Digital Radio Modes

Brooke Clarke, N6GCE 2001 - 2013


Background

By using a digital mode where the signal bandwidth is narrower than the standard 300 Hz to 3,000 Hz voice bandwidth the signal to noise ratio can be improved thus working where voice will not work.

Modes

Continuous Wave (CW)

This is the oldest digital mode also knows as On Off Keying (OOK).  This mode suffers from a number of problems.  For example when the transmitter is turned off no information is being sent, so modes that are always transmitting are better.  There is no synchronization in terms of the on/off edges or in terms or timing relative to the carrier.

Coherent CW (CCW)

The idea is for both the transmitting station and the receiving station to have a precise clock and the start and stop edges of all the symbols fall at known points in time.  This allows automated dectection of CCW at lower signal to noise ratios than ordinary CW.

High Speed CW

During WWII the enemy could use Direction Finding (DF) receivers to locate transmissions that were being made by spies.  In order to reduce the time it takes to transmit a message High Speed CW sets were developed and later used as late as View Nam.
GRA-71 and and English version of the GRA-71 use a magnetic tape that is recorded manually and then played to the transmitter at about 300 Words Per Minute.

Ratio Teletype (RTTY)

Also knows as frequency shift keying (FSK).  Involves transmitting on one of two frequencies.  This mode has been around for a very long time.  Problems: If the shift is too wide the signal takes up more bandwidth than is needed relative to the keying rate.  Minimum Shift Keying (MSK) if better in this regard.  The early demodulators required both Mark and Space signals in order to work and if one or the other was garbled the output was garbled.  Frederick Electronics and I assume others offered advanced demodulators that would work with either Mark or Space signals.

Note that the transmitter is always transmitting something so more power is being useed per bit than in a mode like CW where the transmitter has off time.

By using a number of different forms of diversity the reception of HF RTTY signals could be radically improved.  A common system was to use two receiving antennas, two receivers and two demodulators.  If the antennas have different polarity or are separated by a distance that's a few wavelengths then the signals tend to fade at different times on the two systems and a combiner automatically selects the best signal so the output is the best of both signals.  Another form of diversity is to use a common antenna but have the tow receivers tuned to different frequencies that are both transmitting the same message.  Another form of diversity is time.  The MD-1142 sends a number of versions of the same message offset in time.  The idea is that fades only last a short time and the message can be reconstructed.

The CV-89A/URA-8A is a Military tube type FSK converter with a CRT type tuning indicator.
The CV-483/URA-17 is a Military solid state FSK converter with a CRT type tuning indicator.
The Frederick 1203 a solid state FSK Demodulator that works with either Mark or Space or both
The MD-1142 uses time diversity and multiple audio channels

PSK31

This is a digital mode that's optimized for human generated characters, very narrow bandwidth and the transmitter is always on.

Olivia

Is similar to PSK31 but includes multiple copies of the message offset in time, similar to the MD-1142 and is more robust.

Chirp

Although chirp transmitters are intended for measuring H.F. propagation they do have a MSK like mode where a short message is sent over and over again and the transmitter sweeps from 2 to 30 Mhz.  If there's a path somewhere in that range the message gets through.

 Automatic Link Establishment (ALE)

This really is not a mode, but rather a protocol for linking HF radios automatically.  Each radio in a net peridocially polls the other radios in the net and keeps track of what frequencies work for which radios in the net at different times of day.

Digital Voice

The military uses Continously Variable Slope Delta (CVSD) as a method of cnverting voice into a digital signal.  This is the signal that is sent over the wires of the TA-1042 Digital field phone.  For use in the KY-57, KY-58, or KYV-2 Secure Voice Module the CVSD data is enciphered prior to sending and then descrambled at the receiving end.  This is a symetric system with the same key used for both sending and receiving.

I started looking into what it would take to make a digital voice interface to work with the military radios that have "wide band" capability, i.e. the VHF and UHF radios.  The CVSD chips used in the early military voice encryption units are now obsolete.  There was some amateur radio work done using the AD73311 CODEC, but it has a 16 bit word output with a framing pulse.  This makes it very hard to send over the air without also using some type of VOCODER chip to reduce the bandwidth.  By using a CVSD type of codec the output is just a single bit data stream with no framing required, making it relativity easy to send and receive.

I recently saw a nice distinction made between voice scrambling and voice encryption.  The implication was that scrambling involves mixing up the audio and sending that audio, whereas enciphering implies using a digital data stream to carry the voice message.  From what I've read it's possible to reconstruct any mixed audio scheme, but very much more difficult to break a digital cipher.

The  KY-38 came out during the latter part of the Vietnam era and worked with the PRC-77, so probably used a digital enciphering method rather than an analog scrambling method, but this is all speculation since not much has been published about it.

Frequency Hopping

One way to make direction finding difficult is to have the transmitter change frequency frequently.  Note that the hopping itself may not provide any security since a simple crystal radio that had a wide band input would detect all the transmissions.  This is the case no matter in what order the frequencies are changed.  So voice encryption is needed in addition to the hopping and that's what is done in the COMSEC part of the SINCGARS radios, externally with a KY-57 for the early radio and internally in the later radio.

