Courtesy of Priyom.org: Enigma HM01 Cuban Spy Numbers Station "The digital mode used by HM01 is RDFT (Redundant Digital File Transfer). It was originally developed for amateur radio use; however, it was adopted by the DGI in 2007. The data bursts, created using the DIGTRX software, encode binary files with 8-digit file names and either.TXT, .F1C, or .F1G as the file extension.
"The spoken message headers consist of the last 4 digits of the respective filenames, followed by a bogus number. The last digit previously acted as an age counter, increasing between 1 and 8 inclusive every day, after which the message was replaced on the given slot; on newly added messages, the digit 1 was sent for two consecutive days.
"HM01 uses the same facilities as the international broadcaster Radio Havana Cuba. It is not uncommon to hear the Radio Havana Cuba audio feed being broadcast on the HM01 frequencies by mistake. HM01's audio feed comes from a computer running Windows XP. Windows XP log-out and log-in sound prior to a late transmission, 3 June 2014, 21:34 UTC, 16180 kHz."