Filter settings are not at all connected to simulcast issues. They are for trying to stop signals from other frequencies to interfere and mix with your reception.
The initial problem are a low quality receiver solution and the use of too wide filters that are 10MHz wide when they need to be as narrow as possible down to 100KHz but those are impossible to manufacture and even much wider ones are super expensive.
The 85 cent receiver chip are designed to be used with already amplified signals from a satellite dish or to receive high signal level FM broadcast signals and high level terrestrial TV signals. To be able to use the receiver chip in a scanner its gain need to be maximized and Uniden also put in a preamplifier in the scanner to make it as sensitive as other scanners.
In most user cases the receiver are constantly overloaded which creates intermodulation where one signal frequency can be heard on many other frequencies.
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Unidens filter settings are a IF shift function that moves the 10MHz wide bandpass filter up or down in frequency. If it gets shifted up in frequency the lower edge of the 10Mhz filter are moved closer to the frequency you are monitoring, that will filter out any signals that are lower in frequency but will also open up to more signals above the frequency. That's the Normal and Invert settings. The filter can also be moved halfway up or down in frequency, that's the Wide settings. If the filter isn't moved at all and you receive your frequency at the middle of the filters 10MHz range then that are the Off setting for the shift function as any normal scanner uses.
How you are suppose to use the filter settings are if you monitor a 850MHz signal and you have a strong local signal at 855MHz that then could interfere with your reception making it distorted or even making the receiver loose sensitivity. If you enable a filter settings that shift the 10Mhz filter down in frequency, it will stop to receive signals above 851MHz and that interfering signal at 855MHz will no longer be received and can no longer interfere. But then you could have an equally strong local signal at 841MHz that now starts to interfere. Then you choose the wide filter settings that only shift the filter halfway and both the 855MHz and 841MHz signal will be outside of the filters range where it pass signals to the receiver.
It's a global filter settings in the scanner where you set a filter type and that type will be used wherever Global have been set as the filter type for trunked sites and conventional departments. If you change the filter in the Global setting it affects all settings that use Global as their filter.
For sites and departments you can choose to use individual filters, Off, Normal, Invert, Wide Normal, Wide Invert and Wide Auto and Auto, that will not be affected from what filter you have set in Global.
To actually evaluate what filter to use, if any, are easiest with digital signals. You set a field in the display to show Digital Error Count and you watch that number while receiving and try to get as low number as possible when testing different filter settings. In a site it could be the control channel frequency or any of the voice frequencies that are interfered and when changing a filter to fix a bad reception of one frequency it could make it worse for other frequencies in that site.
For analog you have to listen to the audio quality and also set a display field to show Noise. You want as low noise level as possible and a clean signal but you can only go by the noise number when there are no one talking, it has to be a silent carrier as any audio modulation will make the numbers go up. For analog the filter affects all frequencies that belong to that department, so fixing one problem could create another problem at another frequency.
/Ubbe
wiki.radioreference.com
