Crown CM-30 User guide

CROWN MICROPHONE APPLICATION GUIDE:
TELECONFERENCING & DISTANCE LEARNING
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are registered trademarks of Crown International.
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CROWN MICROPHONE
APPLICATION GUIDE
FOR TELECONFERENCING
AND DISTANCE LEARNING
Thanks to teleconferencing, we can hold meetings
with people in another location without having to
travel there. Distance learning lets a professor teach
thousands of students in various locations, all at the
same time, in a cost-effective manner. It’s also used
for corporate training.
Both teleconferencing and distance learning can save
you or your company thousands of dollars in travel
fees, not to mention the cost of hotels, meals, etc.
Teleconferencing System
Here is an overview of a typical teleconferencing
system (Fig. 1). Two meetings are set up around
conference tables. One meeting is in your company,
and the other meeting is in another company.
Figure 1. Teleconferencing system.
People at your meeting are picked up by micro-
phones. The sound of their voices is sent over stan-
dard telephone lines, or a satellite link, to the other
meeting location. There, the people can hear you
through a loudspeaker.
Similarly, mics at their location pick them up. Their
voices are sent over phone lines or satellite to your
location. You hear them through a loudspeaker. You
can talk back and forth, almost as if they were with
you at the conference table.
There might be cameras and video monitors set up so
that you can see the other people, and they can see
you.
A teleconferencing system is made of the following
components:
• Microphones
These pick upthe voice of each participant. Mics can
be on the conference table or on a lectern. The mics
might be switchable: each user can turn his or her
mic on and off.
• Mixer
This electronic device combines all the mic signals
into one audio signal.
• Telephone coupler or codec
Short for coder-decoder, a codec takes the audio sig-
nal from the mixer, and sends it over the telephone
lines. On the sending end, it digitally codes the audio
into a telephone signal. On the receiving end, it
decodes the telephone signal back into audio.
Some codecs also send and receive video. Video sig-
nals must be sent via satellite, which relays the signal
to the distant location.
Some systems combine a mixer and codec into one
unit. Other systems combine mics, mixer and codec
into one unit.
• Telephone lines
Supplied by the telephone company, standard phone
lines (or high-speed phone lines) carry your audio
signal to the distant meeting location. There, a codec
converts the phone signal back into audio, which is
fed to a speaker. The meeting participants can hear
you through this speaker.
• Loudspeaker
In your meeting room, a loudspeaker plays the voices
of the people from the distant meeting.
• Video cameras and TV monitors
This is an option. Cameras and monitors let you see
the people at the distant meeting, and let them see
you. In many systems, the cameras automatically
switch to show the person speaking.

Distance Learning System
Now let’s explain a distance learning system (Fig. 2).
Typically, students will be in one or more classrooms,
and a professor will be in another location. The pro-
fessor talks to the students through a clip-on or lava-
lier microphone. The mic signal is sent by phone
lines or satellite to the classrooms. There, several
loudspeakers play the professor’s voice to the stu-
dents. The students can see the professor over a TV
monitor.
Figure 2
In the classroom, each student or pair of students has
a desk mic or hanging mic. Students can switch on
the mic when they want to ask the teacher a question.
The mic signals are sent by phone lines or satellite to
the teacher. The teacher and student can talk back
and forth, almost as if they were in the same room.
Figure 3
The parts of a distance learning system are the same
as in a teleconference system. Crown makes one part
of these systems: the microphones.
You should contact a consultant, codec manufacturer
or system integrator to design and install your com-
plete system.
How to Send Clear Audio
When you transmit audio to the distant meeting, the
audio should be clear and easy to understand. But
there are some problems that prevent clear sound:
Reverberation
This is the sound reflected off the room walls, floor
and ceiling. Too much reverberation makes the
speech sound hollow, distant, and blurred.
Background noise
This is noise from ventilation ducts, fluorescent light
ballasts, video equipment, and equipment cooling
fans. Too much noise makes the speech hard to hear
and understand.
Feedback
This is the squealing or ringing sound you hear when
the mics pick up the sound of the loudspeaker. The
speech sound picked up by the mics is fed to a loud-
speaker, and the loudspeaker sound re-enters the
microphone. This creates a feedback loop and makes
an annoying ringing sound.
Here are some ways to prevent all these problems.
The result will be clear, intelligible audio:
• Place mics close to talkers. Put table-top mics
within arm’s length. Talk into lectern mics about 8
inches away. Wear a lavalier mic on the chest.
• Use directional mics. Some examples of directional
mics are cardioid, supercardioid, and hypercardioid.
These mics are designed to reject feedback, noise and
reverberation.
• Use boundary mics on conference tables. A bound-
ary mic is a low profile unit that lies on the confer-
ence table surface. It picks up less room acoustics
than a conventional mic on a desk stand, so the
boundary mic sounds clearer. Also, a mic on a desk
stand picks up table-top sound reflections. These
reflections cause phase interference, which may give
the voice astrange tone quality. The boundary mic is
designed to eliminate phase interference, so it sounds
natural.

