The history of digital radio
The transmission of digital radio calls has been a much-discussed and controversial topic since the beginning of the 1990s, especially in the field of public safety radio. Often criticized were both the high investment costs for infrastructure and equipment and the severely limited possibility of use in buildings. In the meantime, the "authorities and organizations authorities and organizations responsible for public safety" work digitally. Digital radio is also already established in many places in professional radio.
In conventional broadcasting, DAB (Digital Audio Broadcasting) has been firmly established for some time and is being further expanded.
For radio amateurs,"digital voice" has offered a wide range of activities since around the turn of the millennium. In the course of time, D-Star,DMR and C4FM have become established as standards. But also TETRA,the DMR derivative NXDN, P25 etc. are also interesting, even if they are not widespread.
- High voice quality, better filtering of background noise
- Possibility to transmit data (GPS, texts, ...) without disturbing the subscribers
- More flexible connection of participants
- Connection to the Internet / Hamnet enables bridging of large distances
- Eavesdropping security
- Configurability of the devices
- Complex technology with high maintenance requirements
- Dependence on network infrastructure
- New entry / use of the devices sometimes difficult
Where is digital voice used in amateur radio?
In amateur radio, digital voice is mainly used on VHF/UHF.Here is an overview of the three common standards on the market with a comparison of the differences.
|"DMR Digital Mobile Radio"||D-Star – Digital Smart Technology for Amateur Radio||C4FM/System Fusion
Continuous 4 Level FM
|Procedure||TDMA (Time-Division Multiple Access)||FDMA (Frequency-Division Multiple Access)||FDMA (Frequency-Division Multiple Access)|
|Programming required||Codeplug||Repeater lists||–|
|Display of Call Sign||✓||✓||✓|
Unfortunately these standards are not compatible with each other, so no DMR operation is possible with a C4FM device. However, there is a small ray of hope in the form of so-called "bridges": These are the networking of rooms from different "networks".
Currently, the most widespread is the DMR standard, which originated in professional radio. Here, the costs of entry are manageable. A large number of different devices are commercially available. In addition, the worldwide coverage with DMR-capable repeaters is very good. Due to the high number of participants, not only a QSO partner can be found at any time, but also assistance with problems.
In the beginning, mainly professional radios, originally designed for professional radio, were used. These had beside the high purchase price however also the disadvantage that it concerned almost exclusively pure 70cm monoband devices. Thus you had to rely on an additional device for the operation on 2m. Many manufacturers have dualband devices for amateur radio in their range today, which can be used analog and digital in both bands. The operating concepts are also increasingly oriented to the needs of radio amateurs, for example, it is possible to work in VFO mode.
D-Star and C4FM have been specially developed for use in amateur radio; newcomers may (but do not have to) succeed more smoothly under certain circumstances.
Digital radio on shortwave
It is also possible to communicate digitally by voice on shortwave. In addition to the existing SSB transceiver, the following is needed:
Special software, e.g. "freedv". Free available software and further information (in English language) can be found here:To FREEDV
PC mit 2 Soundkarten (RX + TX) or alternatively a Digital Voice Adapter.Since no network or server infrastructure is required, there is no need to register or apply for an identification number.
How does Digital
The central feature of all Digital Voice processes is - unsurprisingly - the digitization of analog voice data. This is still a simple step that has been mastered for a long time. The real know-how is in the subsequent compression and coding of the data. Here, advances in semiconductor technology have meant that more and more elaborate processes can be applied without requiring much power (or space) for the device. The result is better compression, i.e. better utilization of limited bandwidth, and better intelligibility at high compression. The chip that performs these tasks is called a codec (coder/decoder). The algorithms used here are manufacturer-specific and often tailored to a specific speaker group (gender, speech characteristics). There are also open procedures for codecs, but these have not been able to establish themselves generally. One system often used in amateur radio is the AMBE codec (Advanced Multiband Excitation) from the manufacturer Digital Voice Systems.
One of the outstanding development goals for digital voice transmission was, in addition to better utilization of the scarce frequency resource through compression, to achieve the same range as with analog methods. In other words, it must be possible to decode the received signal with the same signal-to-noise ratio (SNR) as reliably as with traditional analog methods.
