Mix-Minus Speech Reinforcement With Conferencing 电路图.pdf

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1、 1997 Rane Corporation Mike Slattery Rane Corporation INTRODUCTION Large conference rooms require speech reinforcement so people at all locations can adequately hear each other. To perform speech reinforcement without acoustic feedback is difficult, add Conferencing and it becomes complex. This Rane

2、Note gives insight into acoustic proprieties of speech reinforcement and applications using Rane Conferencing products. RaneNote 140 Mix-Minus Speech Reinforcement with Conferencing Controlling Acoustic Levels Speech Reinforcement Zones Using Delays to Enhance the Sound System ECS with Zones Two Zon

3、e Mix-Minus using the Aux Out of the ECM 82 Six Zone Mix-Minus using the Post-Gate Outputs of the ECM 82 Applying NAG in the Real World Noise Masking Feedback Eliminator RaneNote 140 MIX-MINUS SPEECH REINFORCEMENT WITH CONFERENCING RadioFans.CN 收音机爱 好者资料库 Controlling Acoustic Levels The following op

4、erations neglect any effect of room echo or room acoustics. Terms used for calculating properties: D0 - Distance from talker to the farthest listener. D1 - Distance from the source mic to the nearest loudspeaker. D2 - Distance from the listener to the nearest loudspeaker. Dn - Distance from the talk

5、er to the nearest listener. Ds - Distance from the talker to the microphone. NOM - Number of Open Mics When audio travels from a source, its Sound Pressure Level (SPL) attenuates by half for every doubling of the distance. The formula for calculating the SPL attenuation is known as the inverse squar

6、e law and is stated as: Without speech reinforcement. Inverse Square Law: SPL Attenuation = Dn SPL 20Log(D0/Dn) When applying sound reinforcement to a large conference room, first you need to know the rooms Potential Acoustic Gain (PAG ). This allows you to determine the maximum amount of sound rein

7、forced, in decibels, achievable before feedback occurs. The PAG formula: PAG = 20Log(D0 * D1)/(Ds * D2) If NOM is greater than 1 then: PAG = 20Log(D0 * D1)/(D2 * Ds) 10LogNOM Figure 1. PAG and NAG When using PAG to setup system gain, it is customary to add 6 dB of Feedback Stability Margin (FSM). Sy

8、stems that operate at 6 dB below their PAG are usually free of feedback problems. PAG = 20Log(D0 * D1)/(D2 * Ds) 10LogNOM - 6 dB How much sound reinforcement is needed to achieve an average SPL at a distant listeners position relative to the non- reinforced SPL at a near listeners position? This Nee

9、ded Acoustical Gain or NAG is the gain in decibels required by sound reinforcement to achieve an equivalent acoustic level at the farthest listener equal to what the nearest listener would hear without sound reinforcement. The NAG formula: NAG = 20Log(D0/Dn) NAG must be less than or equal to PAG to

10、avoid feed- back. Example: D0 - 20 feet D1 - 10 feet D2 - 6 feet Dn - 4 feet Ds - 2 feet RadioFans.CN 收音机爱 好者资料库 Calculating PAG and NAG Loudspeakers are placed throughout the conference room. The closest loudspeaker to the talker is placed 10 feet from the talkers microphone. The closest loudspeake

11、r to the farthest listener is placed 6 feet from the listener. 1. How much acoustic gain can the sound system supply without causing feedback if only 1 microphone is on? (PAG) 2. How much acoustic gain is required for the farthest listener to hear at an equivalent level to the nearest listener? (NAG

12、) 3. Can the farthest listener receive enough gain without feedback? 1. Using the PAG formula: PAG = 20Log(D0 * D1)/(D2 * Ds) - 10LogNOM - 6 dB PAG = 20Log(20*10)/(6*2) - 6 dB PAG = 18 dB (to nearest whole dB) 2. Using the NAG formula: NAG = 20Log(D0/Dn) NAG = 20Log(20/4) NAG = 14 dB 3. Since PAG is

13、 the maximum calculated level that is obtain- able without feedback and it is 4 dB greater than the calculated NAG level, the sound system should operate without feedback. Speech Reinforcement Zones To perform speech reinforcement in a large conference room or auditorium, it is best to use automatic

14、 microphone mixers and divide the room into zones. The zones are made up of both microphone and loudspeaker groups. A micro- phone group is a mix of the post-gate outputs from the individual microphones within a zone. Each loudspeaker group has it own amplifier to allow selective audio sources to be

15、 played within a zone. Grouping microphones within a zone allows a microphone group to be played at selected loud- speaker groups. Zoning allows microphones within a zone to be played on loudspeakers of other zones while disabling them from being played within their own zone. This is typically calle

16、d a mix-minus zone system. Some system designers use relays to disable a loudspeaker group when a microphone within its zone gates-on. Do not use this type of design with acoustic echo cancellers. Changes in the acoustic properties of a room, caused by switching loudspeakers, will cause the echo can

