Creating Text in Audio Waterfall Spectrum Display

This short program uses gnuradio to create an audio signal which, when displayed in a scrolling, waterfall spectrum display, makes text messages appear. The process is similar to OFDM where an inverse FFT of a vector of numbers is taken. Here, the vector of numbers is a scan line of 'video' created by a text message and a character/pixel map. With this character set (6574) be sure to try out the lower ASCII values for special symbols (see char_rom_2.c for details) by setting the line: tline = ' \x04\x05\x06\x07\x08\x09\x0a' etc.

The result:

The above is PSK31 Deluxe with a slowly scrolling display of the audio spectrum from 100 to 2900 Hz. The code:

sptext_ifft_2.py complete program
charset_6574 character set
char_rom_2.c program to create the character set, courtesy Jim Battle of the classiccmp mail list.

This project was inspired by a sound file found in HAMPAL - appearently it's fashionable in some circles to make your call sign appear in a waterfall display before sending a picture file. In fact, I was trying to figure out what software some guys were using to send DRM like signals bearing pictures, when 'HAMPAL' scrolled down the screen! Well, that solved that mystery.