Stl And Exciter Overshoot; Using Lossy Data Reduction In The Studio - Orban OPTIMOD-FM 5500 Operation Manual

1-19
OPTIMOD-FM DIGITAL INTRODUCTION
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 5500 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 a 5500, the ex-
ternal 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
5500's remote control feature offers the means to reduce the peak level of the
5500's audio output as necessary. This way, you can still use the 5500's line-up tone
to adjust the steady-state deviation to 75 kHz. Yet, the reduced peak level of the
audio emitted from the 5500 ensures that the carrier deviates no further than 75
kHz after overshoot. This overshoot reduction can be selected on the input/output
screen, and the remote operation can be selected in System Setup: Network > Re-
mote.

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 cascaded
before the material is finally presented to OPTIMOD-FM's input.
All such algorithms operate by increasing the quantization noise in discrete fre-
quency bands. If not psychoacoustically masked by the program material, this noise
may be perceived as distortion, "gurgling," or other interference. Psychoacoustic
calculations are used to ensure that the added noise is masked by the desired pro-
gram 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.
In addition, 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
originally computed have changed.
Accordingly, if you use lossy data reduction in the studio, you should use the highest
data rate possible. This maximizes the headroom between the added noise and the
threshold where it will be heard. Also, you should minimize the number of encode