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The Art And Technology Of Listening, Part 2
Acquisition of Audio
By Carmen Borgia

Nothing matters more to the sound of your film than to record the sound properly to begin with. Plug a great mic into a great recorder, stick it in front of a great performance in a great room and get out of the way.

  1. Microphone
    Microphones are analog devices that turn a continuously vibrating pressure wave into voltage. A good mic does this precisely, so use a well-made, accurate instrument. When you spend from $50 to $400 on a mic, allow for some variation in manufacturing quality. I have a pair of $79 studio condensers that sound ok individually, but donít match up well when used as a pair. Iíve seen people flinch when I suggest that they spend upwards of $500 on a mic, but a mic is analogous to the lens on a camera. For a given application, it is normal to spend between $100 and $1,500 on a good microphone. One cool thing about a good mic is that it is future-proof. It should serve you well for years to come.

    In film work, you will mostly encounter condenser and dynamic mics. Dynamics are sturdy, a little noisy and capture less detail than condensers. They are useful when you can get the mic right in a subjectís face, such as a news interview. Condensers are more sensitive, and suited for grabbing sound from a distance. We further categorize mikes by pickup pattern, which is to say that they are directionally sensitive. Cardioids favor the area in front of the microphone. Hypercardiods such as shotgun mikes pick up sounds at a distance. Omni directional mikes pick up sound equally in all directions.

    It is deceptive to imagine that a shotgun mic is somehow reaching out to the distant subject and grabbing the sound. A mic only picks up sound that is present where you place it. Whatís really happening is that the mic is rejecting areas of the sound field - to the sides and to the rear - that it is immersed in. As such, directional mikes color the sound a bit, even the good (expensive) ones. This arrangement is a limitation of the physics of sound.

    Lavalier mics may be wired or wireless. Expect to pay a premium for a decent wireless mic. The spiffy ones go for upwards of $2,500, and are worth it. The perils of lavís fall into two categories: wireless transmission and size issues. When a signal becomes a radio frequency, look out for noise that creeps in through the airwaves like motor whine, dimmer hash and interference from radio stations. Good wireless mics allow you to switch stations to find the cleanest spot in the radio spectrum. Size issues relate to the poor little mic stuck to the subject to pick up clothing rustle, bumps and sweat. Any of the preceding issues may make your recording useless, so it is good practice to back up your lavalier recording with a boom track, just in case.

    A very good mic picks up sound with a minimum of coloration. It is sturdy and reliable. It passes its signal to the recorder in a sturdy and reliable manner, which means that it uses an XLR or similarly professional connector. The connector will lock into the recorder, the mixer or the camera with a satisfying click, and will not come flying loose in the heat of battle.

    Additional features add to the versatility of the microphone. A high pass filter switch cuts low frequency sounds like wind or the rumble of a vehicle. These filters remove that portion of the sound permanently, so be choosy when you use it. Some studio mics have switchable pickup patterns that make them either more or less directional. A very useful switch is a pad, which makes the microphone less sensitive. It is indispensable for recording loud sounds without distortion. It is normal practice to use a pad when recording a scene with screaming actors, artillery or the documentary where you burst into the safe house with a phalanx of cops. When embedded, keep it padded.

    This brings us to distortion. Sound takes up more physical space than recorders are capable of recording. When the sound exceeds the available space, you get distortion. The sound comes flying at the system and it wonít fit in. The system canít budge, and the sound is crammed willy-nilly into the available space. Think of standing up quickly and finding that the ceiling is only four feet high: OUCH! So, donít do that. If your digital recording system has a meter, never exceed that zero at the top of the scale. In the old analog days, zero was the part of the meter where you wanted your audio to live. In digital, zero is the absolute ceiling. You may add distortion at any device in your audio chain: the microphone, the preamp, the mixer and the recorder itself. Once you record distortion, you cannot change it. The distortion remains.

    To avoid the dreaded zero, turn it down! I hear you thinking, ďIf I do that, it will add noise.Ē This is true. This is why you must have a recorder with the lowest self-noise available. This is why you must have a microphone with the lowest self-noise available. This is why you must place the mic as close to the subject as the shot will allow, and to get close coverage as wild sound if that is not possible. This is why your recording environment must be as quiet as is possible when you are recording.
     

  2. Recorder
    There are all kinds of good recorders:

    Analog tape Ė Nagra. There are still a few of these out there. The Nagra IV stereo is the last one. It has time code. They sound great and when well maintained are bulletproof. The analog tape has pleasing warmth. There is a bit of hiss that comes with the tape, but when you record audio properly, it will be of no consequence. Once you record, you must transfer the audio into an editing system in real time to synchronize it up.

    Digital audiotape Ė DAT. This one is always 16 bit and 44.1 or 48 Khz resolution, which matches editing systems and master tape formats. DAT is a miracle of modern engineering, but it does have flaws. The tape is narrow, thin and travels within the DAT machine along a tortuous path, which means that while the sound can be very good, the resulting audio on the tape is vulnerable to dropouts, mangling and other physical damage. Does it sound better than a Nagra? Itís up for debate, but the linear (stuck to a long piece of tape) format of audio storage means that you have to load it into your editing system in real time.

