet spot" or poke your head into the room and press "Calibrate" wait a few seconds, hear a few blips and blops and done; now the DSP knows the room and speaker layout and can in theory map anything to it. And it will adapt to anything from 1 to 128 physical speakers easily.

Small test microphones are very cheap (just look on ebay) and can be hidden behind the grille/fabric of a speaker.
Using test/brief tones and analyzing the audio (the key is for the software/player to locate directly opposite speaker pairs where possible), basically one speaker "talks" and the others "listen", this way you can create a room acoustics map, you may be able to calculate distance/placements as well, if not then I'm not sure what tech should be otherwise used to determine speaker placements, all the speakers do this talk and listen dance, theoretically only one or two speakers would need a mic but considering how cheap those mics are I can't help to think how much easier it would be if all had them (more room profile data).

Anyway, with such a calibration feature all you would need to do is place the speakers roughly spherical and they could be placed somewhat discreetly/out of the way or almost hidden out of view. The calibration would adapt. And if the software is good enough even a crappy speaker arrangement/layout would still reproduce a music, film, game soundscape very well. It's all in the software really.
Small loud and good speakers are not an issue, there are some small PC speakers that are rather cheap that sound almost as good as mid range Hi-Fi speakers, place a few of these tiny boxes around a room and calibrate it all and number of speakers does not matter.
If a car crashes through the left wall on the screen then it will sound like it crashes through your left wall. If the ambisonics recording captured people moving behind in a concert seat then it would sound like they are moving behind you.

What is a reasonable number of channels

Reply #20 –
Quote from: Rescator on
Though I do see value in 4 "corner" speakers and one speaker under and one speaker above you, if you manage to put them in a a sphere they should do a impressive job.

PSR seemed to get height rendition with only horizontally displaced speakers (5), but with height info encoding via 2 extra mics.
Some of the new schemes (Atmos, NHK22.2, etc.) seem to have assigned channeling everywhere. 
Great for movie theaters I suppose, but not mine.

cheers,

AJ
Loudspeaker manufacturer