1. Spatial Perception of Audio vs. The Home Theatre
James D. Johnston
Chief Scientist, DTS, Inc.

2. _______________________
FOR THE HOME
_______________________
CINEMA IS DIFFERENT

3. First, some geometry
Home
12x19
Cinema
100x80
So what?

4. First, time delay and 1/r^2 falloff for the cinema
For someone in the center, 10’ back from the screen, speakers in the center and at +-30 feet from that. (sticking to front speakers for now)
The center speaker is delayed by 10 msec.
Right and Left are delayed by sqrt(10^2+30^2)= 31 msec
Difference in level at arrival is (10/31)^2 or 10dB (to the nearest dB)

5. And for the home theatre
Center speaker in middle of room, L and R at +- 4 feet, listener distance 8 feet from center
Delay to center 8 msec
Delay to side speakers 9 msec
Difference in level of 1dB

6. The Ear’s Tradeoff in terms of interaural energy vs. time delay
Up to 1 millisecond or so, the HAS can trade off signal energy for time delay, in terms of imaging and localization. My big example of a home theatre room shows that home theatre is within this bound.
N.B. If the listener is farther away, the problem gets even smaller
The many msec and many dB in the cinema offer no chance of the same

7. The worst Case
In Cinema, one can be easily sitting outside the left or right speaker, leading to a near/far delay of 10 msec for near, and 60 msec for far, and the accompanying (large) falloff
In the Home Theatre, one can find one’s perhaps 10 msec from one speaker, when one is 8 msec from the far speaker. This, and its falloff, is much more acceptable.

8. About that worst case
The “dialog in center” will save the Cinema dialog.
In the home theatre, the delay and gain from center and right (for the left listener) will be of a level that will pull the image back toward the middle of the screen.
Yes, it is necessary to have a center speaker in all of this.

9. The goals in cinema Get out the dialog
Don’t make the watcher look back
Envelop the watcher/listener in something similar to what’s on the screen
This brings us to the use of back channels
In cinema, they are presently intended for information that is to be perceived as diffuse, that is to say, without imaging information, and will be some large number of small loudspeakers intended for that purpose, perhaps dipole, bipole or other indirect radiator.;
In the home theatre they don’t have to be limited, and will be 2 or 4 speakers. In many cases these will be direct radiators.

10. The goal in home theatre
Everything we do in cinema
Provide stable imaging on the couch in front of the TV
Provide a sense of envelopment in a small room with a small t60
Avoid focus on rear and back speakers

11. In other words, Home Theatre is more demanding
The good news is that it is also possible to do a much, much better job in home theatre than it is in cinema, because of the much smaller differences in relative listener vs. speaker position.
You can accomplish things in the home that you could not do in the theatre, in regard to surround and imaging.
That’s the good news.

12. The Bad News Production and mixing for the home theatre
Needs to use all 3 front channels for all front signals
Needs to mix part of the front into the rear channels
Needs carefully defined, situationally appropriate to the MOVIE SETTING design of diffusion and reverberation to sides and back
Yes, sides AND back. That’s how the HAS works.

13. What do we want to get out of multichannel audio?
Enjoyment
A sense of envelopment, inclusion, and presence
A wide sweet spot
High tolerance to speaker setup, equipment setup, and playback space
Ease of setup and use

14. What do we hear in a real space?
Direct sound
Correlated at the two ears
Provides directional cues
The good news:
You get things like stereo imaging and location cues, even when sitting far beyond the critical distance in the room.
The bad news:
The first arrival from a loudspeaker locates the loudspeaker really, really well. Even when you don’t want it to.

15. Reflected sound Provides hall character
Sometimes is correlated, but shouldn’t be
Diffuse sound
Provides distance cues
Provides envelopment
Helps with front-back disambiguation
T60 is typically reasonably high (>2 seconds) but may vary for special circumstances
Good news: This is a means to overlay the listening room venue with the desired venue.
Bad news: Direct loudspeakers really don’t help with the diffuse sound field, in fact they can hurt.
You can’t overlay a small T60 on a longer one.

16. Good news: These work even when overlaying one space with another.
Distance cues
Direct/reverberant ratio
Floor reflections
Good news: These work even when overlaying one space with another.
Bad news: Standard 2-channel stereo overwrites some of these cues with its own synthetic cues.

