ITU-RBS775-1有无图像的多通道立体音响系统

RECOMMENDATIONITU-R BS.775-1^**

Multichannelstereophonic sound system with and without accompanying picture

(1992-1994)

TheITU Radiocommunication Assembly,

considering

a) thatit is widely recognized that a two-channel sound system has seriouslimitations and improved presentation is necessary;

b) thatthe requirements of cinema presentation differ from those that applyin the home, particularly with respect to room and screen size anddistribution of listeners, but that the same programmes may bereproduced in either the cinema or the home;

c) thatbroadcast HDTV signals, and those delivered by other media, should becapable of giving appropriate sound quality with a wide range ofdomestic loudspeaker configurations, including compatibility withtwo-channel stereophonic and monophonic listening;

d) thatfor multichannel sound it is desirable to separate the requirementsof production, delivery and domestic presentation, though these aremutually interacting;

e) thatinvestigations about multichannel sound transmission and reproductionassociated and not associated with accompanying picture are beingcarried out with the basic requirements as laid down in Annex 2;

f) thatone universal multichannel sound system applicable to both sound andtelevision broadcasting would be beneficial to the listener;

g) thatcompromises may be necessary to ensure that the system is asuniversal and as practical as possible;

h) thata hierarchy of compatible sound systems for broadcasting, cinema andrecordings is useful for programme exchange and up- and down-mixingdepending on the programme material (see Annex 1);

j) thatancillary services such as those for the visually impaired andhearing impaired are desirable;

k) thatadvances in digital audio coding currently allow the delivery ofmultiple audio channels in an efficient manner,

ThisRecommendation should be brought to the attention of theInternational Electrotechnical Commission (IEC) and the Society ofMotion Picture and Television Engineers (SMPTE).

RadiocommunicationStudy Group 6 made editorial amendments to this Recommendation in2002 in accordance with Resolution ITU-R 44.

recommends

1 oneuniversal multichannel stereophonic sound system, with or withoutaccompanying picture, within a hierarchy given in Annex I;

2 thefollowing reference loudspeaker aiTangement (see Fig. 1):

threefront loudspeakers combined with two rear/side loudspeakers (Note 1);

一theleft and right frontal loudspeakers are placed at the extremities ofan arc subtending 60° atthe reference listening point (Notes 2 and 3).

Wherefor reasons of available space, it is preferred to place the frontalloudspeakers on a straight line base, then it may be necessary tointroduce compensating time delays in the signal feed of the centreloudspeaker;

一bothside/rear loudspeakers should be placed within the sectors from 100°to 120° from the centre front reference. Precise location is notnecessary. Side/rear loudspeakers should be no closer to the listenerthan the frontal loudspeakers, unless compensating time delay isintroduced (Note 4);

一thefrontal loudspeakers should ideally be at a height approximatelyequal to that of the listener^ ears. This implies an acousticallytransparent screen. Where a non-acoustically transparent screen isused, the centre loudspeaker should be placed immediately above orbelow the picture. The height of side/rear loudspeakers is lesscritical;

3 theuse of five reference recording/transmission signals for left (L),right (R), centre (C), channels for the front, and left surround (LS)and right surround (RS) channels for the side/rear. Additionally thesystem may include a low frequency extension signal for a lowfrequency extension (LFE) channel (see Annex 7).

Incircumstances where transmission capacity or other constraints apply,the three front signals can be combined with one (mono surround, MS)or zero rear/side signals. In the case of mono surround, the MSsignal is fed to both LS and RS loudspeakers (see Fig. 1);

4 compatibility,if required, with existing and low cost receivers by using one of themethods given in Annex 3;

5 down-mixingcapability, if required, for reduction of the number of channels,either prior to transmission or at the receiver, by employing thedown-mixing equations given in Table 2;

6 upwardconversion where an increase in the number of channels is desired,either prior to transmission or at the receiver, by employingupwards-conversion techniques described in Annex 5;

7 overallquality to the requirements of Annex 2;

8 provision(but see also § 9 below) for the following if necessary:

一alternatemultiple language principal services;

oneor more independent channels carrying descriptive information for thevisually impaired;

一oneor more independent channels for the purpose of supplying improvedintelligibility to the hearing impaired;

9 additionaldata transmitted with the audio to enable the flexible use of thedata capacity allocatable to audio signals (see Annex 6).

FIGURE1

Referenceloudspeaker arrangement with

loudspeakersL/C7R and LS/RS

B

ScreenI HDTV 一Referencedistance = 3 H (2p! = 33°) Screen 2 =2/7(2p2=48。)

H:height of screen

B:loudspeaker base width

Loudspeaker	Horizontal angle from centre (degrees)	Height (m)	Inclination (degrees)
c	0	1.2	0
L, R	30	1.2	0
LS,RS	100... 120	> 1.2	0... 15 down

DOI

NOTE1 - Optionally, there may be an even number of more than tworear/side loudspeakers which may provide a larger optimum listeningarea and greater envelopment.