17、celler to loose its adaptation and return echo. The simplest example of using zones for speech reinforce- ment uses only two zones, with each zone having one microphone and one ceiling loudspeaker. A microphone and ceiling loudspeaker are placed at each end of a conference table. Zone 1 contains the

18、 left microphone and ceiling loudspeaker and Zone 2 contains the right microphone and ceiling loudspeaker. To perform speech reinforcement, Zone 1s microphones audio signal is feed only to Zone 2s ceiling loudspeaker and Zone 2s microphones audio signal is feed only to Zone 1s ceiling loudspeaker. U

19、sing Delays to Enhance the Sound System Sound travels through 70 F air at about 1.13 feet per millisecond. In a large conference room using speech rein- forcement, this delay causes the listener to perceive the direction of the talker from the ceiling loudspeakers and not directly from the talker. T

20、his delay-related phenomena is one aspect of the Haas Effect or precedence effect. The Haas Effect is described as: When two loudspeakers are referenced with the same signal, the sound image direction is centered between the two loudspeakers. As one of the loudspeakers is delayed up to 10 millisecon

21、ds, the sound image direction is shifted towards the non-delayed loudspeaker. For the sound image to be restored to the center position, the delayed loudspeaker level must be increased by 10 dB. Increasing the level of the delayed loudspeaker also adds to the loudspeakers SPL. If the delay is betwee

22、n 10 to 30 milliseconds, the delayed loudspeaker contributes a sense of liveliness but not direction. Increasing the delay by 50 milliseconds or more causes the listener to become aware of the delayed loudspeaker. By utilizing the Haas Effect in a speech reinforcement system, the sound image directi

23、on is maintained from the talker and not the loudspeakers. This is achieved by using delays in the sound system to align each microphone group to its reinforcement zone. To determine the amount of delay required between each microphone group and its loudspeaker groups, measure the distance from the

24、center talker of one microphone group to each center listener of the zones that the Zone 1Zone 3 Zone 2Zone 4 Mic Group 1 Loudspeaker Group 2 Loudspeaker Group 4 Loudspeaker Group 3 Delay 1 Delay 2 Delay 3 Mic Group 4 Loudspeaker Group 1 Loudspeaker Group 3 Loudspeaker Group 2 Delay 10 Delay 11 Dela

25、y 12 Figure 2. Delays talkers microphone is reinforced. Add to the measurement the distance from the listener to his closest loudspeaker. Using the measured distance for each zone, calculate the delay and add 10 milliseconds (See above formula). Apply the calcu- lated delay time to each zone for the

26、 microphone group. The number of delays required to perform this task is staggering for a large number of zones. For example, a system with four zones may require three delays per zone (a delay for each microphone group within a zone) for a total of twelve delays! Another method for aligning a micro

27、phone group to a zone is to use programmable delays for each zone and speech detection for each microphone group. For this to work, the programmable delay must have fast recallable memories without producing audio artifacts during delay changes. Each delay must have a memory setting stored for each

28、microphone group. When speech is detected within a microphone group, all delays must recall their setting for that group. This type of system is achievable with ECS and the RPM 26v by using the contact closure outputs on the ECM 82 mixer and program- mable delays available in the RPM 26v. ECS with Z

29、ones ECS provides three methods for creating microphone groups within a zone. In choosing the type of method, first determine the number of microphones within a zone to create the microphone group. If a microphone group can be placed on one ECM 82, then the Mix or Aux output of an ECM 82 can be used

30、 to create the microphone group. If a single microphone group cannot be produced from one ECM 82 mixer, then the Post-Gate Output of an ECM 82 must be used with a separate matrix mixer to create the microphone group. Using the Mix output of the ECM 82 provides for a Mixer Gate feature, which improve

31、s system stability. Unfortunately, if an echo canceller is placed in the ECM 82 then the Mix out has about a 40 millisecond delay. Therefore, when using an acoustical echo canceller with distances between zones of less than 40 feet, it is not wise to use the Mix out. The Aux Output is processed befo

32、re the echo canceller and only requires the internal Aux Output switch be placed in Post- Gate mode (factory default). Two Zone Mix-Minus Using the ECM8s Aux Out Figure 3 illustrates how two ECM 82As and an ECB 62 can perform a simple two zone mix-minus system using the Aux out of the ECM 82A and tw

33、o ports on the ECB 62 to create the zones. This is performed by assigning Mixer 1s Aux output to Zone 2s loudspeakers and Mixer 2s Aux Output to Zone 1s loudspeakers. To play the Program audio (audio from the VCR and Hybrid) at the two zones the audio matrix must include these two inputs. Connecting

