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

(jj points across the room to Tom Holman)

What do we want to get out of multichannel audio? 1.Enjoyment 1.A sense of envelopment, inclusion, and presence 2.A wide sweet spot 3.High tolerance to speaker setup, equipment setup, and playback space 2.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. The bad news: The first arrival from each loudspeaker locates the loudspeaker really, really well.

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.

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.

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 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 1.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 2.Make sure your reverberation is diffuse 3.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 4.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.

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.