Alesis ADAT-XT Digital Recording Concepts, Analog Recording Basics, Digital Recording Basics

Digital Recording Concepts: Appendix A

APPENDIX A

DIGITAL RECORDING CONCEPTS

ANALOG RECORDING BASICS

Digital recording works very differently from analog recording. With analog recording, tape containing millions of tiny magnetizable particles move past a record head. The magnetic field around this head fluctuates according to the audio signal present at the tape recorder’s input. These fluctuations permanently rearrange the particles on the tape to form a pattern that is analogous to the original audio signal.

On playback, the patterns on tape are read by a separate playback head (or from the record head, set up to read instead of record signals) that converts the magnetic fluctuations back into an audio signal.

The biggest problem with analog recording is that the tape itself alters the sound originally recorded on tape. Tape hiss is one problem; it superimposes a low-level rushing noise onto the audio signal. Although there are ways to minimize noise, such as noise reduction circuitry, this colors the sound in the process of masking the noise. Tape’s frequency response is also an issue. Tape has a hard time “absorbing” higher frequencies, which can dullen the sound. Moving tape faster allows the heads to magnetize more particles and extends the high frequency response, but the tradeoff is increased tape costs, and heftier transports to move bigger reels of tape.

DIGITAL RECORDING BASICS

With digital recording, the technology is very similar — tape moves past a record head, and plays back through a playback head. However, the signal recorded on tape is very different.

Audio signals entering each channel of the XT first pass through an analog-to-digital (A/D) converter, a device that takes 48,000 samples or “snapshots” of the signal level every second 1. Each sample is assigned a specific numeric value that corresponds to its level.

Analog Signal Digital Signal

A/D

These numbers, which represent coded audio, are then converted into an audio signal that can be recorded on tape. Fortunately, recording a number on tape doesn’t degrade the signal.

During playback, a digital-to-analog (D/A) converter reads the numbers from tape and outputs a corresponding level. This creates a “stairstep” reconstruction of the original signal, which is close to (but not exactly) the same as the original signal. To complete the process, this stairstep signal is smoothed by a low pass filter. The result is natural-sounding audio that sounds virtually unchanged from what was originally recorded.

148,000 samples are taken per second at a sampling rate of 48 kHz; 44,100 samples are taken per second at a sampling rate of 44.1 kHz.

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