Many people have the misconception that digital recording breaks up an audio signal into little slices, so that some of the signal is missing. Nope — all of the analog signal is captured and reproduced. Here’s what actually happens in the most common digital recording method called Pulse Code Modulation or PCM:

1. The signal from your mixer (Figure 1-A) is a varying voltage. This signal is run through a lowpass filter (anti-aliasing filter) which removes all frequencies above 22 kHz (if the sampling rate is 44.1 kHz).

2. Next, the filtered signal passes through an analog-to-digital (A/D) converter. This converter measures the changing voltage of the signal several thousand times a second (Figure 1-B).

3. Each time the waveform is measured, a binary number (made of 1’s and 0’s) is generated. This number is the voltage of the signal at the instant it is measured (Figure 1-C). Each 1 and 0 is called a bit, which stands for binary digit.

4. Those binary numbers are stored on the recording medium (Figure 1-D). The numbers can be stored on tape, hard disk, compact disc, or flash-memory card.


Figure 1. Analog-to-digital conversion (during recording)

The playback process is the reverse:

1. The binary numbers are read from the recording medium (Figure 2-A)

2. The digital-to-analog (D/A) converter translates the numbers back into an analog signal made of voltage steps (Figure 2-B).

3. An anti-imaging filter (lowpass filter) smooths the steps in the analog signal (Figure 2-C), and the smoothed signal leaves the D/A converter. The original signal’s waveform is reproduced.


Figure 2. Digital-to-analog conversion (during playback)

The curve or shape of the analog waveform between samples is re-created by the anti-imaging filter. Nothing is lost. With recordings made on a CD, the process does filter out signals above 22 kHz, but we can’t hear that high anyway.

The harshness of some early digital recorders was not caused by slicing and dice-ing the signal. It was due in part to excessive phase shift of the anti-aliasing and anti-imaging filters. Those filters have been much improved, so current digital audio is generally much smoother and more like analog.

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A member of the Audio Engineering Society, Bruce Bartlett is a microphone engineer (www.bartlettaudio.com), recording engineer, and audio journalist. His latest books are “Practical Recording Techniques 6th Ed.” and “Recording Music On Location.”

Originally posted 2010-11-10 14:34:34.