Spatial Perception of Audio vs. The Home Theatre

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

2. FOR THE HOME _______________________ THEATRE IS DIFFERENT (jj points across the room to Tom Holman)

3. 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

4. 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. The bad news: The first arrival from each loudspeaker locates the loudspeaker really, really well.

5. 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 soundfield, in fact they hurt. You can’t overlay a small T60 on a longer one.

6. 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 cues.

7. 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.

8. 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

9. 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

10. 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

11. 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.

12. 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

13. 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…

14. 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.

15. 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.

16. 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.

17. 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 them 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.