OPTIMOD-FM OPERATION
3-11
Nevertheless, we are aware that many engineers are fond of composite clipping. We
therefore undertook a research project to find a way to peak-control the composite wave-
form without significantly compromising separation, pilot protection, or subcarrier pro-
tection, and without adding the pumping typical of simple gain-control “look-ahead” so-
lutions.
We succeeded in our effort. The 8400 offers a composite processor that provides excel-
lent spectral protection of the pilot tone and SCAs (including RDS), while still providing
approximately 60 dB separation when a single-channel composite waveform is clipped to
3 dB depth. To ensure accurate peak control, it operates at 512 kHz sample rate.
Like conventional composite clipping, the new algorithm can still cause
aliasing distortion between the stereo main and subchannels. However, this
is the inevitable cost of increasing the power-handling capability beyond
100% modulation above 5 kHz—the characteristic that makes some people
like composite clipping. This exploits the fact that the fundamental fre-
quency in a square wave has a higher peak level than the square wave itself.
However, any process that makes squared-off waveforms above 5 kHz cre-
ates 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 infor-
mation, and the decoded harmonics end up at new frequencies not harmoni-
cally related to the original frequency that generated them.
The 8400’s composite processor has a defeatable 19 kHz notch filter to pro-
tect the pilot tone. While the processing never clips the pilot tone, the extra
spectrum generated by the processing can fall into the 19 kHz region, com-
promising the ability of receivers to recover the pilot tone cleanly. The notch
filter will cause overshoot with some program material because it removes
spectral energy that would otherwise help control peak levels. We therefore
allow the user to trade off overshoot against pilot tone protection by choos-
ing whether the 19 kHz notch filter is in or out.
We still prefer to use the 8400’s main clipping system to do the vast majority of the work
because of its sophisticated distortion-controlling mechanisms. This means that the 8400,
unlike some of its competitors, does not rely on composite processing to get loud. There-
fore, people using its left/right-domain AES3 digital output can enjoy the loudness bene-
fits of the 8400’s processing—the 8400 gets very loud without composite clipping.
ITU-R 412 Compliance
ITU-R 412 requires the “average multiplex power” to be limited to a standard value. The
8400 contains a defeatable feedback multiplex power limiter that constantly monitors the
multiplex power according to ITU-R 412 standards. The power controller 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 processing 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 distor-
tion while controlling multiplex power.
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