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4
transmission. This noise is generally worse for listeners who are some
distance from the broadcasting station.
If a low frequency tone of, say, 300Hz is applied to the transmitter at a
level which yields full carrier modulation (100% or ±75kHz deviation),
a high frequency tone of 10kHz applied at this same input level would
overmodulate the carrier (400% or more!) because of the transmission
pre-emphasis equalization. Normal voice and music programs have
comparatively low energy at the higher frequencies, and actual
statistics of program spectral composition were taken into account
when pre-emphasis characteristics were established years ago.
Nevertheless, occasional high energy, high frequency peaks (sibilants,
cymbals, etc.) can still cause carrier overmodulation, even when
program peaks are broadband-limited to 100%-modulation values.
This is especially true when modern-day recordings of contemporary
music constitute the program source.
Split-Spectrum
Peak Control
A pre-emphasis network
ahead
of a broadband limiter, and a de-
emphasis circuit
following
it, will deal with this situation, although the
overall level will “duck” whenever a high-end peak occurs. This
imparts a “choppy” sound to the program and reduces both
intelligibility and perceived loudness. What is instead required is a
program limiter operating as a dual-band device; a
broadband
section
to cope with normal program peaks, and an
independent
high-
frequency limiter (with proportionally faster time constants) to deal
with those program components accentuated by the transmission pre-
emphasis characteristic. A limiter of this type, now common in both
FM and AM broadcasting systems, has negligible audible effect on most
program material while providing absolute protection from carrier
overmodulation. Comprehensive FM audio processing systems are
available from Inovonics, and from a number of other suppliers.
Internal
Overmod
Protection
Overmodulation protection circuitry internal to the 708 starts with a
complex, active peak clipper integral with the patented low-pass filter
overshoot compensator. Because of its unique mode of operation, a
good deal more peak clipping may be tolerated than with more simple
audio clipping circuits. The 708 may, in fact, even be used “barefoot,”
or with minimal audio pre-processing, yet still yield acceptable
performance.
The 708 also features a built-in Composite Processor which can be
adjusted to clip as much as 3dB into the multiplex signal. This clipping
is performed
prior
to injection of the 19kHz stereo pilot and any
auxiliary subcarriers. While this “composite clipping” technique is
routinely used to increase perceived loudness, it is at the expense of
introducing spurious harmonic products above the 54kHz upper band-
edge of the composite stereo signal. Please refer to the further
discussion on Page 17.
