Chapter 15:
Test/Alignment Procedures - Audio
Input Termination Relays
Change both channels to unbalanced inputs. This should result in the audio precision
indicating a drop of approximately –6dB on both channels, to approximately +12dBu.
Restore the channels back to balanced inputs, and then change the termination to
600Ω. The reading on the audio precision should drop approximately –0.6dB to
+17.4dBu. If the results are not as expected, the termination relays or their driver
circuits are suspect. Restore the termination relays and the audio precision to the
default configuration
Output Driver Calibration
Assign both channels to test tones, 1KHz at –18dBFS. All other settings should be as
per the default values. The audio precision should indicate levels of approximately
0dBu. Adjust the appropriate output driver trim controls (CR4, CR37, CR65, or
CR98) until the audio precision reads 0dBu, ±0.005dB on both channels. Remove the
test tones and switch back to analog inputs.
Input Receiver Calibration
Configure the audio precision generator for 0dBu output levels. The audio precision
analyzer levels should indicate approximately 0dBu. Adjust the appropriate input
receiver trim controls (CR2, CR33, CR64, and CR95) until the audio precision reads
0dBu, ±0.005dB on both channels. The analog processing section is now ready for
specification testing.
Specifications
For all specification measurements, the DPS-475 headroom parameter should be set
to 24dB, and the Audio Precision output level should be set to +23dBu. All other
parameters are set to the default values.
Phase: The phase measurement may be read directly from the audio precision
display screen.
THD+N: The audio precision analyzer Function Reading section should select
THD+N Ratio. The BW limits should be set to 22Hz and 22kHz. Two
measurements should be made per channel; first with no filter selected, and
second with the A-Weighting filter selected.
Full Scale Output: Set the audio precision output level to +23.1dBu. Verify that
the audio precision analyzer levels are at least 23.1dBu, and that the THD+N
(un-weighted) measurement is less than 0.01%. If the analyzer indicates
levels are slightly low, adjust the audio precision output levels accordingly.
When the test is complete, restore the output level to +23dBu
Signal to Noise Ratio: The audio precision analyzer Function Reading section
should select Amplitude, with the A-weighting filter selected. The analyzer
dBr A and B fields should be set to the level reading of channel A and B
respectively. This value should be approximately +23dBu. The measurement
units of channel A should be set to dBr-A, and the measurement units of
channel B should be set to dBr-B. Disable the outputs of the audio precision
generator. The signal to Noise Ratio of each channel is read as the
Amplitude measurement. The measurement is a negative value, and should
be interpreted as a positive value for the purposes of this test. When
completed, restore the Audio Precision outputs.
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