Raytheon SNV-12, Инструкция по установке и эксплуатации

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SNV-12 Operations Manual
3-71
SW3-7, 8 Line Equalization
These switches may be used to compensate for frequency response differences between
different sites. The SNV-12 votes based on the Signal Quality (high frequency noise
measured) or SNR of received audio for the FM and AM/HF modes, respectively. Noise is
measured above 2200 Hz, where relatively less speech energy is present. The Signal is
measured from 300 to 800 Hz where a large percentage of speech is present. Pilot tones are
used between 1900 and 2200 Hz. The frequencies between 800 and 1900 Hz, which contain
varying amounts of both speech and noise energy, are excluded from the calculation.
Leased lines, particularly long ones, can act as Low Pass Filters, rolling off the frequency
response in such a way that the Noise Measurement is reduced relative to the Signal
Measurement. Equalization may be helpful for a site that uses a much longer line than the
others, or in particular, when a combination of leased-line and microwave-linked sites are used.
Microwave links may have better frequency response in the 2200 to 3000 Hz audio pass band
than leased lines and less LPF effect, so more of the high-frequency noise reaches the SVM
module to become part of the noise measurement. If a transmitter is picked up by a group of
receivers that send the audio back to the SNV-12 by a mixture of these two methods, both
microwave and actual wires, there may be a bias created in favor of voting the leased line sites
(due to the lower amount of noise that reaches the SNV-12 from these receivers). The leased
line site may have noticeably inferior audio quality but is still consistently voted due to its
smaller total high frequency noise content (caused by the frequency response differences).
Equalization is likely to be required for installations that mix leased line and microwave links
unless conditioning is added to the leased lines. We recommend that when leased line and
microwave and/or RF links are used that the leased line circuits have C2 conditioning specified.
This may eliminate the need for using the SVM equalization feature and will also ensure not
only that the voting process functions properly, but also ensure the audio characteristics of all
sites are more equal. See Section 3.10 for more information.
If C2 conditioning is either impossible or insufficient, an SVM module‟s DSP software can be
used to modify the SNR measurement to account for the different frequency responses. SVM
switches SW3-7 and SW3-8 add a multiplier to the noise portion of the SNR measurement.
Three multiplier steps are available. Using the line equalization on an SVM biases the voting
calculation against that SVM. It essentially restores the noise that was lost because of the roll-
off in the frequency response. A higher setting pulls in a higher noise multiplier. (This effects
only the Signal Quality calculation that takes place in the DSP and has no effect on actual voted
audio quality).
If equalization is required, the best way to determine the correct setting is to key a transmitter
that will provide a strong signal both to a site that requires equalization and a site that does not.
The site requiring equalization is the site that is being voted but has poorer audio quality. The
other site for the setup test is one that has the better sounding audio but is not being voted. (If
necessary, use the SVM‟s front panel switch to disable other sites.) Start with the lowest
equalization setting, and increase the setting until the non-equalized site is voted instead. The
correct setting is either this setting or the next lower setting (the perfect equalization lies
between the two settings). Remember to power the unit off and back on when changing the
switch settings as the internal processors read the dipswitches only at power-up.