Instruction manual

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21
difference” to balance crosstalk measurements, sacrificing a null at
one frequency to improve that at the other.
When this part of the Calibration Procedure has been successfully
completed, the sub-channel measurement should remain below –50dB
(relative to 100% modulation) as the Audio Generator is swept from
20Hz to 15kHz.
Because the DAVID-II utilizes digital techniques (instead of analog
multiplication) to generate the subcarrier, crosstalk is symmetrical;
that is, sub-to-main crosstalk will be identical with the main-to-sub,
which was optimized in the foregoing Procedure.
Separation Trim The traditional method of adjusting and verifying stereo separation is
to visually monitor an oscilloscope display of the composite multiplex
signal (one channel driven and stereo pilot off), and adjust for the
straightest baseline of the DSB waveform. Although this technique
does permit a rough separation calculation, even today’s best ‘scopes
will show baseline distortion when overdriven to the extent necessary
to resolve separations of 40dB or more. For the high figures of
separation which modern stereo generators can achieve, a precision
stereo demodulator will give much greater measurement and,
consequently, adjustment accuracy.
It is entirely feasible to trim the equalization and phase adjustments in
the DAVID-II output circuitry to correct for shortcomings elsewhere in
the transmission system; for instance, in a “composite” STL, or even in
a long run of coax cable. This procedure does not consider this option,
however, assuming instead that, since a chain is only as strong as its
weakest link, it’s better to fix the weak link than to compensate for it
at another point. The procedure which follows will leave the DAVID-II
adjusted for its own best stereo separation.
1. Cable the COMPOSITE OUTPUT of the DAVID-II directly to the
composite input connection of the Mod-Monitor. Use a short cable
of known characteristics, preferably 50-ohm or 75-ohm coax.
2. With the DAVID-II in the STEREO mode and with the PILOT
switched ON, apply a 10kHz sinewave signal to the left channel
input only. The signal level should be identical with that used in
previous procedure steps. If in doubt, check for 6-volts p-p on R22.
3. At this point, check the Mod-Monitor’s display of Total Peak
Modulation. It should read just about 100%. Adjust the DAVID-II
OUTPUT LEVEL control for an exact 100% reading.
4. Also check the Mod-Monitor indication of Pilot Injection Level. If
necessary, adjust the DAVID-II PILOT INJ. control for a 9% reading.
5. Switch the Mod-Monitor to display left and right channel levels.
The left channel should measure about –1dB. Ideally, the right
channel should read –70dB.