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2-40
INSTALLATION
Audible noise in processed audio
Shrill, harsh sound; excessive sibilance
Verify that the source material at OPTIMOD-PC's audio inputs is clean. Heavy proc-
essing can exaggerate even slightly distorted material, pushing it over the edge into
unacceptability.
The subjective adjustments available to the user have enough range to cause audi-
ble distortion at their extreme settings. Advancing the F
inevitably cause distortion. (Distortion is very probable if gain reduction in the final
limiter frequently exceeds 8 dB.) Setting the L
cause audible distortion of some program material.
If you are using analog inputs, you must correctly match the headroom of the unit's
analog-to-digital (A/D) converter to the peak audio levels expected in your system
(using System Setup). If your peak program level exceeds the peak level you have
specified on setup, OPTIMOD-PC's A/D converter will clip and distort. See step 1 on
page 2-27.
Be sure that the output meters for the processor mixer and Direct Mixer never go
into the red. If they do, turn down all active input attenuators by an equal amount.
(See also "RFI, Hums, Clicks, or Buzzes" on page 2-37.)
Excessive compression will always exaggerate noise in the source material.
OPTIMOD-PC reduces this problem with its compressor gate, which freezes the gain
of the AGC and compressor systems whenever the input noise drops below a level
set by the G
T
(Gate Threshold) control, preventing noise below this level from
ATE
HR
being further increased.
If you are using OPTIMOD-PC's analog input, the overall noise performance of the
system is usually limited by the overload-to-noise ratio of the analog-to-digital con-
verter used by OPTIMOD-PC to digitize the input. (This ratio is better than 100dB.)
It is important to specify the A
page 2-27). You should specify the level as the highest peak level that OPTIMOD-PC
will receive under normal operation. If, in an attempt to build in a "safety factor" or
increase headroom, you specify a higher level than this, every 1dB of extra head-
room that you gain will increase OPTIMOD-PC's noise floor by 1dB.
OPTIMOD-PC's AES3 input is capable of receiving words of up to 20 bits. A 20-bit
word has a dynamic range of approximately 120dB. OPTIMOD-PC's digital input will
thus never limit the unit's noise performance even with very high amounts of com-
pression.
In digital radio applications, if an analog studio-to-transmitter link (STL) is used to
pass unprocessed audio to OPTIMOD-PC, the STL's noise level can severely limit the
overall noise performance of the system because compression in OPTIMOD-PC can
exaggerate the STL noise. For example, the overload-to-noise ratio of a typical ana-
log microwave STL may only be 70-75dB. 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 bet-
ter than any analog STL. Section 1 of this manual has a more detailed discussion.
Excessively high settings of the HF G
caused by excessively high settings of the B5 T
-M
ESS
I
R
NALOG
NPUT
EFERENCE
control can cause this problem. It can also be
AIN
HRESHOLD
ORBAN MODEL 1100
control too far will
L
INAL
IMIT
control beyond "9" will
ORE
correctly (see step 1 on
L
EVEL
(Band 5 Compression
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