20
THE 261 AS AN FM AIRCHAIN PROCESSOR
The Independent
High Frequency
Limiter
A pre-emphasis protection limiting function was added to the 261
with the introduction of Rev. 2 firmware. When the 261 is used as
an FM airchain processor, this function ensures that overmodula-
tion will not occur as a result of the normal 50- or 75-microsecond
transmission pre-emphasis characteristic.
NOTE:
The HF limiter is to be used only with systems that employ
transmission pre-emphasis and complementary de-emphasis in the
receiver. This includes FM broadcasting and analog L/R-mode STL
links.
The independent high-frequency limiter may be enabled only when
the broadband limiting function is turned on. With
DYNAMIC
REDUCTION
set to
PEAK LIMIT ONLY
, or to
LIMIT AND COMPRESS
, scroll
down
q
one more screen to this menu:
With the
t
and
u
select buttons, the HF limiter can be set to pro-
tect either the 50µs or 75µs pre-emphasis transmission curves:
When either characteristic chosen, another programming option be-
comes available.
Flat or Pre-
Emphasized
Output
With the HF limiter engaged, the output of the 261 may be set to
have either a flat or a pre-emphasized output characteristic. This is
set by scrolling down
q
to
OUTPUT:
and making a selection with the
t
and
u
buttons.
Set to
FLAT
, the 261 output will exhibit a frequency-flat overall char-
acteristic below the HF limiting threshold. Above threshold the lim-
ited output ceiling will fall at the rate of 6dB/octave, beginning at
the pre-emphasis turnover frequency. This is the proper setting
when pre-emphasis is supplied by the stereo generator or digital
stereo-gen in the FM exciter. In this case, the
falling
response of
the 261 is complemented by the
rising
characteristic of transmission
pre-emphasis to maintain carrier deviation at full modulation at all
frequencies.
