Spatial Perception of Audio vs. The Home Theatre

Spatial Perception of Audiovs.The Home Theatre James D. Johnston Chief Scientist, DTS, Inc.

FOR THE HOME _______________________ CINEMA IS DIFFERENT

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

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)

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

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

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.

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.

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.

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

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.

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.

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

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.

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.

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.

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.

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

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

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 1940. 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

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.

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

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…

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.

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.

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.

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

Good news, bad news • Bad: • 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.

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.

Spatial Perception of Audio vs. The Home Theatre