Spread Spectrum

Altough Frequency Hopping is a form of spread spectrum, the term is used more often to describe what is called a direct sequence.  This is where the data signal is mixed with a high spreading signal with a dat rate considerably higher than the data signal so the the final signal that's transmitted has a bandwidth larger than what would be used for just the data.  For example the C band satellite television system spreads the normal 4.5 MHz video signal out to 36 MHz thus adding process gian to the system allowing much lower transmitter power int he satellite.  The Magnavox Procom hand held radios have a very long range for a low power radio becasue of the spread spectrum signal. The GPS satellites use spread spectrum and the signal fallingon the Earth is so weak that it is below the background noise level.

The Black Box uses pulse modulation but it's transmitted spectrum looks very much like spread spectrum.  I assume that there's a receiver optimized to receive this signal.

Military Designations

MIL-STD-188-110A/B/C  (Wiki, MARS) Interoperability And Performance Standards For Data Modems (Tactical Data Link: TADIL, Link 11)

110A-1991: 75bps through 2400bps coded and 4800bps with no FEC or interleaving
110B-2000: 9600bps coded in a 3Khz channel and
110C-2011: from 3Khz up to 24Khz in increments of 3Khz channels while providing user data rates up to 120,000bps coded hroughput with multiple HF Independent Sideband (ISB) channels.
STANAG 5066 Profile for HF Radio Data Communications (Data Link Protocol (DLP) (5066-ARQ))

MIL-STD-188-141B/C Interoperability and Performance Standards for Medium and High Frequency Radio Systems

STANAG (Wiki)

STANAG 4197 - Edition 1, Modulation and Coding Characteristics that must be Common to Assure Interoperability of 2400 b/s Linear Predictive Encoded Digital Speech Transmitted over HF Radio Facilities

FED-STD-1015 - Analog to Digital Conversion of Voice by 2400 bit/second Linear Predictive Coding (LPC)
STANAG 4198 Edition 1, Parameter and Coding Characteristics that must be Common to Assure Interoperability of 2400 b/s Linear Predictive encoded Digital Speech

STANAG 4203 - (Edition 3), Technical Standards for Single Channel HF Radio Equipment

STANAG 4285 - Characteristics of 1200/2400/3600 bit/s single tone MODEMs for HF radio links,
FED-STD-1052 Appendix B Telecommunications: HF Radio Modems (Data Link Protocol (DLP))

STANAG 4415 Characteristics of a Robust, Non-hopping, Serial-tone Modulator/Demodulator for Severely Degraded HF Radio Links

STANAG 4481 - Edition 1, Minimum Technical Standards for Naval HF Shore-to-Ship Broadcast System

STANAG 4529 - Characteristics of single tone MODEMs for HF radio links with 1240 Hz bandwidth

STANAG 4538 - (Edition 1) Technical Standards for an Automatic Radio Control System (ARCS) for HF Communication Links

STANAG 4539 C3 (Edition 1), Technical Standards for Non-hopping HF Communications Waveforms

STNAG 4591 - Edition 1, 600 b/s, 1200 b/s and 2400 b/s NATO Interoperable Narrow Band Voice Coder


STANAG 5065 MSK waveform

Link 11 MIL-STD-6011 (Wiki) -> Link 22 (Wiki)

U-229 AUDIO/DATA connector pin out.

DS-101 Smart Fill Devices

DS-102 Common Fill Devices

Other

There are a lot of digital modes in addition to those mentioned above.  Some are for transmitting text and some are for transmitting images.  For example see:
Worldwide Utility News (WUN) -

Harris 5710 & 5720

These are modems in a box that support many of the military data modes.
4365338 Technique for high rate digital transmission over a dynamic dispersive channel, Harris Corporation, Dec 21, 1982, 375/230, 375/254 -  referenced by 121 other patents - optimized for HF data - uses known preamble
4599732 Technique for acquiring timing and frequency synchronization for modem utilizing known (non-data) symbols as part of their normal transmitted data format, Harris Corporation,
Jul 8, 1986, 375/346, 375/367, 375/355 - referenced by 184 other patents - makes use of I & Q processing - preamble type known information is interleaved with the data so the receiver can dynamically re-equalize.

Related

The CV-89A/URA-8A is a Military tube type FSK converter with a CRT type tuning indicator.
The CV-483/URA-17 is a solid state FSK converter with a CRT type tuning indicator.
The Frederick 1203 FSK Demodulator
The Frederick 600A BER test set
Digitech also made a modular rack mount Digital Data Generator
MD-1142 Modem
Cryptographic Machines
Cryptography
Cryptographic patents
FS5000 cold war embassy secret coms 2-way HF radio set
PRC138 ALE capable HF manpack radio
Micom ALE capable HF vehicle radio
GRC-109 HF radio works with GRA-71 high speed Morse transmission (but not receive high speed Morse)

Test Equipment

In order to test any digital mode you need a test signal and a way to evaluate how it gets through the modulation - demodulation system.
Test equipment for early RTTY equipment was designed to test the mechanical RTTY machines and has special functions for them.  These machines were used in a current loop configuration where all the equipment was in series.  Included was a power supply of around 100 Volts and the current was adjusted for typically 60 ma and later 20 ma.
Digitech made a modular rack mount Digital Data Generator and analyzer

The Frederick 600A BER test set uses RS-232 levels rather than the older current loop type of interface.

When testing a communication path that will be used for encrypted messages the bandwidth of the test set needs to match that of the encrypted signal.  For testing both wire line and radio systems utilizing the VINSON protocol the F91120 test set is suitable.
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