This article is based upon three previous articles in which we learned that in order to convert analog to digital, we need to use an analog to digital converter (ADC). An “Analog to Digital Converter” is nothing more than a chip (Integrated Circuit) that has the ability to measure analog audio voltages along many points of the wave and assign a numerical value to each sample. The more samples and the greater the precision of each sample, the more accurately the analog signal will be represented in the digital world.
Now, here comes the best part and the one that will give you the greatest insight as to how digital works! The sample of the analog wave that’s taken by the analog to digital converter is expressed as a numerical value in binary mathematics rather than in the decimal system that we are familiar with. Unlike the system of counting that you’re used to in conventional mathematics where we use ten numerals (0-1-2-3-4-5-6-7-8-9), we use only two numerals (namely 0 and 1) in binary. This is because in electronics the easiest and most basic circuit that we have is a switch. A simple switch is either on or off. Binary is the simplest system in mathematics because it uses only two digits, namely a zero or a one. If we assign a zero(0) in binary as “off” and a one(1) as “on”, all we need to do is transfer this serial on/off information to an analog carrier, and we’ll have succeeded in placing a digital signal on an analog carrier. Any number in decimal mathematics (the numbering system that we all know and love) can be expressed in binary. As an example, the number 9 in decimal is 00001001 in binary. for purposes of this article, namely understanding how digital works, we need go no further in the math department. All we need to know is that if we can place “00001001” on an analog carrier, we will have taken a giant step in attempting to send the value “9” from one place to another. So, how do we do that?
There are many ways to place digital information (a succession of ones and zeros) on to an analog carrier. One common way is to convert the succession of ones and zeros to two tones, one of which represents a zero and the other, a one. When the tones are placed on an analog carrier, the form of analog modulation that results is called Audio Frequency Shift keying (AFSK). Packet, which is one of the basic modes of digital communications in amateur radio, uses AFSK and a TNC (terminal node controller) which is the device that converts the ones and zeros from the packet terminal to audio tones for your transmitter. The tones are sent by means of a cable that connects the TNC to the microphone or line input of your transceiver. Another method of transmitting digital information is to build a device that shifts the analog carrier frequency of your transmitter slightly in one direction or the other, according to whether a zero or a one is being sent. Such a modulation scheme is called Frequency Shift Keying (FSK.) There are other methods of modulating an analog carrier with digital information, which explains why there are so many kinds of digital schemes in amateur radio. Basically they all work in similar fashion, varying only in the way the software involved handles the information.
In our next article, we’ll wrap up this series (it’s about time!) by explaining how digital signals are received, and in turn converted back to analog. In addition, we’ll talk in more detail about the various digital forms of communications that are used in amateur radio. I would be remiss if I didn’t include some personal editorial comments about the advantages as well as the disadvantages of digital communications, so I’ll do that too. Stay tuned!
Rick Fearns, K6VE