• Use an automatic mixer (gated mixer). You adjust
the automatic mixer so that it turns all mics off
except for the one in use. One mic by itself sounds
much clearer than several mics on at once.
• Acoustically treat the conference room. To absorb
reverberation, use acoustic-tile ceiling, carpet, and
drapes. Hire a sound contractor to design or modify
the air handling system for low noise.
• To prevent feedback, place the loudspeaker far from
the mics, and behind them if possible. Use an acous-
tic echo canceller (AEC) with howling rejection.
Another problem with sound transmission is echo.
Your audio signal is sent on a phone line, or via satel-
lite, to the distant receiving site. It takes time for the
signal to travel. At the distant site, the signal might
be picked up by the mics there, and bounce or echo
back to you after a short delay. If you hear this echo,
it confuses the sound of speech.
The solution is an acoustic echo canceller (AEC), a
device that senses the echo and cancels it out. Many
codecs come with an echo-canceller circuit built in.
Crown Mics for
Teleconferencing and
Distance Learning
All Crown mics are condenser mics, which need
power to operate—either a battery or a phantom-
power supply . A phantom power supply sends power
to the mic along its mic cable. Two phantom supplies
from Crown are the PH-4B (powers up to four mics)
and the PH-1A (powers one mic). Most modern mix-
ers provide phantom power for microphones, so that
no external supply is needed.
Here are some Crown mics that work well for tele-
conferencing:
Table-top or desk-top mics
These microphones lie on the surface of a conference
table or a desk. They pick up speech with a clear,
natural sound.
All these models are boundary mics. They eliminate
the effect of reflections from the table top that would
otherwise blur the sound.
An omni boundary mic picks up sound in all direc-
tions, so you put it in the middle of the table. Use
one for a square table and two spaced apart for a long
table. This works for conference recording, but has
too much feedback for teleconferencing use.
Supercardioid or cardioid boundary mics pick up
sound mainly from in front. These mics sound
clearer and have lessfeedback than an omnimic. But
theycost more because you must use several of them:
one per person, or one for every two people.
Listed below are the Crown boundary mics. We’ll
suggest which models to use later in this guide.
PCC-170: supercardioid
PCC-170SW: supercardioid with on/off switch
PCC-170SWO: supercardioid with switch and two
additional remote sensing leads.
PCC-130: cardioid, smaller than PCC-170
PCC-130SW: cardioid with on/off switch
PCC-170
Mini Boundary mics
These are very small mics with a half-supercardioid
pattern. Five models are available:
MB-1: Plugs into a brass cup in the table.
MB-2: Plugs into a jack in the table.
MB-3: Tubular; mounts in ceiling, wall or table.
MB-4: For temporary use. Has a thin cable with an
XLR connector.
MB-4E: Lowest cost. Cable fits through small hole
in table.

Models MB-1, MB-2 and MB-4E require a Crown
MB-100 or MB-200 interface. The MB-200 inter-
face allows remote sensing of switch closure, so it
can be used with a video switcher. Each interface can
take up to four mics.
An example of an MB mic: The MB-2.
Hanging mics
Normally, these mics are hung over the students in a
distance learning setup.
The Crown hanging mics are the CM-30 and CM-31.
Each mic rejects sound from the rear, and rejects
muddy-sounding room acoustics.
CM-30
CM-30 electronics mount in an electrical box in the
ceiling. CM-31 electronics are in a tubular interface
which you plug into a microphone cable.
Mics hanging over a conference table do not work
well compared to table-top mics. Hanging mics are
too far from the talkers for clear pickup without feed-
back.
Lectern mics (gooseneck mics)
These mics are permanently installed on a lectern,
student desk, or conference table. Compared to
table-top mics, lectern mics pick up less paper shuf-
fling and less room acoustics because the mics are
closer to the user. Lectern mics, however, are more
conspicuous and are more susceptible to breath pops.
LM-300A and LM-301
Listed below are the Crown lectern mics:
LM-300A: Miniature gooseneck. Can be shock
mounted with LM-SM shock mount.
LM-300AL: Same as LM-300 but 5” longer.
LM-201: Rugged ball and socket swivel, built-in
shock mount.
LM-301A: Fastens to a flange on the lectern; allows
cable to exit down or out the side.
LM-SM: Shock mount for models LM-300A, LM-
300AL, and LM-301A.

