HFDL 
HF Datalink / HF Acars 

ARINC specification 635-2 27/02/1998 

This module complies, at due time, with ARINC 635-2. Transmission is on a subcarrier 1440 Hz USB. Symbol speed is always 1800 baud. Modulation is 2psk, 4psk or 8psk. Effective bitrate: 300, 600, 1200, 1800 bits/sec.

This screenshot shows a BAUDOT decoder together with an HFDL decoder, oscilloscope shows the baudot signal