Boards, Consoles, and Mixers
Various sources of audio must be carefully controlled and blended during a production.
You will recall we said if analog audio levels are allowed to run at too high a level, distortion will result, and if levels are too low, noise can be introduced when levels are later brought into the normal range.
Beyond this, audio sources must be carefully and even artistically blended to create the best effect.
The control of audio signals is normally done in a TV studio or production facility with an audio board or audio console. A sophisticated version, similar to what you would find in many TV stations, is shown in the right.
Audio boards and consoles are designed to do five things.
Sophisticated audio boards or consoles also allow you to manipulate specific characteristics of audio. These include the left-to-right "placement" of stereo sources, altering frequency characteristics of sounds, and adding reverberation.
For video field production smaller units called audio mixers provide the most basic controls over audio.
A simplified block diagram of an audio mixer is shown below. The input selector switches at the top of each fader can switch between such things as microphones, CDs, file servers, and satellite feeds.
The selector switch at the bottom of each fader typically switches the output of the fader between cue, audition and program.
Cue is primarily used for ▲ finding the appropriate starting point in recorded music. A low-quality speaker is intentionally used in many studios so cue audio is not confused with program audio.
Audition allows an audio source to pass through an auxiliary VU meter to high quality speakers so levels can be set and audio quality evaluated.
And, of course, program sends the audio through the master gain control to be recorded or broadcast.
Even though audio boards, consoles, and mixers can control numerous audio sources, these sources all break down into two main categories:
Mic-level inputs handle the extremely low voltages associated with microphones, while line-level inputs are associated with the outputs of amplified sources of audio, such as CD players.
Once they are inside an audio board, all audio sources become line-level and are handled the same way.
Using Multiple Microphones in the Studio
Most studio productions require several mics. Since the mics, themselves, may have only a 5 to 10 meter (15-30 foot) cord, mic extension cables may be needed to plug the microphone into the nearest ▲mic connector.
Studio mics use cables with three-prong XLR or Cannon connectors, as shown on the left.
Since things can get confusing with a half-dozen or more mics in use, the audio operator needs to make a note on which control on the audio board is associated with which mic. A black marker and easily removed masking tape can be used on the audio board channels to identify what mic is plugged into what channel. Mic numbers ("lav 1") or talent names ("John") can be used for identification.
In the studio mic cables are normally plugged into three-prong XRL or Cannon connector receptacles mounted in the studio wall as shown in this six-connector array.
Because mics represent one of the most problem-plagued aspects of production, they should be carefully checked before the production begins. Unless you do this, you can expect unpleasant surprises when you switch on someone's mic, and there is either no audio at all, or you faintly hear the person off in the distance through another mic. Either way, it's immediately clear that someone goofed.
There is another important reason that mics should be checked before a production: the strength of different people's voices varies greatly.
During the mic check procedure you can establish the levels (audio volume) of each person by having them talk naturally, or count to 10, while you use a VU meter to you set or make a note of the appropriate audio level.
Of course, even after you establish an initial mic level for each person, you will need to constantly watch (and adjust) the levels of each mic once the production starts. During spirited discussions, for example, people have a tendency to get louder. Monitoring audio gain will be discussed below.
It is also good practice to have a spare mic on the set ready for quick use in case one of the regular mics suddenly goes out. Given the fragility of mics, cables, connectors, etc., this is not an unusual occurrence.
An elaborate digital audio console (board), such as the type you would find in a major production studio, is shown above. Note that many of the setting and monitoring status displays are in the form of small LCD screens at the top of the board.
Mics in the Field
If only one mic is needed in the field, it can simply be plugged into one of the audio inputs of the camera. (The use of the internal camera mic is not recommended except for capturing background sound.)
When several microphones are needed and their levels must be individually controlled and mixed, a small portable audio mixer will be needed. The use of an audio mixer generally requires a separate audio person to watch the VU meter and maintain the proper level on each input.
