5-4
TROUBLESHOOTING Orban Model 8400
The 8400 has two systems that fight this problem. The compressor gate freezes the gain
of the AGC and compressor systems whenever the input noise drops below a level set by
the threshold control for the processing section in question, preventing noise below this
level from being further increased.
There are two independent compressor gate circuits in the 8400. The first af-
fects the AGC and the second affects the Multiband Compressor. Each has
its own threshold control.
In the Multiband structure, dynamic single-ended noise reduction can be used to reduce
the level of the noise below the level at which it appears at the input. Both the compressor
gate and dynamic single-ended noise reduction features are explained in Section 3 of this
manual.
If you are using the 8400's analog input, the overall noise performance of the system is
usually limited by the overload-to-noise ratio of the analog-to-digital converter used by
the 8400 to digitize the input. (This ratio is better than 105 dB.) It is important to cor-
rectly specify the
AI CLIP level in the System Setup Analog I/O screen to optimize the
noise performance available from the analog-to-digital converter. You should specify the
level as the highest peak level that will be presented to the 8400 under normal operation.
If, in an attempt to build in a “safety factor” or increase headroom, you specify a higher
level than this, every 1 dB of extra headroom that you gain will be accompanied by a 1
dB increase in the 8400's noise floor.
The 8400's AES/EBU input is capable of receiving words of up to 20 bits. A 20-bit word
has a dynamic range of approximately 120 dB. The 8400's digital input will thus never
limit the unit's noise performance even with very high amounts of compression.
If an analog studio-to-transmitter link (STL) is used to pass unprocessed audio to the
8400, the STL's noise level can severely limit the overall noise performance of the system
because compression in the 8400 can exaggerate the STL noise. For example, the over-
load-to-noise ratio of a typical analog microwave STL may only be 70-75 dB. In this
case, it is wise to use the Orban 8200ST Studio Chassis to perform the AGC function
prior to the STL transmitter and to control the STL's peak modulation. This will optimize
the signal-to-noise ratio of the entire transmission system. An uncompressed digital STL
will perform much better than any analog STL. Section 1 of this manual has a more de-
tailed discussion.
Whistle on Air, Perhaps Only in Stereo Reception
The most likely cause is oscillation in the analog input or output circuitry. If the oscilla-
tion is in the output circuitry and is between 23 and 53 kHz, it will be detected in a re-
ceiver’s stereo decoder and translated down into the audible range.
If you encounter this problem, check the analog or digital outputs with a spectrum ana-
lyzer to see if the spurious tone can be detected here. If it appears at both outputs, it is
probably an input problem. If it only appears at the analog output, then it is likely a prob-
lem with the left/right DACs or other analog circuitry. If it appears only when you use the
composite output, then it is likely a problem in the composite DACs or output amplifiers.
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