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X-Curve History by Tomlinson Holman A History of the X Curve

Posted on February 28, 2010 at 12:54 AM

X-Curve History by Tomlinson Holman A History of the X Curve

The X curve celebrates nearly a quarter century of helping interchange in the industry.

In the history of multichannel sound, the standardization of theelectroacoustic frequency response for monitoring film stands as one ofthe most significant developments. It was standardizing the monitorfrequency response at the ear of listeners that provided for betterinterchangeability of program material, from studio-to-studio,studio-to-theater, and film-to-film. Work started on formalstandardization of the monitor frequency response for large rooms forfilm in 1975 on both the national and international levels. The workresulted in the standards ANSI-SMPTE 202 in the U.S., the first editionof which was officially published in 1984, and ISO 2969 on theinternational level. Actually, the standardized response was in use forsome years before the formal standards were adopted.

The X Curve: The measured electro-acoustic frequency response presentedto the ears of listeners in a dubbing stage or motion picture theater.The curve is to be measured under specific conditions, and is to beadjusted for room volume as specified in the standards referenced inthe text.

The background behind this work began with Texas acousticians C. P. andC. R. Boner, who established in the 1960s that a "house curve" was aneeded concept. They showed that a flat electroacoustic frequencyresponse in a large room sounds too bright on well-balanced programmaterial. This was subsequently found to be correct by otherresearchers, such as Robert Schulein and Henrik Staffeldt, as well.While Boner's practice was for speech reinforcement systems that didnot require theater-to-theater uniformity in the same way that filmdoes, nonetheless the concept of a house curve traces back to them.This development paralleled the introduction of 1/3 octave roomequalization, since there would be little point in establishing a housecurve if sound systems could not be adjusted to it.

Ioan Allen of Dolby Laboratories realized that the idea of a housecurve was a valuable one after applying Dolby A-type noise reduction tooptical soundtracks and extending the bandwidth of the track. While wethink of Dolby A as principally noise reduction of between 10 and 15 dBdepending on frequency when used in a tape context, in the case of theapplication to optical soundtracks, most of the advantage in dynamicrange was taken to extend the bandwidth. The ordinary mono Academy-typesoundtrack had sufficiently low noise for its time only by impositionof a strong high-frequency roll-off that made the effective bandwidthof soundtrack reproduced in theaters about 4 kHz. If such a track wasreproduced with a wide-range monitor, the noise was excessive. Byextending the high-frequency bandwidth of the monitor, and applyingDolby A NR to tame the noise, a very useful extension of bandwidth fromabout 4 to 12 kHz was achieved, while lowering the noise a fairly smallamount.

Then came the question of the best frequency response for the monitor.In an English dubbing stage, Allen did an experiment with a nearfield,flat hi-fi loudspeaker vs. the farfield film monitor loudspeaker, aVitaVox. He adjusted the frequency response by equalizing the filmmonitor until the balance was similar, although the monitorloudspeakers of the day only extended to about 8 kHz before giving upthe ghost. The electroacoustic response curve Allen found measured witha microphone was flat to 2 kHz, then down 1 dB per one-third octave, to-6 dB at 8 kHz, and falling beyond. This was named the X curve, foreXtended response, whereas the older Academy curve got dubbed the Ncurve, for Normal response (although one wouldn't consider it normaltoday).

When extended-range compression drivers and constant directivity hornsbecame available around 1980, the question became, "How should the Xcurve be applied to this new development?" The new systems had a fulloctave of high-frequency bandwidth over older systems, but deliverednearly the same output response across a range of angles, rather thanconcentrating the response on axis as frequency went up as the olderdriver-horn combinations did.

One theory floated in the middle '70s was that the need for a housecurve was based on an artifact of the method of measurement rather thana real need for sound to be rolled-off at high frequencies in largespaces. This was because the quasi-steady-state pink noise stimulusmeasured by a real-time analyzer in a room is time blind, lumping thedirect sound, reflections, and reverberation togetherindistinguishably. If the different soundfields had differentresponses, the pink noise stimulus plus RTA could not sort out thedifferences and would basically average all the responses. Since themicrophone is in the farfield of the loudspeaker where reverberation isdominant, then the response with a collapsing directivity horn vs.frequency could be expected to be rolled-off at high frequencies, sincethe contribution of all the off-axis angles would dominate over thedirect sound. Nevertheless, in this condition, the direct sound couldbe flat, and we might respond to the flat direct sound and ignore thelater-arriving response as listeners.

