Orban OPTIMOD-FM 8600, Operating Manual

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OPTIMOD-FM DIGITAL OPERATION
3-13
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 pro-
cess 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 8600’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 8600’s main clipping system to do the vast majority of the
work because of its sophisticated distortion-controlling mechanisms. This means that
the 8600 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 8600’s processing—the 8600 gets competitively loud without composite
clipping. However, it is also possible to reduce the drive level to the 8600’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 compo-
site clipper is active, stereo separation will decrease.
ITU-R 412 Compliance for Analog FM Broadcasts
ITU-R 412 requires the “average multiplex power” to be limited to a standard value.
The 8600 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 pro-
cessing setting would otherwise exceed the multiplex power limit, the power con-
troller automatically reduces the drive to the peak limiting system. This action re-
tains the compression texture but reduces distortion while controlling multiplex
power.
The 8600 gives you control over the Multiplex Power Threshold (in the Input/output
Utilities screen). This allows you to compensate for overshoots in the signal path up-
stream from the 8600, preventing excessive reduction of the multiplex power.
Power control is applied to all outputs, not just the composite output.
See ITU-R Multiplex Power Controller on page 3-80.