McIntosh MPI 4, Руководство пользователя, Страница: 16 / 24

F I G . 25 OUT OF PHASE
FIG. 26 LEVEL DISPLAY
amplifier outputs are displayed, and their heights can be
adjusted by the GAIN control. For other positions of the
POWER LEVEL selector, the GAIN control is disabled and
power output from the power amplifier is indicated on
the scale in ranges from 100 watts to 0.1 watt. The
level columns respond to the peak voltage present in
the supplied power amplifier signal. The power calibra-
tion is in average watts for sine wave signals. Each step
of the POWER LEVEL switch indicates a difference of
5 dB in power output. Turn the POWER LEVEL switch until
you discover a position which completely contains the
peaks of the program material. To convert the display
readings to actual power amplifier output in watts, use the
Power Table, Fig. 31.
Example: Assume the POWER LEVEL control is set so
the red indicator is opposite "10" on the WATTS scale.
While playing program material, assume the trace reaches
a height of "+1" on the LEVEL SCALE (center of the screen).
Consult the POWER TABLE and find "+1" in the LEVEL
SCALE column at left. Next, read to the right until you
reach the "10" column headed under the WATTS heading.
The figure given there is the exact wattage output of the
power amplifier. In this example 6.3 watts.
LEVEL MODE
The top panel LEVEL mode is activated when the MPI-4
LEVEL button is pressed. With the LEVEL mode switch in
the NORMAL position the height of the two columns, as
read on the center dB scale, is an indication of the audio
level. The vertical lines on the screen rise and fall in step
with the program material. The columns rise very quickly,
(250ms rise time) but decay somewhat slowly (½ sec.
decay rate).
With the LEVEL mode switch in the PEAK mode the
two columns indicate the "average" peak value of the
audio waveform. The two columns rise slower (approx.
½ sec.) than in the NORMAL mode and they also fall
slower (25 sec. decay rate). For sound that continuously
changes in amplitude this mode will give an indication
of the "average" peak level. For a sine wave (a continuous
tone) the NORMAL and the PEAK will give the same dB
reading.
The RESET position of the LEVEL mode switch quickly
discharges the "average" Peak timing circuit for making
another "average" Peak measurement.
In some instances, it is desirable to stop the action
and observe the highest point achieved by the audio
signal over a period of time. This permits close compari-
son between the highest levels attained by left and right
channels and simplifies their measurement against the
scales on the screen. This may be done by placing the
LEVEL MODE switch in the RESET position. This allows
the changing signal to build up and display its highest,
or peak indication. If a higher peak comes along, while
the switch is still held, the two columns will go up to the
higher value. If the power level switch is in a watt position
the two columns will indicate the "equivalent peak average
power" being delivered to the loudspeakers.
The decay time for "Peak" readings while in the RESET
position is 100 seconds. (For a sine wave the peak value
as read in the RESET position will be slightly greater than
in the NORMAL and PEAK positions.)
Return the switch to NORMAL for conventional display.
SWEEP MODE
In the SWEEP mode the MPI 4 operates as a standard
audio oscilloscope. In this mode it is possible to view in-
dividual audio cycles in the program. Since most program
material consists of many tones, the MPI 4 may be ad-
justed to "trigger" at nearly any frequency in the audio
spectrum. This provides a clear display of the range of
interest. If you wish to view tones in the lower end of the
audio range, press the 20-200 pushbutton on the front
panel. Also press the L or R button under the MODE SELEC-
TOR. As the program plays, you should be able to freeze
one or more cycles on the screen by further adjustment
of the SWEEP control (which acts as a fine tuning control).
A single trace display is shown in Fig. 27.
It is also possible to roughly estimate the frequency of
the audio tone displayed on the screen. Assume the SWEEP
FREQUENCY button is set to 20-200 and the SWEEP
control is turned fully counterclockwise. At this time, the
screen is "sweeping" 20 times per second. If one audio
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