Last fall, I ran across the Kickstarter for the Teensy 3.5 and 3.6. The designer, Paul Stoffregen, bills his Teensies as “powerful microcontrollers for making awesome DIY electronic projects.” The Teensy 3.5 uses a 120 MHz ARM Cortext M-4 processor with a floating-point unit. The Teensy 3.6 has a 180 MHz processor. And, they are Arduino compatible.
“Hmmmm,” I thought to myself, “I bet these processors could do some digital signal processing.” I got really interested when I read about the Teensy Audio Library, which is a toolkit of dozens of audio processing components.
So, I joined the Kickstarter, and in due course a Teensy 3.5 arrived in my mailbox. Of course, I haven’t yet done anything with it.
Well, yesterday, my friend, Quentin, KD8IPF, sent me an e-mail posted to the 817ND mailing list. It was posted by Gareth, GI1MIC, and includes a couple of links to an open-source project that uses an earlier version of the Teensy to implement an audio DSP add-in for the FT-817. The links include:
- Code and instructions
- A video showing the Teensy DSP in action
Gareth notes, “The project is easily modified to run on other rigs or could be fitted inside an amplified speaker.” The latter is what I had in mind.
On the code and instructions page, Gareth links to a tool called TFilter. The web page describes this tool as “a web application that generates linear phase, optimal, equal ripple finite impulse response digital filters. It uses a pure javascript implementation of the Parks–McClellan filter design algorithm.” This tool generates the coefficients used by the Teensy program.
I haven’t really looked at the code yet, but Gareth’s program will switch between a CW filter, a SSB filter, and a third type that I couldn’t make out from the video. In Gareth’s application, the Teensy is embedded into the FT-817, and somehow he is able to use one of the radio’s controls to switch between the different filter types.
Another thing about Gareth’s project is that it uses a Teensy 3.2. The Teensy 3.2 uses a 72 MHz ARM processor, and apparently, that’s quite enough power for audio DSP. The Teensy 3.6 costs $30, while the Teensy 3.2 costs only $20.
I always thought that I’d build mine into a speaker. Not only that, I would like to have a PC application, somewhat like TFilter, that would allow a user to build his own filter on the fly. The program would have to calculate the coefficients on the fly and then download them to the filter.