Portable AC (standard Alternating Current) or battery-powered audio mixers, such as the one shown here, are available that will accept several mic- or line-level inputs.
The output of the portable mixer is then plugged into a high-level video recorder audio input (as opposed to a low-level mic input).
Most portable mixers have from three to six input channels. Since each pot (for potentiometer) fader or volume control) can be switched between at least two inputs, the total number of possible audio sources ends up being more than the number of faders. Of course, the number of sources that can be controlled at the same time is limited to the number of pots on the mixer.
There is a master gain control generally on the right of the mixer that controls the levels of all inputs simultaneously. Most mixers also include a fader for headphone volume
Although handheld mics are often used for on-location news, for extended interviews it's better to equip both the interviewer and the person being interviewed with personal mics.
Whereas the mixer shown above will probably require a special audio person to operate, the cameraperson can operate the simple two-mic mixer shown on the left. The output from the unit is simply plugged into the camcorder. A slightly different approach to this was discussed in Module 39.
Audio mixers and consoles use two types of controls: selector switches and faders. As the name suggests, selector switches simply allow you to select and direct audio sources into a specific audio channel.
Faders (volume controls) can be either linear or rotary in design. As we've noted, faders are also referred to as attenuates, gain controls, or pots (for potentiometers).
A rotary fader is shown here.
Linear faders (shown on the right) are also referred to as vertical faders and slide faders.
It's important to maintain optimum levels throughout a production. This is commonly referred to as riding gain.
You will recall that, depending on the production facility, digital and analog audio signals typically require different optimum levels. However, to reduce confusion in the following discussion we'll use the analog standard of 0dB to represent a maximum level.
Normal audio sources should reach 0dB on the VU or loudness meter (next to the 100 in the illustrations) when the vertical fader or pot is one-third to two-thirds of the way up (open).
Having to turn a fader up fully in order to bring the sound up to 0dB indicates that the original source of audio is coming into the console at too low a level. In this case the probability of system background noise increases.
Conversely, if the source of audio is too high coming into the board, opening the fader very slightly will cause the audio to immediately hit 0dB. The amount of fader control over the source will then be limited, making smooth fades impossible. In either case an adjustment should be made in the output of the originating audio source.
To reflect the various states of attenuation (resistance), the numbers on some faders are the reverse of what you might think. The numbers get higher (reflecting more resistance) as the fader is turned down. Maximum resistance is designated with an infinity symbol, ∞, which looks like an "8" turned on its side.
When the fader is turned up all the way, the number on the pot or linear fader may indicate 0, for zero resistance. Even so, just as you would assume, when the pot is turned clockwise or the fader control is pushed up, volume is increased.
Level Control and Mixing
Audio mixing goes beyond just watching a VU meter. The total subjective effect as heard through the speakers or earphones should be used to evaluate the final effect.
For example, if an announcer's voice and the background music are both set at 0dB, the music will interfere with the announcer's words. Using your ear as a guide, you will probably want to let the music peak at around -15dB, and the voice peak at 0dB to provide the desired effect: dominant narration with supporting but non-interfering background music.
But, since both music and voices have different frequency characteristics (and you'll recall that, unlike VU meters, our ears are not equally sensitive to all frequencies), you will need to use your ear as a guide.
During long pauses in narration you will probably want to increase the level of the music somewhat, and then bring it down just before narration starts again.
In selecting music to go behind (under) narration, instrumental music is always preferred. If the music has lyrics sung by a vocalist (definitely not recommended as background to narration) they would have to be much lower so as not to compete with the narrator's words.
Using Audio From PA Systems
In covering musical concerts or stage productions a direct line from a professionally mixed PA (public address) system will result in decidedly better audio than using a mic to pick up sound from a PA speaker.
An appropriate line-level output of a public address (PA) amplifier fed to a high-level input of a mixer can be used. However, don't connect a high-level or speaker level PA signal to a mic input. It can damage the amplifier.
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