JBL Technical Note - Vol.3, No.3 电路原理图.pdf

1 Technical Notes Volume 3, Number 3: JBL Professional Studio Monitors for Multichannel Sound Applications 1. Introduction: Mixing for multichannel sound has become a very important aspect of audio postproduction. There are three driving forces:Video DVD, Audio DVD, and advanced digital TV.The introduction of the video DVD in 1996 has opened up many opportunities in smaller work environments, such as those normally used in video post activities. It is here where most of the films with multichannel digital soundtracks are being transferred into the DVD Video format. Later in 1998, specifications for the Audio DVD will be formalized, and that will open up another avenue for multichannel remix activities, both in the video post environment as well as in the traditional music recording environ- ment.The prospect of digital TV, which will require a new terrestrial broadcasting infrastruc- ture, will probably not get underway until the start of 2000. Just as JBL Professional helped “write the book” for exhibition and dubbing theater loud- speaker technology, we propose now to do the same thing for Video and Audio DVD production for multichannel sound presentation. In this Technical Note, we will present a history of multi- channel sound, leading up to current cinema for- mats.We will then examine the means that have been put forth for translating the cinema experi- ence into the home environment, emphasizing JBLs important role in this area. We will then cover listening and acoustical aspects of small to midsize production environ- ments, underscoring the desired loudspeaker performance attributes and JBLs solutions, with recommendations for JBL Professional loud- speakers for various size installations.We will then discuss future directions in multichannel sound as they apply to the computer work envi- ronment. Finally, we will present a complete glos- sary of terms pertinent to multi-channel sound. 2. A Short History of Multichannel Sound: The earliest studies in multichannel sound were carried out by Bell Telephone Laboratories during the early 1930s, when three-channel transmissions were made over long telephone lines connecting the Academy of Music in Philadelphia and Constitution Hall in Washington D. C. Much new technical ground was broken in these experiments, and the HF drivers used in JBL Professionals cinema products are in fact direct lineal descendants of those used in the Bell Labs experiments. The first public demonstrations of multichannel sound were mounted by the Disney company for the soundtrack that accompanied “Fantasia,” in 1939.This was of course a road show, and when its run was over, the sound developments that supported it were retired. A later road show, “Cinerama,” was introduced in the early 1950s, and shortly thereafter the Cinemascope process was developed, along with a number of other anamorphic scoped 35-mm film formats for wide- screen presentation. Later, the 70-mm format was introduced, supporting six magnetic channels on film, with five channels placed behind the screen along with a single surround channel (see Figure 1). By the mid-fifties, the high fidelity industry had introduced two-channel stereo, and by 1957 the stereo disc was introduced. FM stereo was stan- dardized in 1961, and the public was well on its way to an appreciation of spatial sound reproduc- tion in the home as well as in the cinema. 2 make a presence in film sound. First was Optical Radiation Corporation (ORC) with their Cinema Digital Sound. Like many who are first to explore new technology, ORC paid the price with a num- ber of costly engineering and marketing mistakes. They have been succeeded in recent years by Dolby (SR-D), Digital Theater Sound (DTS), and Sony Dynamic Digital Sound (SDDS), the major players of today in cinema digital sound. While cinema installations worldwide number in the tens of thousands of screens, a far greater number of multichannel sound systems are to be found in the homes of consumers around the world who have invested in the so-called Home Theater market.This audience, which numbers in the millions of installations, is the target of todays international DVD initiative. 3. A Close Look at Todays Multichannel Formats: Throughout the 1980s, Dolby Pro-Logic stereo was the mainstay of cinema surround sound. Figure 2 shows the basic working principles. Essentially, dialog is recorded as an in-polarity signal in both left and right channels.When this signal is detected, even momentarily, the matrix playback (demixing) coefficients are altered to favor the center playback channel. As a rule, dia- log information and on-screen effects are treated in this fashion. Off-screen effects are recorded as anti-polarity signals; when this signal is detected the demixing coefficients favor the surround sig- nal output, which is then fed to multiple loud- speakers placed on the side and rear walls of the theater. In order to enhance separation of the sur- round