Analog & Digital -
With digital voice, this is achieved by redundancy of the signal. The actual data stream of the digitized voice is thus overlaid with a layer that provides security for the transmission. Here, too, there are various system-specific procedures that differ only slightly. Overall, it can be said that with similar level ratios, the range and reliability of digital voice is about the same as that of analog transmission.
Although digital voice requires completely new equipment technology, it also offers certain advantages. For example, the bandwidth is greatly limited by compression. In principle, channel spacing of 6.25 kHz is possible (compare FM with 25 and NFM with 12.5 kHz).
DMR uses this newly available bandwidth to transmit two calls simultaneously on one frequency in the 12.5 kHz grid. This is done by the so-called time slot method. One or the other channel is transmitted alternately for a short period of time (time slot). The procedure is called TDMA, Time Division Multiple Access. The DMR system uses TDMA with two time slots per RF frequency.
A competing method is FDMA, Frequency Division Multiple Access. Here, different channels are transmitted simultaneously on different frequency sections. D-Star and C4FM use a similar method. Here, voice data is transmitted on one section, and additional data such as the current GPS position or other sensor data is transmitted on another section. This combination is well suited for tactical situations where a control center receives reports of other vital data in addition to the voice channel.
An amateur radio license is required for active participation (=transmitting).
Those who want to operate worldwide, but do not have the means to install large antenna systems or are afraid of the effort and expense, can still reach the whole world with digital radio. The radio weather and propagation conditions play a rather subordinate role.
That depends on a number of factors, including the type of operation, the nature of the local infrastructure and, of course, the budget. The first thing to check is whether a digital repeater is available nearby and what standard it operates with. If it is a D-Star repeater, for example, the purchase of a C4FM device makes little sense.
If the repeater is only a few km away, the purchase of a handheld radio is a good idea. If digital operation is preferably to be operated from a vehicle, a mobile unit with a vehicle antenna is recommended. However, a mobile device in the shack can also be helpful if the nearest repeater is further away and a connection via handheld radio is difficult or impossible to establish. The higher transmit power in combination with an outdoor antenna should provide a good connection in most cases.
To answer this question, one should first consider various aspects:
- Which digital standard is supported by the repeaters in my area?
- What do the colleagues I mainly want to talk to use?
- How much training time do I want to invest?
- Do I prefer a particular device manufacturer?
- Assuming that only Yaesu is to be considered as a brand, the determination is automatically made for C4FM/System Fusion.
- If the possibility of image transmission is an important selection criterion, there is no way around D-Star or C4FM. DMR does not offer this function (yet).
This depends on the device and the standard used. Yaesu C4FM devices can be put into operation immediately without prior registration, programming and waiting for a subscriber number - just enter your callsign into the device at the first start and you are QRV.
DMR devices, on the other hand, require "programming" or a "code plug", i.e. a software file containing the device-specific settings as well as repeater lists, contact lists, etc.
The application is done online.
No. Digital calls via repeaters usually use the Internet to connect subscribers over longer distances. Shortwave radio does not require this. Shortwave also offers many more fields of activity than voice-only communication. However, voice can also be transmitted digitally via shortwave.
A hotspot does nothing else than a repeater: It connects the radio to the Internet. So if there is no digital repeater within range, a hotspot can be integrated into the domestic LAN or WLAN. However, cell phone WLAN and mobile data can also be used to participate in digital radio at all locations with a mobile network, provided that an appropriate data tariff has been booked.
There are many repeater lists and maps on the Internet, but also useful smartphone apps (repeaterbook, DMRepeaters, ...).
An interactive map with digital repeaters can be found here:
Most D-Star devices from Icom already contain pre-programmed lists of digital repeaters. So it is possible by means of GPS to display the nearest repeaters at any time and to connect directly.
New and very well equipped DMR devices are already available for less than 200 EUR. Also the very compact FT-70DE for C4FM from Yaesu allows the entry into digital radio for a small price. If you want to buy a new D-Star radio - for example from Icom - you have to expect prices starting at about 600 EUR.
For a possibly necessary hotspot one should plan approx. 160 EUR.