guide to good audio in distance learning

8/6/2019 Guide to Good Audio in Distance Learning

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2000 ASPI Digital. All Rights Reserved. 1

CONTENTS

I. Introduction............................................................................ 2

II. Its (Almost) All in The Acoustics............................................. 3

III. Correct Microphone Usage..................................................... 5

IV. Correct Loudspeaker Usage.................................................... 8

V. Acoustic Echo......................................................................... 9

VI. Background Noise................................................................. 12

VII. Transmission Echo................................................................. 13

VIII. Sound System Problems........................................................ 17

IX. Telephone Audio Problems.................................................... 18

X. Summary............................................................................... 20

About ASPI Digital................................................................ 20

This PDF file is also available in printed form. Contact ASPI Digital for a copy.


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2000 ASPI Digital. All Rights Reserved2

INTRODUCTION

If youre like most educators, distancelearning represents a fun and interest-ing challenge for you. The logistics ofdealing with multiple locations and theproblem of keeping everyone interestedin your topic are your main concerns.And, if the distance learning network isdesigned correctly, they should be youronly concerns.

The problem is that in a large number

of distance learning networks, carefulattention is paid to the video side camera locations, ability to view moni-tors, etc. but little attention is paidto the audio, which carries the bulk ofyour message (unless youre conduct-ing a final exam on sign language or lipreading!).

Does your distance learning networkhave any of these problems?

Hollow, boomy-sounding au-dio

Feedback / squealing in the PAsystem

Feedback / squealing wheneveryou bring a phone line into theconference

Voices are distorted or garbled (dif-ficult to hear or understand partici-pants)

Fans or blowers overpower voices Some voices are too faint, others

too loud Background noise is really annoy-

ing Acoustic echo (you hear your own

voice coming back to you, or yourstudents hear their own voices com-

ing back to them) Transmission echo (people in your

distance learning network hear theirvoices coming back to themselveswith a long delay)

Telephone based audio: the over-all sound is thin, tinny, noisy, diffi-cult to listen to for long periods oftime

YYYYYou do not have to put up with bad au-ou do not have to put up with bad au-ou do not have to put up with bad au-ou do not have to put up with bad au-ou do not have to put up with bad au-

dio in your distance leardio in your distance leardio in your distance leardio in your distance leardio in your distance learning network!ning network!ning network!ning network!ning network!

This paper will discuss the most commoncauses of the problems encountered withaudio systems, and practical suggestionsfor correcting the problems.


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ITS (ALMOST) ALL IN THE ACOUSTICS

Here is a bold (but true) statement: Most

audio problems in your distance learn-ing network are caused by poor acous-tics in the classrooms.

With only a few exceptions, the majorityof your problems can be traced to thehard surfaces youll find in the classroom walls, floors, ceiling, windows, andtables. A room with lots of parallel hardsurfaces is said to be highly reverberant,

meaning a sound introduced into theroom will bounce around for some timebefore decaying beyond your ability tohear it. This bouncing effect creates aboomy or hollow sound in the room, andyou may hear echoes of your voice whenyou talk. This bouncing effect is also re-ferred to as multipath.

But, you say, weve had reverberantclassrooms ever since schoolhouses werefirst designed. What makes them a prob-lem in distance learning?

Reverberant classrooms only cause minorproblems for the people in them (the big-gest problem being fatigue) because theposition of our ears, combined with theprocessing power of our brains, permitus to ignore the bouncing audio and con-centrate on the source of the originalsound (hopefully, the teacher). In other

words, we hear three-dimensionally. A mi-crophone, on the other hand, hearsone-dimensionally it simply picks up ev-erything it hears and sends it to theother classrooms. The original sound isgiven the same treatment as a reflectionof the sound because the microphone can-not differentiate between the two. Thestudents at the other sites do not have thebenefit of 3-D hearing because ALL soundsare reaching them from a single location

(the loudspeaker). The result is that thestudents in the distant locations hear ev-erything - original audio plus reflections -at one level (the loudspeakers volume)which gives them distorted, garbled sound.Even when voices sound reasonably good,a reverberant room can cause a deeprumble which is heard at all other sites inthe network.

