Appendix Iii Signal Analysis Of Thd+N - Lavry 3000S Operation Manual

Appendix III
Appendix III: Signal Analysis of THD+N
Introduction
Lavry Engineering's latest revision to the Model 3000S Sample-Rate Data Format Converter consists of
THD+N (total harmonic distortion and noise) measurement capabilities. The new function expands the
usefulness of this "digital audio all in one tool". The new feature offers the high performance and ease of
use found in all the other features (data format conversion, sample rate conversion, Acoustic Bit
Correction (TM), reference meter bridge and test tone generation). 24 bit tone generation and THD+N
measurement accuracy in excess of 122 dB provide sufficient margin for the next generation of digital
audio equipment.
The Model 3000S's test tone generation and Signal analysis are independent of each other. This flexibility
allows performing either function separately or both simultaneously.
Distortion and noise
Real hardware generated tones contain both distortion and noise. Harmonic distortion is referred to as
energy residing at multiples of the tone frequency. Noise is all other undesirable energy.
Some common sources of distortion:
• Amplifiers: non-linear signal processing causes distortion. While mostly corrected for by use of
negative feedback, amplifiers tend to degrade at higher frequencies.
• Component imperfection: distortions occur when component values depend on the signal. For
example, capacitors tend to counteract a changing signal (dielectric absorption).
• Digital truncation: limitations of word length (number of bits) is a non-linear process. Such distortions
increase at lower signal levels.
By definition, noise is not all random (tones occurring at non harmonic frequencies are considered noise).
Well known and understood is the critical noise requirement associated with signal amplification. Often
overlooked is the accumulated noise due to connecting of many units in series.
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