Discriminator tap work around

[moonbounce]

I have a discriminator tap going from my scanner to to my computer to decode AP 25 digital signals. The frequencies I listen to have both analog and digital signals. As I am sure you already know when the analog signal is active you won't hear any digital broadcast signals until the analog signal stops.

So I was wondering if anyone has tried cutting the signal after the discriminator tap which would mean tinkering with the circuitry as in cutting the signal path after the tap. This would I think, stop the analog signal from tying up the scanning operation as all signals would be routed through the tap to the computer. I have a schematic for my scanner and I just thought I would throw this out there and see if anyone can see any problems with doing this. Thanks in advance.

Moonbounce

 

[krokus]

Which scanner?

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[moonbounce]

Which scanner?

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Pro 2006

 

[Voyager]

Likely any point after the discriminator will have processed the signal enough to distort it and result is poor decoding.

 

[SCPD]

I think MB is saying the scanner uses squelch to know when to stop on a channel but it can't tell digital from analog calls and he would like it skip the analog calls.

MB - you'd need something to bump the radio off the analog call to resume scanning. If you're signed up for the DSD+ Fast Lane - ask them about doing this.

 

[slicerwizard]

I think MB is saying the scanner uses squelch to know when to stop on a channel but it can't tell digital from analog calls and he would like it skip the analog calls.

Yes, he wants his analog, non-trunking scanner to scan Fleetnet (mixed mode SmartZone) channels and not stop on the analog (EMS) traffic. The scanner has no way of differentiating the two, nor, I assume, does it have any sort of PC interface other than its discriminator tap. One would have to add a circuit to detect the 300 bps data riding on the analog calls and use its output to block squelch opening. A 0-150 Hz lowpass filter fed with flat audio is the heart of what he needs. A quad op amp IC is the obvious route - two op amps for the active filter and one for controlling the squelch defeat output.

flat audio --> LP filter --> diode --> smoothing cap --> op amp with trigger threshold --> squelch circuit

150 bps data passes through LP filter and charges cap; third op amp blocks squelch opening

static and P25 digital data blocked by LP filter; cap discharges through drain resistor (not shown); third op amp unblocks squelch opening

Hm, I'd probably use the extra op amp to condition the flat audio, e.g. provide the active filter with a low impedance source signal.