In most video programs it's the audio portion that organizes and makes
the visual intelligible. For some types of programs the absence of
sound would make the production completely useless even with the best
visuals. Ideally, though, if attention is given to high values in both
audio and video, each serves to compliment the other. The result is a
program that communicates powerfully and effectively.
In some ways, the general lack of good audio values in video might be
attributed to the design of the video recorder. With most small,
portable video units, a microphone is built into the camera, and sound
synchronized to the action in the scene is automatically recorded along
with the video. It's so easy that people tend to forget about the
limitations in this setup and alternative techniques that are
available.
Connections for composite video are pretty straightforward. If it says
"video" then it's one volt peak to peak with an impedance of 75 ohms.
Period. Audio connections are not so simple. To make connections from
one piece of equipment to another with confidence, you need to know
some basic audio terminology, and a little theory.
Technical Terminology
Unbalanced Audio Cable
Unbalanced lines are cables with only a single conductor and a grounded
shield. They're used for "mic in" jacks rated at 600 ohms impedance and
"line" or "auxiliary" in jacks rated at 10,000 ohms or higher.
Unbalanced lines are subject to interference, especially in runs of
twenty feet or longer. Unbalanced lines can usually be identified if
they end in "RCA" jacks, mini-plugs or phone jacks.
Balanced Audio Cable
Balanced lines are those with two conductors and a grounded shield. The
two conductors are used to carry signals which are identical except
that one is inverted and is opposite in polarity from the other. Any
interference picked up on the two conductors will have the same
polarity on both. When the two signals are recombined in a transformer,
the interference cancels itself out. This makes balanced lines the best
choice for long cable runs. Balanced lines are generally used for 600
ohm "line" or 50-250 ohm microphone inputs. Generally, if the cable
ends in a cannon (XL) connector it's balanced.
NOTE: Coupling a balanced line to an unbalanced line directly without
using a transformer unbalances the entire length of the balanced line
and defeats the purpose of using it.
Transformer
Impedance is a technical term that refers to the apparent resistance a
circuit presents to an alternating current. This apparent resistance is
measured in units called ohms. The maximum signal transmission between
devices with the lowest distortion occurs when the input and output
impedances are the same. Most audio devices fall into one of three
categories; microphones generally have low impedance (50 to 250 ohms),
balanced lines are rated at a nominal 600 ohms, and unbalanced lines
(used in consumer stereo equipment) have high impedance (10,000 ohms
and higher). Don't worry about an exact match. Simply connecting
outputs to inputs in the same impedance range is sufficient.
Cables connecting high impedance devices are more susceptible to losses
and interference as the length of cable increases. Cables connecting
low impedance devices are less likely to have problems. Low impedance
balanced cables can be run hundreds of feet with minimal problems.
Level - The sensitivity of audio inputs is another important variable.
The standard line level is one volt peak-to-peak (.775 volts RMS) at
600 ohms. Microphone outputs and inputs vary. Specifications are given
either in millivolts or in decibels. Using established convention, two
typical microphone outputs are 1 millivolt (-60dB) and 100 millivolts
(-20dB).
Signal to Noise Ratio is the difference in amplitude between
unintelligible noise generated within the device and the maximum signal
output of the device, again expressed in decibels. Most video recorders
should be capable of an audio signal to noise ratio of forty to fifty
dB. (Digital audio devices are capable of more than 100dB.) You may
have noticed that the range of output signal levels exceeds the signal
to noise ratio of video recorders. If you put a mic level signal into a
line level input, the signal is so low that it falls in the range of
noise. You won't even hear it.
Equipment
Microphones
Microphones capture sound and transform it into electrical impulses
that are sent to the video recorder. Although there are a number of
different microphone designs, only two are used with most video
equipment. From the user's point of view, the main difference is that
condenser microphones need a power source (battery or external) and
dynamic microphones don't.
Dynamic microphones are generally less expensive than condenser mics.
