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Basic Audio http://www.tv-handbook.com/Basic%20Audio.html
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Television Production Handbook
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?1980-2006 Roger Inman & Greg Smith. All rights reserved.
Basic Audio
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
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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
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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.
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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
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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
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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.
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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.
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