34、 Port 1s output to Port 6s input allows for the Program Audio level to be changed without changing the speech reinforcement level. Feedback Warning: Connecting an output port to another input port may result in feedback if the matrix routing is not set properly. (See Fig. 4) The RPM 26v performs aud

35、io processing of the speech reinforcement system. Some of the audio processing features include bandwidth reduction, equalization, delay compensa- tion and compression. DSP program 9 is chosen for its large number of parametric filters. Although this program is normally used as a two-way crossover,

36、its crossovers can be bypassed using the advanced mode by a right mouse click. Placing the RPM 26v between the Aux outputs of the ECM 82As and the ECB 62 allows for audio processing of the speech reinforcement material without affecting the program material. (See Fig. 5) To create zones: 1. Make a m

37、ap of the microphone and loudspeaker place- ments: (See Fig. 3) 2. Divide the table into the number of required zones, or that you are able to work with. 3. Determine which microphones go with each ECM 82A mixer to create a microphone group. 4. Assign the loudspeakers to a zone. 5. Setup the matrix

38、routing for the ECB 62. (See Fig. 4) Figure 4. RaneWare ECS Port ScreenFigure 5. RaneWare RPM 26v DSP Program 9 Figure 3. 2-Zone Mix-Minus Six Zone Mix-Minus Using ECM 82s Post-Gate Outputs Figure 7 illustrates how the Post-Gate outputs of the ECM 82 are used for speech reinforcement. This system ha

39、s superior program and speech reinforcement audio perfor- mance above the method using the Aux output of the ECM 82A. This improved performance is achieved by using a separate program loudspeaker and a matrix mixer. Separating the program from the speech reinforcement audio allows for audio processi

40、ng to be performed on the speech reinforce- ment audio without affecting the program audio. In this application delays were not required, thereby eliminating the need for microphone grouping. This allows for Post-Gate outputs to be fed directly to a 16 x 8 audio matrix mixer. This mixer is then used

41、 to create the speech reinforcement zones using the matrix routing shown in Fig. 6. Since microphone grouping is not used in this example, performing the Haas Effect with delays is not practical. Without individual microphone groups the audio received at a zone contains a mix of all of the microphon

42、es fed to the loudspeakers for that zone. This will cause problems if delays are used. Using a matrix mixer in conjunction with the Post-Gate outputs of the ECM 82A reduces the number of microphones in a zone and increases PAG by allowing a greater distance between microphones and loudspeakers. This

43、 method also allows for individual NAG level adjustments for each microphone within a Zone. (See Fig 7) Since there are six zones, three RPM 26vs are used to perform audio processing for the speech reinforcement system. Some of the audio processing features include bandwidth reduction, equalization

44、and compression. DSP program 9 is chosen for its large number of parametric filters. Although this program is normally used as a two-way crossover, its crossovers can be bypassed using the advanced mode with a right mouse click. To create zones: 1. Make a map of the microphone and loudspeaker place-

45、 ments: (See Fig. 7) 2. Divide the table into the number of required zones, or that you are able to work with. 3. Determine which microphones go with each ECM 82A mixer to create a microphone group. 4. Assign the loudspeakers to a zone. 5. Setup the matrix routing. (See Fig. 6) 6. Setup the NAG leve

46、l for each input of the Matrix Mixer. Applying NAG in the Real World Using the system of Fig. 7, the following procedure describes a method to apply NAG. Since room acoustics is a major contributor of the level settings for PAG and NAG, you Figure 6. Matrix Routing Chart Pod = Podium Prog = Program

47、Zone SpeakerMic/InputInput Route 115, 6, 7, 8, 9, 10, Prog12, 13, 8, 9, 10, 11, 14 227, 8, 9, 10, Prog8, 9, 10, 11, 14 33Pod, 1, 11, 12, Prog7, 1, 5, 6, 14 44Pod, 1, 2, 3, 4, 11, 12, Prog7, 1, 2, 3, 4, 5, 6, 14 55Pod, 1, 2, 3, 4, Prog7, 1, 2, 3, 4, 14 665, 6, 7, Prog12, 13, 8, 14 Figure 8. RaneWare

48、RPM 26v DSP Program 9 Figure 7. 6-Zone Mix-Minus Rane Corporation 10802 47th Ave. W., Mukilteo WA 98275-5098 TEL (425)355-6000 FAX (425)347-7757 WEB might find it easier to set the NAG levels of a microphone to a loudspeaker zones by using a sound level meter and a portable pink noise source with lo

49、udspeaker. The following is a step-by-step procedure that first determines a system level set at the power amp using the microphone and loudspeaker that are farthest from each other (Zr). This will be the maximum level required by the speech reinforcement system. After determining the system level, the other routing levels for this microphone to a zone (Zt) can be set by using a sound level meter or a version of NAG called Route Attenuation Level (RAL). In this system the Podium is 30 feet from the chair at Zone 4 and the microphones are 4 feet apart. Zr