Digital Radio is a rapidly growing area for amateur radio applications. Digital systems are a different form of communications, when contrasted against traditional methods of transmitting audio via analog means.
Whereas, traditional/legacy radio sends your voice along a frequency-modulated analog carrier wave, digital radio converts the audio of your voice, through a codec (think of it as a modem), and transmits the audio of your voice as digital information, which is then decoded by the codec on the receiving radio as the audio of your voice. While it is still sent as a radio signal, the codec is using variations in the frequency as means of representing the 1’s and 0’s of the digital information.
There is a common misconception that digital radio and digital modes are one in the same; that is definitely not the case. Digital modes, or programs, piggyback on top of your existing analog radio. Digital modes will require an interface or Terminal Node Controller (TNC) between a computer and your analog radio. Typically, the digital modes are used for sending e-mail, text, or even custom html forms in the case of FLDIGI.
Digital radios are able to operate in a similar manner; however, the digital operation is done natively through the codec built into the radio. Digital radios are capable of sending a digital form of audio, unlike many of the digital modes you may be familiar with on traditional systems.
Being natively digital, these radios have an additional suite of capabilities completely unavailable to their legacy analog counterparts. While both are a means to remotely communicate when the PTT button is pushed, that’s where the similarities end. The digital nature of these devices, means no interface is required, for performing tasks such as messaging, data-transfers, etc.
There are several digital protocols, currently in common use among radio operators:
APCO Project 25, Phases I and II
Icom D-STAR
Yaesu System Fusion
Kenwood NXDN/dPMR
DMR/MotoTRBO
Echolink
RoIP
Of those protocols, DMR is the only open-source system in widespread usage. From my perspective, that is a huge advantage to the system. While D-STAR is *technically* an open standard, if you know of a manufacturer other than Icom, be sure to let me know. All of the other competing digital systems are proprietary to their respective manufacturers.
For example, there is one manufacturer supporting System Fusion: Yaesu. DMR hardware is available from 42+ different manufacturers. So while, there will be tooth-gnashing, and general attitude from anyone who has invested heavily in either D-STAR, or System Fusion, the numbers don’t bode well for the amateur community, especially given the somewhat limited adoption rates of digital technology by hams, coupled with the fragmented implementation of these digital systems. Yaesu’s WIRES system has had recurring problems throughout its various generational lifecycles. As well, some repeater owners have been having issues with the dedicated repeater needed for the System Fusion protocol.
DMR was formally approved by the FCC for amateur radio use in late 2014. As a relatively new system, the amount of support, and repeater growth has been extremely impressive.
If you look at the system as a hybridization of conventional radio, and cellular telephony, many of the system’s characteristics are easy to understand. Contrary to common misconception, Motorola did not invent DMR. MotoTRBO is a branding of Motorola products designed for compatibility with the DMR standard.
Why digital? Why DMR?
DMR doesn’t require any sacrifice on the analog end of things, meaning that, by migrating into DMR, you will still be retaining access to any analog systems you have been using. All DMR radios are capable of both digital, as well as analog operation. The performance advantage of Digital radio, fills in coverage gaps, providing consistent reception throughout the repeater’s signal area.
Digital vs Analog Performance Differential in Intermediate areas
Whereas, Analog has a linear progression of signal degradation, as the receiver moves further away from the source, in this case, the repeater; Digital radio, by using Forward-Error Correction, maintains a consistent received signal, until the “Digital Cliff” is reached, and the reception goes to zero at the edge of repeater coverage. With Analog, at the near-edge of repeater coverage, the audio transmission may be difficult to differentiate from noise; the Digital signal will be consistent throughout the repeater’s footprint, regardless of distance.
Visual representation of Digital vs. Analog Signal Degradation
DMR uses Time Division Multiple Access, or TDMA. TDMA breaks a singular 12.5 kHz channel spacing down into two segments/slots. By doing this, TDMA allows two simultaneous operations in one narrowband spread/channel by assigning each of the segments in the signal a “Timeslot,” and alternating between Timeslot 1 and Timeslot 2 in rapid succession. Unlike other systems, DMR can send and receive Data on Timeslot 1, while simultaneously passing voice traffic on Timeslot 2. It can double-down on data on both Timeslots, or, it can allocate both Timeslots to voice. All of this occurs at the same time. Technically, there is a 30ms delay; however this delay is imperceptible.
