Adventures in Sound Editing

MixerImagine the tape you made of your kids' last camp out. It's got some crickets on the sound track.

At night, didn't those crickets seem to chirp louder as campers grew quieter and the sky grew darker?

When Bobby started telling Billie [o the story of the cricket that ate sisters, didn't those chirpings seem even louder to  her? With some equalization, reverb and creative mixing of your original sound  track, you could let your viewers hear those monster crickets the way Billie heard them.

Sound editing can turn commonplace video events into adventures that seem larger than life. With some simple elec-tronic equipment,some of which you may already have on your stereo system, you can polish up any raw sound track. Here's how.

Know Your Sound

To make good video, you need to under-stand light; likewise, to make good audio, you will need to understand sound.

Webster's defines sound as "mechanical vibrations traveling through the air or other elastic medium." How many times these vibrations occurin a second is the frequency of the sound. A tuning fork vibrating back and forth 1,000 times per second generates sound (with a frequency of 1,000 cycles per second or Hertz (Hz). If it vibrated 200 times per second, we'd hear a frequency of 200Hz.)

Variations and combinations of frequencies account for all the sounds we hear. At best, the human ear can recognize frequencies between 32 and 22,000Hz. We can perceive frequencies below 32 Hz,but as vibration, not sound. This range deteriorates with age or abuse. It is not uncommon for senior adults to top out at 9,OOOHz, while losing all low-frequency sounds below about 150Hz.

People with ear damage will adjust sound to match their deficiency. For example, a sound engineer at a rock club may set up the PA to produce ear piercing highs to compensate for such a loss of his high frequency sensitivity. So if you plan to use the services of a studio, make sure your sound engineer can hear.

The interaction of two or more frequencies creates a third sound called harmonic. Harmonics, also known as overtones, give sound its life, allowing the human ear to distinguish one voice from another. Poor recordings reduce or eliminate harmonics, turning sound into an unintelligible mess.

Good recordings recognize the har- monics of a sound track, and enhance them-with the help of some snazzy audio post-production devices.

The Mixer

The single most important tool for audio production is the mixer.

A mixer's number of input channels determines how many different signals-read sounds-you can work with simultaneously. You need at least two for mono recordings and four for stereo. More inputs allow you to control more sound sources.

Remember that the tape speed and audio recording format will determine the frequency response ofthe recording medium. A VHS tape recorded on the linear track at EP speed may yield a frequency response no higher than 7,OOOHz. So stick with hi-fi audio if possible.

The first step is to hook up the right cables to the proper inputs and outputs. Do not scrimp on cables, because bad connections can create a lot of buzz and noise. A good mixing console will allow an input signal to be either microphone or line level. This compensates for low-level microphone signals and high-level line inputs, such as those arriving from VCRs, cassettes or CD players. Output will be mono or stereo, depending on your VCR.

Mixers have sliders, or faders, that control volume for each channel. A master fader controls overall volume. Mixers will also feature all or some of the following: attenuation, equalization, cue sends, pan pots, solo switch, monitor control, Volume Unit (VU) meters or Liquid Element Displays (LEDs) and echo/reverb.

Attenuation cuts down high input signals to prevent overloading.
Equalizers allow precise tone adjustment of selected frequency ranges.
Cue sends can send the signal to the video tape, headphone or monitor speaker.
Pan pots control the spatial position between right and left stereo channels.
The solo switch "listens" in on any individual channel without interfering with the recording process.
Monitor controls adjust headphone or speaker volume. We'll look at echo and reverb later.

The Equalizer

An equalizer (EQ) is the most common piece of signal processing equipment. It allows you to divide the entire (human) 32-22,OOOHz audio spectrum into separate bands you can adjust independently. With an EQ, you can raise or lower the levels of these bands to change the tonal characteristics of a sound, reduce noise and even create certain audio effects.

Many mixers have a simple EQ built-in, allowing you to adjust high and low frequencies independently. This is fine for broad tonal changes, but for more control, you'll need a graphic equalizer. Though a handful of mixers offer this level of tonal control, most graphic EQs are external units.

Whereas simple EQs use knobs-one for treble, one for bass-graphic equalizers have vertical sliders that show the amount of correction you applied to each band of frequencies. Thus you can tell, at a glance, what the graphic EQ is doing to the audible spectrum. Hence the termgraphic.

