analog (idea)

Nearly, but not quite.

In terms of signal processing, an analog signal is one which can take any value within a range. It contrasts with a digital signal, which can only take predetermined values, usually equally-spaced within the same range.

In the extreme case, a digital signal can only be on or off (zero or one) (true or false). In less extreme cases, it might be allowed to take one of any of 256 (or 65536, or 97, or any other number) different levels, but, crucially, nothing in between. Any such digital system can be reduced to the extreme case of On or Off, simply by describing each level in terms of its binary value.

As a digital signal moves from the lowest value to the uppermost, it steps though all 256 (or whatever) different levels, remaining at each discrete level for a finite time.

The analog signal, by contrast, can swing continuously from the minimum value to the maximum.

Digital is not necessarily better than analog (or the other way around), it's just that the science of digital signal processing has developed very quickly over the last couple of decades, and modern computers are based on digital systems, so they can handle digital data more easily than analog data.

I have a digital TV, that's better than my old analog one, Right?

Err, well, kind of. In fact the television, whether the label says digital or not, is a combination of analog and digital systems built into a single box. The digital bit of that phrase 'digital TV' usually refers to the broadcast signal, which carries digital data in an encoded form from the TV mast to your home. Your 'digital' TV is capable of detecting and de-coding that signal. All the rest of the electronics inside the TV is just the same as a conventional TV system, in which some circuits are analog and a few are digital. Most are still analog.

In the old days, all TV signals were broadcast as analog signals carried on UHF carrier waves. Modern technology allows them to encode the digital data on a similar carrier wave, but with the digital data, you can carry more channels in the same bandwidth. Broadcasters want to use digital systems to send their programmes over the air because you can offer people a lot more channels that way.

Oh. How about my radio? That's a digital PLL system. Much better than those old analog radios!

Hmmm, fascinating. PLL means phase-locked loop. It is not really a digital system. I know the buttons and fancy numerical display make it look like one, but it's still really an analog system hiding behind a digital façade. All it means is that the radio holds the broadcast frequency better than older models. Those relied on capacitors and coils to set a resonant frequency. If those components heated up, that would change the resonant frequency, and the radio would drift off the station you were trying to listen to. The PLL stops that happening. Again, all the circuitry inside the radio is still analog.

Hmmph. But my digital watch is far more accurate than my old wind-up one.

Now that's a true digital device. Nothing analog in a quartz watch at all! But the accuracy is not because of the digital/analog thing, it's down to the frequency of vibration of the gizmo driving the watch. Even a Swiss-made analog chronometer depends on its escape wheel as the basis of its accuracy. That wheel swings back and forth maybe a few times a second. A quartz watch, by contrast, relies on a quartz crystal vibrating millions of times a second. It's a lot easier to make an accurate timekeeper from a very fast vibration than from a slower one. That's all.

So why do people go on about digital this and digital that as if it's so much better?

Good question! It's basically all about computers and how they deal with information and data. In the old days of electronics (back in the 50s and 60s, and even up to the 80s) everything was analog. Circuit designers used their capacitors and resistors and inductors to make circuits which modelled reality. Change an input here, and watch the output change. If the designers had done a good job, then an input corresponding to (say) the rainfall on a mountain resulted in an output equivalent to the rise in height of a river.

It was a slow, laborious process, and people needed great skill to design good circuits. And even the best designers could only manage simple models.

Move on to the 21st century, and we have powerful processors which are capable of many millions of calculations per second, but they can only handle digital-type data. If we want to model something, it is much easier to build a model (using software) in our digital computer, and let it do the analysis for us (think about FEA or computational fluid dynamics). As the computers get more powerful, we can divide the original analog data into smaller and smaller digital fragments, until it is all-but indistinguishable from the original analog signal.

Another reason is that there has been a great deal of analytical work on digital signal processing, which allows people to compress more and more real information into less and less data. So instead of sending every single piece of information about a TV picture down the airwaves (as an analog signal does), a digital signal can use fancy tricks (often called data compression) to reduce the amount of data needed to completely describe the full picture. This (and other factors) means you can send more channels down the same piece of wire, or over the same frequency space.

Perhaps the most well-known difference between analog and digital is that a digital signal does not degrade as it is copied and re-copied. Compare a photocopier (analog device) with a computer file (digital format). If the computer file is copied many times over, it still remains a perfect copy of the original. We all know what happens to a photocopy of a photocopy, of a photocopy.

Combine these three factors: data compression, perfect reproduction and fast and convenient processing and we have the reasons that signal engineers like to deal with digital signals rather than analog ones.

OK, but I still have some mint-condition vinyl LPs and I play them on my Linn Sondek, sending the output through a valve amplifier to my electrostatic speakers. And why does my Strad sound better than the Yamaha electronic synthesiser?

Ahh. The joys of analog signal processing. Richer, fuller sounds, none of that harshness, and all the harmonics fully intact.

To get the sound of a Piano or a Strad into that electronic music box, it has to be sampled. That means converting the sound into an electronic signal with a microphone (analog), and then sending the electronic signal to an Analog to Digital Converter (ADC). The ADC chops up the sound into tiny pieces in terms of time and frequency. To play the sound back, all those pieces are re-assembled, and then sent to an (analog) loudspeaker. No matter how finely you chop the sound up, something always gets lost, and if the ADC misses a fast change, nothing on earth can put it back in. Secondly, in the playback, there are always transitions from one level to the next, and analog systems don't respond well to those transitions. They introduce harshness and extra harmonics. That makes digitised sounds harsher and less flowing than fully analog sounds.

A digital signal can only approximate to the full analog signal, no matter how finely you chop it up into bits and bytes. Just as a live concert sounds better than a recording, the purists will always want to hear a recording which uses only high quality analog systems throughout the record/store/playback chain. Because by digitising the signal, you are automatically losing some of those subtle effects.

Each Strad is a unique instrument, made by a craftsman using wood from that part of this tree. Each instrument gives a slightly different sound, depending on the thickness of the wood, the amount of glue and many other factors (not to mention who is playing the instrument). Digital reproduction is not yet good enough or fast enough to truly copy all these nuances. At this point, the signal processing engineer has to start talking Fourier analysis, transients, transfer functions and lots of other techy stuff, but the point is that we still don't really understand how our ears and brains interact to give us the sensation of sound, and because we don't really understand it, we cannot design digital systems to properly reproduce those sounds. For the best listening experience, analog will remain the best for some time to come.