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OPTIMOD 6300 DIGITAL
INTRODUCTION
Some microwave links may be modified such that the deviation from linear phase is
less than +10 20-20 kHz, and frequency response is less than 3dB down at 0.15Hz
and less than 0.1dB down at 20 kHz. This specification results in less than 1% over-
shoot with processed audio. Many such links have been designed to be easily con-
figured at the factory for composite operation, where an entire FM stereo baseband
is passed. The requirements for maintaining stereo separation in composite opera-
tion are similar to the requirements for high waveform fidelity with low overshoot.
Therefore, most links have the potential for excellent waveform fidelity if they are
configured for composite operation (even if a composite FM stereo signal is not ac-
tually being applied to the link).
Further, it is common for a microwave STL to bounce because of a large infrasonic
peak in its frequency response caused by an under-damped automatic frequency
control (AFC) phase-locked loop. This bounce can increase the STL's peak carrier de-
viation by as much as 2dB, reducing average modulation. Many commercial STLs
have this problem.
Analog landline (PTT/post office line)
Analog landline quality is extremely variable, ranging from excellent to poor. (For-
tunately, they are largely obsolete, having been replaced by digital links.) Whether
landlines should be used or not depends upon the quality of the lines locally avail-
able, and upon the availability of other alternatives. Due to line equalizer character-
istics and phase shifts, even the best landlines tend to veil audio quality slightly.
They will certainly be the weakest link in a DAB broadcast chain.
Slight frequency response irregularities and non-constant group delay characteristics
will alter the peak-to-average ratio, and will thus reduce the effectiveness of any
peak limiting performed prior to their inputs.
Using Lossy Data Reduction in the Audio Chain
Before the 6300
Many broadcasters are now using lossy data reduction algorithms like MPEG-1 Layer
2, Layer 3, or Dolby AC2 to increase the storage time of digital playback media. In
addition, source material is often supplied through a lossy data reduction algorithm,
whether from satellite or over landlines. Sometimes, several encode/decode cycles
will be cascaded before the material is finally presented to OPTIMOD 6300's input.
All such algorithms operate by increasing the quantization noise in discrete fre-
quency bands. If not psychoacoustically masked by the program material, this noise
may be perceived as distortion, "gurgling," swishing, or other interference. Psycho-
acoustic calculations are used to ensure that the added noise is masked by the de-
sired program material and not heard. Cascading several stages of such processing
can raise the added quantization noise above the threshold of masking, making it
audible.
In addition, at least one other mechanism can cause the noise to become audible at
the radio. OPTIMOD 6300's multiband limiter performs an "automatic equalization"
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Troubleshooting
