Audio Advice: Digital Audio Basics
The big buzz--from the highest end professional studios to the latest prosumer camcorders--is "digital." And not just digital video, but digital audio as well.
Most consumers are familiar with digital audio, thanks to the popular audio compact disc. Although some audiophiles will argue the finer points, CDs have resulted in an increase in the quality of sound people can now hear in their homes. Does this mean that all digital audio is superior? Not necessarily, though digital audio certainly has the potential to be an improvement over what we now hear from consumer videotape formats.
Let's go over exactly what digital audio is, and how it's going to change what we actually hear when we play back our video masterpieces.
How We Hear
Before we can fully understand the implications of digital audio, we need to understand analog audio so we have something to compare it to. Videomaker has covered this subject before in numerous places, so I'll go over it quickly.
Sounds are nothing more than vibrations in the air. These vibrations tickle our eardrums, which our brain then translates into what we know as "sound." The larger these vibrations, the louder the sound; the faster the vibrations, the higher the pitch (or "frequency") of the sound. Each unique sound has a unique pattern that it vibrates the air with; our brain is able to decode these different vibrations and match them up with our memories of what different things sound like.
Our ears can hear a pretty wide range of frequencies: usually from 20 to 20,000 "cycles per second." These numbers reflect how fast something--speaker cones, vocal cords, etc.--are vibrating the air. Each cycle per second (also commonly represented as "Hz" or Hertz) equals one complete back-and-forth movement. The higher the number, the higher the apparent frequency or pitch of the sound. We often refer to the spread between these numbers as the "bandwidth" of the sound.
The unique timbres (or sound quality) of individual sounds are actually the result of combining numerous different frequencies of vibrations together in a particular way. This means that even if a sound appears to have a relatively low pitch, it may have higher-pitched components present which help us identify it from another sound at the same pitch. Therefore, it is important that our ears--and any recording system, analog or digital--be able to accurately translate a wide range of frequencies (i.e. have a large bandwidth).
The higher the level of a sound (or audio "signal"), the louder it appears. The dynamic range of a sound is how much it can vary from its quietest to its loudest. Related to these numbers is the "signal-to-noise ratio"--how much louder a sound is than the background noise. The larger this difference, the easier it is to hear the sound.
For example, as I write this, I am sitting in front of a computer with a noisy fan and several whirring disk drives. The level of this whir and hum is the background noise level in my room. If I wanted to listen to some music over it while typing, I would have to increase the level of that "signal" until it was sufficiently louder than the noise, so my brain could make it out.
This is the same reason you place a lavalier microphone on the person speaking, rather than relying on the microphone on your camcorder. You're hoping to improve your signal (speech) to noise (surrounding environment) ratio.
Analog Audio
To record sounds and listen to them later, we need to capture those vibrations in the air in some way. Since it's not very practical to freeze them in mid-air, we use devices such as microphones to convert them to electrical currents that fluctuate in proportion to the air vibrations. To store these currents on tape, the recording head converts them into a magnetic field that mimics the change in current. This changing magnetic field is then trapped in tiny metal particles on the tape. So the end result is a magnetic pattern on tape that actually follows the original air vibrations--an analogy, if you will, of the original sound. Hence the term analog audio.
To play it back, we reverse the process. We drag the metal particles past a playback head that responds to the magnetic field and generates an electrical current. We amplify this current and run it through speakers or headphones to re-vibrate the air in exactly the same way the air originally vibrated when the sound was first made. Whew!
At least, that's how it's supposed to work. There are many ways that these vibrations can lose their accuracy during their travels. The electronic circuitry may not perfectly reproduce them, or the metal particles may not retain the exact details of the magnetic fields impressed upon them. Perhaps the user tried to push the electrical or magnetic levels higher than the equipment could handle--this results in extreme distortion. Or perhaps the signal levels were too low, making the desired recording nearly indistinguishable from the random noise present on magnetic tape.
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