Time-Division Multiple Access – Breaks down a single Narrowband Channel into two Segments
DMR uses Zones for organization. These Zones are simply a collection of Talkgroups. As well, DMR uses Color Codes. Color Codes behave like Network Access Codes or CTCSS(PL-tones).
So, for example, John and Tom have their radios programmed to 446.00 MHz, and are conducting a conversation over the repeater. Alice and Bob are transmitting data, on the same repeater, at the same time, with their radios programmed to 446.00 MHz. The same principle can be assigned to simultaneous data/voice on both Timeslots.
So, to recap, DMR repeaters will have Timeslot 1 and Timeslot 2. If it helps, you can think of a DMR repeater as two repeaters, Timeslot 1 being a repeater, and Timeslot 2 being a seperate repeater. The Timeslots each contain Talkgroups.
Talkgroups can be thought of as individual channels, that are configured on either Timeslot 1 or Timeslot 2. The SOP for repeater usage is to assign Strategic/Regional Talkgroups on Timeslot 1, and Local/Tactical Talkgroups on Timeslot 2. These Talkgroups are organized into collections, called Zones. Color Codes are the DMR equivalent of Network Access Codes, or the CTCSS/DCS tones you are familiar with from analog radio.
By doing this, two users in San Antonio, Texas can be sharing a conversation on the repeater, on a local Talkgroup on Timeslot 2 using the local repeater. At the same time, a third local user can have a conversation with another user in Casper, Wyoming on Timeslot 1. To break this down further, the first two, local users, are essentially using the DMR repeater like its analog predecessor. The third user is connecting to the same repeater, on the same frequency, at the same time; however, his transmission is routed, using Radio-over-IP (RoIP), through the Internet, and back out through the local repeater in Casper, Wyoming.
DMR System Architecture / Repeater Organization
DMR radios are all monobanders, being either VHF, or UHF, but not both. That is the biggest negative to the system. The reason being? DMR is a professional standard that wasn’t intended for Amateur usage. Most professional users don’t have a need for using a large chunk of spectrum, nor for broadband capabilities. Despite being a professional/commercial standard, the radios are commonly available in a band-split that is larger than the 70cm or 2m spectrum allocations for hams. This means, pick a band that is in line with your needs/uses/preferences/terrain, and run with it.
Some of the advantages of DMR over traditional FM voice are:
Traffic doubling. Because TDMA splits the 12.5 KHz channel spacing into two sections, you can have two simultaneous, separate voice conversations on a repeater on the same frequency. This scalability affords enhanced traffic-handling on an individual repeater system, or amongst a system of networked repeaters.
Longer, consistent coverage. Because of the Forward-Error Correction, the audio maintains the same quality, without regards to range. Whereas, some analog voice may be lost as the the noise-to-signal ratio increases, DMR coverage allows for the audio to remain constant until the radio moves beyond the threshold of repeater coverage.
Superior audio when contrasted to analog. In my experience, there has been zero static, and the audio, for all intents and purposes, is 100% intelligible, with little or no background noise, until the threshold of repeater coverage is crossed.
Battery life, and energy efficiency. Because the transmitted signal is pulsed in rapid succession, unlike analog, DMR is only actually operating/transmitting at approximately 40-50% of the time when the PTT button is pressed. While the battery may have a 2800 mAh capacity, its effective performance is as though it were a 5600 mAh battery. This, in effect, is a doubling of operational cycle for field-use on battery power.
Interoperability with legacy analog systems. DMR radios are capable of operating in either digital or analog mode. This ensures backward-compatibility with any analog end-users you might encounter, as well as analog-only repeaters in your area.
Data transfer. Depending upon equipment chosen, DMR systems are capable of text-messaging, GPS, Remote Telemetry Information, Etc. Text messaging is extremely efficient, both from a duration-of-transmit perspective, as well as on the Forward Error Correction side. Because text messaging is more efficient in data use, it will, like its cellular counterpart, exhibit greater reliability in adverse conditions.
Enhanced Security. DMR, by its digital nature, is inherently more secure than analog radio, through obscurity by minority. Scanner hardware capable of decoding DMR isn’t readily available, at affordable prices. It can be decoded by plug-ins and Software-Defined Radio (SDR) programs, but few existing hardware decoding devices are available. While unlawful to use on the amateur bands, many of the Professional DMR radios from Motorola, Hytera, and Simoco all include a 40-bit basic encryption option, up to 256-bit AES encryption, and, in the case of Hytera, an alternate, proprietary 256-bit encryption protocol, or even a user-defined Crypto algorithm.
