Audio for Music Videoconferencing: A Set-Up Guide

Transcription

1 Audio for Music Videoconferencing: A Set-Up Guide Prepared by the Manhattan School of Music Distance Learning Program to support best practice in Music Distance Learning. 1 P a g e

2 Dear Colleagues, This guide is designed to help prepare your organization for music videoconferencing sessions with the Manhattan School of Music Distance Learning Program. Understanding that music videoconferencing requires more technology set-up than traditional talking head videoconferencing, we have created Audio for Music Videoconferencing: A Set-Up Guide to facilitate an optimal learning experience for our partner organizations. With over 15 years of experience in delivering music videoconferencing programs, we are pleased to offer our best practices in audio for this purpose. Please note that this Guide addresses an audience with varying degrees of technical knowledge and experience, therefore fundamental audio concepts as well as applied best practices in audio for music videoconferencing are covered. In addition, our distance learning technical staff is prepared to offer individual guidance and assistance in preparing your organization for the learning exchange. We would like to thank René Carver, Distance Learning Coordinator for GST Boces in New York and Gordon Roethemeyer, Distance Learning Director for the ESUCC of Nebraska for their peer review of this best practices guide ensuring we are meeting the needs of our distance learning partners throughout the globe. A special note of thanks goes to Bruce Bertrand, MSM Distance Learning Systems Engineer for his technical knowledge and expertise in creating this Guide. We will be updating our Audio for Music Videoconferencing: A Set-Up Guide on a regular basis to incorporate feedback and current experiences from the field. We welcome your input and look forward to our musical journey with your organization. Sincerely, Dean Christianne Orto 2 P a g e

3 Table of Contents Audio Set-up for Music Videoconferencing...1 Microphone and Audio Speaker Placement: Quick Set-Up Diagrams...1 Microphone and Audio Speaker Placement Techniques...2 o Achieving a natural balanced sound o Minimizing Echo o Controlling environmental room sound o Getting the most Direct Sound o Listen! Ways to Test your Sound Audio Mixer Set-Up Guidelines...6 o Audio Connections: Avoiding Echo (again!) o Connecting Microphones to the Mixer o Adjusting Audio Levels on your Audio Mixer Recommended Audio Equipment for Music Videoconferencing...8 Videoconferencing Systems: Vendor-Specific Audio Notes...9 Polycom...9 Cisco-Tandberg...10 Lifesize...11 Glossary of Common Audio Terms P a g e

4 Audio Setup for Music Videoconferencing Below are examples of basic microphone and audio speaker placements for music videoconferencing. In general, following placement practices for live sound will yield good results for musical video conferencing. Please see some examples of those practices below. Microphone and Audio Speaker Placement Quick Set-Up Diagrams Microphones always point away from audio speakers. The less sound from the audio speakers being picked up through the microphones the better. For larger groups or ensembles, spot microphones placed close to the sound source may be required in addition to the main stereo pair of microphones to achieve proper balance. The microphones should be relatively close to the performers, and relatively far from the audio speakers. 4 P a g e

5 Microphone and Audio Speaker Placement Techniques: Achieving a natural balanced sound: The position of the microphones relative to the performers and to the audio speakers is a major consideration when preparing your room for music videoconferencing. The microphones should be placed so that the performers sound is captured with maximum tonal accuracy and good spatial imaging. The performers should sound clear and natural, and all sections of ensembles should be heard with relative volumes that closely approximate what is heard in the room. Minimizing Echo: Another important aspect of your local microphone placement is that pickup of the sound coming in through the audio speakers from the remote side should be minimized to avoid creating echo. For example, when someone on the remote side of the videoconference speaks or performs, their sound is played through your local audio speakers. Sometimes, that incoming sound can be picked up by your local microphone and sent back to the remote side. The result is that the remote side hears an echo of themselves, which is generally not tolerable for performance or conversation. Videoconference systems or endpoints try to minimize the potential echo effect electronically. For instance, videoconference endpoints use algorithms that try to detect in the audio input signal what was just played through the audio outputs, and remove it when present. This is referred to as Acoustic Echo Cancellation, or AEC. While AEC must be turned on or enabled in your videoconference system to have a usable music videoconference, it uses methods that may alter the sound in ways detrimental to musical performance. The undesirable effects of AEC can be best mitigated by careful placement of the microphones relative to the audio speakers. Basically, less sound from the audio speakers getting picked up by your microphones will cause the AEC to do less to the sound. The good news is that Manhattan School of Music collaborated with Polycom, Inc. in the creation of Music Mode, which allows for high-fidelity acoustic musical sound to be transmitted via videoconferencing 1. With Music Mode enabled in your Polycom system, the AEC has been tweaked for the transmission of live, musical sound. Controlling Room Sounds: Less Noise, more Music: Controlling ambient or environmental sound is important and is best achieved through careful microphone placement. Microphones should be positioned to minimize pickup of your room's air handling; sound from projectors and computers; and any other noises your space may have. Noise from any of these sources can make the AEC perform poorly. Getting the Most Direct Sound in a Room: Also, controlling the ratio of direct sound (e.g., sound directly from a sound source) to reflected sound (e.g., sound bouncing off of 1 Both videoconference endpoints must be Polycom units in order for Music Mode to be functional. 5 P a g e

