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SOLIDYNE 462
dsp - broadcast digital processor Page 35
7.3 SC100 STEREO C ODER
Fine calibratio n p rotoco l
Normally the stereo coder don’t need calibration
because it’s a full stable (after many years) design
based on digital oversampling 16x technology.
If you wish to make a control or fine tuning,
please follow the following steps.
PLEASE! IF YOU DON’T HAVE A GOOD KNOWLEDGE OF
THE MATRIX STEREO THEORY OR DON’T HAVE THE
APPROPRIATE MEASUREMENT SET, DO NOT INTEND TO
CALIBRATE THE STEREO CODER.
7.2.1 TEST POINTS DIAGRAM
components side
7.2.2 ADJUST PILOT LEVEL
a) Connect to ground the test point
TP1
(R246) in
order to eliminate the 19 KHz pilot tone. Use a
good DC-20 Mhz calibrated oscilloscope at the
MPX output connector. Using the display control,
select the MPX output to 4 V peak to peak.
Connect a sine wave generator at the LEFT input
of the audio processor, at 1 Khz, + 4 dBm output.
Verify that the output waveform looks like FIG-1.
a) Replace the audio generator at the processors Left
input for audio program (music or voice). Please
carefully take note of this peak to peak value; let
name it Xpp.
b) Disconnect the audio program from the LEFT input
and disconnect the ground of the test point TP1.
Then, the 19 Khz pilot tone will appear.
c) With the oscilloscope measure the peak to peak
value of the 19 Khz pilot tone; let name it Ppp. The
percentage level of pilot tone will be:
Pilot Level [%] = 100.Ppp/Xpp
d) Using the control P4, adjust Ppp to the desired value
(normally 9 to 11 % is a good value)
Note:
If you have a
Solidyne VA16
modulation monitor or
BELAR FMS-2 measurement set, you will be able to measure
pilot tone level from on-air transmission and you can correct it
using P4.
7.2.3 PILOT PHASE ADJUSTMENT
This method is an absolute system not dependent
from the calibration of the Modulation Monitor. You
will need a very good DC-20 Mhz oscilloscope
(Tektronix preferred) with vertical 10x undistorted
span. Proceed:
a)
Change the
SC-100 PHASE jumper
(JF1) from the
Normal “N” position to calibrate “C” position.
b)
Input a sine wave generator at 1 KHz, +4 dBm, at
Left Channel of the audio processor. Adjust the level
of wave generator to get 4 Vpp MPX output.
c)
Connect the oscilloscope to the
MPX output
in
mode DC.
Use the sine wave output of audio generator, to
synchronize the oscilloscope sweep. Adjust the
sweep to 5 uS/Div. Modify with trigger level, the
center of the wave in order to have it centered at
screen.
Adjust vertical sensitivity to 50 mV/Div. You must get
a signal like FIG-2.
If not, you must do a slight change in the frequency
knob of the audio generator in order to avoid integral
multiplication factors that do not allow you to
appreciate the correct image.
Calibrate the P3 Phase Control to get zero phase
error. The phase error is indicated by the tilt of the
imaginary line between H1-H2 points. When this line
is full horizontal, the phase is correct.
FIG.2 Pilot Phase 10X