Operating instructions

LOCAL OPERATION
3-45
AF input level measurement
The measured level of signals applied to the AF INPUT connector can be displayed in volts, dBm,
dBV or mW. The selection is made from setup page 2, as described under Audio Level Measured
in: on page 3-13. When the input level is displayed in dBm or mW, the signal is assumed to be
measured across 600 Ω.
The 600 Ω interface unit allows the Service Monitor to provide a 600 Ω termination. (See Audio
Input Impedance: on page 3-13.) With the input impedance set at 600 Ω, the indicated level is that
of the input to the Service Monitor. With the input impedance set at High, the indicated level is
that of the input to the Service Monitor with an assumed 600 Ω load.
AF filters
The AF signal to the AF INPUT connector on the front panel can be filtered using high pass, low
pass or band pass audio filters and with weighting filters as required.
Press the [AF Filter] key. The left hand soft keys change, to allow selection of AF filters as
shown below. Press the [Return] key after selecting the required filter.
High Pass Low Pass Band Pass
CCITT
†
or
C-MESS
‡
†
Only if option 23 (CCITT filter) is fitted.
‡
Only if option 24 (C-MESS filter) is fitted.
The high pass filter options are 50 Hz or 300 Hz.
There are five low pass filter soft keys. The 50 kHz LP filter can be considered to be the
‘unfiltered’ selection as this represents the normal full bandwidth of the audio circuits.
The other four low pass preset filters can each be set to any cut-off frequency within the range
250 Hz to 1.0 kHz and 3.0 kHz to 20.0 kHz.
The band pass preset filters can be customized by selecting either of the high pass options in
combination with any of the four preset low pass filters.
The preset filters are set from the HELP/SETUP facility. The AF Filter setup explanation starts on
page 3-10.
*OSCILLOSCOPE*
The input to the oscilloscope is taken directly from the AF input connector on the front panel, not
through any of the AF filters. Therefore measurements obtained on the AF voltmeter will show
the effects of filtering, whereas those made using the oscilloscope will not.
Audio distortion
The Service Monitor is able to measure signal noise and distortion against the following
parameters:-
Signal to
noise ratio
SINAD
level
Percentage
distortion
Distortion levels introduced into the audio signal path can be measured using the distortion
measuring module.
Pressing the [Audio Meter] key, to change the soft keys as shown in fold out diagram (3-3), then
pressing the [Dist/S-N]soft key, accesses the distortion measurement menu shown in fold out
diagram (3-3-2). The function of each soft key on this menu is as follows:-