Sounds of the Modern Commercial and Military Digital Station.
|Welcome to the Sounds of the Modern Commercial and Military Digital Station's page. Included on the following pages are information and digital audio samples of various modem modulation types. This includes old familiars such as RTTY and some of the newer standards like that of the various NATO STANAG waveforms and MULTI/39-TONE modems. Also included are the sounds from Aeronautical Stations such as SELCAL.|
Note: You will require aPlayer® plug-in in order to listen to the sample files.
FSK (frequency-shift keying) is still widely used for RTTY (Radio Teletype) in the HF radio spectrum. Some of the more common modes are those known as Narrow Shift (85 Hz), 170 Hz shift, 425 Hz shift, and Wide Shift (850 Hz). In general, 85 Hz shift is associated with a signaling rate of 45.5 or 50 bps. Other mark and space shifts generally employ signaling rates of 50 or 75 bps for the 5 bit Baudot alphabet. Other speeds like 100 bps is employed with error correcting codes such as with FEC (Forward Error Correction) and ARQ (Automatic ReQuest).
|85 Hz Shift [2805 Hz ± 42.5 Hz] (87.5k)||425 Hz Shift (58.8k)|
|170 Hz Shift [2000 Hz ± 85 Hz]||850 Hz Shift [2000 Hz ± 425 Hz] (60.3k)|
During the past years, RTTY modems have been slowly replaced by serial modems. Serial modems offer greater data speeds (75 to 4800 bauds) and error correction. These modems are also adaptable to a wider range of applications from use on HF, VHF, UHF, Satellite and wireline circuits. Many "flavours" of standards are in existence. Below are some of the most common ones found in use by commercial, government and military stations. It is also important to note that these new serial modems are not restricted to the 5 bit Baudot alphabet but in fact, can employ ASCII (in it's many possible forms) and furthermore, can also interface to such devices as cryptographic equipment, FAX machines, modern day battle field imaging systems and Advanced Narrowband Digital Voice Terminals (ANDVT).
Below are some of the more common serial tone modem standards.
The first one presented is the single-tone, adaptively equalized mode, compatible with MIL-STD-188-110A. The single-tone waveform supports data rates of 75, 150, 300, 600, 1200, (Serial Tone) 2400 coded and 4800 bps uncoded. The Serial waveform requires 3 kHz of bandwidth. Its Narrow-Band companion only requires 2.7 kHz of bandwidth.
|Serial: Uncoded (3.2k)||Serial: Coded, Short Interleaving (5.6k)|
|Serial: Coded, Zero Interleaving (4.7k)||Serial: Coded, Long Interleaving (20.5k)|
|Serial Narrow-Band: Uncoded (5.6k)||Serial Narrow-Band: Coded, Short (10.6k)|
|Serial Narrow-Band: Coded, Zero (8.5k)||Serial Narrow-Band: Coded, Long (40.7k)|
The Robust Serial waveform supports 75 bps.
|Robust Serial: at 75 bps, Short Interleave (215k)|
The NATO STANAG 4285 waveform supports data rates of 75, 150, 300, 600, 1200, (Serial Tone) 2400 coded and 1200, 2400 bps uncoded.
The 39-Tone parallel mode is complient with MIL-STD-188-110A App B and supports data rates of 75, 150, 300, 600, 1200 and 2400 bps.
Typical Modem Configuration
In a typical application, the configuration for the above serial modems looks like this.
The SELCAL (Selective Call) paging system in one that is mainly employed between aeronautical ground stations and aircrafts. The system is widely used on commercial aeronautical HF radio circuits and by some military aeronautical stations, Canada being and example. The SELCAL code is comprised of a series of 4 letters representing a specific sequence of audio tones that much resemble those of a TouchTone telephone. The SELCAL is transmitted by the ground station to an aircraft's HF radio set. Upon successful reception of a SELCAL page, the aircraft's SELCAL visual indicator and annunciator alert the pilot/communications officer that the aircraft's HF radio has been addressed and that a ground station wishes to communicate with it.
ICAO SELCAL (5k)
Notice: For security purposes and to avoid fraudulent use of SELCAL, the code for the above example has been omitted.
Waveform Spectral Analysis
Click here to view the associated spectrum displays for the various waveforms presented above.
The following modems were employed in the creation of the sample digital audio files.
|||The FSK tones and the various serial tones were digitized using a Harris RF-5710 HF Modem.|
|||The Multi/39-Tone signal was digitized with a Harris RF-3466A Universal HF Modem.|
|||The SELCAL signal was digitized from a Polestar SELCAL ICAO encoder.|
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Updated: February 05, 1998
© 1998-2014, Richard Lacroix