Operating instructions

SPECIFICATIONS & TESTING
Arrakis Systems, inc. --- 6604 Powell st., Loveland, CO 80538 --- 970-461-0730 --- www.arrakis-systems.com
SIGNAL TO NOISE RATIO (continued) 5-2)
5.3
IMPORTANT - The IC used for the VCA (SSM2018) has a noise floor that, due to
slew rate optimization, increases with frequency above 45kHz. This increase in the
noise floor occures outside the audio frequency band and can (should be) ignored. A
broadband noise measurement would includes those frequencies above 45kHz hence
this type of measurement would be misleading. Therefor a 20kHz or 30kHz band lim-
iting filter MUST be used during SNR measurements to obtain accurate results.
Additionally:
1) A peak reading meter will measure 1.1dB better than an RMS reading
meter.
2) A 20kHz filter will read approximately 1.8dB better than a 30kHz filter.
3) Stray 60 cycle fields and RFI will effect the test.
5-3a) DEFINITION
This test measures the nonlinear response of the signal chain to a fixed frequency
sinusoidal tone. A tone is applied to an audio input. A measuring device at the output
cancels (nulls) the input tone then measures the total energy of the remaining har-
monics (harmonics are multiples of the frequency of the applied tone) and any noise
present. The ratio of the total energy of the harmonics plus noise to that of the funda-
mental (applied tone) x100 is the percent harmonic distortion +N. It is generally
accepted that a good ear can hear 0.1% THD.
5-3b) TEST
A low distortion audio signal generator is connected to a console input, balanced or
unbalanced as outlined above in the SNR description. A distortion analyzer is con-
nected to a console output, balanced or unbalanced. The distortion analyzer will null
out the fundamental frequency and read distortion in percent (%) THD+N
5-3c) CONSOLE PERFORMANCE
IMPORTANT - a distortion test is limited by the noise in the system. As an example, a
60dB S/N ratio means that the noise is down only 1000 times from the signal. This
would measure .1% THD+N of .01%. To find true THD, where noise limits the test, a
spectrum analyzer must be used.
Once again, attention must be paid to filtering and test setup since excess noise will
result in excess THD+N.
5-3) TOTAL HARMONIC DISTORTION