#1 2014-12-21 19:25:57

From: Kuala Lumpur, Malaysia
Registered: 2014-12-01
Posts: 65

Episode 180: Clarifying the Amateur Bell 202 Modem - Modem Modulation

Episode 180: Clarifying the Amateur Bell 202 Modem - HamRadioNow

Any comments and feedback on this matter ?

I always gets feedback that the MicroAPRS modem I'm using could not be decoded by some TNCs like NET51TNC ...

I never experience this issue before but my friend that was testing both my SVTrackR with BH4TDV X2A tracker, the iGate running NET51TNC can "hear" the X2a nearby but not my SVTrackR / MicroAPRS modem at the same distance...

Another user feedback that the modem sound is "too short" ... I'm not sure what it means ..

Referring to the schematics below, only hook up to the LINE_LEVEL_OUT instead of the MIC_LEVEL_OUT ..

Will using Line Level out give too much/over voltage / audio signal or over modulation to the handy transceiver Mic input ??

16072264252_e06b3a346f_n.jpgMicroModem Schematics by stanley_seow, on Flickr



#2 2014-12-22 12:30:42

Registered: 2014-12-01
Posts: 129

Re: Episode 180: Clarifying the Amateur Bell 202 Modem - Modem Modulation

Hi Stanley!
That's a very interesting talk on AFSK modulation and APRS. I agree with many of the points he is putting up, especially about standards and documentation, which is essentially missing regarding the APRS networking stack. I think I will do a blog post sometime after the holidays with comments on this talk.

I don't exactly know what he means by "short", but it might be that the preamble / tail is too short. If the preamble is not long enough to allow the receiving radio to open up it's squelch, the packet will obviously get lost. Maybe a first try would be to increase the squelch to around 500ms (it's long, but some slow radios actually need that high a preamble to receive correctly!). The defaut preamble in MicroAPRS is 350ms, which I believe is a rather sensible value.

Also, you can try hooking up the radio directly to line level output, it might or might not distort the signal more than do any good, it really depends on the radio. Or you could add a trim-potentiometer in place of R5, as I do on the newer MicroModem hardware, which will allow exact adjustment of the output level!

But I would definitely go with trying a higher preamble first!


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