Both are fine for general use. It's hard to find a quality microphone
for less than a hundred dollars. Audio purists will spend hundreds of
dollars on a microphone.
Microphones are also classified according to the shape of the area of sensitivity around the microphone.
Omnidirectional Pattern
An omnidirectional microphone picks up sound equally well from all
directions. It's very flexible in that you can place an omni almost
anywhere in most situations and pick up usable sound. Omnis are made
for various purposes. One kind of omni is the microphone built into the
camera. This microphone allows one person to handle both the audio and
video. Another kind is called the lavalier, a small mic that clips to a
lapel or other part of the wearer's clothing. It can be hidden and
leaves the performer's hands free.
Cardioid Pattern
Directional microphones are designed to be more sensitive in some
directions than others. One example of a directional microphone is the
cardioid. Think of looking down over a microphone. A cardioid picks up
the best sound in an area in front of the microphone. The shape of its
coverage is like a heart. It's great for recording events on a stage
where you don't want audience noise to be picked up. Bidirectional mics
pick up sound on either side, but not in front or back.
Shotgun Pattern
Shotgun mics have a narrow range that can pull in sound from a distance in one direction.
Parabolic Dish Pattern
An omni can be used with a special parabolic dish to pick up sounds
from great distances. You've seen them on the sideline at football
games.
Boom
A boom is a long pole to which a microphone is attached. Usually
there's a special rubber or foam shock absorber between the pole and
the microphone so vibrations in the pole can't be picked up by the mic.
Windscreen
A windscreen is a small cover, usually of foam rubber, that fits over
the top of the microphone. The wind screen is used outside and reduces
(but doesn't eliminate) the sound of the wind. The windscreen doesn't
cut down on the sensitivity of the mic very much. There's no need to
remove it if you're working where it isn't needed.
Mixer
In some situations you'll find that certain sound sources will be
louder than others. Some sources may be farther away from you than
others and so they may not be picked up as clearly. Or, some sounds may
be emitted in the presence of and overshadowed by other sounds. In
these cases, the use of one omnidirectional mic won't let you record
all the sounds you want to capture clearly. To resolve this problem
you'll need more than one microphone. This is where you use a mixer, a
device that takes the inputs of a number of microphones and combines
them into one output for the recorder. The advantage of this technique
is that a microphone can be placed in the optimum position to capture
the sound of each source. In addition, the mixer enables you to adjust
sound levels from each microphone. If, for example, you were recording
a man explaining the skill involved in playing a tuba while the tuba
was playing in the background, you might use a mixer to make sure that
the speaker's words weren't drowned out.
On the front panel of the mixer are gain controls that adjust the sound
level for each microphone. By turning some down and others up, you can
get the right "mix" of sounds. After the sounds are mixed, the level of
the combined sounds can be controlled by a master gain control so that
the sound that goes on your videotape can be adjusted.
Transmitter Mics
Where you can't have microphone cables lying around, but you still need
to mic specific sound sources, many people are using radio or
transmitter mics. Actually, this involves a microphone plugged into a
radio transmitter that sends the signal to a receiver, which is
attached to the recorder. While it's a good idea, problems with
interference, reflections, and obstacles can turn a good idea into a
nightmare. The better systems operate in the VHF band, from 150 to 170
MHz.
Receivers for wireless microphones come in two basic types. Non
diversity receivers have a single antenna and single receiver for each
microphone. There is no protection against reflection or
obstacles. Diversity receivers use multiple (2) antennas and may
use one or two receivers for each microphone. The received
signals are compared hundreds of times each second and the better
signal is used. While diversity systems will help with
reflections and obstacles, radio frequency interference will
still cause problems.