There are a number of ways to control re-

verberant rooms. The best fix is acous-tic treatment: acoustical ceiling tiles, car-peted floors, sound-absorbing panels onthe walls, and draperies over the windows.If you dont have the budget for soundpanels, carpets or draperies, try tackingblankets to the walls and tossing somethrow rugs on the floor. Even though thiswont fix all of your problems, youll besurprised how much quieter the roomgets. You should invest in draperies nomatter what else you do: they will dampensound, and your video will look betterwithout interference from sunlight.

Making a few smallchanges in your room

- such as rugs orcurtains - can greatlyreduce reverberation.


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There are some electronic fixes that canhelp reverberant rooms:

1. An automatic (or gated) micro-phone mixer reduces the numberof times reflected audio can reach

a microphone by turning idle micsoff (the mixer must have athreshold setting that sets theon level above the level of thereflected sounds). Even if youhave acoustic treatment in theroom, you may want an automaticmixer to keep background noiseat a minimum. Automatic mixerswill be discussed in more detailbelow.

2. Microphone processors can distin-guish between the original voicelevel appearing at a microphoneand lower-level reflections, andadapt to prevent the lower levelaudio from being passed through

the system. Mic processors canalso remove background noisefrom the audio. (However, youmust have a processor for EACHmicrophone in the room, whichcan get expensive.)

THE ASPI DIGITAL SOLUTION

Background noise cancellers, such asthose manufactured by ASPI Digital forour EchoFree product line, are very ef-fective in removing the rumble that iscommon in reverberant rooms. Peoplein the originating room will still hear therumble, but it wont be transmitted to

other rooms. If you are using an auto-matic microphone mixer and want tocontrol the rumble that is heard by otherooms, try installing an ASPI product in-line with your automatic mixer.

Reverberation in the classroom

acoustics cont.


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CORRECT MICROPHONE USAGE

Because you cant see microphones ontelevision programs, you might believeyou dont need to place microphonesclose to participants in your classroom.The truth is that microphones are VERYclose to announcers and actors on TV, andthere are a lot of mics in use.

On most television programs, good cam-era angles are used to conceal boom mi-crophones just over the actors heads. Ifyou look closely at a news announcer, heor she may be wearing a color-coordi-nated lavaliere microphone. In television,great care is taken to conceal microphonesbecause the sight of them would distractfrom the desired appearance of the setsand actors. This generally isnt a concernin distance learning classrooms! However,you need to make sure microphones areplaced in locations where students wont

hit or bump them, or where papers andbooks wont be put on top of the micro-phones.

There is another very important reasonwhy commercial TV sounds better thanyour distance learning network: sound iscarefully controlled in TV studios (withacoustic treatment, heavy drapes, non-parallel surfaces, etc.) and sound techni-cians are always on hand to make sure

microphones are in the right positions.Television studios also use highly direc-tional microphones to pick up only thedesired sounds while blocking out extra-neous noise.

It probably isnt practical to convert yourclassroom to a television studio, but you

can incorporate certain aspects of one,especially when it comes to microphoneusage.

Make sure you have an adequate num-ber of microphones in the room. If youdont have enough microphones, it will bedifficult to hear everyone (some peoplewill sound faint and far-off while othersare loud). A common mistake is to try touse a small audioconferencing system in alarge classroom. A few microphones sim-ply wont do the trick when youre deal-ing with a large room. A good guidelineis to have one microphone per 2 to 3 stu-dents (an automatic mixer is importantwhen more than 4 microphones are used:see below).

Get microphones off of the desks andtables. Low-profile boundary micro-

phones that sit on a tabletop are attrac-tive and can work well for a corporateconference room, but in the paper-ladenclassroom, these microphones are boundto get covered (or rustling papers will belouder than the students voices). Po-dium (or gooseneck) microphones area better choice for distance learning: thesemicrophones involve a small standmounted to the desk, and a flexibleneck with a small microphone posi-

tioned about a foot above the table.

What about ceiling microphones?

Ceiling microphones are attractive forclassroom use because they keep mics outof students reach, and out of the way. Itis important to know, however, that as

WHYDONTWESOUNDASGOODASTHEPEOPLEON TV?

1. Use an adequatenumber of micro-

phones.

2. Get mics off desksand tables.

3. For more than fourmics, use an auto-

matic mixer.