NOTE2 - Optimum sound reproduction requires use of wide angular spacingbetween the left and right loudspeakers of two or three frontloudspeaker channel stereophonic systems (see Fig. 1). It isrecognized that the television pictures accompanying stereophonicsound having such an angular width cannot, with current techniques,be displayed to the same wide angles, but are often restricted

to33° horizontal subtended angle at the reference distance, althoughcinema images may be displayed at such angles (see Fig. 1). Theresulting mismatch between picture and sound image width leads todifferences in mixing technique for cinema and television. It isexpected that larger television displays will lead to bettercompatibility of mixes for cinema and television display.

NOTE3 - The size of the loudspeaker basewidth, B (see Fig. 1), is definedfor reference listening test conditions in Recommendation ITU-RBS.1116 一Methodsfor the subjective assessment of small impairments in audio systemsincluding multichannel sound systems.

NOTE4 - If more than two rear/side loudspeakers are used, then theloudspeakers should be disposed symmetrically and at equal intervalson the arc which measures from 60° to 150° from the centre frontreference (see Fig. 2).

NOTE5 - If more than two rear/side loudspeakers are used, the LS signalshould be fed to each of the side/rear loudspeakers on the left sideof the room and the RS signal should be fed to each of the side/rearloudspeakers on the right side of the room. In doing so, it will benecessary to reduce the signal gain such that the total power emittedby the loudspeakers carrying the LS (or RS) signal is the same as ifthat signal had been reproduced over a single loudspeaker. For largeroom reproduction, it may also be necessary to delay, or otherwisedecorrelate, the feeds to some or all of the side/rear loudspeakers.Further studies on such decorrelation is necessary.

FIGURE2

Optional引4loudspeaker arrangement (3 front and 4 surround)

B

C

LS)	60°	60°	60°	RS]
	150°		150°

ANNEX1
Hierarchyof compatible multichannel sound systems for broadcasting andrecording

(l)In the case of mono surround the signal feeding LS and RS shouldpreferably be decorrelated.

ANNEX2
Basicrequirements
Thefollowing requirements are related to the specified multichannelsound system with and without

accompanyingpicture.

1 Thedirectional stability of the frontal sound image shall be maintainedwithin reasonable

limitsover a listening area larger than that provided by conventionaltwo-channel stereophony.

2 Thesensation of spatial reality (ambience) shall be significantlyenhanced over that provided by conventional two-channel stereophony.This shall be achieved by the use of side and/or rear loudspeakers.

3 Itis not required that the side/rear loudspeakers should be capable ofthe prescribed image locations outside the range of the frontloudspeakers.

4 Downwardcompatibility with sound systems providing lower number of channels(down to stereophonic and monophonic sound systems) shall bemaintained (see Annex 1).

5 Real-timemixing for live broadcast shall be practicable.

6 Incases where the number of delivered signals is smaller than thenumber of reproduction channels upward conversion should be ensuredto an acceptable degree (see Annex 5).

7 Thebasic audio quality of the sound reproduced after decoding must besubjectively indistinguishable from the reference for most types ofaudio programme material. Using the triple stimuli with hiddenreference test implies grades consistently higher than four on theITU-R impairment 5-grade scale. The most critical material must notbe graded lower than four.

8 Forthe objective quality parameters Recommendations ITU-R BS.4 andITU-R BS.5 shall be the basis, superseded by new measuring methodsfor digital techniques. (These matters are under study by the ITU-R.)

9 Listeningtest conditions are currently under study in the ITU-R.

10 Forsubjective assessments see Recommendation ITU-R BS.l 116.

11 Thesynchronization of sound and vision signals are currently under studyin the ITU-R.

12 Optimumeconomy shall be pursued in all respects, including both cost andtransmission bandwidth.

ANNEX3

Compatibility

Backwardcompatibility with existing receivers

Inthe case that an existing 2/0 channel format is extended to a 3/2channel format, two methods have been identified to assure backwardcompatibility with existing receivers.

Onemethod is to continue providing the existing 2/0 channel service andto add the new 3/2 channel service. This approach is referred to as asimulcasting operation. The advantage of this approach is that theexisting 2/0 service could be discontinued at some point in thefuture.

Anothermethod is the use of compatibility matrices. The matrix equationsshown in Table 1 may be used to provide compatibility with existingreceivers. In this case, the existing Left and Right emissionchannels are used to convey the compatible A and B matrix signals.Additional emission channels are used to convey the T, Qi, and Q2matrix signals. The advantage of this approach is that lessadditional data capacity is required to add the new service.

