Front Panel information
FIG. 3 FRONT PANEL
MODE SELECTOR
The MODE SELECTOR pushbuttons select the type of
display seen on the screen of the MPI 4. Each of the five
pushbuttons selects a trace—a glowing line of green light—
which shows a variety of information about signals from
the tuner, preamplifier or power amplifier. To assist the
evaluation of each display, the reticle in front of the
screen is marked with calibration points which are used to
measure signal characteristics. As MODE SELECTOR
buttons are pressed, the screen reticle is illuminated in
these colors:
Mode Color
Multipath Blue
Stereo Red
Level Amber
Sweep Green
MULTIPATH
When properly connected to an FM tuner the MPI 4
will detect and display multipart reception. Multipart
reception is the result of a reflected signal arriving at the
tuner antenna slightly later than the direct signal. By
rotating or repositioning the FM antenna it is possible to
reduce multipath reception. The MPI 4 makes it easy to
know when the FM antenna is oriented for the best recep-
tion of any station.
To show multipart reception the MPI 4 displays in-
stantaneous signal strength versus frequency deviation.
Signal strength is shown as vertical deflection of the in-
dicator display beam. Frequency deviation is shown as
horizontal deflection. Multipath reception appears as a
peak or valley in the display.
Multipath reception degrades FM tuner performance in
several ways:
1. Usually there is an increase in background noise
level.
2. Distortion is often heard in the program signal
3. Stereo separation may be reduced.
4. The stereo effect may be completely lost.
5. Stereo indicators may fail to function, or function
erratically.
To overcome multipart reception it is usually necessary
to turn the antenna to receive the FM signal by one pre-
dominant path. Rotating a directional antenna is effective
at correcting multipath reception. In areas where a simple
antenna such as a dipole is used repositioning the antenna
can achieve similar results.
In the MULTIPATH mode the MPI 4 is a very effective
tuning indicator:
1. Signal strength is shown by the vertical position of
the display trace. The higher the position of the
trace the greater the signal strength.
2. Correct tuning occurs when the display trace is
centered horizontally on the screen. Since the dis-
play trace effectively follows the tuner IF response
curve, centering the trace tunes the detector to
the center of the IF curve.
The display of the signal, when there is no multipath
reception affecting the program, appears as a smooth
curving line, (Fig. 4). The calibration points are the two
100% marks at the bottom of the screen. As the program
material grows louder or softer, the trace expands and
contracts. The 100% points indicate the highest modula-
tion percentage normally transmitted by an FM station.
(A more detailed description appears in the "How to Use
Section.")
STEREO
A stereo program causes a complex display which
varies between a circular shape and an elliptical shape.
A mono program causes the display to be a slanted line.
With an in-phase mono program, the signal falls between
the L + R boundaries; in out-of-phase mono program the
trace falls along the L — R boundaries.
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