So I’ve been using Xastir (linux) to provide a sound-card modem for packet radio with quite a bit of success, however, it is kind of hackish and inconvenient to lug around a netbook for the sole purpose of APRS tracking.
To solve the problem of the kludgy netbook I tried a TNC-X which is a hardware TNC. I was quite happy to assemble the kit but quickly noticed a number of disappointments. With what appeared to be such a well thought out design, how in the heck did the author overlook galvanic isolation of the PTT circuitry?! Well, sure enough … the PTT no longer works so until I bother digging around in the circuit, I have a receive-only TNC. Total crap!
I got to thinking that perhaps I could just build my own damn TNC using an Arduino. I looked for some Bell 202 Modem ICs and keep coming up dry (or looking at a WAY over-priced MX614 IC). I know that it is possible to use the processing power of the Arduino to do “zero-crossing” frequency decoding, however, I am not sure how much I like that thought. Perhaps I’ll look at this sort of project as an academic endeavour.
Then the solution leaped out at me! I was staring blankly at a Raspberry Pi development board. If you haven’t heard of these yet, you surely will soon enough. The embedded ARM processor will happily run Linux thus allowing for a ton of flexibility – for free! In a form-factor little larger than a credit card, and at a price point of $35 you get the following:
- 700MHz processor
- 256MB ram
- HDMI & composite video outputs
- 2 x USB ports
- Ethernet connectivity
- SD Card (“hard drive”)
- A ton of GPIO pins for hardware development
So, pop in a USB soundcard and it looks like I have a very capable soundcard modem TNC which can perform a myriad functions. Oh, and it draws a measly 3.5W (full load – ie. network, etc)
Holy crap! So, $35 for the embedded computer, $15 for a USB soundcard and $10 for an 8GB SD card. That is a $60 TNC-extraordinaire!
I’ll report back with the details soon enough!