TABLE1

Fivechannel surround: encoding and decoding equations

Encoding equations

A = L R C LS RS 1.0000 0.0000 0.7071 0.7071 0.0000 B = 0.7071 0.0000 0.0000 1.0000 0.7071 T = 0.0000 0.0000 0.7071 0.0000 0.0000 Qi = 0.0000 0.0000 0.0000 0.7071 0.7071 Q2 = 0.0000 0.0000 0.0000 0.7071 -0.7071

Decoding equations

L'= A B T Qi Q2 = L R C LS RS 1.0000 0.0000 -1.0000 -0.5000 -0.5000 1.0000 0.0000 0.0000 0.0000 0.0000 R‘ = 0.0000 1.0000 -1.0000 -0.5000 0.5000 = 0.0000 1.0000 0.0000 0.0000 0.0000 C‘ = 0.0000 0.0000 = = 0.0000 0.0000 1.4142 0.0000 0.0000 1.0000 0.0000 0.0000 LSZ = 0.0000 0.0000 0.0000 0.7071 0.7071 0.0000 0.0000 0.0000 1.0000 0.0000 RSZ = 0.0000 0.0000 0.0000 0.7071 一0.7071 = 0.0000 0.0000 0.0000 0.0000 L0000

Downwardcompatibility with low-cost receivers

Twomethods have been identified which provide downward compatibilitywith low receiver complexity. The first requires the use of thematrix process described in § 1. A low-cost receiver then onlyrequires the A- and B-channels as in the case of the 2/0 system i.e.a system which does not use a backwards compatibility matrix.

Thesecond method is applicable to the discrete 3/2 delivery system. Thedelivered signals are digitally combined using the equations in Annex4, which enable the required number of signals to be provided. In thecase of low bit-rate source coded signals, the downward mixing of the3/2 signals may be performed prior to the synthesis portion of thedecoding process (where the bulk of the complexity lies).

ANNEX4

Downwardmixing of multichannel audio signals

1 3/2source signals

Table2 shows a set of equations that may be used to mix the five signalsof the 3/2 system down to

TABLE2

Downwardmixing equations for 3/2 source material

Mono - 1/0 format	c‘ = L R c LS RS 0.7071 0.7071 1.0000 0.5000 0.5000
Stereo 一 2/0 format	1/ = L R c LS RS 1.0000 0.0000 0.7071 0.7071 0.0000 R'= 0.0000 1.0000 0.7071 0.0000 0.7071
Three channels 一 3/0 format	Lz = L R c LS RS 1.0000 0.0000 0.0000 0.7071 0.0000 R'= 0.0000 1.0000 0.0000 0.0000 0.7071 C'= 0.0000 0.0000 1.0000 0.0000 0.0000
Three channels 一 2/1 format	L'= L R c LS RS 1.0000 0.0000 0.7071 0.0000 0.0000 R'= 0.0000 1.0000 0.7071 0.0000 0.0000 S'= 0.0000 0.0000 0.0000 0.7071 0.7071
Four channels 一 3/1 format	L‘ = L R c LS RS 1.0000 0.0000 0.0000 0.0000 0.0000 R'= 0.0000 1.0000 0.0000 0.0000 0.0000 C'= 0.0000 0.0000 1.0000 0.0000 0.0000 S'= 0.0000 0.0000 0.0000 0.7071 0.7071
Four channels 一 2/2 format	L'= L R c LS RS 1.0000 0.0000 0.7071 0.0000 0.0000 R'= 0.0000 1.0000 0.7071 0.0000 0.0000 LS‘ = 0.0000 0.0000 0.0000 1.0000 0.0000 RS'= 0.0000 0.0000 0.0000 0.0000 1.0000

Itshould be noted that the overall effect of such downward mixingequations (and compatibility matrixing, see Annex 3) will depend onother factors, such as the panning equations and microphonecharacteristics. It is recommended that further studies on suchinteractions be carried out (see Annex 8).

ANNEX5

Upwardsconversion

Upwardsconversion is needed in cases where the number of production channelsis smaller than the number of channels available for reproduction. Atypical example is a 2-channel stereo programme (2/0) that is to bepresented over a 3/2 reproduction system.

Upwardsconversion involves the generation of the "missing^ channelssomewhere in the broadcast chain. When performing upwards conversion,the following guidelines should normally be respected in order thatthe programme makers have a reference arrangemen匚Theseguidelines do not exclude the possibility, for receivermanufacturers, of the implementation of more sophisticatedtechniques.

