Discriminator Tap

moonbounce · Jul 27, 2015

I did a lot of research for the discriminator tap, starting with Bill Cheek. None of them gave me a good decode while using DSD+ to decode digital signals so I tried my own version and got much better results. What I now have on two tapped scanners is a 10mf capacitor from the test point and a 33k omh resistor in series that is connected to my tape out on one scanner and a plug I put in another scanner.

First attempt was with a straight wire from the discriminator takeoff to the plug at the back of the receiver got about a 40% decode rate, added a 10 mf cap got about a 50% decode signal ( meaning that 50% of the signals got a good decode the rest were poor decodes ). Next I put a 12k ohm resistor in series with the cap and got about a 75% good decode. Next I took the 12K ohm resistor out and replaced it with 33k ohm resistor ( that was the highest value I had ) and now have 100 % clean signal.

Moonbounce

mikewazowski · Jul 27, 2015

Thread moved.

frazpo · Jul 27, 2015

moonbounce said:
I did a lot of research for the discriminator tap, starting with Bill Cheek. None of them gave me a good decode while using DSD+ to decode digital signals so I tried my own version and got much better results. What I now have on two tapped scanners is a 10mf capacitor from the test point and a 33k omh resistor in series that is connected to my tape out on one scanner and a plug I put in another scanner.

First attempt was with a straight wire from the discriminator takeoff to the plug at the back of the receiver got about a 40% decode rate, added a 10 mf cap got about a 50% decode signal ( meaning that 50% of the signals got a good decode the rest were poor decodes ). Next I put a 12k ohm resistor in series with the cap and got about a 75% good decode. Next I took the 12K ohm resistor out and replaced it with 33k ohm resistor ( that was the highest value I had ) and now have 100 % clean signal.

Moonbounce

I use a 10k resistor on my BC890XLT. I have always had good luck with decoding pretty much anything that needed unfiltered audio. I have had different results for different scanners so that may make a difference.

Ive also wondered if you could install a variable resistor on a Disc Tap.

bama9999 · Jul 27, 2015

frazpo said:
I use a 10k resistor on my BC890XLT. I have always had good luck with decoding pretty much anything that needed unfiltered audio. I have had different results for different scanners so that may make a difference.

Ive also wondered if you could install a variable resistor on a Disc Tap.

I used a 10k resistor on a tap I installed on a BC355N and it worked great as well. I know that some use capacitors in addition, but I had great decodes with just the resistor.

moonbounce · Jul 27, 2015

frazpo said:
I use a 10k resistor on my BC890XLT. I have always had good luck with decoding pretty much anything that needed unfiltered audio. I have had different results for different scanners so that may make a difference.

Ive also wondered if you could install a variable resistor on a Disc Tap.

I also wondered what a variable resistor would do, if my setup wasn't working so well I would give it a try.

SCPD · Jul 27, 2015

I use a 47k ohm pot. The load presented by the host PC audio input varies so being adjustable makes life easier.

N0JRD · Jul 28, 2015

What type of cable do I need from my tapped scanner to my computer? Do I plug it into the microphone input of my computer?

bama9999 · Jul 28, 2015

I used an 1/8 inch stereo cable to the mic input of my computer when I was using a discriminator tap. Worked great with no issues.

wkm · Jul 28, 2015

3.5mm mono or stereo cable. I used a RS shield cable with one mono plug on it. Then soldered a stereo plug on the other end after using a foot of it for the d-tap. I did that so the left channel wouldn't be shorted to ground since a d-tap is mono. No need for splitters and can be put in any in-line or mic jack. A mic jack should be mono but with the elimination of in-line on some laptops I suspect they are stereo.

slicerwizard · Jul 29, 2015

moonbounce said:
I did a lot of research for the discriminator tap, starting with Bill Cheek. None of them gave me a good decode while using DSD+ to decode digital signals

Because all of those tap instructions are for feeding an opamp-based data slicer, not a PC sound input.

so I tried my own version and got much better results. What I now have on two tapped scanners is a 10mf capacitor from the test point and a 33k omh resistor in series that is connected to my tape out on one scanner and a plug I put in another scanner.

Yes, you effectively made a voltage divider, which is what's required.

First attempt was with a straight wire from the discriminator takeoff to the plug at the back of the receiver got about a 40% decode rate, added a 10 mf cap got about a 50% decode signal ( meaning that 50% of the signals got a good decode the rest were poor decodes ). Next I put a 12k ohm resistor in series with the cap and got about a 75% good decode. Next I took the 12K ohm resistor out and replaced it with 33k ohm resistor ( that was the highest value I had ) and now have 100 % clean signal.

33k is probably right in the ball park. You should combine them for 45k and give that a test.

DaveNF2G · Jul 30, 2015

If you permanently tap a particular scanner, then you don't need variable components once you have determined the best values for that specific circuit.

moonbounce · Jul 30, 2015

DaveNF2G said:
If you permanently tap a particular scanner, then you don't need variable components once you have determined the best values for that specific circuit.

Determining the best value for the components means a lot of replacing different resistors, a tap would eliminate that.

slicerwizard · Jul 30, 2015

DaveNF2G said:
If you permanently tap a particular scanner, then you don't need variable components once you have determined the best values for that specific circuit.

Until you need to plug it into a different computer or USB sound device. And there is no guarantee that a particular setting is best for all digital protocols; there is a huge difference between NXDN48 and ProVoice signals.

DaveNF2G · Jul 31, 2015

I have not seen any technical explanations or assertions that it makes a difference what sort of slicer is being used. It only seems to matter that the discriminator circuitry in the scanner is not loaded down and is isolated from any stray DC currents, which is what the resistor and capacitor are for.

sameehosen · Jul 31, 2015

HELLO

Thank you, my friend
I had some attempts also reported using variable resistance but failed all
Please portray and explain what you did and attached pictures here of the interest, Thanks

slicerwizard · Jul 31, 2015

DaveNF2G said:
I have not seen any technical explanations or assertions that it makes a difference what sort of slicer is being used.

What sort of slicer? Don't know what you mean.

It only seems to matter that the discriminator circuitry in the scanner is not loaded down and is isolated from any stray DC currents, which is what the resistor and capacitor are for.

Then according to your theory, if a 22k resistor works well, so will a 47k, since it can't put more load on a discriminator circuit, but real world experience says otherwise.

And if 10k doesn't load up the discriminator, it's all good? Even if it's overdriving the PC's mic input?

K5MPH · Jul 31, 2015

moonbounce said:
I also wondered what a variable resistor would do, if my setup wasn't working so well I would give it a try.

One time I used a variable resistor on a old RS scanner and it kept drifting on me while decoding just FYI...........

frazpo · Jul 31, 2015

kb5zcs said:
One time I used a variable resistor on a old RS scanner and it kept drifting on me while decoding just FYI...........

You suppose the resistor was bad? I have to adjust those on some of the equipment I service and it's critical that they hold the setting.

EDIT - Well maybe its a pot that I am thinking of. However, unsure why it would drift like that

DaveNF2G · Aug 1, 2015

pot=potentiometer=variable resistor if wired as one, or variable voltage divider.

K5MPH · Aug 1, 2015

frazpo said:
You suppose the resistor was bad? I have to adjust those on some of the equipment I service and it's critical that they hold the setting.

EDIT - Well maybe its a pot that I am thinking of. However, unsure why it would drift like that

Maybe it was bad could of been it was a long time ago........

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