— 11 —
Program Audio Levels
(Menu Screen 5)
Menu Screen 5 gives a
redundant bargraph pre-
sentation of
monaural
program audio levels.
This is a peak-respond-
ing meter with a floating
peak-hold function.
Although
L:
and
R:
(Left and Right) levels are shown here,
the “stereo” outputs are simply for interconnect conven-
ience, as most broadcast plants are wired for stereo whether
the programming is or not. The two 634 outputs are the
same, and the meters will indicate identical levels as well.
100% carrier modulation is denoted by the large block op-
posite the 0dB marking on the panel. The meter resolves
+1, +2 and +3dB above 0dB. Below 0dB the scale is linear in
0.5dB steps down to –15dB, and then in 1dB steps to –21dB.
0dB represents 100%
symmetrical
amplitude modulation of
the carrier. 400Hz sinewave modulation of the transmitter
to the 100% point would take the meters to 0dB. At higher
audio frequencies the receiver audio response (IF bandwidth
and de-emphasis) must necessarily be factored into the
reading.
Indications above 0dB may be noted when asymmetrical
modulation yields higher positive peaks, as allowed by FCC
(and other authority) rulings, or by impulse noise riding
atop the audio program.
Headphone Monitoring
(Menu Screen 6)
A front-panel
PHONES
jack offers a convenient monitoring
point for 634 setup and casual listening. Whenever a pair of
headphones is plugged into the front-panel jack, the LCD
screen automatically goes to Menu Screen 6.
HEADPHONE
VOL
will begin blinking and the panel knob may be adjusted
for a comfortable listening level.
The LCD shows an arbitrary numerical value and a bargraph
representation of the headphone volume. Once volume is
set, push the knob to save the preference to memory and to
return the screen to the last menu on display.
— 12 —
NRSC De-Emphasis
(Menu Screen 7)
Back in the 1990s, the National Radio Systems Committee
(NRSC) defined a complementary high frequency pre-
emphasis and de-emphasis characteristic for AM broadcast-
ing in the US, similar to what’s used in FM worldwide. The
turnover and slope are actually a compromise between the
50µs (microsecond) European FM standard and the more
precipitous 75µs American standard. It’s called a ‘truncat-
ed’ 75µs curve.
Menu screen 7 is used to set receiver
De-Emph:
to
NRSC
or
to
OFF
. Your decision here should probably be based on
how the audio sounds to you, rather than on orthodoxy, and
on how you set the next variable discussed.
Reception Bandwidth
(Menu Screen 8)
Menu Screen 8 selects
the receiver’s reception
bandwidth, imprecisely
referred to here as ‘IF’
bandwidth, equating it
with the more familiar
analog-radio term.
The frequency choices are
6kHz
,
4kHz
,
3kHz
and
2kHz
.
These are the approximate –6dB points in each case, with a
very precipitous drop above the turnover frequencies as
shown below.
Bandwidth restriction is used almost exclusively to reduce
noise, the bane of AM radio broadcasting.
6kHz
is the clos-
est to a “Hi-Fi” setting for the 634. The rapid turnover and
