Dr. AIX's POSTS — 29 June 2014


David Chesky is a well known in the audiophile community. An accomplished musician, composer, successful entrepreneur (with his brother Norman) and record producer of many wonderful recordings on the Chesky label. We’ve been friends for a long time…every since I started my label 14 years ago and we both would show up at the CES show or other audiophile events. We’ve been on panels together and share a love of contemporary music. It was great to see him again last week during the event at Jungle City Studios. I knew that HDtracks had secured a demonstration area on the 10th floor of the event (along with SuperHiRez) but I was pleasantly surprised to learn that David was also a presenter.

He started his presentation with a discussion of what he’s been working on lately including his collaboration with Edgar Choueiri, a member of the faculty at Princeton that has been working on refinements to binaural recording and playback that he calls “Pure Stereo”. David explained what’s wrong with traditional stereo recording and playback. Live concerts produce sound in front of the listener. We determine spatial location and depth because our two ears receive slightly different information. These include timing, timbral, phase and amplitude differences. Our brains use this information to calculate where an individual sound appears to come from AND how far away it is.

He went on to explain why traditional stereo speakers fail to deliver the information necessary to recreate a “pure stereo” experience. It’s called pure stereo because it maintains the exclusive left and right ear triggers that happen in live sound AND through binaural headphone listening. It’s when the sound that is intended for the left ear “pollutes” the information intended exclusively for the right ear as is the case with stereo speakers that spatial segregation is diminished. David’s right about this. Professor Choueiri has therefore developed a set of filters that eliminate or severely reduce the “crosstalk” between ears.

It’s not that traditional stereo can’t deliver a compelling model of space. Alan Blumlein, the inventor of stereo, showed that a pair of microphones and two discrete channels of information could capture and reproduce a sensation of depth and left-right distribution. The “Pure Stereo” effort attempts to take expand binaural techniques beyond the limitation of headphones.

It turns out that the best way to optimize the full 3D experience using the “binuaral plus” techniques he’s promoting is to have each listener’s pinnae (outer ear) measured and a personalized HRTF (head related transfer function) created. This is very similar to the Smyth Research “Room Realiser” that I’ve discussed here before. David envisions sending a small pair of microphones to each individual user and having them run some sweep tones through their speakers with the mics stuffed in your ears. The HRTFs that result are then used to process a standard stereo recording into a 3D image. Sounds interesting enough…I’ve heard a few of the “binaural plus” recording and not yet been suitably impressed, but I will withhold judgment until the final product is available (remember I received my Ph.D. with a component of my research into binaural recording).

When it came time to play some music, David chose a selection from his own ballet production, Zephyrtine. It was scored to full orchestra and clearly demonstrated high-resolution audio because of the extreme dynamics in the work. It ranged from very quiet to very loud over the 5 minutes that he played. The work was very interesting. The evening was full of 20th century orchestral music between Zappa and Chesky. I should have brought a recording of one of my two symphonic works. Although they were recorded using analog tape over 30 years ago, according to the MQ-A descriptor in the new definition for high-resolution audio they would qualify.

David Chesky is a believer in real high-resolution music. His productions are substantially different from my own, but he does understand the realities of the production process and the challenges of delivering high-resolution music via downloads. It turns out that he also had an HTC M8 Harman Kardon phone at their booth on the 10th floor. He gets it.

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About Author


Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

(4) Readers Comments

  1. Hi Mark,

    One of the important reasons I find Edgar Choueiri’s Pure Stereo concept – and beyond experiencing the actual BACCH filters which very few people have had the chance to listen to -, is that it differs significantly from what the SMYTH Realizer, does, and also from the main premise of the research that Sean Olive and others are developing at Harman regarding HRTF and headphones.

    Both the SMYTH Realizer and the research of Harman into HRTF and headphone target response curves, answer the question regarding how headphones should sound from an equivalent perspective. In the words of Sean Olive, “The underlying premise or hypothesis was quite simple: since stereo recordings are optimized to sound good through loudspeakers in a room, they will only sound good through headphones that simulate the response of a loudspeaker system in a room. This study provides empirical evidence that this premise is well grounded” (http://www.aes.org/e-lib/browse.cfm?elib=16768). With the SMYTH Realizer one can even simulate a 5.1 or 7.1 listening room, that is, again listening to speakers in a room.

