Digital Recording Does NOT Chop Up Your Music
By Bruce Bartlett
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.
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.
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 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.
Bruce Bartlett is a microphone engineer, audio journalist, and recording engineer. A member of the Audio Engineering Society and Syn Aud Con, he holds a degree in physics and several patents on microphone design. He is also a musician and runs a 16-track digital studio specializing in live recording.He has written over 600 articles on audio topics for such magazines as Modern Recording, db, Recording, EQ, Mix, Recording Engineer/Producer, Radio World, Pro Audio Review, Audio, and the Journal of the Audio Engineering Society. He has also written a number of books including Stereo Microphone Techniques, Recording Music on Location, and the best-selling Practial Recording Techniques, all published by Focal Press. The new sixth edition of Practical Recording Techniques has just published!