Brian,

Signal Strength is determined by the noise level of the inputted audio from the receiver.  Normally, with audio from a good discriminator source, calibrated by the radio tune rxnoise command, you'll have a signal strength of around 500 to 600 with NO signal.  This is shown by inputting the command "radio tune rxsquelch".  This number goes up as the noise level is reduced (from an increasing signal level making the FM receiver quiet). 

A level of 999 with the adapter unplugged from the receiver is the expected result.  The adapter/application doesn't know if the receiver has reached full quieting or if the audio has simply disappeared from the adapter.  The result is the same no matter which happened. 

In the case, no signal input results in an audio signal that's full of broadband noise.  It's a loud white noise.  When the receiver quiets, the amount of white noise is reduced, to a point of "full quieting" where the receiver is not outputting much or any noise.  This is why a strong signal into the receiver shows as 999 and no signal results in something between 0 and 500 or so. 

Again, this all points to muted audio from the receiver, and that by definition is NOT discriminator audio.  Real discriminator audio is not muted, or filtered in any way.  Please have a look at the discriminator audio from your receiver and make sure noise is there when the receiver is not hearing a valid signal.

Kevin




On 12/23/2018 9:58 PM, Brian G wrote:

Hi Kevin, thanks very much... I understand 999 is supposed to represent a strong fully quiet signal. That's the issue, there isn't one...but app_rpt thinks there's a signal present and is sending it 100% of the time, unless I use CTCSS decode (or plain vox), which I don't want to do for this node.

I'm using the discriminator output from a Motorola Radius M120, but the same issue occurs on other rigs, and the "signal level" is still at 999 regardless of if there's a radio connected to the dongle or not :( If there's no input, signal level should be at (or close to) zero, shouldn't it?

On Sun, Dec 23, 2018 at 9:29 PM Kevin Custer <kuggie@kuggie.com> wrote:

Brian,

If the application reports 999, my guess is you aren't inputting raw discriminator audio into the interface.  If the audio you are inputting into your radio interface has been Low Pass Filtered or is squelch gated, you cannot use DSP for noise squelch detection.   The application will tell you if there is sufficient squelch noise to utilize DSP noise squelch detection after you do the "radio tune rxnoise" command. If it fails, the audio type, amount, or quality isn't sufficient for DSP noise detection.

When fed with good discriminator audio, the noise squelch detection in app_rpt is nearly as good as a Motorola M6709 (analog) squelch chip, commonly thought to be the high water mark in noise squelch detection.  999 is the total absence of noise, and is the result one would expect with a full quieting signal, or an inappropriately muted RX audio line.

Kevin W3KKC



On 12/23/2018 7:28 PM, Brian G wrote:

Hi all, Merry Christmas

My local club is working on setting up a few ASL nodes, and I've been busy modifying some CM119 based USB sound cards to control the radios. I'm using this card/dongle: https://www.amazon.ca/Syba-SD-CM-UAUD-Adapter-C-Media-Chipset/dp/B001MSS6CS

I've done several so far and have a few nodes working great with CTCSS carrier detect. One issue I'm consistently running into, is when setting the "RX Squelch Level" in the radio-tune-menu, it detects the current RX signal strength as 999. This results in the inability to use anything other than CTCSS for carrier detect. I can make plain old VOX work too, but it results in considerable (2 sec+) hang time.

Using DSP carrier detect won't work because it's impossible to set the squelch high enough (max value 999). It always thinks there's a signal present when there isn't. This happens regardless of whether there's a radio connected to the dongle or not. As a matter of fact it happens even when plugging in an unmodified dongle right out of the package. I need it to work with no CTCSS. VOX would be fine if it wasn't for the hang time, but would rather use the DSP option in usbradio.conf. 

I assume it's just the nature of the beast, and if I want it done properly I need an adapter and radio capable of COS. But maybe someone knows if there's something that can be done or not?  Anyone have any insight on how to either eliminate the VOX hang time, or eliminate that "invisible" signal? I might be overlooking something. Thanks for reading!

--
Brian - VA3DXV

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