channel from the screen channels, the sur- round signal is delayed by up to 20 milliseconds in the rear of the house. Surprisingly, there was little development in multichannel sound during the 1960s. But in the early 1970s, Dolby Laboratories, who had half a decade earlier made fundamental improvements in recording technology via their code-decode noise reduction system, turned their attentions to the problems of film sound, and a new round of development was underway. By standardizing loudspeaker playback equalization and levels in both the creative environment and the public exhibition environment, a new standard of film sound presentation rapidly evolved. By the early 1980s, Dolby paved the way for significant economies in film production. Over the years, the costs of magnetic striped film had escalated to the point where 35- and 70-mm prints had become nearly prohibitive. Dolbys introduction of “Dolby Stereo” prints, a purely opti- cal process, presented a stereo pair of tracks in which left, center, right, and surround program was encoded via a sum/difference phase matrix. The matrix approach (based on the unsuc- cessful consumer “quadraphonic” movement of the early 1970s) provided limited channel sepa- ration in playback, but it was soon found that film sound mixers, if they monitored through the encode/decode portions of the matrix, could pro- duce some quite convincing effects.The cost sav- ings alone mandated that the film industry go ahead with the process, and eventually a vast “Home Theater” industry grew up around Laserdiscs and VHS tapes with their sound tracks encoded with Dolby Stereo. By the early 1980s, digital was beginning to Figure 1.A summary of film formats for surround sound: 35-mm 4-track mag (a), 70-mm 6-track mag (b); Dolby Digital, with SR analog and AC-3 (c); DTS timecode track for maintaining sync with external CD-ROM (d); Sony SDDS (e). Figure 2.Details of Pro-Logic. Encoding (a); decoding (b) 3 The digital formats combine the foregoing pre- sentation layout with discrete digital tracks, while providing a stereo pair for the surround channels. Basically, all three current digital film formats can be treated the same as regards signal integrity. They all work on some degree of perceptual (lossy) coding, in which psychoacoustical mask- ing phenomena are used to reduce the digital bit rate.The basic operation is one of determining just what the instantaneous required bit rate actu- ally has to be in order that the effects of masking (and unmasking) will be inaudible. For example, a loud, dense passage of music need not require the full 16- or 20-bit encoding process; it may turn out that only 4-bit coding will suffice because of the high degree of adjacent band-to-band noise masking potential.When this determination has been made by the systems internal “psychoa- coustical” model, shorter word lengths can be invoked, along with their appropriate scale fac- tors. Decode instructions are sent along with the data-reduced program, and a satisfactory recon- struction can then be made in the theater. (Note:The Compact Disc does not work on these principles; it is an example of linear PCM, in which the program data transmission rate is fixed at 16 bits per channel with a sampling rate of 44.1 kS/s.The CD was on the drawing board nearly two decades ago when data reduction techniques were purely experimental.) The audibility of data reduction is a favorite topic among engineers and audiophiles. It is true that most data reduction transmission systems can be made to “fail” (i. e., to be audible in the form of some reproduced signal artifact) with carefully chosen, or even arbitrary, input signal conditions. But the usefulness of these systems stems from the simple fact that in the cinema their operation is foolproof virtually all the time. Dolby SR-D: SR-D stands for Spectral Recording Digital. The digital signal is recorded at a fixed bit rate of 384 kilobits per second, and that data is placed in the unused space between sprocket holes on one edge of the film.The standard analog sound track space can then carry a Dolby stereo pair of optical tracks encoded with Dolbys Spectral Recording process (an analog noise reduction system), hence the name SR-D. In the Dolby digi- tal process, should there be a failure, momentary or long-term, the system will switch over to the analog sound tracks, and the show goes on as usual. SR-D supports 5.1 channels, that is, five full bandwidth channels for left, center, and right screen channels, a stereo pair of surround chan- nels, and a special effects (below 150 Hz) track. The digital portion of SR.D is known as AC-3. DTS: The DTS process has its digital data recorded on CD-ROMs, which are played through a ROM drive governed by a sync track located on the film.The audio data rate is variable, depending on the demands of the program.The film also carries a stereo optical pair of tracks, so any fail- ure of the digital portion of the system will result in a switch-over