Advanced remote-control features. Like encryption, these are going to be found primarily on the Professional DMR radios. Several options are available for remotely-bricking a radio, verifying that a particular user is within range, status-checks on users, geofencing, patrol and route monitoring, individual or group calling, overrides for emergency traffic handling, remote-monitoring of individual user location, man-down/lone-user warnings. If an in-network user goes down, and there are multiple variables that can be configured for this, including time stationary, a warning is sent out to pre-programmed, in-network, authorized users. This can be further configured to send out e-mails, make phone calls, send text messages, provide telemetry data, or trigger an alarm, to notify other users on the network of a problem. In short, if a dude goes down in the field, under circumstances you prescribe, a warning will be sent to any user you want a notification to be sent to.
Internet-Connectivity. All DMR repeaters are capable of connecting to the Internet. Depending on the feature set available from the repeater manufacturer, you can configure multiple, geographically-separate repeaters through IPSC, or IP Site Connect. This, in effect, allows repeater owners to coordinate a network of repeaters, regardless of whether or not the repeaters are within the Transmit/Receive range of one another. There are several large, public-access, networked repeaters on-line, with worldwide linking. As well, for repeater owners, this allows for remote access and diagnostics of the repeater system.
Intra-Site Connectivity. If multiple repeaters are within the Transmit/Receive range of one another, the repeaters can be linked within a local network, and redundantly linked with an Internet backbone. This topology allows for an individual to move seamlessly to another repeater, in-network, via roaming, without a loss in coverage. The cross-redundancy of internet connectivity at each individual site, as well as the geographic proximity of the repeater sites, means two systems would have to simultaneously fail, in order for a network user to lose access to the rest of the users on the network.
Talk-around. If two DMR radios have been set up for Talk-around, or direct-mode, and are within range of each other, they have the capability of automatically switching to simplex operation on the repeater’s output frequency. This frees up additional traffic handling for the repeater, and also allows the users to automatically switch back to the repeater once the users fall out of simplex range.
Many of the DMR radios available are intended for the Professional market. A byproduct of this, is that, these radios not only have advanced capabilities in terms of control of the radio system, but they are set up for duty cycles of 100%. Contrasted with Amateur equipment, that is a significant, and noteworthy, performance differential alone.
Hytera Realtime GPS-Tracking of Simultaneous Terminals/Client Radios – Initial Setup
Motorola Realtime GPS-Tracking of Simultaneous Terminals/Client Radios – Map View
As well, many of the Professional DMR radios have undergone environmental hardening, such as submersibility-tolerance, dust-intrusion mitigation, vibration/shock-hardening, internal RF shielding, increased selectivity and sensitivity, and extremely robust spurious rejection.
Long and short, the Professional/Commercial radios developed for DMR are built like tanks, have better internal components, and will universally outperform their Amateur peers. That said, these radios come with a cost premium, and a typical ruggedized DMR HT radio will cost, on average, about the same as a dual-band Amateur mobile rig ($300-$700 USD). Quality Amateur DMR-capable radios are available from Connect Systems, as well as Anytone.
Warning: Rant
The “Baofeng” of the DMR-world is the Tytera MD-380. It is a very, VERY poor knock-off of Hytera. (Note: Hytera is one of the top-tier manufacturers of DMR equipment, along with Motorola, Tait, and Simoco. Hytera manufactures the Harris DMR radios, so that should end any confusion concerning their products.) Some repeater owners have had problems with the MD-380 radios encroaching into the adjacent Timeslot. This is the digital equivalent of wicked spurious emissions, and harmful interference.
If you are seriously considering getting into DMR as a communications tool, please, for all that is holy, stay the hell away from the Tytera MD-380.
If you want to pick one up for monitoring DMR, rock on; it’ll do that. But, please, do yourself, and the rest of us a favor, and do NOT hit the PTT on one. You would be much better served by picking up a base-model Hytera PD502, or, cheaper yet, a used Motorola XPR6550. The Connect Systems CS-750, and the Anytone AT-D858 are the minimum acceptable hardware for actual usage. I’ve used a buddy’s Anytone, and received no poor signal reports, nor were there issues with tying up Timeslots. Sample of one, but the Anytone worked well, for the time I had it.
For a an additional $25 more than the Anytone or the Connect Systems, you can pick up a Hytera PD362 DMR radio.
Hytera PD362 Portable Radio
The PD362 has a form factor much smaller than the typical phone, and would make an excellent, high-end DMR HT for EDC. Motorola and Hytera are the two premium manufacturers for DMR, and you would be extremely well-served with any of their new, or used offerings.
