STANAG-4197

STANAG-4197 is  a NATO standard agreement indicated as "Modulation And Coding Characteristics That Must Be Common To Assure Interoperability Of 2400 Bps Linear Predictive Encoded Digital Speech Transmitted Over Hf Radio Facilities".

A description of the STANAG-4197 waveform can be found in radioscanner.ru, below a self-explanatory picture about the four parts of this signal:   

STANAG-4197 waveform (courtesy radioscanner.ru)

The S-4197 modem generates two separate signal formats based on two tone libraries: the 16-tone library is used for the system preamble and the 39-tone library is used for digital voice data. The initial preamble (modem preamble) is used in the receive modem for the detection of signal present, the correction of doppler, and the identification of the beginning of the system preamble. Modem preamble consists of four unmodulated tones followed by three tones simultaneously phase modulated.   
The modem preamble segment is sent on 16 channels at 75 Baud and channel separation 112.5 Hz (~112),  encoded voice (LPC) segment is sent on 39 channels at 44.44 Baud and channel separation 56 Hz: both the segments are formed using OFDM technology. 

This waveform is used in Advanced Narrowband Digital Voice Terminal (ANDVT or AN/DVT) modems that transmit encrypted digital voice over HF, these modems include the ANDVT MINTERM KY-99A modem. Sometimes you may found this signal under the ANDVT name, but it's wrong since STANAG-4187 is the waveform while ANDVT is the modem.

OFDM 16-tone data segment

OFDM 39-tones voice segment