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microphones are moved away from the

people who are talking, backgroundnoise becomes a huge problem. Thereason is that only a portion of thespeech energy actually reaches the mi-crophone when the mic is several feetaway. In order to hear what the talkeris saying, microphone gain (volume)must be increased. As noted earlier inthis paper, the microphone doesnt dis-tinguish between voices and back-

ground noise it just picks up every-thing it hears. Increased gain to pickup the voices will mean an increase inbackground noise as well. Anotherproblem with increasing gain is the like-lihood of feedback some room audiosystems just cant handle high micro-phone gain.

If you suspend microphones from theceiling (dangling from cables or

mounted on stiff rods), you can reducesome of the talker-to-mic distance andincrease intelligibility. If you do this,make sure students dont have to lookup in order to talk into the microphone;position the microphones a couple feetabove and in frontof the desks.

Microphones that are flush-mounted onthe ceiling can work (and provide a nicerappearance in the classroom), but beaware of two major downfalls:

1) ceiling mounted mics are much closerto air handling systems than they are tothe students, and the whoosh of aircould overpower voices; and

2) there is a possibility of mechanical cou-

pling, where the mics pick up vibrationsfrom the ceiling framework, such as peoplewalking on the floor above you. Also, becareful to locate microphones away formloudspeakers to minimize the possibility offeedback.

The ASPI Digital Solution

If youre using ceiling microphones and

youre having problems with noise or feed-back, ask your system integrator to instala multi-channel echo / noise canceller suchas the ASPI EF1210 between the micro-phones and the microphone mixer. Thisshould cancel most of the backgroundnoise and will allow you to crank up thevolume on the mics to make voices muchmore intelligible.

If your classroom has more than four mi-

crophones (and itshouldunless its a smalclass), you need to use an automatic mi-crophone mixer. When a large number ofmicrophones are on at once, especially ina reverberant room such as a classroomthe rooms gain into the audio systemcan become too high. This excessive gainresults in squealing or howling (feedback)Also, as mentioned earlier, an excessivenumber of on mics will increase back-ground noise in the audio system. An ASPDigital EF1210 will help with the noise, butyoull get even better results if you com-bine it with an automatic mixer.

mic usage cont.


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Look for an automatic microphone mixer that will:

be able to handle the number of microphones you will be using (most mixersare expandable or can be cascaded).

not require a specific type of microphone - this can prove costly. provide an automatic gating threshold - this means that a microphone will

only turn on when the sound level exceeds a pre-determined level. Thesmarter mixers can automatically set the gating threshold above the back-ground noise.

permit a chair override on one of the channels. This permits the instructor totake control of the mixer by simply speaking into his or her microphone.

allow one microphone to be always on. If all mics turn off, that rooms audiowill go away, making it sound to the other rooms as though the connectionwere cut off. Leaving one microphone on will provide a more natural soundand assure the other rooms that the connection hasnt been cut.

mic usage cont.


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CORRECT LOUDSPEAKER USAGE

A common mistake in distance learning

or teleconferencing classrooms is the ten-dency to use only one or two loudspeak-ers to carry the audio from the other classsites. This can result in the followingproblems:

When too few loudspeakers are used,its necessary to crank up the volume,which places the room closer to afeedback point.

The students near the loudspeakersare in pain from the volume, whilestudents who are far away from themcan barely hear whats going on.

Using an adequate number of loud-speakers will allow you to keep volumeat a reasonable level throughout, and willminimize the amount of loudspeakeraudio that is picked up by microphones.

The best way to distribute loudspeakersis to place them in the ceiling (or mountthem along the walls) and control themwith a separate power amplifier. The

better amplification systems permit zon-

ing of the loudspeakers, which is usefuin lecture halls for amplifying theinstructors voice along with the audio fromother sites (the loudspeakers directly abovethe instructor are kept at a lower volumeto prevent feedback via the instructors mi-crophone).

One final note on loudspeaker placementtry to isolate loudspeakers from micro-

phones as much as possible. Remembethat a microphone will pick up all soundsin its vicinity, and audio coming out of aspeaker will be treated as just another voicein the room. This can create feedback /squealing, and is one of the causes ofacoustic echo (see next page).

Make sure you useenough loudspeakersto keep the volume ata comfortable level.


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ACOUSTIC ECHOOR: WHYAM I HEARINGMYSELF? THISISREALLYANNOYING!

Anytime two or more sites talk to eachother with open microphones andloudspeakers, the potential exists foracoustic echo. Acoustic echo is causedwhenever a sites microphones pick upreceived audio off loudspeakers and re-transmit the audio back to the originat-ing site.