The VU Meter
Audio recording systems have built-in limitations. When sounds are
below a certain level, they're masked by noise. When sounds are too
loud, the system can't handle the level and distortion results. If
you're in charge of audio during a production, you want to make sure
that the sound fed into the recorder falls in an acceptable range. In
some instances, with some equipment, there is almost nothing you can
do. With some mixers and some audio recorders equipped with VU meters,
though, you can control the sound levels. A VU meter shows you a visual
representation of the strength of the audio signal expressed in volume
units. The maximum allowable sustained level is zero. Most VU
meters range from -20 VU to +3 or +5 VU. Although digital
recorders have considerably more range, any sound much below –20
VU will be masked by the inherent noise in an analog recording
system. It is good practice to keep important audio between
–10 VU and 0 VU.
Monitoring the sound using this meter is fairly easy. Set the audio
level so that the loudest sound of any duration just hits zero VU.
Occasional peaks may go into the red. If you are recording to
analog audio tape you can ignore these fluctuations when setting the
levels. When recording digital audio you have a signal to noise ratio
of up to 100 dB, but you should never exceed 0dB, since there is no way
to capture audio levels that exceed 100% amplitude. Now
you’re all set and should leave the controls alone unless the
sound levels change. For instance, suppose you're using a fixed
position microphone and the speaker suddenly moves away from it after
you set the levels. If the needle on the meter only moves a little at
the lower end of the scale, you'll have to use the gain controls to
boost the level of sound back into the acceptable range. If the speaker
comes close to the microphone and starts to scream, and the needle on
the meter goes consistently into the red, you'll have to reduce the
sound level. Voice should be kept mostly between -7 and 0 VU.
Automatic Gain Control
Automatic Gain Control (AGC) is built into most consumer audio
equipment to make the taping of sound easier. It boosts the sound
signal automatically when the level gets too low and compresses the
sound when it gets too loud. This feature allows one person to handle
both video and audio without having to worry about monitoring the sound
level. In many situations you'll get usable sound with AGC.
Unfortunately, certain characteristics of AGC circuits cause problems
in some situations.
Those problems are sensitivity to background (ambient) noise and the
reaction time of the system. Most AGC circuits won't boost
a sound until it reaches a certain threshold level. The circuit makes
an arbitrary decision as to what is noise and what is signal. In a
noisy environment (such as a room with a window air conditioner) the
ambient sound will be boosted to an unacceptable level. AGC may
boost the hum from an amplifier, turning it into a dull roar.
Some mixers can emit a low level buzz that may also be boosted by AGC.
The other side of the coin is the problem of reaction time. If during a
normal recording there's a sudden loud noise, the AGC circuit often
will drive the recorded audio down below audible levels for three to
four seconds.
If you can take control over your audio levels and operate manually, do it.
Production Tips
1) Scout the area where you want to tape before the actual recording.
Try to visualize the kinds of recording situations that you'll
encounter. If you're alone or short-handed, you might have to settle
for the built-in microphone on the camera and hope for the best. If you
have many sound sources and you can round up someone to handle the
audio, use a mixer and multiple microphones where they're required. If
you don't want the microphones to show in a scene, mount them on booms
or hide them. Lavalier mics provide excellent pickup and can be hidden
easily, but they restrict movement. Where movement is important, booms,
sound parabolas, or transmitter mics might be appropriate.
2) Make sure all plugs and connectors on cables and equipment
fit. Make sure the impedances and levels on all connected devices
match. Do not assume that all microphones, mixers, and recorders are
designed for compatible impedances or levels. Use adapters only
where you are sure devices are compatible.
3) Try to visualize where the equipment and microphones will be placed
and the dimensions of the area that will be used for taping. Use this
information to estimate the amount of audio cable you will need. Again,
it is better to take too much than too little.
4) Once on location, if you are using fixed microphones, set them at
their positions. The optimum placement for a mic is six to twelve
inches from and below the speaker’s mouth. If it's too close
you'll get too much bass response and not enough treble. If it's too
far away, the level of the voice in relation to the noise in the
surroundings may not be high enough. If the mic is directly in the
speaker’s wind stream, you may get popping and hissing when he
pronounces certain consonants.
