Orban Optimod-TV 8382, Operating Manual

Page: 175 / 244

OPTIMOD-TV DIGITAL TROUBLESHOOTING
5-1
Section 5
Troubleshooting
Problems and Potential Solutions
Always verify that the problem is not the source material being fed to the 8382, or
in other parts of the system.
RFI, Hum, Clicks, or Buzzes
A grounding problem is likely. Review the information on grounding on page 2-8.
The 8382 has been designed with very substantial RFI suppression on its analog and
digital input and output ports, and on the AC line input. It will usually operate adja-
cent to high-powered transmitters without difficulty. In the most unusual circum-
stances, it may be necessary to reposition the unit to reduce RF interference, and/or
to reposition its input and output cables to reduce RF pickup on their shields.
Particularly if you are using a long run of coaxial cable between your stereo encoder
and the exciter, a ground loop may inject noise into the exciter’s composite input—
especially if the exciter’s input is unbalanced. The Orban CIT25 Composite Isolation
Transformer can usually cure this problem.
The AES3 inputs and output are transformer-coupled and have very good resistance
to RFI. If you have RFI problems and are using analog connections on either the in-
put or output, using digital connections will almost certainly eliminate the RFI.
Buzz can be caused by Incidental Phase Modulation (ICPM), where the visual carrier
interferes with the aural carrier. This can be caused by poor transmitter tuning
and/or by insufficient bandwidth in the antenna system's notch diplexer. ICPM has
particular severe consequence in the BTSC and EIAJ stereo systems, and the transmit-
ter often requires updating the successfully transmit stereo.
Poor Peak Modulation Control
The 8382 ordinarily controls peak modulation to an accuracy of ±2%. This accuracy
will be destroyed if the signal path following the 8382 has poor transient response.
Almost any link can cause problems. Even the TV aural exciter can have insufficient
flatness of response and phase-linearity (particularly at low frequencies) to disturb
peak levels. Section 1 of this manual contains a complete discussion of the various
things that can go wrong.
Digital STLs using lossy compression algorithms (including MPEG1 Layer 2, MPEG1
Layer 3, Dolby AC2, and APT-X) will overshoot severely (up to 3 dB) on some pro-