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Author Topic: AGWPE vs real TNC, and other AX25 questions  (Read 8096 times)
KC2YWE
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Posts: 14




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« on: August 01, 2011, 07:02:51 PM »

Hey all -

Got my ticket last summer, and worked VHF all day. Back home this summer with a new antenna (from Arrow Antennas, a fantastic J-Pole) and got my shack re set-up.

But I've been having trouble with AGWPE this whole time. I couldn't really afford a proper TNC, and had some spare computers lying around (I'm a computer tech) so I have it set up and working.

However, I'm having a lot of problems with it. With my new antenna, I'm decoding a lot more - and now trying to digipeat them.

That's the problem, but it's more a symptom. The sound-card modem just doesn't do a very good job realizing when a new packet is coming, and it shouldn't be colliding. So whenever I try to digipeat, I collide like crazy. I'm sure nobody appreciates that, so I've turned it off.

If anyone knows how to sort that out (maybe some AGWPE settings?) I'd be more than grateful.

But my real question is, is it worth it? I can afford a TNC now, and I was thinking about picking up a TNC-X (MFJ-1270X) locally. Will it perform better? I appreciate the idea of not being dependent on a program, and my instinct says that I'll do a better job transmitting/receiving. I also suspect that a lot of my problems are due to the fact that I'm not set up properly, which presumably a (relatively-)plug-and-play TNC would fix.

Am I way off course?
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K7RBW
Member

Posts: 398




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« Reply #1 on: August 01, 2011, 09:51:53 PM »

I've had decent luck with AGWPE, but you really have to have a clean audio channel all the way back to the antenna.
- Good, solid antenna
- Good, clean RF
- Good, clean AF (at the proper level)
- Good, clean coupling between the radio and the sound card. Any impedance mismatches, grounding, or level issues will mess up the reception
- Sufficient horsepower on the computer and make sure it's not running something that might prevent clean decoding (any hardware or software that might disrupt the processing)

If you tighten all that up, you should get decent decoding. At least, that's been my experience. But it takes a lot of electromagnetic vigilance to make it work. Many of these issues aren't a problem for standalone or dedicated TNCs (however, you'll still need a good clean RF signal to start from).

Good luck!
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KC2YWE
Member

Posts: 14




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« Reply #2 on: August 01, 2011, 11:09:32 PM »

Thanks for your reply. I don't have any ... anything (isolation, grounding, etc) between the radio and the computer. It doesn't seemed to be causing a problem (subjectively), but I'll be putting in isolation transformers and the attenuation circuit mentioned here: http://www.soundcardpacket.org/1cablerx.htm . However, the AGWPE scope seems to be giving good clean signals. See these images:

http://rmead.com/noise.png
http://rmead.com/packet.png
http://rmead.com/packet-best.png

Both packets were properly decoded, and the second one has clear harmonics from the 1200Hz tone, which says "good signal" to me. Channel noise is minimal (1/9 maybe) with my new antenna. I'm terrified of overdeviating, so I've got my soundcard all the way down (literally one or two clicks above muted). Packets seem to come through fine.

Is it just all in my head? If so, I may get a TNC for other reasons, but I don't want to rush out and buy one without an idea of what good it'll do me.
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K7RBW
Member

Posts: 398




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« Reply #3 on: August 02, 2011, 06:45:38 AM »

The photo you have labeled "packet-best" looks overdriven to me. You shouldn't have all those harmonics in a clean signal. There are only two frequencies sent (the mark and space freqs) and they are never sent at the same time, so those two freqs should not be mixing to create harmonics.

One thing to check is to see if your computer is getting the audio from the Mike-level input or the Line-level input. Some use the same jack for both (and I'm not sure how they differentiate the two). If you have to turn your soundcard volume all the way down to get anything close, it's likely that the audio out of your radio is line level (about 700mv-1v) or higher and your soundcard input is expecting mike level which is about 5mv.

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KC2YWE
Member

Posts: 14




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« Reply #4 on: August 02, 2011, 09:09:15 AM »

I fiddled with the audio that my radio outputs (a TM-V71A), and brought it down a bit to be more suitable for my mic input. The built-in soundcard doesn't have line-in. After I brought it down to the point where there was no clipping, there were still harmonics. I suspect the transmitter is overdeviating?

In any case, I get the best packet results when the noise is clipping (in the oscilloscope view). And since I'm decoding the vast majority of the packets I can hear above the noise, maybe it's not a problem?

I may get the TNC anyway. Being able to simply plug it in, not worry about deviation and line-level/mic input attenuation and isolation and so on is kinda appealing to me right now...
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KD8DEY
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Posts: 352




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« Reply #5 on: September 07, 2011, 09:08:34 PM »

Since a Real TNC is in hardware Vs software such as AGWPE there shouldn't be as much load on the computers CPU/Memory resources which could be an advantage on older computers...... Smiley
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LA9XNA
Member

Posts: 109




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« Reply #6 on: September 09, 2011, 12:22:58 AM »

You can pick up a good used unit for a fer $ at Ebay. Because a lot of folks have equipment laying around from the Packet days.
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