The pick-up path of the audio can be di-rect (straight line from loudspeaker to mi-crophones) or indirect (bouncing around

the room, then hitting the mics). De-pending on the size of the room, or howreverberant the room is (reverberantrooms bounce the audio around for quitea while), a certain amount of delay willoccur between the time the audio ap-pears on the loudspeakers and when itreaches the microphones. This room de-lay, combined with any delays intro-

duced by the transmission path, will resultin a delayed echo that could rival deep can-yons or baseball stadiums. Unless yourea professional sports announcer, you canbecome extremely unnerved by this de-layed repetition of your voice.

Fortunately, its easy to fix acoustic echo!

All it takes is a product called an acousticecho canceller (AEC). The AEC is a bi-di-rectional device which is placed into a

rooms sound system between the micro-phone mixer and transmission system , andthe receive port and loudspeaker system.

Acoustic echo cancellers use digital signalprocessing (DSP) technology to comparereceived audio with the audio being sentback down the transmission system. Any

Removing acousticecho is easy with an

acoustic echocanceller.


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audio that has the same characteristics as the received audio is removed from thetransmitted signal. Recent advancements in AEC technology have made these de-vices very effective in removing both real-time and delayed echoes.

An important note: an AEC in your room will do nothing to keep you from hearingthe return of your own voice, because an AEC benefits the OTHER site(s) by remov-ing their audio from the audio being sent to them. In order to eliminate acousticecho at all sites, you will need to place an AEC at each location in the distancelearning network.

When shopping for an acoustic echo canceller, look for the following features:

Automatic adaptation to room conditions (some older AECs must use a loud,long burst of noise to train the AEC to the room)

A tail time (echo cancellation span) that is sufficient for the size of your roomtaking into account both loudspeaker-to-mic distance and reverberation issues (ifthe tail time is not long enough, not all of the echo will be cancelled, and otherrooms will hear everything from weird blips to full echoes)

Constant adaptation to changing room conditions (some older AECs freeze at acertain setting, and break into echo/feedback when people move around theroom)

FAST adaptation to the room. This is called convergence rate the faster the

AEC adapts, the better.


All ASPI Digital AECs feature very fast convergence rate currently the best in theindustry along with automatic and constant adaptation, and a tail time that issufficient for even large training rooms. Plus, were the only company to includebackground noise cancellation in our AECs.

acoustic echo cont.


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ACOUSTIC ECHO CANCELLATION BETWEEN ROOMS


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MOREON BACKGROUND NOISE

One of the most distracting things in adistance learning network (or any typeof multi-site electronic meeting) is the ex-traneous noise that is picked up by mi-crophones and sent to the other sites. Inaddition to the rumble or boomysounds caused by room reverberation,there are several other types of noise tobe considered:

HVAC / air handling systems

Fans on computers, overhead projec-tors, etc.

Buzz from fluorescent lighting Noises coming into the room from

outside (talking, vacuum cleaners,traffic noise, etc.)

Papers shuffling / rattling in front ofmicrophones

Some of these problems arent fixed eas-ily. You may need to ask the building

contractor to install quieter air handlingsystems or bring in an electrical contrac-tor to change the lighting system. Becareful where you place microphones a mic right next to an overhead projec-tor fan or computer will pick up almostnothing but fan noise. As for noises com-ing in from the hallway, one suggestionis to buy an ON AIR - QUIET PLEASElight and install it next to the classroom

door in the hallway. (If your integratordoes not have one of these lights, it canbe purchased through a broadcast equip-ment dealer.) Traffic noises can be damp-ened somewhat with heavy curtains andother acoustic treatment (see section II).


All products in ASPIs EchoFree line in-corporate background noise cancellationThis patent-pending technology helps toreduce the problems caused by micro-phone pick-up of fans, blowers and otherconstant noises. It isnt a cure-all, though please try to keep those mics away fromthe projector fans if you can.

ASPIs Noise Cancellation Algorithm (patentpending) dramatically reduces the level ofambient noise that is sent to other locations.


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Whether your distance learning networkis using satellite delivery or MCU / landbased systems, you can introduce signifi-cant echo problems even when you havetaken steps to eliminate acoustic echo ateach of the sites.