    The question receives a quite different answer from Prof. Choueiri’s perspective of Pure Stereo, and the way both headphones and speakers should sound is clearly as if the listener was actually there at the performance, as the goal is an accurate reproduction of the 3D soundfield. This is clearly not acheived nor attempted by Surround or multichannel recordings. From the Pure Stereo perspective, the best possible recording technique is obviously binaural stereo: “The stereophonic recording technique that is most accurate at spatially repre-
    senting an acoustic sound field is, incontestably, the so-called \binaural” record-ing method”. But other classical stereo techniques would certainly yield better results and more convincing results than effects a bunch of reverbs.

    The recordings that – from my understanding of the Pure Stereo concept – would work best with Pure Stereo, would be those who actually contain a soundfield that could be subject to be rendered in 3D or Pure Stere, because all the 3D cues have been preserved rather than done away with, and that while distorted by standard stereo playback, would be possible to recover with the use of the BACCH filters. The way we hear would have a significantly more weight than it does from a 5.1 or 7.1 approach, and the stereo recordings you sometimes characterize as “documentary” and too distant would obviously better document the soundfield by preserving all 3D cues better than through multitrack recordings, which then implies adding artificial: “The sound image would be an artificial one and the presence of extreme ILD and ITD values would, not surprisingly, lead to often spectacular sound images perceived to be located in extreme right or left stage, very near the ears of the listener or even sometimes inside of his head” (E. Choueiri)

    The interesting work of Prof. Choueiri moves in a different direction when it comes to 3D audio, clearly differenciating itself from Ambisonics and Wavefield Synthesis, and again, in the concept behind reproducing and reconstructing an accurate 3D soundfield. The concept is ultimately Stereo and also clearly and effectively overcomes the need for multiple microphones and speakers. This is quite interesting, and not from a merely practical perspective of not needing more speakers and more mics to record, but because – I believe – it approaches our human hearing more accurately. It clearly makes a point for classic stereo recording techniques using two microphones and a very significant vindication of binaural stereo recording as the best way to capture the full detail and richness of a 3D soundfield.

    The other aspect that blows my mind in Prof. Choueiri’s paper (https://www.princeton.edu/3D3A/Publications/Pure_Stereo.pdf), is his answer to question 20. As those who read the paper may have objected, it is rather complicated to get the BACCH filters installed and it has to be made in situ, with measurements carried out by a pro, and that limit you to more or less fixed “sweet spots”, etc. The rather pernicious process of getting the Pure Stereo 3D experience in your living room can nevertheless be completely bypassed. Here is question 20 and the answer from Prof. Choueiri’s paper:

    “Can Pure Stereo be experienced without the the Pure Stereo Processor?

    Yes. If a stereo signal is filtered through a Pure Stereo processor and recorded it becomes a Pure Stereo recording and does not require playback through a Pure Stereo Processor. It can then be played back on any normal stereo system and can be heard in 3D with no special hardware or processing. (Such pre-processed Pure Stereo recordings are generally made with non-customized (universal) u-BACCH filters in order to make them compatible with all stereo playback systems.)

    This feature is piquing the interest of a number of leading recording and mixing engineers [you bet!], and recording labels, who are interested in making new audio recordings in 3D or reissuing existing stereo recordings in 3D. The consumer can play these recordings in 3D on a regular stereo system without any specialized equipment.”

    I can’t but help to wonder what would happen if one could implement the filters in the system of a pair of Neumann KM 183 digital omni mics for example? (And a consideration of digital mics as a good advancement towards HRA would be very interesting).

    It appears that the apparently simpler and much less expensive stereo recording techniques still hold pretty impressive stuff in store. Prof Edgar Choueiri’s Pure Stereo concept is indeed very cool!


    • Prof. Choueiri’s work is interesting and certainly contributes to the expansion of possibilities for binaural recording AND playback through speakers and headphones. If the goal of a particular recording is to “document” a alive performance in an acoustically appropriate concert environment then ambisonics or binaural can do a good job. However, I’m not at all interested in making 3D recordings that sound as distant, diffuse and spatially flat as ambisonics or binaural creates. I’ve done hundreds of recordings with distant miking and just prefer being close to the instruments. It’s personal taste.

  2. Is the forthcoming Chesky product meant to be used with headphones or with speakers? If speakers, how many?

    From reading your article, I suspect it is headphone-only technology.

    • The experience works in headphones AND 2 narrow dispersion speakers.

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