Think of the graphic EQ as a vastly expanded bass/treble dial. The more sliders, the greater the selective control.

Say you have some music on your sound-track that sounds flat and unexciting. Try boosting the 100Hz and 10,000Hz sliders, while lowering the 1,000Hz to 500Hz sliders. The graphic pattern on the equalizer will look like a suspension bridge-what you'll hear is rock concert sound.

If you're working with classical music, you may need to elevate the middle frequencies to bring out specific instruments. And Aunt Trudy's squeaky voice may require you to slide down the high and upper midrange frequencies.

Hooking up a stand-alone EQ is easy-simply connect it between the source output and mixer input. Or, if you want to alter your whole sound mix, place the EQ between your mixer and record deck.

On the Level

In the world of audio post-production, the strength or "level" of an audio signal is as important as the way it sounds. We have our ears to tell us that a sound is tonally correct; for clean recordings we need something to tell us about the signal's level.

VU meters and LEDs monitor the levels so you can prevent overloading and distorting the signal. VU meters use a unique scale calibrated in decibels (dB), which measure actual signal strength.

VU meters boast an ascending scale of values that start with negative numbers. Instead of being at the bottom, zero is near the top of the range. Why? Because zero dB indicates a full-strength signal. This way it's easy to remember that if a signal strays much above zero, distortion may result.

Sometimes, simply setting a record level based on the VU meter isn't enough.

Some sounds jump wildly from loud to soft, making them distorted one second and virtually inaudible the next. The difference between these very loud and soft sounds is the dynamic range.

Changes in dynamics, at least within reason, are good. It's the dynamic range of a recording or vocalist that conveys realism and power. Streisand's vocal style is dynamic, my grandmother's isn't.

But when recording sounds, too much dynamic range can be a problem. Thankfully, there's help available from a nifty device called a limiter. A limiter reduces instant signal peaks, such as a loud scream or feedback from an electric guitar. A limiter allows you to set a maximum signal level; the unit holds all sound under that limit.

A compressor works somewhat like a limiter, though its effects are less dramatic. Radio stations use compressors to maintain high signal levels without fearing distortion. The small sacrifice in realism may be worth it if you plan on taping loud live concerts or monster truck rallies. Good production mixers have limiters built in, for controlling sounds at the time of recording.

Reverb

Sound loses energy as it moves through the air. Sound strength drops 6 dB every time you double the distance between a mike (or ear) and the sound source. This is why good microphone technique includes moving the mike close to the sound source during live shooting or narration dubbing.

Sound waves bound off walls and hard objects. Echoes are bouncing waves that follow more than 1/20 second after the original sound; reverb is repeated reflecting waves that sound almost continuous.

Post-production electronics can simulate both these effects. Echo and reverb units can run the gamut from simple tape loops, playing back what you record immediately, to sophisticated digital reverbs simulating rooms of all shapes and sizes.

Reverb units bring fullness to sound. With a reverb you can make a three-piece band playing in the basement sound like a major concert hall event. Reverb emulates the sound of a big hall. But watch out-too much reverb will make things muddy.

Reverb also adds authority to a narrator's voice. Rock and roll's ultimate DJ, Cousin Brucie, uses a lot of reverb.

Reverb, particularly when applied to drums, can make a band sound bigger than life on tape. Now you know how rock bands maintain the ambiance of the concert hall on recorded media.

Once it's on tape, you can't eliminate reverb easily from a recording. That's why recording engineers add it later for maximum control. Use sound-absorbing dampeners-rugs, blankets, egg cartons-in small studios whenever recording to capture a clean dry sound. Then employ electronics in post-production to simulate real room resonance.

Digital delays are similar to reverbs. They electronically delay the input signal for a selected amount oftime. You can produce echoes of extremely short duration. Use digital delays to double a vocal, and you can make it sound as though two people are singing together. And, if you take one output channel from the delay and run it through an EQ, you may even make it sound like two different people. The possibilities are infinite.

Phase shifters delay the incoming signal slightly, causing the delayed signal to partially cancel the original. This causes whooshing soundeffects-good for planes, trains and other speedy objects.


Next month, we continue with Going for the Take in Part Two