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.
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
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.
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) -
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.Back to Brooke's Products for Sale, Military Information, Time & Frequency, Electronics, Home page
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 utilaiing the VINSON protocol the F91120 test set is suitable.
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