There are three ways that transmissionecho can be created:

1. Satellite delivery with telephone

call-ins (see Figure A, page 14)

In this type of transmission (typically usedin business television or distance learningnetworks that span huge distances), mas-ter video and audio are sent to the re-mote classrooms by satellite. To ask ques-tions or respond to the teacher, the re-mote classrooms call in to the master fa-cility via telephone. So that all other class-rooms can hear the question or response

from a remote classroom, the audio fromthat room is uplinked to the satellite. Theproblem with this is that the classroomoriginating the question ALSO hears thataudio, and it has been delayed consider-ably because of the satellite delay. This isnot the same thing as acoustic echo, be-cause this audio is intentionally mixed intothe master feed.

2. Loop-back of audio (see FigureB, page 15)

Even when satellite delivery is not used,redistribution of audio in multipoint net-works creates a similar problem. Becauseall sites in an MCU based network are al-ways on audio-wise and the feed ismultipoint-to-multipoint, each site will

hear their own audio looped back to them.Again, this is an intentional mix, not acci-dental pick-up of audio via loudspeakers.The delay may not be as long as that ex-perienced via satellite, but the loop-backof a classrooms audio at full volume cancreate significant feedback and howlingproblems.

3. Copies of audio caused by dif-ferent types of equipment (see Fig-

ure C, page 15)

An interesting problem occurs when olderCODECS are used in the same multipointnetwork as newer, faster equipment. Thenew equipment (using fiber optic or T1transmission) sends the signal to other sitesvery quickly. When an older CODEC, suchas an ISDN-based product, is introducedinto the network, it sends out its signal(which includes that room plus all othersites) at a slower speed. The result is thatthe rooms with newer equipment will heartwo copies of the same audio transmis-sions from the other fast equipment plusthe transmission from the slower equip-ment.

Until recently, there has not been a satis-factory fix for these problems. Satelliteand loopback echo have been somewhat

controlled by using push to talk systems,which mute the incoming audio in the re-mote room until the student has finishedspeaking. These systems typically place ahold on the mute for about a secondafter the student stops speaking so thatthe tail of the echo does not reach theclassroom. The problem is that the tailmute often cuts off the first few words of

SATELLITEAND TRANSMISSION ECHO


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the instructors response as well. In order for all sites to hear the full response, theinstructor must learn to delay his or her response by a couple seconds, which canlend an awkward feeling to the session and reduce the natural flow of conversation

No fix at all existed for the copies of audio problem until now.

The ASPI Digital Solution (see Figure D, page 16)

In early 2000, ASPI Digital introduced a product called the EF1210TEC (TransmissionEcho Canceller) which incorporates multi-channel acoustic echo and noise cancella-tion along with up to two seconds of transmission echo cancellation. This product is

placed at each remote site in the distance learning network and can handle echoesthat have been delayed up to two seconds by the network transmission system. TheEF1210TEC can be set up to handle either looped-back audio or copied audio.

Transmission Echo Figure A - Satellite Echo

transmission echo cont.


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Transmission Echo Figure B - Loop-Back echo

Transmission Echo Figure C - Echo caused by copies of the same signal



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Transmission Echo Figure D - ASPI Digitals EF1210TEC can remove duplicated audio orlooped-back audio from your distance learning network.

Duplicated

Looped-Back



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SOUND SYSTEM PROBLEMS(SQUEALING / HOWLING)

When a sound system starts squealing or

howling, it is said to be in a feedbackfeedbackfeedbackfeedbackfeedbackstatestatestatestatestate. Feedback is caused by either anelectronic or acoustic signal feedingback to its source, becoming amplified,and going through the cycle again.

Electronic feedback is causedwhen an output signal is erro-neously routed back to the soundsystems input. This can also be

caused by improperly adjustedtelephone equipment (see nextsection).

Acoustic feedback is caused byhaving microphones too close tothe loudspeakers that carry themicrophone audio (local soundreinforcement).

Acoustic feedback can also be caused

when the gain of the audio system istoo high. Frequently, if the other roomscant hear your room, it is because themicrophones are too far away from thetalkers. The answer is to turn up the gainon the microphones so theyll pick up thevoices more easily. The problem with

turning up the gain is that many acous-

tic echo cancellers cant handle an in-crease in gain, and theyll break intofeedback.

The electronic feedback problem can befixed through proper wiring of the soundsystem or replacing telephone equip-ment that may be causing the feedback.Ask your system integrator or consult-ant for assistance.


