1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G03934%-3G0G0!.$G0G0-%$)!42!.3-)33)/.G0G00,!.G0G0!30%#43G0G0/ amended at Melbourne, 1988)The transmission considerations given here are applicable to conference calls set up and operated in accord
2、ancewith Recommendation E.151.1 In order to respect CCITT Recommendations concerning loudness ratings on international connections, highquality bridging equipment shall be used. This equipment shall be designed to provide a nominal transmission loss of 0dB in the direction from whichever participant
3、 is for the moment active (speaking) to all inactive (listening) participants.This loss shall be measured between equal level switching points of national circuits or virtual switching points ofinternational circuits.Note - Some conference bridges employ the use of automatic gain control (AGC) to mi
4、nimize the contrast thatexists between the speech levels of participants on connections having different losses, and the above consideration doesnot apply for such bridges. The transmission consideration for bridges with AGC is a subject for future study.2 A modern conference bridge shall be used wh
5、ich employs techniques to avoid excessive transmissionimpairment from the accumulation of noise and echo at the output of the bridge in a multiport conference arrangement.In a conference connection with two bridges: one bridge has N1 ports including a talker and the other bridgehas N2 ports, noise i
6、ncreases as the number of ports is increased, according to the approximate rule: 10 log (N1 + N2 -1).Talker and listener echoes also increase as the number of ports is increased as shown in Figure 1/G.172.The multi-bridge configuration thus highlights the need for noise and echo control.Note 1 - For
7、 example, a conference bridge which provides voice-activated switched loss or its equivalent maybe used. In such a bridge, 15 dB of loss would be connected in each input to the bridge when the customer on that path isinactive. When a participant becomes active the loss is switched from his talking p
8、ath to his listening path. Thisdifferential action protects the talker from echo and prevents a reduction of singing margin when the switch is operated.The loss which normally exists in the transmit path attenuates weak input signals such as noise before they enter thebridge. With this arrangement t
9、he level of the total signal reflected back to any active port will be the sum of theindividual reflections from all other ports diminished by 30 dB.This bridge can be equipped with about 30 ports.Note 2 - A description of a conference bridge employing voice-activated switched loss is available in A
10、nnex 2to Question 6/XVI in Volume III-3 of the Green Book. The transmission requirements contained in that annex could beused for the design of bridging equipment. Requirements for the design of bridging equipment using other techniques tocontrol level contrast and noise and echo accumulation are th
11、e subject of future study.2 Fascicle III.1 - Rec. G.172a) Talker echo level b) Listener echo levelKi Transmission loss between bridgesN1 Number of #1 bridges subscribersN2 Number of #2 bridges subscribersFIGURE 1/G.172Increased talker and listener echo levels in a conference with two bridges3 Optimu
12、m operation of a conference bridge is obtained when its location is close to the center of theconnection. This tends to equalize loss from the bridge to all conference locations on the connection, thus minimizinglevel contrast. Thus bridging equipment for international calls should be at high order
13、transit centers.4 Bridging equipment should he 4-wire presented and 4-wire switched on both national and internationalcircuits, wherever possible.5 Attention is called to Recommendation G.114 concerning mean one-way propagation time which recommendsthat connections with delays in excess of 400 ms sh
14、ould not be used except under the most exceptional circumstances.To comply with this recommendation, care should be taken in the selection of a connection diagram so as to avoid theuse of more than a single satellite circuit. For some conferences, using a single star network, this could influence th
15、echoice of location for the conference bridge. For other conferences, the use of a multiple star network could be selectedwith a single satellite circuit, equipped with appropriate echo suppressors, linking the conference bridges.6 The conference connections should be carefully chosen so as to minim
16、ize the number of voice-activatedswitched loss devices in tandem to no more than two per conference leg. This includes customer premises conferenceequipment (such as loudspeaking telephones) and network equipment (such as echo suppressors), but excludes thebridging equipment.7 Whenever the conferenc
17、e involves a single person at a location using a subscriber handset the room noiseshould be limited to about 60 dBSPL(A)1)at the user position to provide good quality transmission. Figure 2/G.172shows the mean opinion score of transmission quality versus room noise 1. Failure of the customer to comp
18、ly with thisguideline may cause the conference to be unacceptable.8 When a conference involves more than a single person at each location it may be desirable to use conferencerooms equipped with microphones and loudspeakers. To assure an adequate signal-to-noise ratio and freedom from the_1)Sound pr
19、essure level relative to 20 Pa and using the A-weighting. See Recommendation P.54 for information concerning soundlevel measurements.Fascicle III.1 - Rec. G.172 3effects of conference room reverberation, the microphone and loudspeaker placement guidelines contained inSupplement No. 4, Volume V shoul
20、d be followed2).Note - This curve represents the opinion of customers listening over the telephone in room noise from 37 dBSPL(A) to 72 dBSPL(A).Each point is an average over all values of speech level and circuit noise given at that room noise level.FIGURE 2/G.172Relationship of transmission qualit
21、y to room noiseReference1 Guidelines for improving telephone communications in noisy room environments, Bell System TechnicalReference, PUB 42902, February 1980, American Telephone and Telegraph Co._2)Another problem associated with hands-free conferencing is the likelihood of acoustic feedback betw
22、een loudspeaker andmicrophone. While this feedback is today generally controlled using voice-activated switched loss in the conference room terminalequipment, note is taken of the fact that Study Group XV has proposed new studies to determine how to use echo cancellers to controlthe acoustic feedback.
