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9
PRE-EMPHASIS SELECTION
The
DAVID-II
supports the 75-microsecond FM broadcasting pre-
emphasis standard, common to the Western Hemisphere and parts of
the Orient, and the 50-microsecond standard used in Europe and Asia.
Pre-emphasis appropriate to the shipping destination is jumpered at the
factory, but this is easily changed if necessary.
Beneath the top cover, locate the
second
vertical row of integrated
circuits from the right-hand edge of the chassis. This is the row
next
to
the one behind the input barrier strip. Between the second and third
ICs from the rear panel are two jumper pin strips, each with a push-on
jumper. These are identified as
JMP1
and
JMP2
in the circuit board
legend, and each is designated with a
75
and a
50
marking to indicate
proper jumper placement for the 75µs and 50µs pre-emphasis
characteristics. Figure 3 illustrates the jumpering options
75-MICROSECOND
50-MICROSECOND
Figure 3 -
Pre-emphasis Jumpering
“BASS-ENHANCE” EQUALIZATION
The
DAVID-II
incorporates a unique bass-boost equalizer which may be
enabled at the user’s option.
Normal
DAVID-II
low-end response is flat to within 1dB down to 10Hz.
When the equalizer is jumpered into the circuit, response assumes the
shape graphed in Figure 3 on the next page. The effect of this
equalization is to increase the amount of
perceived
“bottom end,” while
attenuating much of the
sub-audible
program energy.
Very
low-
frequency bass notes are lost in nearly all listening situations, only
serving to sabotage modulation efficiency and program density. Even
the megawatt subwoofers in cars with blacked-out windows don’t
respond to frequencies much below 50Hz; rather, they make a lot of
noise in the rather narrow region between 60Hz and 80Hz.
The few dB of boost just before cutoff more than makes up for any lost
legitimate
program bass information. This is analogous to the
Rheinfelder Effect,
a phenomenon associated with a novel audio noise-
