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PSR600 AU mode

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IXNAY

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I have a question about narrow banding. The new frequency setup is for narrower channels and spacing.
I use AU mode and that is supposed to work for the narrower signals. However, on frequencies using narrow band FM, the audio is weaker than before they switched. P25 frequencies are ok, as are regular FM like NOAA weather.
I think AU decides the bandwidth based on frequency. So do I have to change from AU to FM N setting on
the narrow FM frequencies? I have tried audio boost, which helps a bit, but the audio level is still too low compared to regular FM stations. Also, does any know how AU decides which bandwidth/mode it selects (by frequency,etc).
This all started when Ventura County Fire switched to FM N. The forest service freqs are going narrow also and I think their dispatch audio has started sounding weaker. I monitor both.
 

gewecke

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I have a question about narrow banding. The new frequency setup is for narrower channels and spacing.
I use AU mode and that is supposed to work for the narrower signals. However, on frequencies using narrow band FM, the audio is weaker than before they switched. P25 frequencies are ok, as are regular FM like NOAA weather.
I think AU decides the bandwidth based on frequency. So do I have to change from AU to FM N setting on
the narrow FM frequencies? I have tried audio boost, which helps a bit, but the audio level is still too low compared to regular FM stations. Also, does any know how AU decides which bandwidth/mode it selects (by frequency,etc).
This all started when Ventura County Fire switched to FM N. The forest service freqs are going narrow also and I think their dispatch audio has started sounding weaker. I monitor both.
AU merely selects the mode received at the time. AU is mostly used for milcom where several modes can be used; AM,FM, NFM

73,
n9zas
 

Mike_G_D

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Firstly, the GRE 500/600 models and their Radio Shack variants do not include the "NFM" mode in the "au" "automatic" mode selection. Currently, the only way to implement "NFM" in those GRE units is to activate it in a programmed memory channel manually - do not rely on the "au" mode. It also cannot be activated in service search or in programmed search despite the incorrect information printed in the manuals.

Secondly, the GRE units do not fully implement "narrowband FM" in that they do not correctly alter the discriminator voltage output relative to frequency deviation when used in their "NFM" mode, at least I cannot detect any change using my ears which are rather discerning. The only way to increase apparent loudness or volume when faced with "NFM" signals is through the use of the "6dB audio boost" which is a "band aid" solution in my opinion. Nevertheless, the NFM mode does have one major benefit in that it does activate a tighter IF filter which effectively reduces adjacent channel interference and is, therefor, quite useful when dealing with closely spaced narrowband channels used simultaneously within a relatively confined geographic area as they often are where I am. I now have to use that tighter IF filter selected through the use of the GRE's "NFM" mode because there have been cases wherein I can hear multiple fire incidents around me in which many of the new narrowband air tac CDF channels are being used simultaneously. Without the tighter IF "NFM" filter I would not be able to effectively separate those communications.

This is directly opposite of what the Uniden BCT-15 that I have does. It also has a "NFM" mode but it employs the "other half" of the solution. When used, the Uniden's NFM mode does alter the demodulator FM discriminator voltage output versus input frequency deviation or in some way compensates for the reduced voltage variations ("audio level") in post discriminator amplification but it does nothing, as far as I can tell, in terms of IF filter bandwidth. So, while the Uniden does apparently make some volume level adjustments/tracking for "narrowband" +/-2.5KHz deviation analog FM signals, since there is no change in IF filter response when there are simultaneous adjacent channel communications it is almost useless.

Ideally, the correct way to handle the new +/-2.5KHz "narrowband" FM analog signals would be to BOTH use a tighter IF filter AND correctly adjust for the reduced deviation in terms of apparent "volume" either through higher voltage level output for a given deviation or through post processing in the following amplifier stages. For me, in my location, given the heavy CDF usage of the new tighter 7.5KHz spaced "narrowband" channels and my ability to hear several different areas around me which may be active simultaneously, if I am forced to choose only one half of the full implementation I would choose the GRE's tighter IF filter and deal with the reduced volume (and use the 6dB "audio boost") since, at least, I can effectively separate the conversations on multiple adjacent channels. In my area, the Uniden is, unfortunately, effectively useless for me on these channels.

-Mike
 

IXNAY

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Mike (and others), that explains what is going on here. Ventura County Sheriff and fire have gone narrow, as has the forest service and CDF. It looks like I have to go back and edit all those frequencies. I use PSREdit
software, which is the only way I can program the PSR600. The front panel controls are too complicated.
Right now I have only tried audio boost. So now I'll try both (boost+FM N) and see how it works out.
 

Mike_G_D

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Mike (and others), that explains what is going on here. Ventura County Sheriff and fire have gone narrow, as has the forest service and CDF. It looks like I have to go back and edit all those frequencies. I use PSREdit
software, which is the only way I can program the PSR600. The front panel controls are too complicated.
Right now I have only tried audio boost. So now I'll try both (boost+FM N) and see how it works out.
That's what I do with all of my FMN programmed channels on the 500/600 series units - make them FMN+6dB audio boost. Not ideal but the best that can be done given the inherent limitations of the units' design. Still, like I said, if I can only be given "half" the solution I'll take the tight IF filter part and boost the audio in crude fashion. I do really wish they could make a firmware update that would allow FMN manual selection in search modes though.

One caveat concerning the FMN mode though - if there is a strong signal present on an adjacent frequency to one of your programmed NFM channels, say a nearly constant one from local interference or other nearby source or even simply a very strong real two-way source on and adjacent frequency, you may notice the unit will stop on the NFM channel during scan even when no signal is present actually on that channel and you may hear a very brief spurt of noise - like a very brief "pfft" sound - and then the unit will go quiet and continue to scan after the prescribed delay time. From what I can tell, this is due to another inherent design limitation of the 500/600 series units' FMN mode - there is a slight delay before the narrow IF filter is kicked in when the so-programmed channel is selected either during scan or via manual mode so that the channel is initially selected in standard "wide" mode before the narrow filter kicks in. If your source of adjacent frequency interference is constant, say from some local noise source like some EMI from some nearby electronics (cable box, TV, etc.) and you are using an attached and/or indoor antenna, then this can be very annoying as the unit will continue to sporadically stop on the NFM channels and make the short spurt of noise then go quiet after the narrow filter is switched in and sit on the NFM programmed channel until the delay time passes then continue to scan (assuming, of course, that no actual desired signal is present on the channel). There is no way around this short of moving the unit away from the source of interference and/or using the attenuator. Just FYI.

-Mike
 
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