Lavalier mic
This is a miniature mic that clips onto the talker’s tie
or shirt (Figure 4). It’s useful if the talker moves
around while speaking. The teacher in a distance-
learning system often uses a lavalier mic.
Figure 4. A lavalier mic.
The rest of this guide suggests the best Crown mics
for various applications. It also suggests how to
place them to achieve the best sound quality.
Distance Learning
Applications
In a distance learning system, each mic needs to be
near its user to prevent a hollow, muddy sound.
Desktop mics tend to pick up too much paper shuf-
fling, but gooseneck or lectern mics work well. You
can mount a gooseneck mic on a desk to pick up two
to three people. Or, hang a mic about 2 feet in front
of, and 2 feet above, every two to three people.
Angle the mics to aim at the people (Figure 5). So
that only one mic will be on at a time, run the mics
through an automatic mixer.
In a distance learning auditorium without desks,
mount lectern mics onto the seat backs, or use hang-
ing mics.
For best results consider treating the room acoustics.
Add some carpet, acoustic tile ceiling, and drapes
over the windows. This will greatly increase clarity
and reduce the hollow sound you hear with hard
room surfaces.
Figure 5. A hanging mic.
Teleconferencing
Applications
As we said earlier, the sound picked up at a confer-
ence is much clearer if you use several mics, and turn
off all mics except the ones in use. In a teleconfer-
encing system, there are two ways to switch each mic
on and off:
1. Manually, by each user.
2. Automatically, by an automatic mic mixer (gated
mixer).
Let’s look at both options.
Manual switching of mics
With this system, use a standard non-gated mixer
with several mics. Choose one of these mic models:
PCC-130SW: Small mic with a half-cardioid pattern.
PCC-170SW: Larger mic with a half-supercardioid
pattern for extra clarity.
PCC-170SWO: Like the PCC-170SW, but provides
remote sensing of switch closure. These mics can be
used with a video camera switcher. When a person
turns on his or her mic, the camera aiming at that per-
son is switched on. Then the TV-monitor view will
track whoever is speaking.

Mini Boundary Mics: Very small mics with a half-
supercardioid pattern. Three switchable models are
available:
MB-1: Plugs into a brass cup in the table.
MB-2: Plugs into a jack in the table.
MB-4E: Lowest cost. Cable fits througha small hole
in table.
These models require a Crown MB-100 or MB-200
interface. The MB-100 interface has connections for
an on/off switch near each user. The MB-200 inter-
face also allows remote sensing of switch closure, so
it can be used with a video switcher.
Automatic switching of mics
In this system, use an automatic mic mixer with sev-
eral mics. Choose one of these mic models:
PCC-130: Small mic with a half-cardioid pattern.
PCC-170: Larger mic with a half-supercardioid pat-
tern for extra clarity.
Mini Boundary mics: Very small mics with a half-
supercardioid pattern. Five models are available:
MB-1: Plugs into a brass cup in the table.
MB-2: Plugs into a jack in the table.
MB-3: Tubular; mounts in ceiling, wall or table.
MB-4: For temporary use. Has a thin cable with an
XLR connector.
MB-4E: Lowest cost. Cable fits through small hole
in table.
Models MB-1, MB-2 and MB-4E require a Crown
MB-100 or MB-200 interface. The MB-200 inter-
face allows remote sensing of switch closure, so it
can be used with a video switcher. Each interface can
take up to four mics.
Mic placement
Once you have chosen a suitable mic model, place a
group of them as shown in Figure 6.
Figure 6. Teleconference mic placement.
1. Place one mic at arm’s length from each person, or
place one between every two people.
2. At a lectern, mount a Crown LM series lectern mic
of your choice. If the leader of the meeting moves
around while talking, he or she needs a lavalier mic.
Clip the mic onto the shirt, about 8 inches below the
chin.
3. If your mixer has phantom power, connect the
mics to the mixer mic inputs (Figure 7). Models
MB-1, MB-2, MB-4E connect first to the MB-100 or
MB-200 interface, then to your mixer.
Figure 7. Teleconference system block diagram.
4. If your mixer does not have phantom power, con-
nect the mics to a Crown PH-4B phantom power
supply. Connect the power-supply outputs to your
mixer mic inputs. Models MB-1, MB-2, and MB-4E
connect first to the MB-100 or MB-200 interfaces,
then to the PH-4B, then to your mixer.
5. Connect the mixer output to the line input in the
teleconference codec.
By following these suggestions, you can look for-
ward to clear sound in your teleconference and dis-
tance learning systems.