If we then were to change to a constant directivity horn, with itsoutput more constant over all angles within its coverage, and thesystem is tuned to a "house curve," then it might be expected to soundduller than the older horns, at least on axis at a distance. That'sbecause, under these conditions, both the direct sound and thereverberant sound would be rolled-off and on the same curve. So one ofthe first experiments I did on this combination was to playconventionally mixed program material over constant directivity hornsequalized to the X curve to see if the sound was too dull. It was not;in fact, with the bandwidth extension from 8 to 16 kHz, it actuallysounded somewhat brighter, but this was due to the extended compressiondriver response instead of having to do with the equalization curve.

So what's going on here? This was later explained by Dr. Brian C. J.Moore, author of numerous refereed journal articles on psychoacousticsand the book, An Introduction to the Psychology of Hearing. Therolled-off house curve has a good basis in psychoacoustics, because asoundfield originating at a distance is "expected" to be morerolled-off than one originating nearby. It is a little like opticalillusions in vision that show, despite occupying the same area on theretina, pictures look bigger on a larger screen, even when a smallscreen is closer and takes up the same horizontal and vertical angles.As it turns out, both spectrum and level are affected by the perceptionof the size of space you are in, and "getting it to match" perfectlyfrom large to small room in physical sound pressure level and responsedoes not result in sounding the same.

With the additional octave of high-frequency extended range of moremodern drivers and horns came the need to calibrate the X curve to thehighest audible frequencies. Later editions of the SMPTE and ISOstandards show the roll-off to 8 kHz as originally standardized, butadded rolloff from the extended curve in the bands above 8 kHz. Someusers don't employ this additional roll-off, staying on the original Xcurve to 16 kHz, but in an experiment I did at USC, I found thatfollowing the letter of the standard was an improvement inhigh-frequency balance and interchangeability of program material. Thiswas done in a very sensitive experiment, reported earlier in SurroundProfessional, that involved playing trailers in a large theater exactlyas they sounded in the dubbing stage, with the agreement from thepeople who had supervised their mixes that they sounded correct, andthis involved eight trailers mixed in a variety of studios. Both leveland response standards had to be perfect to accomplish this, and just a1 dB error over several octaves that crept in during setup was heard,and had to be corrected.

Another development of the X curve is how it should vary with roomvolume. Although a variation in the response with room volume waswritten into the original standard, further work shows that theresponse should be "hinged" at 2 kHz, and turned up at high frequenciesin smaller rooms. Curves that extend the range out to higherfrequencies before breaking away from flat do not seem to interchangeas well.

Today, the major factors affecting interchangeability no longer have todo with the target curve, since the X curve is very well accepted, butrather have to do with how the curve is to be measured and adjustedelectroacoustically. The standard calls for such needed items to makegood measurements of quasi steady-state noise as spatial averaging,temporal averaging, and the proper use of measurement microphones. Thelargest variations among different practitioners are in the use ofmicrophones. The problem is that the soundfield seen by a microphone ina large room is a mixture of direct sound, early reflections, andreverberation. Standard measurement 1/2-inch microphones demonstratevery different high-frequency response when measured anechoically onaxis and with a diffuse field. Differences are on the order of 6 dB inthe top octave between the two, and response in rooms is highlyaffected by the differences between these two. Only by the use ofsmall, low-diffraction microphones, such as 1/4-inch or smallerdiaphragm mics, are the differences kept small.

The best usage of measurement microphones today is to calibrate smallones for grazing incidence across the diaphragm rather thanperpendicular to the soundfield, because, this way, the microphone willdemonstrate the most similar response for the direct sound (across thediaphragm) and reverberation (a diffuse field). One of the primary waysin which problems show up in this area is in the difference exhibitedbetween sound originating from a more-or-less point sound screenchannel vs. a surround array: 1/2-inch microphones make serious errorsbetween these two because the soundfields generated under the twoconditions are so different.

The X curve now has nearly a quarter century of use and has absolutelyacted to help interchange in the industry. Combined with levelstandards, and de facto industry standards such as speakerdirectivities, the whole film industry has benefited without a doubt.Problems linger in applying the standards uniformly due to differentmethods of measurement. Also, when heard over a modern flat loudspeakerin a small room, program material balanced on an X curve monitor soundsoverly bright. That's because the original experiment that set thecurve was made many years ago, without the frequency range availablefrom today's components. This is not too important because, so long aseveryone agrees to use the same curve, then the response sounds thesame to the mixer on the dubbing stage as to the audience member in anyauditorium. Interchangeability of X curve material with home video canbe handled with a simple re-equalization. The ATSC television standardrecognizes the differences, sending a flag that tells receivingequipment whether the program material was balanced on an X curvemonitor, or on a flat monitor in a small room, and home equipment cantake appropriate action to re-equalize the program accordingly.

Heres a PDF that goes into more detail...

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