Design Article
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kdboyce
bcarso
Thanks for your clarifications Ken! And yes, I wasn't paying too much attention ...
Generating spatial audio from portable products - Part 1: Spatial audio basics
Ken Boyce, Audio Technologist, Texas Instruments Silicon Valley Labs
1/3/2012 12:20 PM EST
[Part 2 of this article introduces two beamforming techniques and takes a close look at TI’s LM48901 spatial array audio amplifier.]
The soundstage generated by a stereophonic audio system is typically restricted by the physical locations of the speakers, while the sound events perceived by the listener are limited within the span of the two speakers. In small-size stereo speaker systems, such as those found in portable devices, the perceived stereophonic soundstage becomes very limited, almost monophonic.
Portable stereo sound systems have a limited soundstage because their design is limited by the number of speakers and the spacing between them. To overcome this size limitation, spatial audio sound generation techniques can be used to expand the stereo soundstage, achieve better crosstalk cancellation, and enhance certain spatial cues. Texas Instruments (TI) has applied these sound processing techniques in a new family of spatial audio integrated circuits for portable products with two to 16 speakers. These include smartphones, tablets, laptops, sound docking stations, and sound bars.
This two-part article provides a spatial audio tutorial and highlights the audio design techniques employed in the LM48901 quad Class D spatial array IC. The first part defines spatial audio and discusses key techniques such as head related transfer function, crosstalk cancellation and audio beamforming. Part two provides an overview of the LM48901's beamforming technique and its web-based software tool that in a few simple steps allows you to create audio effect programming to convert your product's small soundstage into an immersive cinematic experience.
Spatial Audio
Naturally occurring sound in the space around us is inherently spatial. Sound sources are located at a point in a small region of the total space around us, although some sources such as earthquakes and landslides emanate from a broader area.
The sound scatters off various objects in the environment around us, then we hear the direct and scattered sound binaurally with our ears and after some human auditory processing, the sound is finally recognized. We make decisions about the processed sound, characterizing the sound(s) with labels such as direction, location, loudness, ambience and quality, near, far, tone, fat and thin, among others. In the case of two or more sound sources, the characteristics mix of each sound source (relative to the total sound being received) is also determined.
Spatial audio is simply a term applied to the reproduction of sound by electronic/mechanical methods that attempts to artificially recreate the real world listening experience of sound(s), or alternatively attempts to artificially alter reproduced sound(s) in order to create a perceived spatial environment that may not have originally existed.
The principle of spatial audio reproduction is simple:
If the reproduced sound waves arriving binaurally at your eardrums are identical to those of the real audio source at a particular position, you will perceive the reproduced sound as coming from a source at that particular position. It does not matter that originally the sound source may have been at some other position. The sound 'data' arriving at your ears is what is processed by the brain to ultimately characterize all aspects of the sound.
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EREBUS
1/3/2012 4:47 PM EST
Having tested some of these algorithms twenty years ago, I can honestly say that the technology is amazing. The advancement in inner cranial and exo cranial perception has been a long time coming from the research labs to commercial products. I think everyone will be amazed with how sound can be manipulated to put you into a 3-D sound environment to match the 3-D video available.
This is really neat stuff.
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bcarso
1/4/2012 2:13 PM EST
Although I'm a big fan of such processing in certain applications (for example nearfield "personal" monitors, one needs to note the importance of the room in more traditional audio settings, which generally is neglected and for which is much more difficult to account. And it tends to screw up spatial processing of this sort.
But merely treating the room as a negative, as is suggested in this article, flies in the face of listener preferences and other psychoacoustic results, some of which are sometimes subtle but rather well-understood now, despite continued misunderstandings in the popular press and elsewhere. In particular, it is misleading to state, as the author does, that reflections per se result in reduced "clarity". In fact, however counterintuitive, the reality is quite the contrary. See Toole's book Sound Reproduction for a comprehensive treatment of these issues.
Brad Wood
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zeeglen
1/4/2012 5:12 PM EST
Good observation Bcarso. Here is a fun experiment for a rainy day:
Start with the typical residential room with drywall (acoustically reflective) walls and ceilings and maybe some curtains and floor carpet acoustic damping for good luck.
Drive a single speaker with a sine wave about 300-400 Hz or so at comfortable loudness.
Plug one ear with an earplug.
Move around and see how many places exist where you can completely null the perceived audio tone with the position of the unplugged ear. Caused by reflected (standing) waves and destructive interference, the wavelength IIRC of 350 Hz is somewhere about a meter.
This effect exists with normal listening with both ears, just harder to notice.
