SIGNALS & NOISE (Letters to Editor) (Jan. 1989)

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Forcing the One-Point

Dear Editor:

I have read with interest Edward Tatnall Canby's recent columns concerning stereo microphone techniques and Denon's so-called one-point approach using a pair of distant omnidirectional microphones spaced about a foot apart. I would like to make a few comments concerning the "Audio ETC" column in the August 1988 issue.

A common fallacy into which Mr. Canby falls is the assumption that inter-loudspeaker level differences translate into interaural level differences, and that inter-loudspeaker timing or phase differences manifest themselves as interaural time and phase differences. This is just not so, and, in fact, almost the reverse is true! The reason is basically because stereo reproduction is predicated upon the fact that each ear hears both loudspeakers, as opposed to binaural (i.e., headphone) reproduction, where each channel communicates with only one ear. It can be shown that, over the lower half of the audible spectrum, interchannel level differences produce interaural time differences which duplicate the dominant localization cues used in natural hearing, and so produce stable images between the loudspeakers. (Listening to two separated sources-loudspeakers-radiating highly correlated signals is quite unnatural, and it is not obvious, a priori, what signals should be fed to them to simulate the ear signals which occur naturally.) On the other hand, inter channel time differences produce quite unnatural interaural polarity differences and, consequently, vague and unstable imaging. The conclusion is inescapable: Coincident or quasi coincident microphone techniques using directional microphones are essential if natural localization of the direct sound sources is to be achieved.

So why does Mr. Canby so dislike the coincident microphone recordings he has heard? The reason relates to the nature of their reverberant sound pickup. He is quite correct in pointing out that coincident cardioid microphone recordings reproduce the indirect sound pickup and reverberation in phase in the two channels. This results in all the reverberation appearing to originate from between the two loudspeakers, producing a rather "closed in" sound lacking in spaciousness. But this is most decidedly not a feature of all coincident recording techniques; it is, rather, a consequence of the cardioid patterns chosen. The use of polar patterns with out-of-phase rear lobes avoids this problem while retaining stable imaging on the direct sound. Thus, coincident hypercardioids angled at 105° to 109°, or figure-of-eight microphones angled at 90° (the Blumlein configuration), produce a much more pleasant, open, and spacious sound.

Indeed, the Blumlein arrangement is equally sensitive to sound energy pickup from all angles in the horizontal plane (just as with omnidirectional microphones). It also produces anti phase signals for all sounds arriving from the side quadrants (e.g., reverberation). In my personal experience, I far prefer a Blumlein or coincident hypercardioid recording to one made with coincident cardioids.

If the recording venue is suitable for distant omnidirectional microphones, then a Blumlein arrangement will work well and provide superb imaging--both laterally and in depth-together with convincing, open-sounding, reverberant reproduction of the acoustic. A goodly proportion of the ambient "warmth" so praised by Mr. Canby in spaced-omni recordings is nothing but "phasiness" on the direct sounds due to the inter-microphone spacing. This is the reason for the anomalous imaging produced by such recordings. Stereo is a flawed medium, but until the Ambisonic surround-sound extension of Blumlein's ideas reaches the marketplace, we should try to capture both imaging and ambience. This, spaced-omni recording does not do. It is a pity to throw out the baby in order to preserve the bath water!

Stanley P. Lipshitz

Audio Research Group University of Waterloo Waterloo, Ont. Canada

Pleased with Polk

Dear Editor:

I would like to submit Polk Audio for your Audio Accolades. In this so-called service economy, where the service is usually lacking, Polk came through twice. My first difficulty was an intermit tent problem with my car speakers. I couldn't find the receipt, so Polk willingly called the store to verify the purchase. I was also reluctant to send in my new speakers because of all the other things in the car that could be buzzing due to them. After so much satisfaction with my home models, I also found it hard to believe that bad speakers could make it out of Polk's door. They encouraged me to send them in anyway, and I had new ones in less than two weeks.

