[LAD] A little quiz about audio measurements...

Niels Mayer nielsmayer at gmail.com
Fri May 28 15:30:54 UTC 2010


This experiment was done in Europe with 50Hz AC line current.
When rectified by a non-switching power supply, that ends up being 100hz "hum".
Alternately, it's 50 hz ground loop noise that is being rectified or
has cross-over distortion resulting in 100Hz noise.
This "rectification" could take place through a number of unexpected
means, including oxidized contacts that act as semiconductors (
http://en.wikipedia.org/wiki/Rusty_bolt_effect ) , low-quality
electrolytic capacitors being used for Ac-coupling, and output
transformer hysteresis. (
http://en.wikipedia.org/wiki/Intermodulation#Passive_intermodulation
)

The 100hz signal due to it's regularity, could also be caused by
"periodic jitter"... i think more info is needed to determine whether
the situation you're seeing is caused by jitter or modulation by
external hum (like testing at a bunch of difft freqs and mixing two
sine waves to see the IMD behavior). Perhaps this periodic jitter is
the effect of AC line hum interfering with the sampling clock??

http://wolfsonmicro.com/uploads/documents/en/Specifying%20Jitter%20Performance.pdf
....
Audio ADCs and DACs have three important inputs;
the signal input, the voltage reference, and the clock.
Noise and interference on the voltage reference causes
amplitude modulation, and jitter on the clock causes
phase modulation. The resulting modulation products
look very similar in the frequency domain. One of the
authors once spent several days trying to track down a
low-frequency jitter problem, only to find that it was
in fact a problem of LF noise on the voltage reference.
....

Niels
http://nielsmayer.com



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