NTi Audio Digilyzer DL1 - Manual (Page 48)

48
Measurement Functions
display mode: Giving a better readability the display mode
determines the rapidity of following up the input signal changes.
The available modes are:
• SLOW 3 sec. averaging
• NRM 1 sec. averaging
• FAST no averaging
If averaging is active measurements are smoothed in an exponential
way (exponential time constant) before being displayed.
Application hints:
• Whenever one sample reaches full scale, slight clipping of the
signal is possible so the THD+N value may degrade. Therefore,
try to level the signal so the full scale indication does not
appear.
• An analog to digital (A/D) converter may show the following
errors at the signal conversion:
• The imperfect linearity of the converter adds (hopefully little)
new harmonics to the signal.
• Every analog part generates noise which is added to the
signal during conversion.
• An A/D converter has only a finite resolution (e.g. 16 bit), so
the converter must round each sample value, which results
in an error called quantization noise.
A perfect test signal fed into an ideal A/D converter causes a
THD+N of the digitized signal of theoretically
-N * 6.02 dB - 1.76 dB (N ... bit resolution of the converter)
E.g. a 16 bit converter has a theoretical THD+N of -98.08 dB.
In practice good converters (even 24 bit) do not achieve better
values than -110 dB. With such measurements the input signal
is often the limiting point. To measure THD+N down to -100 dB
a generated sine wave with a THD+N better than -100 dB is
required. Such a sine wave is often generated only by expensive,
special audio analyzing equipment.