Then one of my home units was stolen. As you know, Polk speakers are symmetrical sets, and my model was discontinued. First they checked to make sure they didn't have the parts to build another unit. Then they provided me with documentation for the insurance company. Polk's personnel were courteous, prompt, and followed through on my requests.

These are things that it's nice to expect from any company but are difficult to get. I believe that any firm that provides good service should be acknowledged and patronized, even if the cost is slightly higher. In the long run, it may even save you money.

-Name withheld

Profile Praise

Dear Editor:

I wish to compliment Audio on "An Informal History of Solid-State Amps" (June 1988). It is not often that readers get to see circuit topology discussed.

Bravo! Also, I was very happy to see the review of the Onkyo T-909011 tuner (July 1988). I was pleased that Leonard Feldman made an adjacent-channel selectivity measurement. I wish he could come up with some sort of cross-modulation immunity measurement-a step beyond spurious rejection. From my experience (two tuners), Onkyo has done some excellent work in designing r.f. front-ends having low distortion: they got rid of my cross modulation problems.

Ronald J. Brey Rockford, Ill.

Mats Made in Heaven

Dear Editor:

In response to Edward M. Long's fine review, "Mats & Clamps: By the Numbers" (April 1988), I wish to offer a few observations and recommendations.

Audio's continued coverage of record/playback hardware and accessories is most reassuring for those of us yet to be persuaded by the musical qualities of the Compact Disc format.

The ascendancy of the CD has, however, released many thousands of LPs into the second-hand market-invigorating byproducts for the collector.

Avid vinyl hounds can look forward to years of poring over record collections abandoned in favor of the shiny, antiseptic discs. Recently, I discovered a clean copy of Rachmaninoff's "Piano Concerto No. 3" with Byron Janis-a Mercury Living Presence issue of some rarity-in a New York City shop not known for carrying classical titles.

This deep-maroon, margin-control copy cost $2.00!

Returning to clamps and mats: Two years or so ago, I purchased a Thorens Model TD 321 turntable and had a Premier MMT tonearm mounted on it.

This duo, combined with the venerable Grace F-9E Super cartridge, contributed a very politely neutral-almost proper-yet tuneful presentation. The elusive magic, however, was missing for me. With classical material especially, there was a sameness to all recordings. It was, in some cases, unacceptably difficult (for me, anyway) to differentiate between various orchestras playing an identical piece. After much experimentation, including the use of a dedicated wall stand (which helped considerably), I hit upon a mat and clamp combination not covered in Mr. Long's article. Thorens provides a thick rubber mat with their better players which, I'm convinced, masks the capabilities of a player much more closely approaching the fidelity of the Oracle/VPI/Linn/SOTA league than has been previously written about in hi-fi journals. To top it all off, the Thorens TD 321 costs less than half as much as those other turntables.

I placed Sumiko's acrylic mat and clamp combination atop a generic felt mat, the felt/acrylic sandwich approximating in thickness the discarded rubber mat. Only a small adjustment in tonearm height was required. Gloriously (I assume all other setup problems had been addressed properly), the right stuff emerged, particularly in the areas of dynamics, pace, and bass articulation. At last, I was free to concentrate on the changing textures on each record or the complexion of the music, and not the system's response: I also approximated Premier's internal damping in their more costly FT-3 arm by wrapping a few turns of fiber tape on the MMT. This can result in better focus and sound-stage stability, but this option is, I believe, cartridge-dependent. Thus, empirical trials are called for. Try a turn just back of the headshell and another just before the junction at the pillar. In my listening, this modification has helped delineate inner-groove choral material and similar tracking complexities. This added weight will also change the resonance frequency and the Q of the arm and cartridge, which in this case added to the tracking ability.

Perhaps the most welcome result of these exercises has been that I no longer have the nagging desire to upgrade to one of the dearer turntables mentioned above. I write in the hope that other Thorens owners will discover the hidden delights inherent in this mass-marketed design.

- John Hallenborg; New York, N.Y.

(Source: Audio magazine, Jan. 1989)

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