I've been working on open-source balloon control software [1] for my own balloon project that I've been building on/off for about six years. I am hoping to use a software modem to bridge between the RF and my AX.25 packet reader/writer [2] because I wanted to save weight. Payload weight is the name of the game in the balloon hobby--the lighter your payload, the less you have to inflate your balloon and the higher it will fly before it bursts. Even an ounce matters and the competitive altitude record balloons are invariably tiny, single-board, all-in-one payloads that are built around a single low-power microcontroller.

There are a couple of good open source options for software-based AFSK APRS modem/TNCs:

Dire Wolf: https://github.com/wb2osz/direwolf minimodem: https://github.com/kamalmostafa/minimodem

You'll still need a USB sound card though, which adds weight and cabling. I would love to see some open-source SBC hardware a la Beaglebone Black or RasPi that had a high-quality sound card built onto the board and wired through a Molex-type plug that was suitable to the bumps and bounces of flying.

[1] https://github.com/chrissnell/GoBalloon -- This was my first project in Go and while it works well, it now looks pretty ugly to me with a year and a half of Go experience under my belt.

[2] https://github.com/chrissnell/GoBalloon/blob/master/ax25/dec...