In the beginning…
When I got into HAM Radio, in 1999, I bought a Kenwood HT, programmed it up and was talking on a local repeater. Back then, as a new HAM that is what you did. You could meet people from around your area and often had great QSO’s. Then as you learned more about HAM Radio, you would buy a newer and bigger radios, then you could talk to people farther away. Then you would upgrade you HAM Licence so you could use even more bands and reach even more people.
Several years ago a system called EchoLink and IRLP was developed. They would allow you to talk to people using the Internet. A Repeater owner would put EchoLink or IRLP on their repeater and with a few DTMF Tones you could link that repeater to other repeaters or reflectors. On EchoLink you could download a free application for your computer and phone and use it to get into the EchoLink network and chat with your friends. Now you could talk to people all around the world with a handheld radio or computer.
Enter the Digital Age
In the late 1990’s the Japan Amateur Radio League developed a digital system called DStar for the Amateur Radio. That system is available for HF, UHF and VHF. As long as your signal strength is above the minimal threshold, then the data received was better then any analog signal.
Digital modes in the Amateur Radio is not new, HF have been using RTTY, PSK31 and other modes for years. They are mostly used to send data from one station to another. That data could be just about anything from Text messages to pictures.
Amateur Radio Hobbyist are always looking for new and exciting ways to use radio communications. DStar now allowed you to send voice as the data instead of Text Messages. When you use the Internet as the backbone network, you can now link Repeaters to each other and send voice packets from one repeater to another, same as you could with EchoLink or IRLP. The big difference is the packets in digital mode are small and faster then packets in an analog mode.
In the Commercial Radio, Digital Modes have been available for awhile. Modes such as APCO P25, DMR, and NXDN are popular and used by public safety around the world. As with most things the Amateur Radio Hobbyist wanted to use the same technology as the Commercial Radio users. That brings us to the focus of this Post.
DMR (Digital Mode Radio)
So what is DMR? DMR is an international digital radio standard developed by the European Telecommunications Standard Institute (ETSI) in 2005. It allowed for improved voice quality, improved functionality such as location information, improved security, and improved channel efficiency (2 slot TDMA). The aim of DMR was to provide an affordable, low-complexity digital standard to replace analog radio system in public safety and other services including HAM Radio.
ETSI DMR defines three different standard tiers:
- Tier I (unlicensed): DMR equipment works in Direct Mode (simplex) on public frequencies. Tier I DMR devices are best for individuals, recreation, small retail, or other situations the do not require wide area coverage.
- Tier II (licensed conventional): This Tier is aimed to be a direct replacement for the analog conventional radio system. DMR systems operate under individual licenses working in simplex mode or repeater mode, and has a degree of wide-area coverage.
- Tier III (licensed trunked): DMR trunking systems operate under individual licenses with a controller function that automatically regulates the communications. This Tier is not used in the Amateur Radio band.
The DMR standard also specifies the use of TDMA (Time Division Multiple Access) technology. TDMA provides two logical channels in each 12.5 Kilohertz channel space. This provides a useful doubling of capacity within the same analog channel space.
DMR is an “open standard” and is not proprietary to a single manufacturer such as DStar and Fusion. That means that competition is possible between manufactures. You can get a DMR Transceiver for under $100.00, which you can not do with the other digital modes.
The Digital Audio Quality for the end user is one of the key benefits of DMR. With an analog signal, as you leave the coverage area your signal will gradually weaken and you will hear an increased amount of “hiss and crackle” until the signal is lost. With a digital signal you will remain clear to the edge of coverage. DMR Systems use a device called AMBE+2 vocoder to convert voice information into digital data. During the digitization process background audio noise is reduced.
DMR is not only for voice communications. DMR also enables data applications such as text messaging, GPS, SCADA, telemetry, and IP data over the air. One of the key drivers for users to switch to digital is the ability to add enhancing data services and applications to the radio system.
One of the major advantages of DMR is the use of TDMA. TDMA allows for 2 time slots in the same bandwidth as the narrowband FM channel used by analog systems. During a call, your radio will use 1 of the 2 time slots, which means that half the time your transmitter is actually turned off, which in turns (in theory) doubles your battery life. For repeater owners this means that 2 conversions can be in progress at the same time with no interference.
Getting started in DMR can be as simple as getting a DMR Transceiver and connecting to a local DMR Repeater. Don’t have a DMR Repeater in your area? No problems you can then pickup a DMR Hotspot and be on the air in a matter of minutes.
In an upcoming post I will walk you through picking the right radio for you and getting a Hotspot and setting it up.
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This post is in no way a be all end all to DMR, the goal was to give you a brief out line of what DMR is and why you should use it as an Amateur Radio Operator. There are more in depth posts on the internet that can give you the nuts and bolts of DMR.
until next time 73’s
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