to analog. DTS is also a 5.1 channel system. SDDS: SDDS supports a total of eight full bandwidth channels. Five of these are normally assigned to the screen channel group (although relatively few theaters outside of the professional houses are equipped to do this).The remaining channels are then used for surround and special effects.The digital data is written on both edges of the film, which also carries a stereo pair of optical tracks. The is a provision in the systems front-end to allow five screen channels to be down-mixed to three channels for performance in commercial theaters that have not been outfitted with left- extra and right-extra screen channels. Omnibus Prints: Since the three digital systems use different sections of film real estate, it is possible to pro- duce a film print that contains them all: Dolby between the sprocket holes, SDDS on the edges, the DTS time code track, and the standard ana- log stereo optical pair. 4. Multichannel Systems for Video Purposes: Of the systems listed here, only Dolby AC-3 and DTS have application on DVD product. Dolby is the de facto standard for NTSC based video product, but DTS is an option for the producer of the disc as well. Systems for Music Only: At the present time, standards for the Audio DVD are still in the process of being formed, and a final set of recommendations are due in mid- 1998. However, a number of non-standard CDs have been made that are encoded with DTS 5.1 surround.These require a special DTS decoder which is connected to the digital output of a stan- dard CD player.The output of that decoder is a 5.1 set of analog channels. A small number of Video DVDs have been made which contain 5- channel surround music only segments (e. g., music without moving picture.) 4 5. Bringing the Cinema Experience into the Home and Small Workspaces: Figure 3 shows a typical 5.1 channel setup in a cinema. Note that there are a number of surround loudspeakers arrayed side and back of the patrons.The actual number of surrounds depends on the size of the house and the maxi- mum sound levels that are desired in the listening space. In any event, there must be enough sur- round loudspeakers to provide what has been called “a significant number of insignificant sound sources.” What this means is that any listener in the house should not be able to localize a sound source at any one loudspeaker, but rather be immersed in a thoroughly diffuse soundfield. Expectations in the cinema are that each sur- round channel be capable of matching the acoustical output capability of a single screen channel, and this usually requires up to 12 high sensitivity surround loudspeakers for each sur- round channel, adding to a total of 24 for the stereo pair of surround channels. When the home theater market first got under- way, it was quickly determined that the typical liv- ing room or den could not possibly handle multi- ple surround loudspeakers, and the problem of creating a diffuse field in the home was not to be solved for some time. Many feel that it still has not yet been solved, but an excellent step in the right direction is afforded by the dipole surround loudspeaker. The dipole can be approximated by an unbaf- fled loudspeaker or by back to back loudspeakers in an enclosure connected in opposite polarity. These are both shown in Figure 4. Note that the output of the dipole is minimal along its 90 bearing angles. If a pair of these loudspeakers are placed on the side walls of the listening room with their 90 “null planes” facing toward the pri- mary listening area, a fair approximation of a dif- fuse field can be made. Under this condition, the majority of the sound from the dipoles that reach- es the listener has undergone a reflection from the front or back of the listening space, and local- ization of the dipoles becomes ambiguous.This of course is desirable and in an important part of the approximation of the cinema experience. Another important step in correlating the small room listening experience with the standard cine- ma is overall signal equalization.The cinema and dubbing theater are normally equalized to the ISO 2969 curve, shown in Figure 5a. Also shown here is the typical on-axis high frequency loss caused by the film screen at Figure 5b.When a film is brought into a video mastering studio, it will sound quite bright, inasmuch as there is no screen loss function to match the sound to that which the film mixers heard.This must be com- pensated for in the processing of film product for home video tape, Laserdisc, or for Video DVD. The final EQ must take into account the fact that high quality home loudspeaker systems are now being designed for fairly flat on-axis response out to 8 or 10 kHz. JBLs suggested guidelines for a target surround loudspeaker room EQ are shown in Figure 5c.This equalization is empirical and represents an average of consumer installations in which low frequencies may be slightly boosted by diminished sound absorption at low frequen- cies and