6 walls, floors, etc.) may be achieved with microphone placement. Generally speaking, more direct sound from the performers and less sound reflected from surfaces in the room will result in a better learning experience during a music video conference. Remember that the sound captured by your microphones is being reproduced in a space that may have an entirely different room acoustic. Reducing the amount of roominess will help with keeping the AEC in check. For individual instruments and very small ensembles, we use close miking techniques to achieve this. For larger ensembles close miking is generally not practical. In these situations, finding microphone positions that produce the proper sound, tone, balance, and sufficient rejection of audio speaker and room sound is the goal. You may find that in reverberant rooms controlling the ratio of direct to reflected sound is more difficult. Choosing a room that is less reverberant may produce better results from AEC. Listen! Ways to Test your Sound In all situations, listening to the sound you are sending to the remote side is essential. 1.) If possible, connect via videoconference to your own room. (NB: This requires two videoconference systems). Be sure the connected location has high quality audio speakers and is an acoustically isolated room. 2.) As this isn't always practical, you can listen with a high-quality set of sealing headphones connected to your audio mixer or a remote videoconference unit. This immediate feedback will let you quickly match the sound you're sending to what you hear in the room, and will make you immediately aware of problems that would otherwise only be heard by the remote side listeners. As a best practice, listen to your audio mixer's output before making the videoconference connection. Audio Mixer Setup Now that your microphones, audio speakers and room are set up, let s turn to the audio mixer. In setting up your audio mixer for music video conferencing, the following guidelines will help you achieve good results. Audio Connections: Avoiding Echo (again!) The audio output from your videoconference system or endpoint must never be routed back into your videoconference audio input. This is the electronic equivalent of placing a microphone directly in front of an audio speaker, which will result in feedback. The symptom of this feedback will not be the squeal or hum you may associate with live sound feedback, but will be a full echo heard by the remote side. Acoustic Echo Cancellation (AEC) algorithms are not designed to handle this situation. Additionally, the audio outputs from your videoconference endpoint should bypass your local audio mixer and be sent 6 P a g e

7 directly to your audio speakers or speaker amplifier. Connecting your Microphones to the Mixer Each microphone should be connected to the microphone preamplifer or preamp at the top of a channel strip on the audio mixer. Condenser microphones will need power, which can usually be provided by the audio mixer through a feature called phantom power. Dynamic microphones do not require phantom power. Most small audio mixers have phantom power that is switchable globally or in groups, so you may not be able to turn phantom power on for some preamp channels and off for others. This is not a problem for dynamic microphones, as they are able to safely handle phantom power. Additionally, if you are using ribbon microphones, you must not plug them into a preamp that is providing phantom. Doing so may damage the microphone. If you are not sure what type of microphone you have, consult with an expert in your organization, the manufacturers website, or your MSM technical contact. The output of the audio mixer should be connected to the input of the videoconferencing endpoint, making sure that the left and right channel levels are matched. See the section on videoconference vendor-specific notes at the end of this document for more on connecting your mixer and particular endpoint. Adjusting Audio Levels on your Audio Mixer When setting up your audio mixer, make sure that all unused channels are muted and all faders and preamp gains are set to minimum. All Equalization or EQ should be set to flat or bypassed, and all auxiliary sends and returns should be off. Stereo channels should be panned left and right, with other channels being centered or panned appropriately. All compression should be off. The master fader should be set to unity or 0. For each microphone, adjust the preamp gain (sometimes called trim) to just lower than the point where its peak light illuminates during the loudest sections of music. This will ensure that the preamp will not be a source of distortion. A stereo pair of microphones should have their gain set identically. After all microphone preamp gains have been set, turn up the channel faders for each microphone to around unity, adjusting each so that the proper balance is heard through your headphones or isolated audio monitors. Again, the level for stereo pairs should be set identically. The mixer's master meter should not reach the clip or peak indicator. If it does, adjust the master fader down. At this point you should look at your videoconference endpoint's audio metering screen, usually found in the videoconference onscreen menu options or via a web-based interface. During the loudest portions of the music the audio levels should not reach the top of the meters. If the levels from the audio mixer to the videoconference system are too high, 7 P a g e

