Orban OPTIMOD 5750 Operating Manual page 110

5-3
Orban 5750 Technical Manual Troubleshooting
The 5750 has two systems that fight this problem. The compressor gate freezes the gain of the AGC and compressor
systems whenever the input noise drops below a level set by the threshold control for the processing section in
question, preventing noise below this level from being further increased.
In the Multiband structure, dynamic single-ended noise reduction can be used to reduce the level of the noise below
the level at which it appears at the input.
If you are using the 5750's analog input, the overall noise performance of the system is usually limited by the
overload-to-noise ratio of the analog-to-digital converter used by the 5750 to digitize the input. (This ratio is better
than 108 dB.) It is important to drive the 5750 with professional levels (more than 0 dBu reference level) to achieve
adequately low noise. (Clipping occurs at +27 dBu.)
The 5750's AES3 input is capable of receiving words of up to 24 bits. A 24-bit word has a dynamic range of
approximately 144 dB. The 5750's digital input will thus never limit the unit's noise performance even with very high
amounts of compression.
If an analog studio-to-transmitter link (STL) is used to pass unprocessed audio to the 5750, the STL's noise level can
severely limit the overall noise performance of the system because compression in the 5750 can exaggerate the STL
noise. For example, the overload-to-noise ratio of a typical analog microwave STL may only be 70-75 dB. In this case,
it is wise to use the Orban 8200ST Studio AGC to perform the AGC function prior to the STL transmitter and to control
the STL's peak modulation. This will optimize the signal-to-noise ratio of the entire transmission system. An
uncompressed digital STL will perform much better than any analog STL.
Whistle on Air, Perhaps Only in Stereo Reception
The most likely cause is oscillation in the analog input or output circuitry. If the oscillation is in the output circuitry
and is between 23 and 53 kHz, it will be detected in a receiver's stereo decoder and translated down into the audible
range.
If you encounter this problem, check the analog or digital outputs with a spectrum analyzer to see if the spurious
tone can be detected here. If it appears at both outputs, it is probably an input problem. If it only appears at the
analog output, then it is likely a problem with the left/right D/A converter or other analog circuitry. If it appears only
when you use the composite output, then it is likely a problem in the composite D/A converter or output amplifiers.
A whistle could also be caused by power supply oscillation, STL problems, or exciter problems.
Interference from stereo into SCA
A properly operating 5750 generates an immaculately clean baseband, with program-correlated noise below –80 dB
above 57 kHz even when the composite limiter is used aggressively. If the 5750 and the rest of the transmission
system are operating correctly, subcarriers should experience no interference.