Un análisis alternativo

One of the first efforts to create and internationally accepted theatre B-chain response curve happened at the 8th Congres del' Union Internationale des Associations Techniques Cinematographiques in Brussels in 1968 (UNIATEC, 1968). The curve reported is represented in figure 2.28.

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Figure 2.28: B-chain for mixing rooms from UNIATEC proposal September, 1968; from (Ljungberg, 1969)

The concern amongst researchers such as Lungberg was that this curve, when added to the Academy curve, produced a high frequency roll-off of approximately 30 to 38dB at 8kHz. Work on an international standard had started with an ISO meeting in Moscow in May of 1969 which brought together studies from both the U.S. and the U.S.S.R. This meeting was an early attempt to standardize the response of the B-chain and its measurement processes. Initially it received a basic agreement from the U.S., the U.S.S.R, and most of the European countries who had been working on parallel studies of the B-chain. The three studies referenced in section 2.5 were published individually in the December 1969 issue of the SMPTE Journal along with a reprint of the draft international standard for the B-chain response prepared by ISO TC36 (SMPTE, 1969), (Vlahos, 1969).

The studies that appeared in the December 1969 SMPTE Journal all referenced the use of noise for B-chain measurements, typically played off film or tape. A tape recorder allowed direct insertion of noise into the B-chain while noise printed to film would require the subtraction of any A-chain characteristics from the measured response. Buckle and Lumkin utilized white noise in 1/3 octaves, Rasmussen and Ljungberg used both whole and 1/3 octaves, and the draft

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ISO document cited the use of pink noise in sequential 1/3 octaves, typically measured with a sound level meter and an RMS millivoltmeter (Buckle and Lumkin, 1969), (Rasmussen, 1969), (Ljungberg, 1969), (SMPTE, 1969). The standard response curve presented in the ISO draft included a high frequency roll off of about 14dB at 8kHz, with a low frequency roll starting at 100Hz in an effort to provide spectral balance. However, this curve made up only the B-chain characteristic which would then need to be added to the old Academy Curve for the A-chain, as shown in figure 2.29.

Figure 2.29: Standard curve of characteristics of monitoring chain, ISO/TC 36/WG 3 July, 1969; from (SMPTE, 1969)

During this exploratory phase of the first ISO draft it was revealed that not all of the countries involved had used the roll-off of the Academy curve, with some refusing to use it while other chose to use a 'flat' characteristic instead. The first draft of the ISO proposal was never approved as it was felt that the heavy Academy high frequency roll-off needed to be maintained to accommodate both films currently in production and those that were still in circulation and would be for several years (Rasmussen, 1976), (Vlahos, 1975). Vlahos additionally pointed out that, in practice, international standard committees attempt to reach agreements of some kind but, if aspects are not accepted by all parties involved, then sub-elements" are selected so that

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some agreement could be reached. He went on to acknowledge that compromise on certain aspects is an acceptable outcome.

After a meeting in London in June 1971, a modified draft was disseminated to member countries in April 1972 and an editorially modified version was circulated for voting in November 1972. Eleven countries voted yes, while three countries disapproved: Germany, Italy, and the U.S. Germany felt that its theatres had a much flatter characteristic than shown in the document. The primary reason for the U.S. to vote "no" was the belief that only the combined A and B-chain document would be of use. The argument from the U.S. was that one country could have a steeper A-chain roll-off and a flatter B-chain than another, yet demonstrate the same over all response. This case was further argued in a comment by Petro Vlahos, U.S. correspondent to ISO TC36, circulated in August 1973 (Vlahos, 1973). In an effort to solve this impasse, ISO TC36 then agreed to start work on an A-chain document to augment their work on the B-chain. The U.K. drafted a new version of ISO DIS2969, which defined a 3dB per octave slope starting at 2kHz. It was apparent that in some countries, this curve was much brighter than current practice, and it was decided that ISO member countries should be asked to survey some of their typical theatres. A new draft of the document was circulated in February 1975, with two curves listed. Both were brighter than the previous 16 dB down at 8 kHz. One was 11 dB down at 8 kHz, and the other, the wide-range curve, down 6 dB at 8 kHz and were labeled 'Y' and 'X' respectively. A note in the draft stated: “This proposed draft includes two characteristics for the response beyond 4 kHz. The response and tolerances are identical up to 4 kHz. Depending on the replies by Preparatory Working Group 3 Specialists, only one of these characteristics will be chosen.”

During a meeting in London in June of 1975, a compromise was reached and it was decided that both curves should be in the next draft document. The 'Y' curve was to support films currently in circulation while the 'X' curve was to support new soundtrack formats such as the hue-modulated color soundtrack. These curves are shown in figure 2.30. This draft was refined and a new version was circulated for comments in September 1975 (Rasmussen, 1976), (ISO, 1975b).

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Figure 2.30: Proposal for B-chain response curve, agreed to by ISO preparatory working group at London meeting, 1975; from (Rasmussen, 1976)

Vlahos had raised the question of whether the old Academy curve should be abandoned and noted that the issue had been argued among member countries but countered that doing so would overlook pictures that were currently being made and that would be in theatres for years to come.

During this same time frame, a B-chain acoustic response curve for dub-stages was submitted to SMPTE for consideration as an American standard with the thought that it may become a U.S. proposal to the ISO (Vlahos, 1975). This proposed curve is shown in figure 2.31.

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Figure 2.31: Dub-stage B-chain acoustic response, showing proposed standard and tolerance (solid lines) and average of 9 Hollywood dub-stages (dashed line); from (Vlahos, 1975)

Of interest to note, this curve was for dub-stages working with the extended frequency characteristics of magnetic prints and it was dependent on the mixer to lower the high frequencies level (follow the Academy curve) to compensate for optical releases. Also worth noting is the extended low frequency characteristics and the tighter tolerances in the mid and high frequencies of this curve. This curve never became part of a published SMPTE standard. A possible reason that the curve depicted in figure 2.31 was not adopted was implied in a paper presented by Allen (1975). He commented that, though despite the improvement made by magnetic soundtracks, it seemed certain that optical soundtracks will continue to be used because of production cost savings.

With final ratification in 1977, the second ISO draft became the first published version (ISO, 1977). This document was mimicked in the U.S. and became SMPTE 202M Dubbing Theatres, Review Rooms, and Indoor Theatres – B-Chain Electro-Acoustic Response, in August of 1978.

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