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17
4. Switch the Modulation Monitor to display the injection level of the
19kHz stereo pilot and confirm 9% injection. Touch-up the
PILOT
INJ.
control, if necessary, then re-confirm 100% carrier deviation.
5. If an RDS or an SCA subcarrier is to be broadcast, re-connect it to
the 708 at this time. Using the Mod-Monitor, or other means of
reading the auxiliary subcarrier injection level, verify proper
injection.
6. Reset the Mod-Monitor to read total carrier modulation. If
necessary, trim the
OUTPUT LEVEL
control to maintain carrier
deviation within legal limits. Depending on current operating rules
and practices, when either an SCA or an RDS subcarrier is
broadcast, total peak carrier deviation
may be permitted
to exceed
the customary 100%-modulation figure. The station (or consulting)
engineer should be aware of legal guidelines in effect and assume
responsibility for any adjustment which deviates from this
procedure.
7. Reset the
COMPOSITE PROCESSING
control to the full-
counterclockwise,
OUT
position.
IMPLICATIONS OF COMPOSITE PROCESSING
In the FM “loudness war,” the ultimate tool in the broadcaster’s arsenal
is
composite processing
. This is more appropriately called
composite
clipping
since linear gain-reduction techniques cannot easily be applied
to the “interleaved” multiplex stereo signal. The historic justification
for composite clipping reflects the need to control the overshoots of
uncorrected low-pass filters in early stereo generators.
Despite refinements in filter technology, which all but eliminate
overshoot as a potential overmodulation problem, it seems that a small
gain in perceived loudness can always be achieved by clipping peak
excursions of the multiplex signal just before it is fed to the exciter.
This clipping action is not without certain tradeoffs, however, among
which are modulation of the 19kHz stereo pilot and the generation of
out-of-band distortion components.
Pilot Modulation
The 19kHz stereo pilot, already at a level some 20dB below 100%
modulation, is subject to amplitude perturbations by program peaks
when overall composite clipping is employed. With more than about
1dB of clipping, the pilot can actually be lost for the duration of the
clipped peak. The receiver’s stereo decoder may even lose phase-lock;
this was a serious problem with early add-on composite clippers.
More sophisticated stand-alone composite clipping devices strip the
19kHz stereo pilot from the composite signal, clip the remaining
multiplex stereo components, then re-apply the pilot. The Inovonics
708 simply performs its clipping function prior to initial pilot injection.