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Appendix III
Common sources of random noise:
• Resistor noise (flat frequency distribution) increases for larger value of resistance.
• Semiconductor noise, mostly flat frequency distribution. Increased noise levels at very low frequencies
usually occurs below audible frequencies.
• Capacitor noise, inconsequential for higher values, has become a performance limiting factor with the
introduction of very small capacitor values incorporated in modern semiconductors such as
oversampling sigma delta converters.
• Digital truncation: limitations of word length (number of bits) in A/D converters, signal processors and
more. The problem grows with increased amount of processing.
Common sources of non-random noise:
• Coupling to analog signal path: AC power line, RFI/EMI, coupling of digital signals to analog path,
inadequate power supply rails and more.
• Digital truncation: limitations of word length (number of bits) in A/D converters, signal processors and
more. The problem increases for low level signals.
• Limit cycles: cyclical patterns behavior in feedback based digital signal processing (such as sigma
delta converters and IIR filter structures).
Listening tests and measurement of individual equipment in the audio chain does not guarantee optimum
THD+N performance. Setup optimization is a very complex subject. Top recording engineers blend artistic
considerations and engineering know-how into the process. The following discussion does not deal with
artistic aspects. We acknowledge the great importance of artistry in music production, but are bound to
limit our discussion to measurable and objective phenomena.
Optimizing THD+N (some engineering considerations):
Analog amplification: good signal to noise ratio requires "early" signal amplification, but with careful
attention to tradeoffs between distortions and noise.
Analog attenuation: undesirable from noise standpoint, may be required to accommodate signal range
limitations of various gear.
Jitter: clock jitter in A/D, D/A and Sample Rate Converters degrades THD+N. Reference D/A clock jitter
in a studio may have little to do with the end product quality, but may make the monitoring process difficult.
Sample rate converters perform best with low jitter on both incoming and outgoing clocks.
Configuring proper digital chain: whenever unit A may drive unit B or visa versa. A good "rule of thumb"
is to have the better performer drive the lower performer. A quality digital device utilizing 24 bit words is
limited to 16 bits when driven by a 16 bit device. The compounded outcome is that of "2 x 16 bit devices".
Reversing the order allows the first process to retain its high accuracy, leaving a compounded outcome of
one 16 bit device.
Measuring THD+N
The common method for measuring THD+N is based on feeding a "device under test" with a quality
reference test tone and measuring the undesirable energy (THD+N) at its output. Lavry Engineering's
Model 3000S provides the user with a reference test tone. The processed tone (or any other source) may
be fed back to Model 3000S input. The input signal is filtered by a very sharp notch to separate the
desired signal component from the undesirable energy (THD+N). The undesirable energy (THD+N) is then
displayed in dB (referenced to full scale).
LE3000S
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