ITU-T P 30-1988 Transmission Performance of Group Audio Terminals (GATs)《群组音频终端(GATs)的传输性能》.pdf

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1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 0 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G042!.3-)33)/.G0G015!,)4935“3#2)“%23 G0G0,).%3G0G0!.$G0G03%4342!.3-)33)/.G0G00%2 when applied by asource with a matched internal resistive impedance, it provides a level of -22 dBV.Both test signal

2、s are filtered in the transmission system bandwidth.Note - The preferred acoustic signal to be used in the measurements for the audio alignment is defined inRecommendation P.50. However, other signals such as speech-shaped noise or pink noise may be used in someapplications.2 Interconnection specifi

3、cationsThese specifications are the basic requirements for a GAT to be connected to a network and to allowcommunication between several locations.2.1 Sending sensitivity2.1.1 Wideband GATsFor wideband applications, the transmission characteristics of the audio-channel shall be in accordance withReco

4、mmendation G.722.2.1.1.1 Send side alignmentThe sound source is positioned over the edge of the conference table on the centre line of each confereesposition, as defined in Recommendation P.34 (see Figure 3/P.34), and delivers a signal which complies withRecommendation P.64 i.e. -4.7 dBPa at the mou

5、th reference point (MRP).During the send side alignment the microphones of the GAT shall be positioned on the table as in real use.Volume V - Rec. P.30 3The microphone gain controls must be adjusted to achieve, for each position of the source, an output linelevel of - 22 ( 2) dBV at point X (see Fig

6、ure 1/P.30), assuming the signal recommended in Recommendation P.50 isused. This value takes account of an 18 dB peak factor of the speech signal and 6 dB for the variations betweenspeakers and the variations due to conferees movements.2.1.2 GATs connected to the public switched telephone networkSuc

7、h terminals must comply with Recommendation P.34.2.2 Stability testThe GAT shall have a minimum stability margin of 3 dB when the microphone and loudspeaker paths arelooped at reference point X in Figure 1/P.30 and the sound source is activated as described in 2.1.During the measurement, the volume

8、control shall be in maximum position.3 Transmit quality specificationsThese specifications limit the degradations induced on the network by a GAT.3.1 Electro-acoustical specifications3.1.1 MicrophoneThe electro-acoustical characteristics of the microphones should conform to IEC Publication 581-5.3.1

9、.2 Octave band measurementsIn situ measurement of the overall transmission frequency response characteristic is recommended. It isdefined as the difference between the octave spectra of the electrical signal at the X interface and the acousticexcitation at the MRP. The artificial mouth is positioned

10、 as in 2.1.1.In order to prevent excessive fluctuations of the frequency response of the system, and since themeasurements are performed on site, octave band measurements are recommended in the range 125 Hz to 4 kHz.The sum of the absolute differences between the measured values and their average sh

11、ould be as low aspossible. A practical target of 10 dB is achievable.3.2 Echo performance3.2.1 Acoustic echo controlTo get satisfactory suppression of acoustic echoes it is necessary to provide the audio processor with eitheran echo canceller or an echo suppressor. The echo cancellation technology i

12、s recommended if highest possible speechquality performance is aimed at. However, it is recommended always to complement echo cancellation with a mildecho suppression, in order to prevent the undue transmission of room background noises when no talkers are active inthe room. This condition should pa

13、rticularly be met in multi-conference environments.3.2.2 Echo return lossThe echo return loss of the audio system shall be measured at reference point X of Figure 2/P.30, with thevolume control in maximum position. When the electric test signal, as specified in 1, is applied to the input port(receiv

14、e in), the level measured at the output port (send out) shall not be higher than -62 dBV.An acoustic echo loss of 40 dB includes a margin of 5 dB in order to provide an echo return loss of 35 dBwhen several GATs are used in a conference situation. This value of 35 dB should be understood as a minimu

15、m value.The long-term target value for the acoustic echo loss must be considered as being 45 dB (especially, to take intoaccount the case where a handset is connected to a hands-free terminal). This value is known to prevent any subjectivedegradations due to delayed acoustic echo 1, 2. The level mea

16、sured at reference point X will then be -72 dBV.4 Volume V - Rec. P.30Note - The echo canceller shall permit double-talk with negligible speech quality degradation (under studywith Question 2/XII).3.3 Electrical noiseThe electrical noise emitted by the GAT at the reference point X should be less tha

17、n -55 dBm, within thetransmission bandwidth. No component outside the band should exceed 20 dB above the noise level in the band.The measurement must be done with no conferees in the room and without incoming signals on the receivingside of the equipment in order not to activate the microphone circu

18、its.The noise emitted by the GAT at the reference point X when the microphones are active should be no morethan -50 dBm. It must be measured by forcing the system into the emission mode as if one speaker were active in theroom.3.4 Reverberated field picked up by the microphoneFor this measurement, t

19、he sound source is positioned in order that the distances between the sound sourceand all the microphones greater than three times the distance between the microphone and the position defined for thesend side alignment. It is also recommended that the source be, at least, one meter from the walls. T

20、hen the signalmeasured at point X shall be not more than -29 dBV (this accounts for a direct-field over reverberated-field ratio of 6dB 3). It must be measured by forcing the system into the emission mode as if one speaker were active in the room.The test must be performed for each microphone in the

21、 room.Basic requirements for the choice of the conference room, for its acoustical treatment and for the positioningof microphones and loudspeakers can be found in Supplement No. 16.4 Near-end quality specificationsThis part of the Recommendation tests the minimum specifications intended for the loc

22、al users.4.1 Electro-acoustical specifications4.1.1 LoudspeakersThe electro-acoustical characteristics of the loudspeakers should conform to IEC Publication 581-7.4.1.2 Octave band measurementsIn-situ measurement of the overall reception frequency response characteristics is recommended. It is defin

23、edas the difference between the octave spectra of the acoustic signal delivered by the loudspeaker(s) at the listeningpositions and the input electric signal at the X interface.The sum of the absolute differences between the measured values and their average should be as low aspossible. A practical

24、value of 12 dB is achievable.4.2 Receiving sensitivity4.2.1 Volume controlThe audio conference terminal shall be provided with a volume control. The gain at maximum positionshould conform to 4.2.2. The volume control should ideally be linked to the echo control mechanism.4.2.2 Receiving side alignme

25、nt4.2.2.1 Wideband GATsThe electrical test signal is connected to the input port of the system. The receiving gain shall be adjusted inorder to reach a sound pressure level of at least 65 dB and 20 dB above the acoustical noise level at the MRP. Thealignment procedure should be performed with the vo

26、lume control in the maximum position.Volume V - Rec. P.30 54.2.2.2 GATs connected to the analogue public switched telephone networkSuch terminals must comply with Recommendation P.34.References1 CCITT - Contribution COM XII-No. 170, Study Period 1985-19882 CCITT - Contribution COM XII-No. 171, Study Period 1985-19883 CCITT - Contribution COM XII-No. 172, Study Period 1985-1988