8 distortion may occur at the endpoint's inputs. Distortion at this point will not be audible through your headphones, but will be audible at the remote side of the conference. If your mixer's meter indicated an appropriate level, but your endpoint's meter shows the level as being too high or too low, adjust the input level on your endpoint through its administrative interface. Consult the user documentation for your endpoint for the procedure to do this. Recommended Audio Equipment for Music Videoconferencing The audio equipment that ships with most video conferencing endpoints is designed for speech, and generally is not suitable for musical sound. In particular, we recommend that you use microphones designed for music. Below are a few examples of audio equipment recommended for high quality music videoconferencing. This list is by no means exhaustive, but is representative of commonly available and affordable options from well-known vendors. Speaking Microphones Shure SM58 - handheld dynamic microphone used for talking Crown PZM-6D - condenser PZM microphone that can be placed on a table Microphones for Music Audio-Technica AT8022 single-point stereo mic Studio Projects C4 stereo pair Studio Projects C3 se Electronics se1a stereo pair Audio Mixers Mackie 1202-VLZ Pro Soundcraft COMPACT 10 Allen & Heath Zed 10 Headphones Audio-Technica ATH-M50 Sony MDR P a g e

9 Videoconferencing Systems: Vendor-Specific Audio Notes The following two pages include notes on the setup of videoconferencing equipment from the most common vendors. The notes and recommendations listed are intended to help you properly interface your audio mixer and your videoconferencing system. Understanding and applying the information in the section relating to your equipment will provide a baseline setup, and will make troubleshooting much easier. Polycom General Audio Setting Information a. For all Polycom HDX systems, the audio output level should be set to fixed with the level at 4. This allows the system to output its maximum level without distortion or compression. If you find the playback level too soft, the volume should be adjusted using the amplifier for your speakers. b. Ensure that the setting labeled Enable Polycom Live Music Mode is on. This is found in the audio section of the Admin Settings section of the system's settings menus. HDX 7000 & HDX 8000 The HDX 7000 and HDX 8000 series have audio inputs and outputs that use consumer grade signals on RCA connectors. Many small mixers, including those listed above, have RCA outputs in addition to professional quarter-inch TRS outputs. These outputs, often labeled as tape out, will provide an appropriate signal for interfacing with these HDX systems. Also note that the inputs of the HDX 7000 & 8000 labeled Aux are the appropriate inputs. The inputs labeled DVD/VCR do not have AEC, as they are not intended for input from microphones. HDX 9000 The inputs and outputs of the HDX 9000 series are 3-wire terminal block connectors, often branded as Phoenix connectors. These are not common in audio equipment, but can provide high quality connections for audio. 9 P a g e Custom cables can easily be made for interfacing the terminal block connectors with professional level XLR and TRS connectors, as well as for consumer level RCA

10 connectors. Details for wiring these connectors can be found in the HDX administrator s guide. Audio input 1 on the HDX 9000 has Acoustic Echo Cancellation and the other audio inputs do not have Acoustic Echo Cancellation. Note that the HDX 9000's inputs can be set to either line or microphone level, and are capable of providing phantom power. When connecting a mixer to the HDX, you must set the inputs to line level. Cisco & Tandberg General Audio Setting Information Tandberg was acquired by Cisco in Tandberg-branded video conferencing models are now sold under the Cisco brand. Generally speaking, the capabilities of the VC endpoints have remained the same. If your ensemble is of a size suitable for only one or two microphones, you may want to connect your microphones directly to the VC endpoint, eliminating the need for a mixer. The 95/75/3000 MXP series line inputs operate with AEC in mono only. To avoid phase cancellation issues, channels on your mixer should be panned center, and mic placement should be adjusted for the most accurate timbre and balance, without consideration for spatial image since there is no stereo imaging. AGC should be disabled on all inputs and outputs. Echo Control should be set to On, not On+NR. Mixer Mode should be set to Fixed MXP & 95/75 MXP The 3000/95/75 MXP has four audio inputs, including two microphone inputs (inputs 1 & 2), and two RCA line level inputs. The microphone inputs are intended to be connected directly to supplied condensor microphones, and as such, always supply phantom power. If you intend to use this system with a dedicated audio mixer, you should disconnect the microphone inputs, connect the mixer to RCA input 3, and designate input 3 as a microphone input in the administrative interface. This causes RCA input 10 P a g e