For monitoring or scanning DMR the MD-380 would fill a role. For communications? Not just no, but hell no.
DMR is an optimized system for using maximum spectral efficiency. Save up the extra $50-$100 and get a radio that doesn’t completely trash the system it was supposed to work with.
Seriously, just…don’t.
Communicating via the MD-380 is for dick-squeezers. If you’re using a MD-380 on a connected network, or a repeater, brush the Cheeto cheese out of your neckbeard, and get a clue. By saving a couple of bucks, you’re going to push repeater owners into going to restricted access (RAS), and you’ll end up with a $130 paperweight.
/endrant
With that out of the way, the MD-380 would serve an excellent role as a cheap, portable, DMR Scanner. An experimental firmware update has been released, that will open up multiple, simultaneous Talkgroups. This allows for the MD-380 to listen to all the Talkgroups on a given Timeslot at the same time. Considering the lack of commercial scanners for decoding DMR, the MD-380, fills the position well.
The other alternative to the MD-380, for DMR scanning, would be using Software Defined Radio, and a software program like DSDPlus. DSDPlus can decode all the currently fielded digital modes, with the exception of APCO Project 25, Phase II and System Fusion.
DSDPlus Software. Through a Virtual Cable Interface to a Signals-Monitoring Program, Various Digital Protocols Can Be Decoded
Despite the rage-fueled critique of the MD-380, it could also serve adequately as a dedicated DMR simplex tool. The problems presented by the unit are only apparent when interacting with repeaters. If you had a desire for digital Simplex operation, the MD-380 would function for that limited purpose.
It was mentioned earlier in this writing that users are able to communicate between two geographically-distant repeaters. Since we’ve touched on what hardware to avoid in the above rant, it’s a good time to go over SOP, and Best-Practices for responsible DMR-linked repeater use.
Timeslot 1 is typically configured for Strategic-Level communications. So, for example, if you key up your local repeater, and choose a Worldwide Talk Group, you will simultaneously activate every repeater on the planet, in that network. In the case of the DMR-MARC network, you’d activate 2,000+ repeaters at once. As well, the same would apply to your Continental talk group, for example, all of North America. While it is acceptable to use these Talkgroups as a calling channel, extended conversations will tie-up the repeaters. Once two parties have made contact on a wide-area Talkgroup, the procedure would be to move to another Talkgroup on that Timeslot. On the DMR-MARC network, this would be the TAC310 Talkgroup. By moving to this Talkgroup, if we use the example from earlier, connection works as follows:
User 1 in San Antonio, Texas, throws out his call sign on the North America Talkgroup. His transmission is, “XXXXXX, North America, MONITORING.” All the linked repeaters in North America retransmit his audio. Note that he brought up that he was on the North America Talkgroup, indicating that he is not local traffic.
User 2 in Casper, Wyoming, throws out his call, in response to User 1. His transmission is, “XXXXXX, this is ZZZZZZ in Casper, Wyoming. Reading you 5 by 5, OVER.”
User 1 responds, “ROGER, ZZZZZZ. If you’re available for QSO, I will be switching over to TAC310, and MONITORING.”
User 2 confirms, “ROGER that, XXXXX. ZZZZZZ, OUT.”
User 1 ends transmission, “XXXXX, OUT.”
At this point, both operators would switch over to the Talkgroup, TAC310. By doing this, they are communicating directly through the network, but only on their respective local repeaters in San Antonio and Casper.
Recap: Operators link up through the North America calling Talkgroup. Once contact is made, they behave professionally, by not tying up unnecessary hardware, and transfer the QSO over to the Talk-Around Channel, only using their local repeaters.
Earlier, Internet connectivity was mentioned as an advantage. If your QTH isn’t within range of DMR repeater, you’re not totally out of luck. You can obtain a DV4Mini.
The DV4Mini is a personal hotspot for the various digital systems available. It is, for all intents and purposes, a self-contained mini-repeater for your own personal use. Simply plug it in to either a computer, or Raspberry Pi, and configure the software. By doing this, the DV4Mini will receive your transmission, and jack into the respective DMR network you want to connect to via Internet linking. The DV4Mini will work with most of the digital protocols (D-STAR, System Fusion, etc.) in addition to DMR.
