Thursday 2 May 2013

DSD and Analog

There is a lot of buzz about DSD these days, and how it sounds more like Analog than PCM does.  Well that’s just opinion of course, but a lot of people seem to be lining up behind that idea.  So here’s a tidbit just to get you thinking...

What exactly is Analog anyway?  Lets look at an analog electrical waveform.  When we transmit an electrical signal from one place to another (such as from an amplifier to a loudspeaker, or from one place on a circuit board to another), that transmission takes place in the form of electrical current flow.  The variations in the magnitude of the current flow represent the information that is being transmitted.  When we play music through an electronic device, the variations in current flow, backwards and forwards, represent exactly the variations in air pressure, up and down, that we perceive as sound.  In a loudspeaker these variations in current are converted into corresponding variations in air pressure, and thus we get to hear the music.

Electrical current flow is carried by fundamental particles called electrons.  These, together with protons and neutrons, are the building blocks that together comprise atoms.  Electrons carry an “electronic charge”.  Just like balloons do when you rub them against a nylon sweater.  But unlike the balloon, an electron’s electric charge is built right into it.  Electrons cannot acquire or lose their electric charge, they always have the same amount of charge and they keep it permanently.  So when electrons flow along a metal wire, for example, their electric charge is also flowing along the wire.  It is this flow of electric charge that we define as an Electric Current.  The more charge that flows along the wire, the bigger the current.

Now, it is a very interesting property of electrons that the amount of electric charge that each one carries is a fixed and very precise amount.  There is absolutely no variation whatsoever – not even the smallest imaginable amount – in the magnitude of the charge on an electron.  Every single one is absolutely identical.  Electrical charge can only exists in amounts which are whole-number multiples of this fundamental quantity.  This has an interesting consequence on current flow.  An electrical current measured in Amperes is, to all intent and purpose, a measure of the number of electrons flowing through the wire in question.  And that number is a whole number – there is no such thing a “part of” an electron.  Either the whole electron has flowed through the wire or none of it has.  Current flow therefore has a fundamental “granularity” to it.

Can we detect this granularity?  Yes we can, but in ordinary electronic circuits the answer is no.  The number of electrons that flow down a wire per second when it is carrying a current of one Ampere is so big that, if you wrote it down, it would have eighteen digits.  Try writing down an 18-digit number and see if you can make sense of it!  (During the Iraqi war, Condoleeza Rice enters the Oval Office and informs George Bush that two Brazilian soldiers were killed in Baghdad that morning.  President Bush instantly turns an ashen shade of grey and slumps into his office chair, holding his head between his hands.  Finally he looks up through red eyes and asks, “Just how many, exactly, is a bazillion?...”).

But the concept does have some relevance to the line of thought that caused me to write this post.  Because any electrical current flow comprises a discrete flow of electrons, I can represent that current flow by recording when each and every electron arrives at the detection point.  Since an electron has either arrived or it hasn’t, I can indicate this by writing “1” every time an electron arrives, and “0” every time an electron doesn’t arrive!  If the current flow is large, I will be writing more 1’s than 0’s and vice-versa if the current flow is small.

Strange thing, but isn’t that exactly how DSD works?...