If your system is breaking into feedbackwhenever you increase the gain on themicrophones, chances are your acousticecho canceller isnt robust enough tohandle the higher volume. We suggestyou try installing an ASPI Digital EF1210between your microphones and micmixer. The EF1210 is designed to handle

much higher room gain than competingproducts. Plus, itll cancel the back-ground noise that inevitably comes upwhen volume is increased.


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If your sound system starts squealing orhowling whenever a phone call is intro-duced, the problem is not in the soundsystem but in the interface to the phoneline. Most often, the wrong device hasbeen used for bringing the phone call intothe audio system. Telephone couplers,devices normally costing $300 or less,simply will not work for your applicationbecause they cannot adequately isolate

the two sides of the telephone call. Thisinadequate isolation results inbleedthrough of audio from the sendside of the coupler to the receive side,and when this audio is amplified throughyour sound system, electronic feedbackresults.

Luckily, phone coupler problems are alsoeasy to resolve. Throw out the couplersand replace them with digital telephonehybrids or digital phone-adds, avail-able through your systems integrator or

a broadcast equipment dealer. Thesedevices are the same products used atradio and TV stations to bring callers intotalk shows. They are considerably moreexpensive than the couplers (they cost

about $800 on the low end and $2500on the high end) but wont introducefeedback into your audio system.

There is only one trick to using tele-phone hybrids: the audio sent down thetelephone line cannot contain any of thecallers own audio or feedback will resultYou need to use what is called a mix-minus feed to the caller, which is a mixof all of the audio in your system minusthe callers audio. Conferencing phone-add products, such as the ASPI Digita

EF200, have been designed to work withthe echo canceller so that this problem iseliminated. However, if your system iscomplex and involves mixing of a num-ber of audio sources, you may need tohave your integrator create a sub-mixor clean feed to send to the callers onthe phone line.

OTHER TELEPHONE SYSTEM

CONCERNS

If your distance learning network is anaudio-only system based on standard

TELEPHONE CALLSAND TELEPHONECONFERENCING PROBLEMS


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phone lines (rather than digital or satellite transmission), a significant problem candetract from the efficiency of your class: listener fatigue. Simply put, when youlisten to a phone call for a long time, your brain has to work overtime just to processout the line noise and compensate for the thin, tinny sound of the line.

The best cure for phone line quality is to upgrade your distance learning network toanother transmission system such as ISDN, ATM, or fiber optic. These systems offerhigher bandwidth, meaning voices sound natural, and transmission noise is elimi-nated because digital technology is employed. If your budget does not permit anupgrade of this type, you should at least try to maximize your system through the

use of digital telephone hybrids combined with microphones, loudspeakers, andacoustic echo cancellers each site. Also, when looking for a digital telephone hybrid(or phone add), try to find one that incorporates background noise cancellation(such as the ASPI Digital EF200). This will remove the hiss that makes phone calls sohard on the ears.

telephone problems cont.


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2000 ASPI Di it l All Ri ht R d20

SUMMARY

Yes, it is hard to achieve good sounding audio in distance learning networks. Thereare many obstacles beginning with the room acoustics, continuing with the elec-tronics, and ending with user technique (speaking directly into microphones andlearning the etiquette of multi-site operation). However, audio problems are notinsurmountable. Once your audio system has been correctly tuned, you will finda dramatic increase in productivity and enjoyment.

ABOUT ASPI DIGITAL

Since 1981, ASPI Digital has specialized in

the development of digital signal process-

ing (DSP) products for voice and audio.

ASPIs vision is to enhance the way people

communicate by building innovative DSP

sound products. ASPI understands that au-

dio is critically important to the total audio /

videoconference experience. ASPI is using

its expertise in audio and DSP to develop

high-quality, room based audio systems for

the A/V and sound contractor markets.

ASPIs EchoFree products allow users to

experience full-duplex communication in

situations never before thought possible,

such as distance learning, telemedicine and

courtroom applications.

1720 Peachtree St. NWSuite 220

Atlanta, GA 30309Toll-Free (North America):

800-932-2774404-892-3200

404-892-2512 faxwww.aspi.com

The ASPI Digital Logo, The Sound of DSP and EchoFree are registered trademarks of ASPI Digital.

guide to good audio in distance learning

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