fldigi software package provides digital modem program for hamradio operators, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install fldigi
fldigi is installed in your system.
Make ensure the fldigi package were installed using the commands given below,
$ sudo dpkg-query -l | grep fldigi *
You will get with fldigi package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Ham Radio |
| Package | fldigi |
| Version | 3.23.14-1 |
| Description | digital modem program for hamradio operators |
| Sub Section | hamradio |