Nac decoding ?

[DaveNF2G]

I just caught some conventional P25 traffic using a tapped BC-235. I feed the audio from the tap to kNACk via soundcard input. Result = no decode.

My previous setup, direct earphone output from an untapped BC-296, produced apparent decoding activity, with a NAC display of 000 during silent periods and FFF during transmissions.

 

[grem467]

Im also getting a greyed out 000

setup is a motorola maxtrac 16 pin, and works fine on the MDC program as well as unitrunker via the mic port. Windows XP, and on-board sound.

 

[SCPD]

Try this version - it will allow choosing any available sound input - not just the windows default. It should also run on Windows 98.

 

[SCPD]

Here's a software oscilloscope view of a relatively clean (mid to high 90's) C4FM signal. Compare this to mancow's sample (on the previous page of this thread) - I think you'll see the waveform here is smoother. This particular screenshot was taken from a BC780XLT with a cheap microphone pre-amp mounted inside. I'm normally not close enough to this particulare site to get such signal.

Hope this helps.

-rick

 

[carbineone]

Rick thank you . Your abilities are amazing.

 

[SCPD]

I was told about this program last week and have been playing around.
Works fine for me on my windows XP machine. No complaints there.

I also have an old laptop which runs win98 SE, and this is mainly used for radio software. The Knack runs but the first version doesn't decode. I was getting the same start up errors as others have. The latest version runs, but does not decode. As someone has stated i Get FFF only as a first decode and it stays stuck like this. I know the NAC on the transmissions I am monitoring use a totally different NAC from decoding them on the XP machine.

I am using a discriminator tapped 780xlt and the transmissions are line of site to me, and not to far away, so they come in well.

Thanks anyway for 'The Knack' Handy little program and can't wait to see it with more features. Who knows, maybe this will lead one day to freeware APCO25 voice decoding software!!!!

 

[SCPD]

The latest version runs, but does not decode. As someone has stated i Get FFF only as a first decode and it stays stuck like this.

Obviously there's something different. Grab a software oscilloscope program and compare the waveform between the two computers. It may be variations in load impedance vs. the XP machine (which can be fxied with a buffer amp) or the '98 box needs the signal level adjusted up or down - or both.
-rick

 

[mancow]

Rick, where do you get dibscope? I searched but haven't found anything about it on google.

 

[SCPD]

I wrote it. It was the prototype for the tiny oscilloscope window in T4Win. It's very limited compared to the other free 'scope programs other there. I can email a copy to you if you like.
-rick

 

[DaveNF2G]

Here are my test results with the latest kNACk.

(Same results for both sound source choices, so I won't duplicate them):

Tapped analog* scanner: No decode - stays on 000.
Analog scanner earphone: No decode - stays on 000.
Digital scanner earphone: ? decode - starts on 000, goes to FFF on first transmission.

* I haven't tapped any of my digital scanners yet. The type of scanner being tapped should make no difference if the processing of the signal begins after the discriminator anyway.

 

[SCPD]

Tapped analog* scanner: No decode - stays on 000.

To avoid any confusion - the program requires discriminator audio. Speaker or earphone audio won't work. (I'm sure Dave knows this - he's just being thorough.)

-rick

 

[mam1081]

You might check the quality of the received signal. I could not get a decode using a Pro-94, but could with a 780. Maybe try a different radio?

 

[wa8pyr]

Some options:

2. using the datasheet for a common a JFET op-amp - build a voltage follower.

Rick,

Unity gain or with some amplification?

Tom WA8PYR

 

[wa8pyr]

Some options:

2. using the datasheet for a common a JFET op-amp - build a voltage follower.

All;

Attached is a PNG snapshot of the unity gain voltage follower from the TL071/072/074 datasheet. This low noise JFET op amp should do the trick. It requires a dual-rail supply up to +/- 18vdc. I'm still looking for a easily available low-noise JFET op amp that only requires a single-rail supply so I can install the buffer right in the scanner and use it's power.

Tom

 

[SCPD]

Unity gain or with some amplification?

If using a single op-amp package - unity gain (you want the hi-Z input). If you have dual op-amp. Use the unity gain stage as the input (like the graphic you supplied - very nice!) followed by a gain stage with small pot / variable resistor.
-rick

 

[wa8pyr]

If using a single op-amp package - unity gain (you want the hi-Z input). If you have dual op-amp. Use the unity gain stage as the input (like the graphic you supplied - very nice!) followed by a gain stage with small pot / variable resistor.
-rick

Gotcha. I'll give it a whirl; I'd like to try UniTrunker on Delaware County's new P25 system, as well as try to snag some NAC info from the TSA at work as well as the fire and security folks at DSCC.

Tom

 

[mancow]

Will a TL081 dual op amp work? If so, how should it be hooked up to use the two stages as mentioned earlier.

 

[SCPD]

The TL081 is a single op-amp. The TL082 package holds two op-amps. For a working schematic ... go to the Wiki and locate the four-level FSK circuit. This one uses a quad op-amp and 3/4ths of a quad comparator. Use the first two stages (the first two op-amps) - where the first stage is a voltage follower and the 2nd is a gain stage. Ignore the rest of the schematic (the peak detectors and comparators). The circuit should run for weeks on a pair of 9 volt batteries.

The TL07X and TL08X are JFET adaptations of the "old school" LM741 family of op-amps. The drawback of these is the need for a bi-polar power supply - hence the two batteries. A more modern op-amp can be run from a single supply (eg. +6 volts and gnd vs. +9 volts and -9 volts).

If you don't mind the batteries - the circuit above definitely works.

-rick

 

[swest90]

After reading these posts, I feel lucky, it has worked for me from the very first try. I am using a 895XLT tapped run straight into the line-in at about 20% volume.

Awesome job once again! This stuff really impresses me.

 

[mam1081]

-snip-
...as well as try to snag some NAC info from the TSA at work....
-/snip-

You get anything on TSA? I get "001" or "000" on them. When I program an ASTRO radio for RX-only with those NACs, I don't get anything. That almost makes me think they are running talkgroups.