3-10 Operation
but also compromise stereo separation—filtering causes the single-channel composite waveform to “lift off the
baseline.”
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 waveform without significantly compromising
separation, pilot protection, or subcarrier protection and without adding the pumping typical of simple gain-
control “look-ahead” solutions.
We succeeded in our effort. The 5950 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 Interpolation” composite limiter, which serves as an overshoot compensator for the composite
clipper when it is active. (Overshoot compensation necessary to remove overshoots 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 beyond 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 process 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 5950’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 5950’s main clipping system to do the vast majority of the work because of its
sophisticated distortion-controlling mechanisms. This means that the 5950 does not rely on composite processing
to get loud. Consequently, broadcasters using its left/right-domain AES3 digital output can enjoy the loudness
benefits of the 5950’s processing—the 5950 gets competitively loud without composite clipping. However, it is also
possible to reduce the drive level to the 5950’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 composite
clipper is active, stereo separation will decrease.
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