Trends and Activities in the Development of Multichannel Sound Systems GQnther Theile Institut fiir Rundfunktechnik D-8000 Munich,Germany The new 3/2-stereo sound format is recommended by specialist groups of CCIR, SMPTE, and EBU. It provides an additional center channel and two surround channels, completing the left and right stereo channels, thereby offering an enlarged listening area and, at the same time, enhanced quality of the stereophonic presentation. This article looks at a number of aspects related to practical applications with and without accompanying picture, and to a final detailed speci- fication of a universal multichannel sound system. Current international discussions and investigations as well as new experiences of the program makers have produced actual results concerning multichannel monitoring in the studio (and home), application of the subwoofer concept and the low-frequency effect channel, downward compatibility, compati- bility with film material, etc. Some of these results may be considered for improvements of the CCIR Recommendation 775, "Multichannel stereophonic sound system with and without accompanying picture," and for the specification of a corresponding universal multichannel Ixansmissionsystem. 1, Introduction to digital sound broadcasting (e.g. DAB) of a classical music concert,without accompanying picture. But also Since many years it has already been widely recognized during the television transmission of the concert the that the conventional two-channel stereophonic loud- viewer/listener will expect the optimum stereophonic speaker technique has serious limitations and that further quality, not the least because when listening to music at developments of loudspeaker stereophony are desirable home there may be frequently a strong desire to close with regard to providing an improved stereophonic image one's eyes and concentrate on the auditory event, thereby for a number of listeners who are not positioned at the avoiding the distraction of a more or less appropriate ideal reference point.of the loudspeaker arrangement (e.g. picture. /1/.../7/). During thc last years a great deal of investiga- tive and development work has taken place throughout Considering this basic compatibility requirement, CCIR the world initially addressing the standardization of a TG 10/I and specialist groups of SMPTE and EBU/22/, suitable multi-channel stereophonic system to accompany /23/, /24/ recently have recommended one additional high-definition television (e. g./8/.../20/). However, it center channel C and two side/rear surround channels Ls, has become clear very soon that the improved presenta- Rs, completing the conventional left/right stereo tion performance is desirable not only for applications channels L, R. This new reference format is refered to as with accompanying picture but also in case of audio-only "3/2-stereo" (3 front / 2 surround channels). It is specified applications/5/,/9/,/13/,/14/,/20/,/21/, in CCIR Recommendation 775 /22/ "Multi-channel stereophonic sound system with and without accom- We may envisage that in a few years the television set panying picture", within a hierarchy of sound formats will be replaced by a "home movie" system which will providing lower numbers of channels and reduced include a relatively expensive multi-channel sound stereophonic presentation performance. Possible alter- reproduction system in addition to the picture display. It native lower level sound formats are 3/1, 3/0, 2/2, 2/1, will be able to overcome some of the weakness of 2/0, 1/0 which may be used in circumstances where conventional two-channel stereophonic systems. Further- economical or channel capacity constraints apply in the more, in the domestic environment there will unlikely be transmission link or where only a lower number of acceptance of, or space for, both the special loudspeaker reproduction channels is desired. arrangement for audio-only reproduction and the special loudspeaker arrangement for television sound, side-by-side In principal, the 3/2- or 3/4-loudspeaker arrangement in the home. offersenhanceddirectionalstabilityand clarity of the frontal sound image and improved realism of auditory CCIR Task Group 10/1 (multi-channel sound systems) ambience. Basic characteristics and considerations of the was therefore faced with the problem of defining a new five-channel stereophonic format have already been universal loudspeaker repro-duction standard which is able discussed in previous papers (e.g. / 17/, / 18/, / 19/).Those to satisfy the requirements of HDTV sound whilst also aspects related to the specification of a corresponding providing optimum stereophonic quality to satisfy the universal multi-channel transmission system are "purist" music-lover when, for example, he is listening presented elsewhere (e.g./26/,/27/). 