Why Digital?

Why Use Digital?

by Walt Fair, Jr., W5ALT

One very valid question might be asked: Why do we need digital communications, when analog methods have served us well for a long, long time? The purpose of this discussion is to show that there are valid reasons to apply digital techniques, but in order to gain real benefits, we need to understand when and why they work well, and also when and why they suffer from limitations.

Introduction

First, we need to establish just what makes a mode digital. For purposes of this discussion, I will define a "digital mode" as a modulation scheme that uses discreet signal characteristics, while an "analog mode" is a modulation scheme that uses continuously varying signal characteristics. Note that the signal characteristics might be amplitude, frequency, phase, etc.

Now, according to those definitions, it is readily apparent that CW is a digital communications technique, since all we need to know to read a Morse code signal is whether or not there is a signal and whether the signal is long or short or absent. In Morse code, no other signal characteristics have any meaning whatsoever. By the same token, most voice modes, like AM, FM, and SSB, are analog modes, since the frequency or amplitude can vary continuously and might be any particular value at any particular time.

If you ask most hams what's the easiest mode to use, a large group will likely say SSB or FM voice, since all one has to do is talk and listen and we are used to talking and listening from our earliest childhood. But, you will also find a staunch group of hams who will steadfastly say that the simplest mode is CW, which happens to be the oldest mode. Why is that?

So which mode is the simplest? In terms of equipment, there is no doubt that CW is simpler in terms of equipment complexity, bandwidth requirements and power utilization in the presence of noise and fading. In terms of effort, there is likewise no doubt that the voice modes are simpler, since little or no training is needed to use them, but to use CW some effort to learn Morse code is required. We are, with few exceptions, all able to talk at an early age, but no one has ever been born knowing CW!

So what's the answer to the question of what is the simplest mode? The ony correct answer that I can come up with is "It depends." It depends on whether you mean simplest in terms of effort or in terms of equipment. Clearly voice modes have an advantage in some areas, while CW has an advantage in others.

What Makes Digital Equipment Simpler?

In designing and constructing digital equipment, it is only neccesary that the equipment function correctly at the signal levels of importance. For example, it is not of any particular importance whether a CW receiver attenuates high or low audio frequencies, since those do not enter into the ability to interpret CW. In generating a RTTY signal, it is not important how the transmitter operates between the mark and space frequencies, since it should never operate there anyway.

In the design of equipment, one important consideration is in the linearity of circuits, but with digital techniques, linearity takes a back seat to other considerations, since it is not neccesary to maintain strict linearity - only to ensure that the equipment generates or responds correctly to the discreet states needed. Analog systems, however, do not have discreet states, but must generate and respond correctly to every signal level that might be encountered, because by definition, they will be encountered.

The discretization of signal characteristics, therefore, allows a relaxing of some (not all!) circuit and design considerations, especially when active devices (diodes, transistors, IC's, etc.) can easily be driven to completely open or completely cut-off states that represent a binary code. And as is shown in mathematics, any other discreet code can be represented in terms of a digital binary code.

What Makes Digital More Efficient?

In terms of communicating, sometimes digital seems to work better. Many hams have documented times when they were not able to continue a SSB QSO due to poor band conditions, so they switched to CW and found the signals 100% readable. There are also quite a few testimonials to having completed QSO's using PSK31 where the signals were not even audible to the ear, yet the information came through just fine.

If we analyze why digital modes sometimes seem better in terms of noise, fading and bandwidth problems, the answer is that they are inherently simpler from a technical and operational standpoint. To receive a CW signal, we just have to decide whether there is a signal or not and if so, was it long or short duration. Nothing else matters. With RTTY we must make a choice at discreet times as to whether the signal was a mark or a space, again, nothing else matters. With PSK31, at every signal transition time, which we know in advance, we just need to decide whether the signal phase changed by 90o or not, nothing else matters. But when we listen to a voice, we have to decide on a much more complicated set of criteria. Did he say "B" or "P"? Was that "VK" or "BK"?

Have you ever noticed that most digital signals sound pretty much the same? Whether it's from Australia or China, Alabama or the Bronx, a CW "beep" is a beep and a PSK31 signal still warbles the same. And when band conditions get bad on the voice modes, most operators resort to using phonetics to spell out information. Why? Because we reduce the choices that must be made from thousands of words to only having to select 1 letter out of 26, which is a much easier task.

That appears to me to be the real reason that digital modes at times appear to work better. With discreet signal characteristics, at every step we (or our software) only has to choose 1 option out of a limited set of possibilities. And that is a much easier task than deciding how to interpret an analog signal that might represent many things.

Is Digital Perfect?

Ha!! Not by a long ways!

Some of the reasons digital modes may be better also show its weaknesses. As in most of life, there's always a trade off.

One potential problem with digital is its "energy density". Digital signals are often squeezed into relatively small bandwidths. That's good, since it conserves spectrum and increases the signal-to-noise ratio. But it also means that digital modes place special requirements on equipment. While transmitting, most digital modes are 100% duty cycle, compared to around 50% for CW and 20 - 30% for SSB. Since most ham gear was designed for CW or SSB, that means it is neccesary to cut the power back to about 1/2 or so of normal to keep from frying the transmitter finals.

In addition, most digital modes these days are computer generated and controlled and computers can do things fast - much faster than a human operator. For modes like packet radio which requires a handshake signal between the stations, that means the transceiver must rapidly switch between transmit and receive mode. That of course slows down the communications due to the switching, but it also places a heavier than normal load on the relays or other circuits used as a T/R switch.

Then we get to the non-technical considerations.

Many hams I've talked with mention that they like voice modes because the other modes seem too impersonal. And to some extent I'd have to agree. There are just too many subsidiary cues in voice communications that convey additional information - often referred to as "personality" or "feelings." You have to admit, if I say "You are an S. O. B." with a grin on my face and a light-hearted voice inflection, most people would laugh. But if I type an email message and say the same thing, most people would automatically assume I was being a nasty old fellow and get upset.

There's little doubt that the voice modes more easily convey human emotions and serve a social need for conversation. But many times we as hams really just need to express ourselves better and make sure the information is exchanged. That explains why the so-called "emoticons" seem so important in written internet exchanges - they serve to express a little bit of the subtle feelings that get lost in the black and white world of written communications.

The truth is that there is likely no perfect mode, either digital or analog. But each mode has its advantages and its disadvantages. Each mode serves well for some applications, but not so well for others. As in all things there's a trade off.

So I would submit that for casual chit-chat we hams should use whatever mode we want to. But, when it is important to exchange real information accurately, there is a very good chance that digital techniques might work better than analog.

If we agree that that is true, then it seems apparent that both analog and digital techniques will likely be used for a long time to come. But it is important that we as ham experimenters keep in mind when the digital modes show an advantage and when they do not.

Of course it's easy to over-generalize, so please take these comments in the spirit they are meant. Just keep in mind that digital and analog modes all have their place in amateur radio. It is our challenge to figure out when each works best and then apply that to further amateur radio.

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