Orban OPTIMOD-FM 8700, Operating Manual

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OPERATION ORBAN MODEL 8700i
carrier protection and without adding the pumping typical of simple gain-control
“look-ahead” solutions.
We succeeded in our effort. The 8700i offers a patented “Half-Cosine Interpolation”
composite limiter that provides excellent spectral protection of the pilot tone and
SCAs (including RDS), while still providing approximately 60 dB of separation when a
single-channel composite waveform is clipped to 3 dB depth. To ensure accurate
peak control, the limiter operates at 512 kHz sample rate.
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 compo-
site 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 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 8700i’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 8700i’s main clipping system to do the vast majority of the
work because of its sophisticated distortion-controlling mechanisms. This means that
the 8700i 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 8700i’s processing—the 8700i gets competitively loud without composite
clipping. However, it is also possible to reduce the drive level to the 8700i’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.