Amazing Stereo Tricks for Next-Gen Tablets And Cellphones

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Stereo sound from a tablet? From a smartphone? What's the point? The speakers are too close together, right? The left and right channels will reach both your ears as if they originated from the same point. 

Stereo does work with headsets or earbuds, of course, and Dolby Labs and others have whole suites of software that take advantage of psychoacoustics to enhance the “naturalness” of the headset -listening experience.

That’s good, but how do you get from there to the design of tablet and smartphone playing through its built-in speakers?  What kind of psychoacoustical magic could be worked with tiny speakers less than a foot apart?  Could you do something with that kind of configuration the way, you can “tune” a home theatre?  

Obviously, the answer is “yes” or I wouldn’t be blogging.

Audio Processing from the Former National Semi’s  Skunkworks

One of the technological gems that Texas Instruments picked up when it acquired National Semiconductor earlier this year was National’s existing R&D. (BTW, National is now known as “Texas Instruments Silicon Valley Analog Division.” I am informed.*) in a number of areas, that research was very close to being productized.  This is one of those instances.

The basic technological idea is easy to grasp: with two speakers, you assume a listening distance and a certain distance between ears, and you add a bit of the left left-channel audio to the right channel, and a bit of the right to the left, controlling the phases so that, at each ear, the sound from the opposite speaker attenuates its contribution AND provides a boost to the sound that ear is supposed to be receiving.  

On top of that, there’s another trick that involves more psychoacoustics than simple-beam-shaping.  If the software detects the characteristics of a sound suggesting some kind of lateral movement of an audio source – a car driving from left to right, say – it calculates a faux Doppler frequency shift  and adds that to the left and right audio signals.

So far, I’ve been talking about two-speaker stereo here, but the National Engineers had always been thinking of ways to extend the potential product reach, so they created a product that would do the trick with 2, 4,  . . . up to 16 speakers n a horizontal array. More speakers, not only provide a greater the sensation of depth; they offer the ability to extend the utility of the product from tablets and phones to big, wall-mounted flat-screen TVs.

Does it Work?

That’s the basic story for the theory.  You’re probably asking yourself how that works for the rinky-dink speakers in a tablet or phone.  (It certainly occurred to me, because the last time I got an audio demo at National, they used a pair of $13,000 Wilson Watt-Puppy speakers to demonstrate the extreme linearity of some op-amps. Now they were talking about speakers about the size of a short stack of quarter coins that probably cost just about that much in large quantities.)

Well, given that the demo involved an array of four one-inch speakers in a plastic enclosure the demo was still impressive.  If you take the best of the one-inch speakers available today and you exercise some care with the enclosure you mount them in, and if you account for the frequency characteristics of the speakers in your design, you’re going to get okay sound.  You can hear it on your iPad and a lot of smart phones. It’s not like a pair of Wilsons, but it’s a far cry from a 1957 AM transistor radio.

For the demo, I positioned the 4-speaker sound bar that TI had created about two feet in front of me.  The signal sources were music videos and movie clips played on a laptop.

The baseline experience, before the magic was turned on, was just half-hearted stereo, using the two outside speakers on the demo sound bar, which were positioned about as far apart as the speakers in my iPad.  If there was any sense of depth, I didn’t notice it.

Then they switched in the demo board, piping sound to all four speakers, and the effect was remarkable.  My perception of an audio space was much wider than the space between the outside speakers and I could point to different apparent sound sources.  My brain could place multiple sound sources within a wedge of virtual space maybe three or four times as as the space represented by the sound bar and with considerable depth.

One thing that surprised me a little was what happened when the two middle speakers were eliminated.  The effect was still, there but not nearly so vivid. Those extra speakers, even though they’re close to the acoustic center line are important.

I was sorry there was no big-screen TV demo.  I’m fascinated by the potential that offers.

GUI Setup

If this were a merely nifty technology demo, it would be one thing, but there’s a wide gap between beam-shaping theory and beam-shaping reality in a product at Best Buy. From the standpoint of a consumer-product manufacturer, how many weeks would a useful implementation of this technology add to the time to market for a new portable device or flat-screen TV? 

Recognizing that as a probable deal-breaker, The TI engineers spent as much effort on crafting a GUI to simplify the programming the EEPROM that sets the psychoacoustic parameters for the device as they did on the psychoacoustics.

The chip, by the way, is the LM4901.  If you go to The TI Spatial Audio site, you’ll find the datasheet, plus a basic video about the technology, along with full a 20-minute slide presentation that describes in detail how to do the programming. That part turns out to be simple filling-in-boxes-with-numbers for the most part, and they don’t require much more than transferring the measurements of the physical end-user product.

A Plug for Electronic Design’s Contributing Technical Experts

TI, of course, has a lot more to say about audio than just this product.  One of the more interesting engineers down in Texas in Dafydd Roche, whom I met at last year’s AES meeting in San Francisco. He’s now doing a quarterly column on audio for Electronic Design, and his first effort, on soundbar-design, can be found at: Soundbar Design From Start To Finish: Setting Design Specifications.  The second installment in that series is here.  It takes a reader from the specs defined in the first column to the possible options in selecting actual parts.  A third will be up early in 2012.

*No one will admit to calling it, “TISVAC.”

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Don Tuite

Don Tuite covers Analog and Power issues for Electronic Design’s magazine and website. He has a BSEE and an M.S in Technical Communication, and has worked for companies in aerospace,...
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