KellyKeeton dot Com v3.0 reverse engineering life

26Sep/080

Using FRS radio for Digital Modes

So I have been getting into digital modes a lot more since HRD really has stepped up the cool factor. However I have a few issues with testing and playing. First I have no good antenna in my RF-Hell area to get out to any fellow nerds who have the time to play. Second I have no radio yet that is good for transmitting hours of testing.

I had a idea for the problem I have a stack of old FRS radios that I never use whynot adapt the VOX of the Motorola FRS to the PC for use with HRDDigital Master 780

I set to work, first thing I learned was that the 2.5 mm jack that motrola uses isthis weird "long jack" you can just hack up some cable for cell phone vox kits, Ihappen to have a texas instruments link cable from the old TI-85's I have sittingaround. I have more then one so hack hack. I hacked up this particular cable becauseof two things 1. it had a nice ferrit coil so I eliminate (if any) RF into my soundcard 2. you can hack away plastic and T.I. actually uses the long 2.5mm plug thatmotrola uses to make you buy their shit.

After some tinkering with the VOX I learned that you must short the MIC to get theradio to power up in VOX mode, however if you short it outright you get PTT mode.Simple fix throw in a resistor, I had a stack of 450kohm resistors sitting on my deskfrom a prior mess so used them. However I assume a 1k will work just fine.

Then just play simple hook up game, plug the radio speaker into the PC MICand radioMIC into PC speaker.

Simple little test shows it works, the volume is high but appears to not be clippingI havent scoped the audio yet to check but rather just went right to a test. hookedup two cables for two FRS radios. Turn on DM780 on two PC's and yup I have communicationin CW. I havent tried any other modes yet but I would assume the only issue is theaudio gain, DM780 will allow you to transmit at less then 0db gain right from theapplication, this is a setting to tinker with. Also volume on the radios I didnt fusswith (it was 2am with a working product I wanted to sleep)

Improvments might come from putting a cap in line or a POT to lower the input to theVOX on the radio, but I assume that I can fix all this in software with no need foradditional hardware.

Next step is to heatshrink it all up and now I have a nice little demo kit for digitalmodes. I can also have some fun at any public space by transmitting oliva and watchingpeople think its space creatures.

I have no idea if this is FCC legal, I couldnt find any data saying that you canttransmit digital modes on FRS bands. Seeing as its public domain frequency space Iam also not very concerned with my transmissions since people also transmit musicand swear there.

I used two Motrola T5420 FRS radios in this test. I also used 500k resistor

Update: I got some feedback in the legality of this - longboring legal document summerised: The most onerous restriction seems to be thatthe data transmission can't exceed one second, and there can't be more than one ina thirty-second period.

So if your super worried plug it into something else ... hell thinking about it youcould just plug it right into another computer ...duh. But where is the fun in that?Legally you also cant swear on the FRS radios or CB radio so keep that in mind.

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