— 7 —
and monitoring purposes, these meters give an expected
visual response only for frequencies below about 400Hz.
Setting the Input Level With a Tone
The most accurate method of adjusting the
MPX INPUT
LEVEL SET
control requires applying a 400Hz sinewave tone
to the rear-panel
COMPOSITE/MPX INPUT
jack. This tone
must have a peak-to-peak (p-p) amplitude identical to the
p-p level of the multiplex broadcast signal required for full,
±75kHz FM-carrier modulation. A p-p (oscilloscope) meas-
urement is essential here because the FM modulation speci-
fication is based on a peak, rather than an r.m.s. or aver-
aged value, of the composite-MPX waveform.
With a 400Hz tone applied as specified above, adjust the
MPX INPUT LEVEL SET
control so that the
LEFT
and
RIGHT
0dB
LEDs are on steadily.
NOTE: The
MPX INPUT LEVEL SET
control is a “multi-turn
pot.” It requires about 15 full screwdriver rotations to trav-
erse from one end to another.
Once the 400Hz tone has been set and the actual compo-
site/MPX signal connected, program audio levels should
peak at about the –1dB point. The stereo pilot and any au-
dio or data subcarriers (such as RDS/RBDS) will not influ-
ence the meters. As these ‘silent partners’ can account for
about 10% of total carrier modulation, actual program audio
will peak about one dB below 100% modulation.
Setting the Input Level With Program Audio
A reasonably-accurate means of setting the input level is
simply to adjust the
MPX INPUT LEVEL SET
control so that
speech and music peaks reach the –1dB point on a fairly
regular basis. This setting will depend in large part to how
heavily the program audio has been processed, but for all
but classical music formats this adjustment method should
prove satisfactory.
Status Indicators and Alarms
The series of three LEDs below the INOmini logo indicate
status and fault modes.
MONO
and
STEREO
will toggle to indicate whether the
transmission includes the 19kHz stereo ‘pilot tone.’ Nor-
— 8 —
mally the green
STEREO
LED will be lighted. But if pro-
gramming is actually being broadcast in monaural, or if the
transmitter defaults from stereo transmission for some rea-
son, or if the input signal is just lost, the yellow
MONO
LED
will come on instead. At this same time, the rear-panel
P
(Pilot-Loss) terminal gives an NPN transistor saturation to
ground. This may be used to actuate a relay, light a remote
LED indicator or tie into a remote control system.
The red
SILENCE
indicator will light when either or both of
the program audio channels goes dead. The technical con-
ditions for this are that: 1) the peak level of the audio in
either channel has to drop approximately 20dB (off the me-
ter scale), and: 2) must remain below this threshold for
about 30 seconds. When these conditions are met, the
front-panel
SILENCE
indicator will light and the rear-panel
A
(Audio-Loss) terminal will give an NPN transistor satura-
tion to ground. This may be used to actuate a relay, light a
remote LED indicator or tie into a remote control system.
Forced-Monaural Operation
A jumper under the top cover of the 514 decoder may be
set by the user to inhibit stereo program decoding. In this
case the analog and digital program audio outputs will de-
liver monaural audio, and the front-panel
MONO
indicator
will remain on, whether a stereo pilot is present in the input
signal or not.
The jumper is located on the circuit board somewhat be-
hind the cluster of three status/alarm indicators. These two
snapshots illustrate the jumper position for the normal and
forced-monaural modes.
Normal (Stereo)
(Jumper to the Left)
Forced-Mono
(Jumper to the Right)
