Analog to Digital: What's the scoop?

by Reggie Huff

Ever since we drifted through the revolution of vinyl records to audio CDs, we have come to accept the astounding notion that the music of the Rolling Stones is somehow, magically, converted into a string of 1s and 0s. Those 1s and 0s are then converted through some kind of alchemy within our CD player back into magnificent sounding stereophonic sound. For most of us, this process of converting the analog music into a digital (1s and 0s ) format and then back again is one of the bewildering mysteries of contemporary life.

Unraveling the mystery
Let's start with the distinctions of Analog & Digital. In the simplest explanation, Digital means that there are only two stable conditions in which something can be maintained. The typical wall switch has an ON (usually up) position and an OFF (usually down) position which allows us to turn a light on or off. This is a digital lighting control system.

The Analog counterpart would be a lighting dimmer. With this analog lighting control system, the brightness can be set to any of a wide range of levels and left there indefinitely.

Using the standard wall switch (digital), the light would always be in either the 0 level of brightness (OFF) or the 1 level of brightness (ON). If we were to graduate the light brightness into say, 10 distinct brightness levels, then with the analog dimmer we could set the light to any of 10 separate brightness levels. Similarly, we could break the brightness range into 100 distinct brightness levels and then carefully turn the dimmer to set the light at 1%, 2%, 73%, etc. The number of distinct levels we break the range into is called Resolution.

The world's a stage
Let's say this lighting dimmer happens to control the lights on a stage in a theatre where some kind of play is being presented. Let's say also that the play is just about to open, and the Lighting Master has developed the lighting sequence by paying keen attention to what's happening on stage. She controls the lights brilliantly, providing ideal lighting control.

Using a copy of the play script, we could create a procedure for the lighting control for others to run by noting the lighting level (1%, 5%, 47%, etc.). These levels could be set at each minute throughout the play. For simplicity's sake, we might just drop the "%" and call the brightness levels 1, 5, 47 and so on. We could mark the dimmer switch accordingly and thereby make it easy for the Lighting Apprentice to follow the lighting scheme created by the Lighting Master.

If the script called for 50, then 52, then 54, then 56 with each successive line in the timeline the effect would be a gradual increase through the period. If the script called for 25, 25, 25, 25 and then 75, the effect would be a striking shift in brightness which would occur nearly instantaneously.

The process of capturing the lighting control sequence by noting what the lighting level is at each point in time and recording that light level as an integer from zero to 100 might be called Analog to Decimal conversion. To get to Analog to Digital (A/D) we merely need to convert each of the levels recorded...

from its decimal for.......into its digital equivalent
1 .............................................0000001
5 .............................................0000101
47 ............................................0101111
etc. ..........................................etc.

With all of its nuances, the lighting brightness, as it changes throughout the play, would actually have been digitized-or stored in the form of 1s and 0s.

Play lighting controller
If we set out to make an automated lighting controller that would adjust the lighting levels correctly throughout the play, we would begin by storing each of these digital lighting-level values into successive memory locations.

Next, we would create a clock, like a metronome, that would tick at the rate of successive lines in the script/timeline. With each clock tick, we would then get a value recorded in the corresponding memory location and turn the light dimmer dial to the value recorded there. In this way, we would reconstruct the original analog lighting sequence for the audience to view by reading back the digitized version of it as it was recorded from the original Lighting Master performance. This is what is called the Digital to Analog (D/A) conversion process.

Recording the Stones
It hopefully becomes clear through this simple but improbable analogy that the music of the Rolling Stones is analog in its original format. We capture the sound as it changes in time by:
• converting the sound into an electrical signal.
• taking samples of that signal's voltage magnitude tens of thousands of times each second.
• storing the captured values as their digital values into subsequent memory locations on a CD.

In addition:
• Our CD player has a clock ticking at the same rate as the clock used for sampling the original music.
• The voltage levels are reconstructed, just as the lighting level was, by referring to the value stored in the memory site.
• Special "smoothing" circuits help round out the edges to deliver the music we're after.

When we take a digital sample, one thing that happens is that there are narrow "peaks" found in the original analog signal that do not appear in the restored signal. These signal elements happened between the clock ticks of the digitizing process, and so they were lost.

We can ensure that these peaks are represented in the digital sample by increasing the digitizing sample rate. The trade-off is that when we increase the sample rate we need much more memory in which to store the sampled values. On the other hand, at the cost of more memory, we can provide an ever more precise reconstruction of the original signal.

"Analog to Digital: What's the Scoop?" first appeared on EBNONLINE.COM on November 22, 1999.

© Copyright 1999, 2000, 2001 Huff Communications. All Rights Reserved.