996 Discriminator

[dbrown2]

Thank you for the help. And it does a little, your right still a little fussy. But from what I got so far. I would only expect to get the control channel for a system I am monitoring. It would give me something like 865.500 or whatever. It does not display any type of text that is transmitted?

 

[SCPD]

David,

Go to the website on my previous post. It has another link that provides some good information on data slicers and trunker.

 

[GrayWolfSA58]

post the mods you make we all want to know,taged

 

[mikewazowski]

Anybody found an internal slicer point yet?

 

[dbrown2]

What is the differance between a 2-level data slicer and a 4-level data slicer? Can both decode MDT's?

 

[ka3jjz]

No problem on the newbie question. All of us were there at one time (and some of us still are). The discrimination tap for most scanners is used in conjunction with what's called a 2-level data slicer for decoding digital information such as ACARS and other information transmitted in FSK format. Most, if not all trunked systems transmit what's called a control, along with the audio channels, which is basically a series of tones at certain intervals that can be understood with a computer. By using the discriminator (base audio unfiltered) from the scanner and a data slicer you can use a program called trunker. This allows you to see the control channels used for trunked systems.

Only one correction I would make here, and it's an easy mistake to make if you're unfamiliar with how a FM receiver works. For ACARS decoding, you don't use the discriminator output; the discriminator is a circuit that is not involved in AM reception; just a shielded connection from a speaker or headphone will do nicely.

I've heard of some folks doing it this way, but I strongly suspect this is due to insufficient isolation between stages, with a little signal leakage getting into the discriminator. You're much better off with the speaker or headphones when decoding ACARS with software such as the well-known ACARSD.

73s Mike

 

[mike_s104]

I'm looking to get a 996T. I would like to mod it. what is the best software to use with the tap?

 

[N9JIG]

I'm looking to get a 996T. I would like to mod it. what is the best software to use with the tap?

Depends on what you want to do with it. Trunker and it's many variants are great for decoding trunked signals for example.

 

[mike_s104]

Depends on what you want to do with it. Trunker and it's many variants are great for decoding trunked signals for example.

thanks...I think that answered my question.

I want to decode a CC for a public safety P25 system and DoD P25 380MHz systems.

 

[mike_s104]

I have discriminator taps in just about all of my scanners using RCA jacks, I use them for both Trunker type applications and PL decoding. I have the caps in most of the radios, as I have read many places that they are needed to prevent bad voodoo if the discriminator is shorted. That said, I don't have them in all my radios and have never had a problem. I also have not seen any difference in performance one way or another...

so, you should solder the cap inline with the discriminator to the center conductor for the RCA jack and of course the outside conductor will be ground. mount the RCA jack in some fashion on the rear of the unit. buymake a datasslicer, connect it to the RCA jack and the PC. use trunker to decode the data. use a second scanner to hear the audio from the decoded control channel. is this correct?

 

[N9JIG]

so, you should solder the cap inline with the discriminator to the center conductor for the RCA jack and of course the outside conductor will be ground. mount the RCA jack in some fashion on the rear of the unit. buymake a datasslicer, connect it to the RCA jack and the PC. use trunker to decode the data. use a second scanner to hear the audio from the decoded control channel. is this correct?

In a nutshell, Yes!

 

[mike_s104]

In a nutshell, Yes!

cool, I wanted to get everything straight before I ordered the 996T. Thanks!!!

 

[DaveNF2G]

I was thinking that it might be better to just connect a straight wire between the tap point and the connector on the radio, then put any resistors or capacitors that might be needed in the slicer circuit.

 

[slicerwizard]

I was thinking that it might be better to just connect a straight wire between the tap point and the connector on the radio, then put any resistors or capacitors that might be needed in the slicer circuit.

Ouch. I'd be careful with that, Dave. You're risking a direct short at the external connector or in the slicer. If the demod chip can't handle that, you've hosed an expensive scanner.

Conventional wisdom suggests that at a minimum, a 5k to 10k resistor right at the tap point will protect against shorts and will isolate that point in the scanner from the capacitance of the long cable run to the slicer.

Personally, I would also add a tantalum cap (1 to 10 uF) in series with the resistor, since I don't like the idea of having that tap point connected to +/-12V without a DC blocker (even through a 10k resistor) if the slicer is miswired. That kind of accident can happen while designing / building / troubleshooting homemade slicers. Since many slicer designs need a DC block anyway, I'd just put it in the scanner.

