ITU-R & DOLBY & DTS多聲道喇叭擺位

以下資料節錄引用自http://hardforum.com/showthread.php?t=1170975

03-20-2007, 11:18 PM

phide Banned, 8.2 Years   Status:

Multichannel Speaker Positioning and You So, here it is. Incomplete, and probably chock full of errors (let me know if you find some), but I wanted to get this thing up to give me a little more incentive to wrap it up. Take it for what it’s worth – I’m pretty God damn busy these days. All illustrations are in PNG24 format. If you’re using an archaic browser, you may as well just drop off the planet. Introduction Quite a bit of confusion has existed over proper speaker placement for multichannel audio in the minds of consumers and, additionally, in the industry as a whole. Questions often arise regarding how different guidelines for certain technologies may or may not be appropriate for other, similar technologies. Is there a difference in speaker placement between Dolby Digital and DTS? What about Dolby Digital EX and DTS-ES? What recommendations should I follow when there's conflicting information? For the most part, these questions are easily answered, but as the questions become more complex, the answers become exponentially more complex. What do I do if I use an X360, a PS3 or a Gamecube? What do I do if I want to play both PC games and watch encoded movies on my PC? Here is where easy answers become riddled with complexity and half-assumptions. The goal of this thread is to define speaker placement guidelines for various uses and technologies in a consistent manner so that users can be made more aware of any potential issues or conflicts and to offer recommendations as to how to bridge certain gaps when deciding upon speaker placement. Additionally, I'll try to reinforce the necessity of proper speaker placement as a means of delivering the maximum effect of surround sound for various technologies. For every end, there's a beginning, and it may be useful to try and understand under what conditions multichannel content is created before delving into the vagaries of specific technologies. Multichannel Mixing Environments for Film and Music: ITU-R, Dolby and THX Typically, a film mixing stage is designed to mimic the acoustic qualities of a theater while maximizing the degree of linearity. Although every mixing stage differs, be it in structural design, degree of isolation or decorative elements, most (if not all) mixing stages/studios follow some sort of standard for monitor placement, some more rigorously than others. The two standards most prominently followed are the International Telecommunication Union Radiocommunication Assembly's BS.775-2 document and THX's guidelines for multichannel production (and associated THX certification). The two are, in fact, quite similar, differing only predominantly in terms of application. Dolby has also published fairly ambiguous guidelines for 5.1 production, but because of the ambiguity, I don’t think it’s really necessary to concern ourselves with those guidelines. Above is a simplification of one of the drawings presented in the ITU-R BS.775-2 document (also mirrored in Dolby’s 5.1-Channel Production Guidelines PDF). This guideline is just that, a guideline, and cannot typically be recreated to an exact degree in real-world applications, but remains the benchmark for how a 5.1 mixing environment should be designed. The specification is fairly self-explanatory, though the ITU-R additionally advises that surrounds be placed one to two feet higher than ear level and aimed downward toward the listening position. When mixing or mastering in a 5.1 environment for film or music, the ITU-R recommendation is referred to by Dolby as a “good starting point”. Of important note here is the ITU-R’s emphasis of a true radial configuration. Each individual speaker (excluding the LFE) should optimally be placed an equal distance from the listening position such that all phase relationships among all channels are proper. One could easily understand the significance of phase disparities possible in a multichannel system, so the ITU-R has made sure that no confusion exists regarding their stance on the proper means of multichannel speaker placement. THX’s recommendation for 5.1 production doesn’t differ substantially, though some of the information they’ve published (the stuff that’s available to me) is nearly as ambiguous as is Dolby’s. For the most part, the THX recommendations and the ITU-R’s recommendations are identical, though certain aspects differ due to some of THX’s proprietary technologies. THX’s published drawings are not nearly as exacting (and feasibly unrealistic) as those in the ITU-R’s BS.775-2 document, but they reflect a more real-world (polygonal) environment, as opposed to an environment that is essentially a cylinder (radial). As far as the real world is concerned, the ITU-R recommendation is that which is probably most referenced by studios and post houses. Again, this comes down to the issue of ambiguity, and there is very little in the ITU-R documents. Multichannel Listening Environments for Film and Music (5.1): ITU-R, Dolby Digital, DTS and THX For the most part, there are no real differences between what Dolby, DTS and THX recommend for the home environment, except for the concept of leniency in speaker positioning. Although Dolby and DTS don’t explicitly define that all speaker should be placed an equal distance from the listening position, the fact that this convention should be followed in optimal conditions is something of a given. Because many HT pres, receivers and sound cards allow for minor corrections by directly inputting speaker angle and distance, the necessity of exact positioning is partially negated. For the best, purest results, however, one should attempt to position speakers in a radial fashion, each placed the same distance from the listening position as the center channel. The only change in the illustration worth mentioning is the “introduction” of leniency over the very stringent ITU-R recommendation. Dolby specifically recommends that the fronts be placed at any angle between 22 and 30 degrees, while also allowing for a great deal of leniency for the surround channels, defining any angle from 90 to 110 degrees acceptable. No mention of surround speaker height is made, though one would assume that surrounds should be placed slightly higher than the fronts and center and oriented so that they fire downward toward ear level. Something to keep in mind is that, in the home, it’s fairly rare for there to be a single listening position. Speakers should be placed in such a manner such that all listening positions are covered equally well; meaning that fronts and surrounds may need to be angled differently from what Dolby recommends. In most cases, good judgment is all that needs to be followed to get reasonably good surround performance. THX recommends that fronts be placed in such a manner as to form a 45 degree angle from the listening position (22.5 degrees for each speaker on a radial plane). THX insists that this results in a “wide sound stage and precise localization of individual sounds”. They also insist that surrounds be placed two feet higher than the listening position or higher. My recommendation for configuring a 5.1 system for a home theater is that the ITU-R recommendation be followed but allowing for some leniency. For the maximum kick-your-ass experience, however, follow the ITU-R recommendation to the letter. Multichannel Listening Environments for Film and Music (7.1): Dolby Digital EX and DTS-ES With 7.1 brings the addition of two playback channels, positioned pretty much directly behind the listener and spaced for optimal imaging. Unlike DTS-ES, however, Dolby Digital EX does not offer discrete back channels, instead relying upon matrixing to create two audio channels. The positioning recommendations between Dolby and DTS, however, are pretty much the same: NOTE: THX 7.1 info is forthcoming. It’s a complex God damn mess and I’m too lazy to make a new illustration right now. Multichannel Listening Environments for Console Gaming: Dolby Digital and The Great Void Unfortunately, there was no real way for me to avoid this topic. There is no real information available regarding how multichannel audio is mixed on consoles that feature Dolby Digital output. Do we follow the Dolby guidelines? One may assume so, but there are reasons why one may believe that Dolby’s guidelines should be ignored. For now, I’m going to leave this one open. I may revisit it later, but my only advice at this point is to do whatever the hell seems reasonable. Multichannel Listening Environments for PC Gaming: DirectSound, OpenAL, Dolby Digital Live, DTS Connect and Cartesian Mixing I’ll get to this later. For the time being, an illustration: