OK, here we go! Based upon the information you learned in articles one and two of this series, let’s “see” exactly how analog modulation is converted to the digital world. Simply put, it’s accomplished by the use of a device called an Analog to Digital Converter also called an ADC in the electronics world. But what exactly is an analog to digital converter, and how does it work?
Let’s begin with a simple sine wave, which is a single audio tone example of analog modulation. The ADC “samples” (or measures) the voltage of the sine wave a multitude of times during each complete cycle. This is referred to as the sampling rate. The accuracy of the measurement of the voltage is referred to as the sampling precision. The difference between the sampling voltage of the analog signal and the actual voltage is referred to as the sampling error. In review, you have two measurements taking place as you convert analog to digital, namely the number of samples taken each second and the precision or gradation of each sample. Don’t be concerned if you are confused at this point. Re-read the sentences above, and if that doesn’t work for you, a minute or so at the chalkboard during a break at the next club meeting will bring this concept into sharp focus. If you miss the next club meeting, let’s talk about it on the air!
Let’s continue. If, for example, the sampling rate is 40 Hz, that simply means that 40 points of the sine wave will be sampled every second. You can now understand why higher sampling rates will generally produce a more accurate representation of analog modulation than lesser ones. In other words, when we convert analog to digital, not all of the analog modulation is transferred to digital. In a typical analog to digital converter, no matter how high the sampling rate or frequency, the higher the sampling frequency the more accurate the conversion will be. The “space” between sampling points will always be lost. In a manner similar to motion picture film where 24 frames per second (30 fps in video) of new information is projected (each frame being duplicated or shown twice), the same principle applies to sound. In other words, the eye can’t perceive the loss of the missing information and “sees” the projected motion picture as continuous. Likewise, the ear can’t detect the missing information and thus perceives the digital representation as uninterrupted audio. In the early days of CD’s, some violinists could actually detect the difference between digital audio CD’s and conventional phonograph records playing their music because they could hear the “stutter” caused by the space between samples of high frequency audio in the digital medium. This “effect” exists to this day!
So in summary, in order to convert analog to digital, we need an Analog to Digital converter that operates fast and has the ability to have sufficient detail (bits)) in order to complete the task accurately. In the next part of this series, we’ll discuss not only how this is accomplished but how digital information is sent out over the airwaves. Again, stay tuned!
by Rick Fearns
K6VE