    Digital hard disk and digital flash recorders. Now weíre talking! Deva, Sound Devices, Fostex, Tascam and others make these. These devices record sound through high quality converters and store it as audio files on either a hard drive or a solid-state memory card of some sort. Since you record the audio data directly to random access digital media, you can copy the sound from the recorder right into your editing system without the hassle of real time transfers. These machines are more robust than DATs in hostile environments (jungles, urban exteriors, press rooms), and the ones with flash cards have no moving parts, which are the components that tend to fail first. They have another advantage in that you record the audio into the same format that it is edited in (WAV & AIFF), so there is no translation from the source recording to the edit stage. These devices come in two track and multi-track designs. They are no more complex, and may be easier to use than DATs because the audio in the units with flash cards can be copied onto your editing system with an inexpensive card reader.

    Camcorders and audio on videotape Ė Mini DV, Dvcam, Hdcam, and D5 record at 16 bits and 48 kHz, the same as DAT. In theory, they should sound as good. In practice, the cheaper cameras use cheap converters that add unpleasant gravel to your low-level audio. This is not so bad if the record levels are good, but low-level noises accumulate when you pile up tracks in post audio processes and degrade the end product. Never rely on the built-in camera mic unless itís a high end camera or youíre doing ENG. Beware of automatic gain controls included in most cameras, they are the audio equivalent of auto focus and should be avoided unless absolutely necessary. Itís hard to beat the convenience and low profile of an all-in one picture and sound recorder, but you will sacrifice some audio quality. Set your priorities and make your choice.

    There are too many recorders for me to list here. If youíre blazing new trails, always do a test to make sure it will hold sync with your picture. Recorders are designed for specific users. Film people need battery powering, low-noise mic preamps, phantom power for condenser mics, clear metering, the ability to bypass automatic gain controls and limiters, time code, high capacity audio storage and excellent reliability. These specifications will help narrow your list.
    Record only to uncompressed audio formats such as WAV or AIFF. Do not record to .mp3 or MPEG, as these are compressed audio formats, suitable only for the final stage of your audio. Compressed formats throw away some of the audio data to save space, and do not hold up well in audio post processes.
     

  3. Mixers
    Many recorders have only two channels on which to record audio. In some cases, you will use more than two microphones and will need a mixer to sum the available mics down to two channels. You may place three or more microphones about the set in the sweet spots, routed into your recorder, and away you go.
    You would think that adding a bunch of microphones to a shoot would pick up more sound, just like adding more lights will make a set brighter. In practice, multiple mics are subject to phase cancellation, which is an artifact of turning a three dimensional sound field into a two dimensional electrical signal. Phasing may make the resulting audio dull, or sound as if you are listening through a tin can. Observe the three-to-one rule: when using two mics, they should never be closer to one another than a factor of three from the source. So, if one mic is a foot from the subject, the other mic must be three feet or more from the subject. Whenever you combine two mics into a single channel of a recorder, listen carefully to be sure that phase cancellation is not happening, something you cannot remove once recorded. If your recording setup is challenging, hire a professional production sound recordist.
     

  4. Analog to digital converters
    If the audio does not make it to and from the digital domain accurately, it matters little what we can do with it inside the system. Itís like a low-resolution photograph. Once you take a picture, you canít make it look better just by increasing the number of pixels. We call the part of a digital audio system that turns the analog microphone or other signal into a digital state a converter. You need at least one to get the audio into the system, and then you need another to get it back out. Converters do not have little lights or buttons or a spiffy graphic interface. Converters do the unglamorous job of making the audio digital and then back again.
    If your converters are not high quality, none of the nifty digital cleanup tools that exist will make them work well. Like so many other things on planet gear, good converters cost more than poor ones.
    Noisy and distorting converters are usually included at no extra charge in consumer electronics such as camcorders, minidisc recorders, and .mp3 recorders. Donít ever use the microphone jack that comes with your computer to record audio that you plan to keep. If it has a little Walkman style jack that you plug an eighth-inch connector into, it probably has poor converters. Professional systems sport those heavy-duty three pin XLR connectors that real microphones use.
     

To summarize:

  1. Use a good microphone in a quiet environment; get it as close to the subject as possible. If the thing you are recording is loud, pad the mic.
     

  2. Record your audio to a good quality recorder with quiet mic preamps and good converters.
     

  3. Always record uncompressed audio as a .WAV or. AIFF file. Do not use .mp3, MPEG or other compressed formats.
     

  4. Use a mixer for more than two mics. Listen up for phase issues in the resulting recording.
     

  5. You cannot remove distortion and phase problems captured in a recording post.
     

Tune in next month for installment three: Editing & Mixing.
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Carmen Borgia is the head of audio services for DuArt Film & Video in New York City. He oversees a post production sound department that provides mixing, sound design, restoration, transfer and printmastering. His department caters to independent projects in all formats from mono optical up to digital 5.1. Visit Carmen's personal Web site at: www.carmenborgia.com
 

 

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