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)
5.2
5-2c) CONSOLE PERFORMANCE
SNR TEST: MIC LEVEL INPUT - most microphones have a low-Z output. These
MICs require, for proper performance, that the input impedance of the console be
matched to the MIC’s output impedance. The input impedance of the console is very
high, therefore and impedance matching resistor must be placed across the MIC input t
erminals. Place a 150 ohm METAL FILM 1% reistor across the (+) and (-) inputs of the
MIC channel. If the signal generator is UN-balanced, then connect the (+) unput to
ground with a wire jumper and feed audio into the (-) input. If you have a balanced
signal generator apply balanced (+) and (-) audio to the MIC input. Do NOT connect
the (+) terminal to ground in the balanced signal generator case. Use a millivoltmeter
to confirm that a -50dBm calibrated setting from the signal generator. This setting is
typically calibrated for a 600 ogm load, NOT a 150 ohm load. Set the channel fader to
normal the operating position ( the ‘thick black line’ or ‘in hand setting’). the input
module should alredy be calibrated for normal bus level (see “Calibration’ section of the
manual). Make certain that only one input channel is on. Adjust a ouput summing amp
rimpot to give a +8dBm (1.95 VRMS) at one of the console outputs (no load is necessary
on the output). Your SIGNAL is therefore +8dBu. Remove the signal generator leads
from the input connector (remove the ground from the (+) input if applicable). The level
that you now read at the ouput is the NOISE. Compute SNR using the formula shown
on the previous page.
SCR TEXT: LINE LEVEL INPUT - Apply a +8dBu (1.95 VRMS) tone from the signal
generator to a line level input module. The connections can be either balanced or
unbalanced as outlined above. An input terminating resistor is not necessary. Set the
channel fader to the normal operating position. The input module should be adjusted
for proper bus level (see ;Calibration” section of the manual). Adjust a output summing
amp trimpot to give a +8dBu (1.95 VRMS0 at one of the console outputs (no load
necessary on the output). Your SIGNAL is therefore +8dBu. Remove the signal generator
leads from the input connector (remove the ground from the (+) input if applicable). The
level that you now read at the output is the NOISE. Compute SNR using the formula
shown on the previous page.
The many variable factors associated with this test make it difficult to accurately com-
pare SNR specifications between equipment manufacturers. Differences in signal lev-
els, input and output impedences, band limiting filters, test setup and other factors have
substantial impact on the test results. Therefor, SNR specs obtained using different test
methods cannot be accurately compared. Furthermore, once the console is installed,
60 cycle hum (even if very, very low) can be the limiting noise factor in this test.