1 Frontalchannels

LIWhena monophonic programme is to be presented over a reproduction systemwith three frontal loudspeakers, the mono signal should be presentedover the centre loudspeaker only. When two frontal loudspeakers areonly available, the mono signal should be presented over both leftand right loudspeakers with an attenuation of 3 dB.

1.2Whena stereophonic programme is to be presented over a reproductionsystem with three frontal loudspeakers, the left and right signals ofthe stereo programme should be presented respectively over the leftand right loudspeakers only.

2 Surroundchannels

2.1 Whenthere is no surround signal in a programme, surround loudspeakersshould not be activated.

2.2 Whena given surround signal is to be reproduced over more than oneloudspeaker, decorrelation between each loudspeaker signal should beperformed. Furthermore, proper attenuation should be applied to eachloudspeaker signal so that the combined sound pressure level producedby these loudspeakers should match that of a single frontalloudspeaker 讦fedby the same signal at a given reference listening position.

3 Datachannel

Auxiliaryinformation describing the mode of transmission (number and type oftransmitted channels) should be transmitted periodically in a specialdata channel in parallel with the programme. This information will beneeded to perform upwards conversion in receivers.

ANNEX6

Additionaldata*

Itis necessary that some additional data are sent to the multichannelsound receiver, to enable it to identify the multichannel soundconfiguration in use, and provide the loudspeakers with the requiredsignals. Implicit in the ability to reconfigure a multichannel soundsystem is the ability to use the available sound channels flexibly,so that a wide range of applications can be covered.

Thedetails of the additional data (bit rate, data format, etc.) have yetto be determined. However, the following applications, which wouldneed to be signalled in the data channel, have been identified:

一thesignalling and controlling of different multichannel soundconfigurations for the main programme and conversion (e.g. 5-channel,3-channel, 2-channel, mono) to other configurations;

一 indicatinga special sound signal for listeners with impaired hearing;

- indicatinga special sound signal for viewers with impaired sight;

一indicatinga separate audio programme (SAP);

Furtherstudies and contributions from administrations are necessary.

conveyingdynamic range control information, to compress or expand the dynamicrange;

conveyingcharacters for a text service;

flexibleusage of the data capacity allocated for audio signals.

ANNEX7

Subwooferchannel or low frequency extension channel (LFE)

Thepurpose of this optional channel is to enable listeners, who chooseto, to extend the low frequency content of the reproduced programmein terms of both frequency and level. In this way it is the same asthe subwoofer channel proposed by the film industry for their digitalsound systems.

Inthe film industry the subwoofer channel carries high level, lowfrequency sound effects which are intended to be fed to (a) specificlow frequency loudspeaker(s). In that way the magnitude of the lowfrequency content of the other channels is restricted so that themain loudspeakers are not required to handle these special effectssignals. The main film sound channels carry normal low frequencysounds but not at such high levels- They are therefore sufficient ontheir own if these special effects are not required by the user. Thiscombination has the other benefit that the coding of the high levelsignals in the subwoofer channel can be optimized without affectingthe coding of the main channels.

Althoughit is recognized that the number of domestic consumers who willchoose to use a subwoofer channel is likely to be restricted, it isalso recognized that there are other future applications of the HDTVsound system under discussion that will make more use of this option,e.g. distribution of signals to cinemas.

Thesubwoofer channel should not, however, be used for the entire lowfrequency content of the

multi・channelsound presentation. The subwoofer channel is an option, at thereceiver, and thus

shouldonly carry the add让ionalenhancement information.

(Ina similar way, the surround channels should carry their own lowfrequency information rather than it being mixed into the frontchannels. This forward mixing of the low frequency sounds is anoption, at the receiver, to decrease the requirements on the surroundloudspeakers.)

Thesubwoofer channel should be capable of handling signals in the range20-120 Hz.

Thelevel at which the subwoofer channel should be reproduced, relativeto the main channels, is still to be studied. It is noted, however,that the film industry is currently coding the subwoofer channel suchthat a positive gain of 10-12 dB is required on reproduction. Acommon standard on this point will obviously be beneficial.

Thecoding of the main channels should not place any reliance on maskingprovided by the subwoofer channel. The coding of the subwooferchannel may, however, assume masking due to sounds reproduced fromthe main channels.

ANNEX8

Compatibilitymatrixing and downward mixing

Methodsfor providing backward compatibility and downward compatibility aredescribed in Annex 3. Annex 4 contains downward mixing equations for3/2 source material.

However,it is recognized that the alternative down-mix coefficients for thesurround signals LS/RS are desirable, depending on the type of theprogramme material.

Fouralternative surround signal down-mix coefficients should be indicatedby the broadcaster.

0.7071
0.5000
0.0000
Reserved
Additionaldata should be transmitted to indicate which coefficient should beused.