There have been suggestions that tantalum caps will distort the waveform reaching the slicer or sound card, but those distortions only affect the very low end of the spectrum, which we don't care much about and they are easily eliminated by oversizing the cap and reducing the current flow through it. Data slicers and sound card microphone inputs have such high impedances, that a 10uF tantalum _is_ very oversized for the application.

I have used 10uF tantalums on discriminator taps for close to 10 years now and haven't had any problems with them. I've looked at the waveforms with a scope and with a WAV file editor and I don't see any distortions.

 

[Dewey]

Since this thread is taking on a discriminator tap design view, I do have a question. I've seen a couple of recommendations of what to add into the discriminator tap, and where. I've been running straight line for years without any problems, but the thought of "protection" does come up in my mind every now and again... especially given the rising cost of the newer radios.

First, if I add an inline cap, I take it that I should connect the negative to the tap point, and the positive to the lead running to the center conductor of the tap jack? While at this point, does it make any differerence if I attach the cap at the tap point or at the jack (I've seen both)? Second, I've seen one recommendation where someone added a connection between the center conductor and the ground of the jack (can't remember if it was a cap or resistor) for protection. If anyone has heard of the, does it add to the "protection"?

I know if I ask ten different people, it is quite possible that I will get nine to eleven different answers, but I'm just looking for a consensus from those in the know.

Thanks,
Dewey

 

[slicerwizard]

if I add an inline cap, I take it that I should connect the negative to the tap point, and the positive to the lead running to the center conductor of the tap jack?

No. Since the tap point will most likely have a positive DC bias relative to ground, you would connect the positive side of a polarised cap to it.

does it make any differerence if I attach the cap at the tap point or at the jack (I've seen both)?

If the connection at the jack end of the jumper wire breaks and comes in to contact with anything (power or ground), would you want the other end of the wire connected directly to the sensitive tap point or to a DC blocking cap and/or current limiting resistor? Given the cold solder joints that some hobbyists create, which would you have me recommend?

I've seen one recommendation where someone added a connection between the center conductor and the ground of the jack (can't remember if it was a cap or resistor) for protection. If anyone has heard of the, does it add to the "protection"?

A very small cap (around 0.001 uF, maybe even less) will keep any high frequency noise that the external cable might pick up from getting in to your scanner. It's good engineering, but unless you plan to run a transmitter nearby, I wouldn't worry about it. The external cable should be shielded anyway, so there shouldn't be much of anything coming down the wire anyway. The cap can also soak up some of the static charge that might be on the cable before you plug it in. It's not going to provide any significant protection though.

I know if I ask ten different people, it is quite possible that I will get nine to eleven different answers

You can count on it.

 

[Dewey]

You can count on it.

Thanks slicerwizrd

Dewey

 

[mtindor]

Look at the picture in this post:

http://www.radioreference.com/forums/showpost.php?p=297579&postcount=7

By looking at that picture, can anyone identify a good point to hook up the ground for a discriminator tap?

I ask because I'm not mounting the audio jack directly to the back of the radio - instead the wires run outside of the radio and the jack will be mounted elsewhere. I want to make sure I have the ground that I need.

Thanks

Mike

 

[slicerwizard]

I see several options - you could loosen one of the screws holding the board in and wrap your ground wire under it; you could solder to the mounting tab points on either side of where the DB9 pins connect to the board; you can probably use those two large pads that are down and left of the DB9 - they look like they hold something big (like a transformer?)

Whatever you choose to use, verify that it actually is grounded; you should measure zero ohms from it to the metal chassis.

 

[mtindor]

I see several options - you could loosen one of the screws holding the board in and wrap your ground wire under it; you could solder to the mounting tab points on either side of where the DB9 pins connect to the board; you can probably use those two large pads that are down and left of the DB9 - they look like they hold something big (like a transformer?)

Whatever you choose to use, verify that it actually is grounded; you should measure zero ohms from it to the metal chassis.

Well, that's the kicker - no multimeter here. So I can't check if it's ground or not. Thought there may be some 'telltale' sign.

Scanner: bcd996t

red wire:
disc --> 10k resistor --> 10uF tantalum capacitor (didn't see a +/- so just installed it whatever direction was convenient) --> audio jack.

black wire: ground screw on chassis --> audio jack.

results: nothing - nada. I don't get anything coming from it.

Wonder if the 'Disc' on the 996 really delivers what we are anticipating? I'm going to use the uniscope and see if I detect anything at all.

I believe I have it pretty much tapped just the way Rich does.

Mike