Actually, yes, as long as the low-frequency cutoff is reasonably close to the inverse of the baud rate, and the DC offset is 0V on the receiver side.
Why Do My Discriminator Taps Always Stop Working Eventually?
- Thread starter CanesFan95
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slicerwizard
Member
You have little to no knowledge or control over the receiver software being fed by an arbitrary user's discriminator tap, so you have no way of knowing what your added distortions will do to decoding, especially for already-degraded weak signals. Any "It'll be fine" assertions are baseless.
And who would sign off on your degraded RS232 circuit design? Likely no-one.
And who would sign off on your degraded RS232 circuit design? Likely no-one.
slicerwizard
Member
Discriminator Tap Plans/Diagram Question
hey all! im getting ready to tap my 106, so im dotting my I's and crossing my T's.. just freshining up on the job and ive had a question now for a while.. ive seen 4 diff styles of taps made, 1 comprised of 2 resistors,1 tant capacitor, another, 1 resistor,1 tant capacitor and another...
And you're ignoring the fact that pretty much every mic/line in circuit has inline capacitors that block DC and everything below 20Hz or so. Some block everything below 100Hz. Cheaper audio ADC circuits typically have a higher low frequency cutoff. And yet they still work...
prcguy
Member
Mic/line is analog audio. ADC circuits are analog to digital converters. The analog side would be very forgiving of a capacitor input and would only worry about low frequency roll off, the digital side not so much. I've had to hand build ultra fast analog to digital converters and sample/hold circuits to 3GHz and have dealt with DC offsets and what they can cover up. The discriminator output of a scanner is not as critical but it still needs some special attention IF the end result is passing digital data.
And they are usually connected together in computers, particularly when being used to record, or process discriminator audio. The analog audio passes through a capacitor that filters out DC and subsonic freqs at a minimum before it is digitized and the digital data sent to dsd+ or whatever.
prcguy
Member
Yes, but when using an analog receivers discriminator output to pass digital data that is essentially 1s and 0s going from a zero volt reference to some higher DC level or multiple stepped levels, you have to treat the path very carefully to keep some resemblance of a square wave with proper rise/fall times and pulse widths.
Arkmood
Member
Thought so too until went without both resistors/capacitors (also non shielded cables) on Pro93/95... can only report what was observed first hand through experimentation with scanners mentioned in post, and to be clear taps were for cc decoding with a variety of MB audio inputs from Supersocket7 to 1150 - some sound card inputs included all worked as expected...
CanesFan95
Active Member
Discriminator Tap Plans/Diagram Question
hey all! im getting ready to tap my 106, so im dotting my I's and crossing my T's.. just freshining up on the job and ive had a question now for a while.. ive seen 4 diff styles of taps made, 1 comprised of 2 resistors,1 tant capacitor, another, 1 resistor,1 tant capacitor and another...forums.radioreference.com
So I put a 2nd resistor connecting the ground and tap point? I thought in electronics it's considered bad to touch a negative and positive, like that would cause a short.
Resistors don't short.
CanesFan95
Active Member
So that 10 uF capacitor, is that not polarized? And the cord to the PC mic input has to be shielded? I haven't seen shield mono / stereo cords before.
KA1RBI
Member
I use an old PRO-2006 and OP25 rx.py -a - that combo absolutely blows the doors off of my USB dongle receivers. There are literally dozens of channels I can't hear on the RTL.
Trying to connect a good outdoor antenna to the RTL is literally a joke. With --gains reduced by 20 dB I can still receive a local VHF control channel -- tuned to an overload/spur frequency a MHz off from the actual frequency.
The PRO-2006 in addition to the disc tap also has a 455 KHz IF tap and a homebrew down converter that outputs a 24 KHz IF (nominal). This feeds into the sound card via one of the DC block caps that slicerwizard is so very fond of. It nips out a 3v-DC offset that seems to ride on the input lines.
Using that setup (OP25 rx.py -A) pulls in LSM including Steuben County's VHF P25 LSM repeater. That's about a 50-mile trip. LSM doesn't work well through a disc tap...
Max
CanesFan95
Active Member
LSM doesn't work well through a disc tap.? But I thought you said the PRO-2006 blows the doors off the dongles.
For raw RF reception, yes. Just not for decoding bits from received LSM.
If you'll reread what KA1RBI wrote he stated "...in addition to the disc tap also has a 455 KHz IF tap and a homebrew down converter that outputs a 24 KHz IF (nominal)."
The disc tap is not what he uses for the LSM system. He downconverts the IF to a suitable frequency that he soundcard can use and then processes that whole signal (no discriminator pre-processing) in the digital domain and likely does an I and Q extraction and further processing. That is significantly different than a simple "disc tap".
A standard FM discriminator will essentially ignore any amplitude components in a signal which can cause problems in LSM CQPSK and more complex modulated signals like pi/4DQPSK, etc. It works ok for signals where the information is only contained in phase and frequency changes though, like 4 level FM such as conventional P1 modulation uses.
-Mike
slicerwizard
Member
Yes, it is.
It's best practice and more important in harsh RF environments. You don't have to.
slicerwizard
Member
You're conveniently brushing aside the facts that (a) users are free to not use devices that have high cutoff frequencies (most USB sound devices are inexpensive and work quite well) and (b) that each undersized series cap in the audio chain increases the cutoff frequency and the distortions introduced by these caps are additive and increase BER. This tap audio stream is already suffering enough without your undersized DC blocker.
So you claim. You've shown no proof that all of these implementations result in clean P25 HDU decodes, for example.
prcguy
Member
I was just reading through the manual for one of my old PL/DPL repeater tone panels. It says to connect the audio in directly to the receivers discriminator, no caps, no resistors, otherwise it may not decode DPL. Similar for connecting the tone panel to the repeater transmitter, you go right to the FM modulator where it will frequency shift key the transmitter for DPL and a capacitor in that circuit would ruin things.
If a receiver or device is designed for you to connect right to its discriminator then it probably has a DC offset adjustment circuit which would be diddled to zero out DC offsets. I would not expect scanner receivers or other consumer grade things to have this.
If a receiver or device is designed for you to connect right to its discriminator then it probably has a DC offset adjustment circuit which would be diddled to zero out DC offsets. I would not expect scanner receivers or other consumer grade things to have this.
That's not what's being done here though. The scanner discriminator tap isn't expecting anything to be connected to it, so ignoring DC bias or overloading the circuit can cause the scanner to quit working when you connect to the discriminator.
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