This is the first part of an occasional series on how you quantify audio quality. It's about much more than distortion!
Proving things is very difficult, not least because it's really hard to agree sometimes about what constitutes proof. This is especially so when you have to get people involved and even more so when the subject is the quality of something as subjective as audio. There is simply no such thing as an 'Audio Quality Meter'. You might think that there was, but it would raise the question "what exactly is it that we're measuring?"
Some would say that the property to be measured would be distortion, which itself would be defined as the amount by which the recorded sound differs from the original.
There are problems with this even before we start. How do we define the original? Do we mean the studio master tape? Do we mean the signal from the microphone? (Difficult when you're recording an orchestra or in the case of a multi-track studio session).
Part of the problem is that neither microphones nor test equipment actually hear anything. How can they? You need a brain and some sort of (probably sentient) being to do that. Hearing implies perception and perception is far from perfect, as anyone who has ever bumped into a lamppost on a dark night will attest.
This means, inevitably, that quality is intertwined with people and not scientific instruments.
It would also be reasonable to say that the only perfect sound is the one present in the air before it is recorded, meaning you can never record a perfect sound. I'm not sure whether this is an example of Heisenberg's Uncertainty Principle, which states that you can't measure a quantum phenomenon without changing it – probably not really. But just as you can't measure tyre pressure without letting out a small amount of air (thereby reducing the pressure), you can't record – or measure – sound without affecting it. The simple fact that any microphone diaphragm has mass and inertia (and that any interpretation of the movement of that diaphragm will have non-linear elements to it) means that all you can ever record is a degraded sound.
Even without recording and measuring equipment, where are you supposed to stand? Two listeners standing in different places will hear different audio. Are we supposed, therefore, to say that the sound of an audible event is in some sense the totality of listeners' perceptions at all possible listening positions? Maybe we should!
Finally, just to complicate matters, even a perfect recording (one where there is zero distortion) wouldn't necessarily be chosen as the best sounding one. You might find that any given audio reproduction system has a pleasing colour to it. You might even find that playing the audio through a valve (tube) amplifier the audio just "makes it sound better."
So where do we go from here? In the next part of this series, we'll be looking at how you can set up a valid test for the quality of audio or for some part of the audio reproduction chain.