180 AES 12th INTERNATIONAL CONFERENCE TRENDSANDACTIVITIEINSTHEDEVELOPMENOFTMULTICHANNESOUNL DSYSTEMS This article looks at a number of aspects related to location of the listener and important for let's say 90 % practical applications with and without accompanying of the dialogue. It is also well-known that the perceived picture, and to a final detailed specification of a universal quality of a centre phantom image (e.g. speaker) is multi-channel sound system. Current international unfavourable with respect to sound colour, "clarity" and discussions and investigations as well as new experiences spatial sharpness when compared with a corresponding of the programme makers have produced actual results centre loudspeaker image /17/,/28/. concerning multi-channel monitoring in the studio (and home), application of the subwoofer concept and the low frequency effect channel, microphone and sound mixing Compatibility with film sound techniques, downward mixing, compatibility matrixing, compatibility with film sound material etc. Some of Another important advantage is related to the compati- these results may be considered for improvements of bility with film sound material. Looking at the common CCIR Recommendation 775, and for the specification of practice in the cinema (resulting from developments for a corresponding universal multi-channel transmission several decades), adequate directional coincidence of system, auditoryand visualeventsis achievedhere by three screen-related loudspeakers (Fig. l, top). L C R 2. Three front channels J=K nj=,,, jml, It has been shown in previous papers/13/,/14/that the directional stability of the frontal image increases W _ _ disproportionatelyOirectiowith nathelstaincreasebility in the number of the ILCI-"' NEMASCREEN ] front loudspeakers. The listening area ensuring suffi- ciently small directional imaging distortion is about three C times wider and deeper when four front loudspeakers are L yn, R used instead of three. Thus, if absolute image location is important, a four-front-loudspeaker arrangement is better HDTV SCREEN [.a_ than a three-front loudspeaker arrangement which is itself better than conventional 2/0-stereo. W However, the three-front-loudspeaker arrangement has I-" B _ i been identified to be the optimum compromise, due to B ' W = 1,2 several considerations presented below: Fig. l: Comparison of three-front-loudspeaker As regards audio-with-picture reproduction, it is reaso- arrangementsfor sound accompanying nable to consider not the listener's perceptibility threshold cinema and domestic HDTVpresoztations to directional imaging distortions (i. e. to directional discongrnity of auditory and visual events as investigated This cinema situation will not differ significantly from e.g. in/10/), but rather his tolerance threshold (i.e. the the future domestic HDTV situation, when we consider acceptable directional imaging distortion). The acceptable further developments of television displays: It is expected extent of angular displacement between visual and audi- that future displays (e.g. home projection systems) will tory images has been investigated using normal pro- lead to large screens (1.5...3 meters wide) and to corres- gramme material. Account was therefore taken of ponding viewing angles much greater than 33°, because distractions caused by the spectator's interest in the the high picture quality of HDTV allows "telepresence", programme content, the association of the visual and similar to that enjoyed in the cinema, as shown in Fig.1 auditory images ("perceptual fusion" phenomenon), the (below). It seems realistic and advantageous to consider width of the sound source and, last not least, the type of simply almost congruity of the listening angle and the programme material. It was concluded from the results of viewing angle for the specification of the reference HDTV corresponding experiments published in /11/ that the sound/picturedisplay arrangement. directional displacement should be less than 15° (tolerance threshold). This means that the directional stability For the production standard and domestic HDTV presen- provided by three loudspeakers is tolerable. Practical tations it is proposed to specify the relation listening experience of several recording teams in different studios angle / viewing angle ("congruity index" ci = B .W). The and with widely differing types of programme confirms this conclusion, congruityindex should be approximatelyci _ 1.2. For instance, a listening angle of 60 ° would imply a viewing Particularly in the case of andio-with-picture presentation angle of about 50°. the most dominant benefit from the center loudspeaker is obviously the "stable middle", which is ensured at any