Itu-R 412 Compliance For Analog Fm Broadcasts - Orban OPTIMOD-FM 8500S Operating Manual

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OPERATION

ITU-R 412 Compliance for Analog FM Broadcasts

For those who prefer the sound of conventional composite clipping, we also offer a
defeatable composite clipper. This also provides excellent spectral protection for the
pilot tone and subcarriers. The composite clipper drives the "Half-Cosine Interpola-
tion" composite limiter, which serves as an overshoot compensator for the compos-
ite clipper when it is active. (Overshoot compensation necessary to remove over-
shoots introduced by the pilot- and SCA-protection filters following the composite
clipper.)
Like conventional composite clipping, the "Half-Cosine Interpolation" composite
limiter can still cause aliasing distortion between the stereo main and subchannels.
However, this is the inevitable cost of increasing the power-handling capability be-
yond 100% modulation above 5 kHz—the characteristic that makes some people
like composite clipping. This exploits the fact that the fundamental frequency in a
square wave has a higher peak level than the square wave itself. However, any proc-
ess that makes squared-off waveforms above 5 kHz creates higher harmonics that
end up in the stereo subchannel region (23-53 kHz). The receiver then decodes these
harmonics as if they were L–R information and the decoded harmonics appear at
new frequencies not harmonically related to the original frequency that generated
them.
While the processing never clips the pilot tone, the extra spectrum generated by the
processing can fall into the 19 kHz region, compromising the ability of receivers to
recover the pilot tone cleanly. Therefore, the 8500S's composite processor has a 19
kHz notch filter to protect the pilot tone. This filter does not compromise stereo
separation in any way.
We still prefer to use the 8500S's main clipping system to do the vast majority of the
work because of its sophisticated distortion-controlling mechanisms. This means that
the 8500S does not rely on composite processing to get loud. Consequently, broad-
casters using its left/right-domain AES3 digital output can enjoy the loudness bene-
fits of the 8500S's processing—the 8500S gets competitively loud without composite
clipping. However, it is also possible to reduce the drive level to the 8500S's left/right
domain overshoot compensators and to increase the composite limiter drive by a
corresponding amount. This arrangement uses the overall composite limiter (with or
without the composite clipper's being active) to provide overshoot compensation. It
has a different sound than using the left/right domain overshoot compensators—the
sound is brighter but has more aliasing distortion (as discussed above). If the com-
posite clipper is active, stereo separation will decrease.
ITU-R 412 requires the "average multiplex power" to be limited to a standard value.
The 8500S contains a defeatable feedback multiplex power limiter that constantly
monitors the multiplex power according to ITU-R 412 standards. The power control-
ler automatically reduces the average modulation to ensure compliance. It allows
you to set the "texture" of the processing freely, using any preset. If a given process-
ing setting would otherwise exceed the multiplex power limit, the power controller
automatically reduces the drive to the peak limiting system. This action retains the
compression texture but reduces distortion while controlling multiplex power.
ORBAN MODEL 8500S