Digital voice modes have been increasing in their use within amateur radio. As technology advances, the advantages of using digital voice systems become more apparent.
One of the issues with digital voice systems, is that there are several from which to choose and this means that there is no single standard for all.With FM, AM and SSB, it is possible to receive them easily on most receivers, but when it comes to digital voice systems for amateur radio, there is no single standard, and often amateur radio equipment for digital voice is not mult-standard.
Ham radio digital voice concepts
Whatever the digital mode that is used, there are several basic concepts that are common to all types, whether used for amateur radio or form commercial or other applications.
Audio enters the transmitter in the normal way from a microphone and is amplified. It is then passed to an analogue to digital converter and as the name suggests it converts the signal to a digital form.
The resulting digital data is then compressed and encoded in a particular format by an item known as a Codec.
Once in the right format the digital data from the codec is fitted into the system data format and then modulated onto the signal.
Obviously the receiver is the reverse of the transmitter so that the incoming signal is demodulated, and data removed from the transmitted signal structure, passed through a codec, and then through a digital to analogue converter to reconstitute the audio.
Digital voice modes have the advantage that they provide much a good signal to noise ratio even in the presence of interference, and they can sometimes be copied at much lower levels that an equivalent analogue system.
One of characteristics of digital systems is that they can operate down to low signal levels and then suddenly disappear. They move from 100% copy to nothing for a small signal change. This is rather different to analogue systems where there is a gradual degradation in performance.
VHF / UHF amateur radio digital voice systems
Although in theory digital voice systems can be sued on virtually any frequency, there are some that tend to be used mainly on the VHF / UHF bands.
A summary of some of the major amateur radio digital voice modes is given below.
- D-Star: D-Star standards for Digital Smart Technology for Amateur radio and it is a digital voice standard that was developed by the Japan Amateur Radio League, JARL in conjunction with a number of universities and amateur radio companies. D-Star is an open standard, although it does use an AMBE codec which is proprietary and it is not possible to radio amateurs to obtain licences for the code development. The first D-Star equipments were mobiles and handhelds from Icom, but others have since joined and offer D-Star compatible equipment.
- DMR: DMR stands for Digital Mobile Radio and it was a digital voice standard developed by Motorola for mobile radio systems, especially those used for commercial applications. In view of its original applications, it has many features that are aimed at commercial / business users. Nowadays there are several manufacturers supplying DMR equipment to the amateur radio market.
- System Fusion: Although properly called System Fusion, it is often referred to simple by the name Fusion. It is a digital voice system for amateur radio designed by Yaesu and as such it is not an open standard. Yaesu is the only manufacturer of radios for this mode.
System Fusion is incorporated into the Yaesu repeaters and as a result it is available on many repeaters. Despite this, many of the users of the repeaters that have Fusion simply use them as analogue repeaters.
Amateur radio digital voice for HF
Although digital voice seems to be more widely sued on VHF / UHF than HF, there are several digital voice systems that can be used on HF.
For HF amateur radio digital voice, the normal approach is to use a PC which links into the HF transceiver. Sound out of the sound card or audio output drives the audio input and is ultimately transmitted.
One of the most important choices is the codec and programme used. There are several options for this
- D-Star: D-Star is an amateur radio digital voice system that is also widely used on the VHF and UHF bands where it is possibly the main scheme. At HF, some find that they want to experiment it the codec to obtain the best signals, however it uses the AMBE codec which is proprietary.
- FreeDV: FreeDV is an amateur radio digital voice scheme that uses OFDM modulation techniques in order to combat HF fading. Low bit rates are sued to digitize the speech and then modulate the signal so that the bandwidth is less than 6 kHz. FreeDV uses an open source codec and this means that it is possible to undertake a lot of experimentation.
Digital voice systems are not as widely used within amateur radio as some of the digital data schemes for communication. However as the technology has some major benefits, it will undoubtedly gain greater acceptance. One of the main drawbacks is that there is a variety of different schemes.