Orban OPTIMOD-FM 8600, Operating Manual

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1-20
INTRODUCTION ORBAN MODEL 8600
ue to use the external AGC to protect the STL when you install the 8600 at the
transmitter.
If you are keeping your analog OPTIMOD-FM as a standby processor, you will proba-
bly want to use the external AGC to drive both the 8600 and the 8100A1 (also called
8100A/1) transmitter chassis in parallel. This is usually practical. However, complica-
tions will occur if you are not using an Orban 8100AXT2 (also called 8100A/XT2) Six-
Band Limiter Accessory with your 8100A1, because, to correctly drive an 8600, the
external AGC must be strapped as if it were driving an 8100A1 (or 8100A/1) +
8100AXT2 (or 8100A/XT2) system. Therefore, if you have only an 8100A1 (or
8100A/1), you will have to re-strap the external AGC for operation
without the XT2
before you can put the standby 8100A1 (or 8100A/1) on the air.
STL and Exciter Overshoot
Earlier in this section, we discussed at length what is required to prevent STLs from
overshooting. There are similar requirements for FM exciters. Nevertheless, in some
installations some overshoot is inevitable. If this is a problem in your installation, the
8600’s remote control feature offers the means to reduce the peak level of the
8600’s audio output as necessary. This way, you can still use the 8600’s line-up tone
to adjust the steady-state deviation to
±
75 kHz. Yet, the reduced peak level of the
audio emitted from the 8600 ensures that the carrier deviates no further than
±
75
kHz after overshoot. This overshoot reduction can be selected on the
I
NPUT
/O
UTPUT
screen and the remote operation can be selected in
S
YSTEM
S
ETUP
> N
ETWORK
/
R
EMOTE
. See step (8.D) on page 2-22.
Using Lossy Data Reduction in the Studio
Many stations are now using lossy data reduction algorithms like MPEG-1 Layer 2 or
Dolby AC2 to increase the storage time of digital playback media. In addition, source
material is often supplied through a lossy data reduction algorithm, whether from
satellite or over landlines. Sometimes, several encode / decode cycles will be cascad-
ed before the material is finally presented to OPTIMOD-FM’s input.
All such algorithms operate by increasing the quantization noise in discrete frequen-
cy bands. If not psychoacoustically masked by the program material, this noise may
be perceived as distortion, an “underwater sound,” or other perceptual degrada-
tion. Psychoacoustic calculations are used to ensure that the added noise is masked
by the desired program material and not heard. Cascading several stages of such
processing can raise the added quantization noise above the threshold of masking,
such that it is heard.
At least one other mechanism can cause the noise to become audible at the radio.
OPTIMOD-FM’s multiband limiter performs an “automatic equalization” function
that can radically change the frequency balance of the program. This can cause
noise that would otherwise have been masked to become unmasked because the
psychoacoustic masking conditions under which the masking thresholds were origi-
nally computed have changed.