17. Performance and Movie “spaces”
Performance usually larger, with larger T60
For Performance, will have “pleasant” character
Will be highly diffuse
For movies, will attempt “realism” or “hyper-realism”
Still usually larger. Very small scenes, except in Dramas, are unusual.
May have unpleasant characteristics appropriate ot the situation. May therefore be diffuse or not.

18. What does a playback space typically add?
Highly correlated reverberation on a much shorter timescale (T60 < 2 seconds or so)
Bass modes due to the smaller room
Badly spaced and located loudspeakers
Sometimes not very good ones
Stacked speakers
Poorly designed/placed/nonexistent center speakers

19. How about production methods for home theatre/small rooms
Typical pan potting
Leads, in multichannel case, to a very small listening area. All channel direct signals are synchronous, first waveform wins every time.
Highly dependent on speaker placement, matching
Time/amplitude panning
Leads to a much wider listening area
Less sensitive to speaker placement/matching
More like what would occur in the intended “simulated reality” space
All signals panned to at least 3 channels (for 5.x) with appropriate gain and delay

20. That center channel Fletcher and Snow
You must have it to get good distance cues
This doesn’t work if you don’t have all relevant program in it
Easily demonstrated by the interaural mixing effects from +-30 degree speaker placement This was known before It’s time we accept it.
Should contain dialog and other sound
Dialog should also show up in left and right for loudness reinforcement, with appropriate time-delay and amplitude cues
Essential for wide listening area
Helps in overcoming playback environment noise

21. Side and Rear Channels
You need side channels to get a sense of envelopment from the sides. The HRTF of the human head pretty much prevents getting this from the back
You need back channels to get sensation from the back. The HRTF problem is the inverse of side channel
Yes, you can try to “compensate” and then the playback room reverberation offers a contrary cue, and your auditory system figures out that something is wrong.

22. What’s this about time-delay panning?
Pan pots are effectively the same as a combination of a tight microphone pattern added to purely coincident microphone placement
This means that even small shifts from the center of the 5.x setup “suck in” to the nearest speaker containing the nearest signal source
Time delay panning is like a nearly-coincident system
It allows moving around, and the time alignment of the extra signals from the extra channels reinforces in a way that greatly widens the listening space

23. Reverberation
Should have different time character but the same frequency profile (T60 as a function of frequency) in each channel in order to envelop
Direct/diffuse ratio can provide wonderful distance cues
Early reflections, despite the arguments, often add unpleasant colorations to a sound source. I prefer to use time cues instead.
N.B. First reflections, or strong side reflection, are appropriate when simulating a “bad” environment, say a cave, aircraft hangar, hallway…

24. Real Capture
Coincident capture leads to ‘must be in the center’ playback
Very wide placement leads to all-envelopment, no-imaging playback. The ear and brain can not make head or tale of the highly decorrelated attacks
Nearly coincident (.5 to 3 millisecond) capture methods provide time and amplitude cues that provide for wide listening areas and realistic playback of both diffuse and direct sound.

25. Loudspeakers
Barring a loudspeaker that has separate direct and diffuse inputs (some exist, but are not in either production or common use) the more independent channels, the better.
Bass management makes speakers smaller, but it also impairs the ability to cope with different room and auditory system issues.

26. The point?
Consider the auditory system function when you are making multichannel signals
The “law of the first wavefront” is real. Make sure the correct thing gets to the correct ear at the correct time for the FIRST ARRIVAL
Make sure your reverberation is diffuse
Make sure that you don’t inadvertently create a plane wave by putting sound from only one speaker, unless you want the listener to localize that speaker
The best reverberation in the world, single-channel, from one speaker, is still a direct signal IN THE PLAYBACK ENVIRONMENT. And that will suck the listener right into the speaker.

27. You can decorrelated signals at the two ears in a small room with properly decorrelated outputs from 5 speakers, better 7.
This won’t work in a large room, both attenuation and delay due to distance work against you
This only works if you have proper capture or synthesis technique

28. Good news, bad news Bad: Good:
You can’t get rid of the reflections in the smaller room
System setup in home theatres is often a problem, especially at the low end
Good:
Earlier arrival from the loudspeakers can help mask or overlay reflections with the desired sensation
The smaller, lower T60 room allows for a much greater range of experiences, and for more verisimilitude for a greater range of auditory scenes.
Room correction can often diagnose the worst of the bad setups.

29. In Summary The rules for home theatre are very different
The difficulties are different
The ability to immerse the listener is actually better, but you have to recall the presentation space as well as the auditory scene.