Good article, but room acoustics do play a big part.
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bcarso
1/4/2012 8:47 PM EST
Indeed. If we were to take the "reflections diminish clarity" assertion seriously, we'd all be seeking out anechoic spaces for our listening enjoyment. For those of you who have had access to a good chamber, you know how absurd that would be.
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kdboyce
1/5/2012 1:02 AM EST
There was no mention of anechoic chambers in the article, and I definitely do not advocate them as a desirable listening environment. . They are, however, a good environment for making specific audio measurements. Anechoic chambers are also known as “dead rooms” while the most pleasant listening environment is a “live room”. The terms refer to the fact that a live room sounds more realistic while a dead room is, well… dead – no life.
There is no perfect technique to accurately reproduce sound. All methods have drawbacks and what one enjoys another will not. From a psychoacoustic point of view, practically all reproduced sounds we listen to don’t faithfully reproduce the original and are enhanced in some way for our listening enjoyment (or tolerance). This does not stop people from using the techniques, however. Witness a whole generation that only knows MP3 music quality and you know what I mean.
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kdboyce
1/4/2012 10:58 PM EST
I suppose the best statement would have been: “While reflections make it easier to hear from any position, the reflections arrive at different times and intensities than the original signal and CAN result in sound that lacks clarity.”
The intent of the statement in the article was not to explicitly “treat the room as a negative”. Rather it was to point out that the room does affect clarity, usually negatively. Because of this fact, much effort, including material in O’Toole’s excellent book, is expended to improve room listening environments for consumers.
As a practical matter, reflections (or echoes) are a very important part of acoustics as they help us estimate the direction, and distance of sound objects. But no reflecting surface is perfectly flat and that fact alone will blur the reflection, e.g. “smear” the sound. It becomes more complex as the number of reflecting surfaces increase.
Reflections can obscure the true source of a sound under certain conditions and reduce intelligibility. Longer echoes are generally a less offensive than shorter echoes. Echoes can cause phase interference which results in either reinforcement or partial cancellation of a sound at a particular frequency. For music, this can occur over a wider frequency range than for speech. When similar complex sounds in different phases interact, the effect is called comb filtering and is almost always undesirable as it obscures detail and harms intelligibility.
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azskibum
1/4/2012 4:12 PM EST
Nice overview Ken. I'm looking forward to reading Part 2.
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Frank Eory
1/4/2012 4:15 PM EST
Oops, wrong login -- that was me. Again, thanks for a good article Ken.
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kdboyce
1/5/2012 1:31 AM EST
I want to address a couple of other points Brad made.
1. Yes... the room situation can screw up spatial audio effects, especially if you are using the technique to try to reproduce a 5.1 or 7.1 audio system. In this case you may want to do some specific beam forming in order to create wanted reflections. Some makers of such systems also advocate having a microphone at the desired listening position, and use the data it picks up as a means of measuring the room characteristics and adjusting the algorithms accordingly.
However, the article premise was on small product use cases where space constraints did not permit even a reasonable stereo sound field. In this case, the spatial audio processing could enhance the listener experience and it was not intended for a multi-person audience.
2. As a closet musician, I am were aware of the improvement of synthesized sounds that can be accomplished with the judicious use of early reflections and reverberation as well as equalization. To me, a "dead" piano (or 'dry' as they say in music lingo) is useless - not real sounding. So reverb and reflection echoes are added (make the sound 'wet') to make the piano sound more realistic as if it were in a real room. All 'dry' or all 'wet' makes Jack a dull boy, so there is always an optimum mix of 'dry' to 'wet'. Whether this is accomplished totally electronically or partially by the room makes no difference so long as the resultant sound sounds more realistic to the average person. If so, then they can enjoy it. If not.... there is always complaints.
Lastly, I want to thank everyone who has read the article and commented on it. I hope my replies have helped.
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bcarso
1/5/2012 2:28 PM EST
Thanks for your clarifications Ken! And yes, I wasn't paying too much attention to the "portable" in the article's title --- my apologies :) I am on edge a bit I guess when I read things about rooms, particularly after some recent pronouncements about how poorly understood loudspeakers and rooms are (AES Heyser lecture by a certain prominent hifi magazine editor).
Floyd is just Toole btw. I joked with him when he was persuaded to leave NRC in Canada and join Harman International, that he had decided to become a capitalist Toole :)
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kdboyce
1/5/2012 4:35 PM EST
My apologies to Fred vis a vis O'Toole vs Toole.
No problems Brad. I am glad you read the article thru and your comments gave the opportunity for further clarifications.
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