11 3 to be treated as mic input 2, using AEC and being routed only to the remote site rather than to the local speakers. Alternatively, you may connect the outputs of a mixer to the microphone inputs if you use both an inline pad and a phantom blocking transformer. Failure to use a pad will result in distortion because the level from your mixer will be further amplified by the Tandberg's inputs. Failure to use the phantom blocking transformer will send phantom power to the outputs of your audio mixer and can damage the mixer MXP The 6000 MXP has six audio inputs, including two microphone inputs (1 & 2), one mic/line input (3), one RCA line input (4) and two RCA DVD/VCR inputs without AEC (5 & 6). LifeSize To connect a mixer to the 6000 MXP, use input 3 or 4 and be sure, through the admin interface to disable phantom power on input 3. Room 200/220 The LifeSize Room series VC endpoints have a 3.5mm line input jack which can be configured to use AEC. The appropriate setting in the LifeSize administrative interface is found at: Administrator Preferences : Audio : Active Microphone Note that when the line input is selected as the Active Microphone, it operates in mono only. To avoid phase cancellation issues, channels on your mixer should be panned center, and mic placement should be adjusted for the most accurate timbre and balance, without consideration for spatial image. 11 P a g e

12 Glossary of Common Audio Terms AEC Short for Acoustic Echo Cancellation. An algorithm or circuit used by video conferencing endpoints to reduce or eliminate echo. AGC Short for Automatic Gain Control. An algorithm or circuit for automatically adjusting the volume of an audio signal so that the apparent volume remains relatively consistent. Balanced Refers to a type of electrical connection that uses three conductors to improve the connections ability to reject noise from external electro-magnetic sources. Condenser Microphone A microphone converts sound into an electrical signal by measuring the variation in capacitance between its diaphragm and a charged plate. These microphones require electrical power to charge the plate, which may be supplied by a battery, phantom power from the mic preamp, or a dedicated power supply, depending on the design of the microphone. Distortion - In the context of digital audio systems, including video conferencing, distortion most often refers specifically to clipping, an undesired effect of the inputs of an audio component receiving a signal with a voltage greater than their maximum design voltage - in other words, sending a component a signal that has been amplified too much. The audible effect is harsh, similar the sound of crumpling paper or tearing fabric. Distortion of this nature is resolved by finding the point in the audio signal chain at which it is being produced and reducing the amplification prior to that point. Distortion may also refer more generally to any other undesired change in the audio signal induced by some component of the signal path. This could be caused by mismatched electrical impedance between analog components, effects of audio processing algorithms like data compression or acoustic echo cancellation, or malfunctioning analog or digital components. Dynamic Microphone A microphone that uses a combination of a permanent magnet and an induction coil to convert sound into an electrical signal. The coil is physically attached to the microphone's diaphragm, a thin membrane that vibrates when excited by sound. Line Level Refers to the voltage output by piece of audio equipment. Typically around 1.5 volts for professional equipment, or around 0.5 volts for consumer grade equipment. Mic Level Refers to the voltage output by a microphone. Typically around 20 millivolts. 12 P a g e

13 Mic Preamp An electrical device that amplifies the signal from a microphone (mic level) to line level for recording or processing with studio equipment. Phantom Power Electrical power supplied to a microphone over the same cable used to carry the audio signal. This is a common feature on professional level mixers and mic preamps. Polar Pattern Refers to the characteristic sensitivity of a microphone. Common polar patterns include: Omni-directional, Cardioid, Hyper-cardioid, Figure 8 Power Amp or Speaker Amp An electrical device that amplifies the signal from a piece of studio equipment to the voltage required to operate loudspeakers. Powered Speakers or Active Speakers Speakers that have an integrated power amp and accept a line level signal. PZM Short for Pressure Zone Microphone, also known as a Boundary Microphone. This type of mic is usually placed with its diaphragm very close to a surface, such as a table top, floor, or wall. RCA A type of connector used in consumer audio equipment. It has two electrical contacts, a protruding center pin surrounded by a cylindrical barrel for the male side of the connector, and provides an unbalanced connection between devices. It is primarily used for line level signals on consumer grade devices. Ribbon Microphone A specific type of dynamic microphone that uses a very thin metallic ribbon as both the diaphragm and as a substitute for the induction coil. These microphones generally have a figure 8 polar pattern. As the ribbon is very thin and lightweight, ribbon mics are generally more fragile than other mics. Also, since the ribbon is actually part of the electrical circuit, phantom power may damage ribbon mics that aren't designed to handle it. TRS Short for Tip, Ring, Sleeve. A common audio connector that is roughly cylindrical and has three contacts. Unbalanced Refers to a type of electrical connection that is not balanced. All other things being equal, an unbalanced connection is more susceptible to noise from external electro-magnetic sources, such as nearby speakers, radio transmissions, and power supplies. XLR A type of connector used in professional audio equipment and microphones. It has three electrical contacts, providing a balanced connection between devices, and 13 P a g e

14 14 P a g e latches to prevent unintentional removal. It is used for both line level and mic level signals.