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Thursday
Jul072011

Crosstalk cancellation, or "Why Dr. Edgar Choueiri is my new hero" PART 1

In case you haven’t heard, there was huge news in the world of binaural audio this spring. Edgar Choueiri, head of Princeton University’s “3D Audio and Applied Acoustics” lab, announced breakthrough audio processing that solves crosstalk cancellation for binaural audio playback through two speakers.

“Great!” you’re probably thinking. “But what the heck is crosstalk cancellation?”

Well, to wrap your head around crosstalk, first we need to check in on the basics of binaural audio, which I’ll quickly go over here.

  • Localization cues ("HRTF") diagramHumans are able to hear and localize sounds coming from all directions because our brains can detect differences in those sounds between our two ears. The most crucial differences are time delay , and  volume .
    • For example: Imagine that a friend calls to you from across the street, in front of you and to your right. The sound pressure waves from their voice travel towards you, reaching your right ear first and your left ear a split second later. Also, since sound losses energy as it travels through the air, and your friend’s voice has to travel farther to reach your right ear, it is slightly weaker when it arrives. Your brain subconciously recognizes these differences, allowing you to locate your friend, even with your eyes closed.
  • Binaural audio is recorded with a dummy head microphone (like “Fritz, our Neumann KU-100), that has two omni-directional microphones embedded in its ear canals. This allows the mics to record and preserve the time/volume differences of sounds that reach its ears.
  • Now here’s the kicker: To be able to properly listen to a binaural recording, you need to make sure that the signal from the left microphone goes only to your left ear, and that the signal from the right mic goes only to your right ear. Doing so allows your brain to decode the localization cues from the recording as if you were hearing them live, giving you the full “3D experience.” Of course, the easiest way to do this is with headphones.

So here’s the problem. When you listen to any stereo recording through speakers, the left/right signal separation isn’t preserved. The signals from both speakers blend and interact with each other as they travel through the room, muddying the localization cues and altering the stereo image. This, my friends, is crosstalk. Sure, things that are panned to one side still seem to come more from the one speaker than the other, but it’s hard to pinpoint a specific location.

It’s not a big deal when you’re listening to a close-mic’ed, traditional stereo recording, that’s specifically mixed to sound good in this way. However, crosstalk can seriously mess up what makes binaural audio so beautiful. (I’m of the strong opinion that binaural recordings made with the KU100 still sound great over speakers, but crosstalk undeniably wipes out the 3D effect.) For that very reason, intelligent audiophiles have been attemping to find ways to minimize and cancel out crosstalk for years, but never very successfully. That is to say, it’s been done, but not without either requiring some serious equipment investment or noticeably affecting the tonal balance of the recording, which sucks a lot of the fun out of listening to it.

But there’s hope! Speaker-listeners and stereo-lovers of the world can rejoice, because thanks to Dr. Edgar Choueiri, binaural 3D audio is available for you too.

But more on that, and why I’d go so far as to call Choueiri my hero, next time.

For now, check out the MP3 demonstration below of an environmental binaural recording (turn up the volume until it’s about as loud as you can recall an average downpour to be). Listen with speakers and sure, it sounds like rain. But listen with headphones, and I bet you’ll duck when the umbrella opens!

_________________

Alex Kall is the owner and head engineer at Kall Binaural Audio, as well as the stage manager and marketing coordinator at Symphony Nova Scotia. His ears may not look bigger than normal, but he listens like he means it.

"November Rain" by Kall Binaural Audio

References (2)

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Reader Comments (1)

Hello from Bob Schulein,
If binaural audio reproduced via headphones or with two loudspeakers using cross talk cancellation is of interest, you may be interested in my work and postings on YouTube.

Both of these technologies are very old, but have benefited from advances in digital signal processing and acoustics over time. Dr. Chouleiri's work with cross talk cancellation is a good example of binaural reproduction using two near-field loudspeakers.

My focus has been on using these advances in conjunction with HD video to create a more exciting and realistic entertainment experience, for either type of sound reproduction.

The following links introduce the technology:

http://www.youtube.com/watch?v=Pi1dKjwFdhU
http://www.youtube.com/watch?v=Lx7IMz5rH4g
http://www.youtube.com/watch?v=yiNKYjKNdcA

A variety of productions may then be then be viewed by searching using the word
ImmersAV on YouTube.

Here are a few links to get started:

http://www.youtube.com/watch?v=J8KeriVSVVE
http://www.youtube.com/watch?v=fRpKIFjiLv8
http://www.youtube.com/watch?v=GtloMssuFsE
http://www.youtube.com/watch?v=9LWoGWvVG00&playnext=1&list=PL08340B1DC0BA1505
http://www.youtube.com/watch?v=OldpSOwOovc

All comments would be appreciated via E-Mail or YouTube comments

Thank you,
Bob Schulein

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