Why A Digital Signal Is Never Accurate
The signal that comes from
the output of your microphone preamplifier is pure and perfect. But as soon as
it is converted to digital, something is lost. Why does that happen?
Let's assume you have a great
performer, with a great instrument in a wonderful acoustic space. You choose the
ideal microphone and use a really good preamplifier.
You have the perfect audio
signal, right there at the output of the preamp.
That is the art you
and your performer have created.
But to record that signal, it
must be converted to digital form. And in that process, something is lost, and
noise and distortion are added.
Why is that?
The process of digital
conversion involves measuring the level of the signal, thousands of times every
So at each of these
measurement points, the signal is 'sampled', which means that its voltage is
held constant for a short time while it can be measured.
The measurement is then
'quantized'. In a CD-quality digital audio signal, there are only 65,536
allowable values. So the actual value of the signal is pushed up or pulled down
to the nearest allowable quantization interval.
This is where the inaccuracy
lies. The signal was almost certainly somewhere in between quantization
intervals, but it has to be approximated by the one that was closest.
This happens at every
measurement point - 44,100 per second in CD-quality audio. So in every second,
there are 44,100 tiny little inaccuracies in the digital signal.
Since these are mostly random,
the effect is heard as noise. But since they are not entirely random but related
to the signal, they also produce distortion. Distortion is where the shape of
the waveform is changed from what it should be and additional frequencies are
So the process of digitization
adds noise and distortion. 'Quantization noise' and 'quantization distortion' we
Fortunately there are two ways
the problem can be lessened.
The first is to add 'dither
noise' to the signal. Oddly enough, adding noise randomizes the quantization
noise and, done properly, removes the harshness.
The second is to sample at a
higher resolution - 24 bits instead of 16, for example. This makes the
Even so, no matter how fine
the resolution, detail is always lost and noise and distortion are added.
Your signal will never again
be as pure and perfect as it was at the output of the preamplifier.
Having said that, the
inaccuracies produced even in CD-quality audio are tiny, and in 24-bit audio
they are tinier still. It is worth knowing about, but most people would find it
difficult to tell the difference between the raw output of the preamp, and the
(Note - the delta-sigma type
of analog to digital converter operates in a rather different way to the
converter described. However it is still subject to similar errors in
David Mellor - Record-Producer.com 2007