Operating instructions

LOCAL OPERATION
3-65
Input filtering
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 right 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.
Distortion measurement
The distortion measuring facilities of the monitor are available within the AF TEST mode.
Pressing the [Audio Meter] key, to change the soft keys as shown in fold out diagram (7-3), then
pressing the [Dist/S-N]soft key, accesses the distortion measurement menu shown in fold out
diagram (7-3-2).
Pressing each of the soft keys will have the following effect:-
Off
Disables any active distortion measuring function.
Hint. By disabling this function, the time taken to measure the remainder of the measurement functions is
shortened thereby reducing the update time.
S/N
Selects signal to noise ratio as the measurement method. The AF generator is disabled and
enabled alternately by the monitor software and the total output level from the circuit under test is
measured by the AF voltmeter at each state. See Fig. 3-10 a & d, Distortion level and signal to
noise level measurements, on page 3-36.
Calculations are performed by the software to calculate the signal to noise ratio using the formula:-
S/N
= 20
1
2
log
V
V
dB
=
++
20 log
()S
N
D
N
dB
The distortion bar chart ranges available are 0 to 30 dB and 0 to 100 dB. The signal to noise ratio
is displayed as a digital read-out above the bar chart. The AF test menu is recalled automatically
after pressing this key.