5) Lay out the cables and attach them either to a mixer or video
recorder, then use gaffer’s tape (or duct tape, if gaffer’s
tape is not available) to tape the cable to the ground every six to
twelve feet. In high-traffic areas either cover the cable with a rubber
mat or tape all along the cable so it's impossible to trip over. This
will reduce the chances of someone hurting himself or damaging
equipment.
6) Whenever possible, set sound levels using a VU meter.
7) Whenever possible, monitor the sound going to the recorder. Listen
for high levels of background noise from the location, hum and other
interference in the lines, and distortion, as well as a good sound mix.
When a problem is discovered, it's generally better to correct it, even
if it means delaying taping.
8) Watch for idiosyncrasies in the performers that might affect the
sound. Tapping fingers or banging fists near table-mounted microphones
are annoying. Some people are nervous on camera and rub their hands
over hand-held mics or microphone cables, causing a distracting
scraping sound. People who tap or scratch their chests while wearing
lavalier mics also can be a problem. Lavalier mics may click against
buttons or jewelry if they're not carefully placed.
9) When using a sound mixer, use tape or a china marker (grease pencil)
to label each control to indicate the source. You don't want to turn
the wrong control at a critical moment.
If you are recording a live event it is important to be flexible and to
be prepared. You probably will not have the only audio system on
location. Musical groups are using sound reinforcement and so are
most public speakers.
This raises an important question. What audio are you there to
record? That answer may change with the nature of the event and
your audience.
If, for example, you are recording a public speaker your interest is in
getting the cleanest possible recording of the speaker. Any
ambient sound is your enemy, particularly the public address
system. You want your microphone as close to the speaker as you
can get it. If there is only one speaker a lavalier microphone is
the best solution. If there will be multiple speakers, a
microphone on the podium is the best solution. If there are
multiple speakers at multiple locations you will need an audio mixer
and an audio operator to keep the active mike up and all of the others
down.
If you are recording a musical performance there is more to
consider. Four acoustic (unamplified) performances you could mic
each instrument or group of instruments, mic the audience for applause
and reactions, and manage it all with a mixing console. You might
not be satisfied with the result if you don’t get enough of the
reverberation in the venue. You could place a couple of
microphones above the front of the stage or performance area. The
individual instruments will not be as clean, but your recording will be
much closer to the experience in the audience. In well-designed
concert halls you can actually get good results placing your
microphones in the middle of the audience seating.
For electronic or amplified performances you will want to place your
mics in the audience seating or at the front of the stage.
There are two problems you should consider in advance. First,
what if the sound reinforcement is actually louder than the source when
the two sounds reach your microphone? And what if the sound
reinforcement is so loud that you literally cannot hear the sound in
your headphones to judge the audio quality? If you anticipate
this sort of problem, your best solution is to isolate your audio
console and operator in an adjacent room or at least use a good headset
designed to reduce unwanted ambient sound.
Equalization
Generally speaking, equalization is a tool which is used to correct
deficiencies in sound. It involves changing the amplitude of narrow
bands within the audio spectrum. It is (and this is important) a form
of audio distortion. In other words, if something sounds all right to
you, don't waste your time trying to improve it through
equalization. To put it another way, any change you make in
the audio system is by definition distortion.
The simplest equalizers are the bass, treble, and loudness controls on
consumer receivers. Graphic equalizers slice the audio spectrum into a
series of narrow bands, while parametric equalizers let you set the
target frequency, the width of spectrum, and the amount of boost or
attenuation you want. While graphic equalizers have a wide variety of
uses, from matching sounds recorded with different microphones or under
varying conditions, Parametric equalizers are better at isolating and
reducing the impact of undesirable background sounds.
Sound is a more important part of most television programs than the
viewers ever realize. Unless you do an adequate job of treating the
acoustical and aesthetic problems involved, the entire meaning of your
program can be distorted or obscured. As in any other area of
television production, experience and common sense prove to be your
most valuable tools.
