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6.3.3 FM and AM S-Meter and Stop, and Stereo Levels
The FM and AM S-meter signals, pin 11 and 12 of the LA1837 respectively, are analog voltage
levels that indicate the FM IF and AM RF input signal levels. These signals are connected to the
A/D inputs of the ST72324. During factory tuner alignment the appropriate test signal levels are
injected into the UUT and the resultant ADC values for FM stop level, FM mono/stereo level and
AM stop level are stored in the EEPROM U7000 [sheet 3, D3].
The stop level is the voltage level above which the signal strength is deemed strong enough to
warrant stopping on a channel during seek. This does not mean that the unit will always stop on
a station if the S-meter level is high enough since an IF count is also performed during to en-
sure that the correct IF frequency has been obtained.
The stereo level is the level above which a channel that has been automatically forced into
mono will return to being decoded as stereo (if stereo material is present).
When first tuning to a station the unit defaults into mono for one half second to prevent a mono
station from coming through as noisy stereo. If the initial S-meter read is greater than the
stereo level the unit switches into stereo. After the initial S-meter read the unit switches be-
tween stereo and mono in the following way. Every 500ms the S-meter is read and the unit
switches from stereo to mono if it reads one S-meter level below the force-mono level. The unit
switches from mono to stereo after ten consecutive readings of S-meter level above the stereo
level. The reasoning for using one sample to force-mono but ten consecutive samples for
stereo is to ensure that a unit with S-meter reading levels close to the threshold do not switch
between noisy stereo and clear mono too often. The set voltage between the force-mono and
stereo thresholds also helps to prevent unnecessary switching between mono and stereo,
mainly due to modulation noise on the S-meter capacitor.
Note: Switching stereo “on” via the On Screen Display will enable the above automatic force-
mono function while switching it “off” disables this automatic function and ALWAYS forces the
unit into monaural decoding. Also note that the stereo icon on the front display and the stereo
status flag on the OSD indicate the state of the ST-LED line, which indicates detection of the
19 kHz pilot tone present in stereo broadcasts. In force-mono (automatic or otherwise) this will
always be low even if the actual broadcast is in stereo.
The nominal FM stop and stereo levels are:
FM stop : 30 dBf @ 98.1 MHz
FM stereo : 42 dBf @ 98.1 MHz (force-mono approx 2dB lower)
THEORY OF OPERATION