When it comes to equipment choices, the options are seemingly endless. On the high-end, you have companies like Motorola, Hytera, Tait, and Simoco. Contrasted with the Amateur offerings, you’ll notice that on the Professional side, there aren’t singular HT offerings. The manufacturers on this end of the spectrum have an entire suite of DMR products for various applications. For example, Hytera has over 15 different DMR HT radios, with a wide variety of configurations and applications ranging from covert tactical radios, standard HTs, to small form-factor radios with high-output. Motorola, as well, has over 10 different HT radios, with varying intended applications. Motorola and Hytera both are the current industry standards for repeater networks, which means, by going with one of these systems, you will have access to additional benefits when using a natively-compatible client radio.
Another benefit of the Professional radios, aside from them being inherently ruggedized, is that they often support advanced feature sets unavailable on Amateur systems. The system management applications alone open up a host of new tools for users, including telemetry, user-status, remote-disabling, unit tracking, coordinated dispatching, standalone file-transfers, etc.
Advanced Control Features May Include Coordinated, Remote-Dispatching and Patrol Tracking
Considering there is little cost differential, regarding the Professional vs Amateur repeater units, by going to a unified system, for example, Motorola Repeater+HTs+Mobiles, the ability to use the DMR system to its full potential is unleashed.
Many of the Professional offerings, contrary to popular opinion, do enable FPP, if so desired. The units can easily be placed into Wideband mode, to enable cross-compatibility between Analog and DMR systems.
In the middle-tier, is Vertex Standard. Vertex Standard offers its eVerge-series radios for DMR. Like MotoTRBO, eVerge is simply the product line from Vertex, designed to be compatible with the DMR standard. These offer some of the features of the Professional radios, and more than the dedicated Amateur equipment. The eVerge radios are ruggedly built, and are rated for submersability. The EVX-539 is the top-dog of HTs from Vertex, and retails for right around $350 USD. This would be a solid choice for someone looking for a ruggedized, DMR-capable radio, without spending $500+ on a Motorola or Hytera. If your radio is expected to function in an EmComm role, the Vertex eVerge series of radios should be the minimum standard to strive for. Vertex also offers DMR repeaters, as well as mobile radios in the eVerge lineup. The product offerings from Vertex, appear to be solidly designed, and built around complete system integration like the offerings from Motorola, Hytera, Simoco, and Tait. If you are familiar with the Yaesu FT-60R, the EVX HT radios are very similar in form. Vertex also has a hotspot DMR device, the EVX-Link, in the eVerge line.
Moving on to the Amateur gear that is available, the “Go-To” radio is the Connect Systems CS-750. The CS-750, and the Anytone AT-D858 are both clones of the Motorola XPR6550, and are both functionally identical. Both offer full compatibility with either Motorola, or Hytera, networks for voice communications. Some of the remote-control features from the Professional radios are present (stun/revive, timed lone-worker, etc.); however many of the advanced features like Geofencing, location-tracking, patrol monitoring, etc. are unavailable for these radios. As well, at least to my knowledge, there are no Amateur DMR repeaters currently being fielded. This places the Amateur equipment at a disadvantage, as there is no way to build a fully-featured system using Amateur equipment. Essentially, the Amateur equipment available will only operate in a client/slave role to the overall DMR network that is being hosted. It still affords the user with all the voice features, and remote connections through IPSC, but the “advanced” operational capabilities of the DMR system are unavailable to the Amateur equipment currently being fielded.
The only exception, in the market, is Kenwood.
Kenwood recently unveiled a suite of DMR radios intended for US Markets. It appears that this product line is for Professional users as an alternative to Kenwood’s P25 and NEXEDGE offerings; however, the availability of another quality manufacturer of DMR products is always a positive development for long-term system life. Once more information is provided by Kenwood, more conclusive assessments can be brought forth. Based on pricing, the Kenwood units appear to be right in-line with similar Amateur digital offerings from Icom and Yaesu.
So, with Vertex Standard (Yaesu) and Kenwood committed to manufacturing a suite of products designed for DMR, that leaves Icom as the only manufacturer from “The Big Three” that isn’t supporting DMR. Icom released a statement that they would continue to pursue the FDMA protocol, and would not be developing any DMR products. When you factor in the existing hardware support from over 42 different manufacturers, including two of the largest manufacturers of Amateur radio equipment, there is, obviously, a tremendous foundation for DMR that doesn’t currently exist with any of the competing digital systems.
Are there disadvantages to DMR? Of course. At the very edges of repeater coverage you will likely experience packet(data) loss on your end, if there isn’t a nearby networked-repeater to roam into. This results in the R2D2-voice data failure that occurs on all digital systems. That said, there are multiple analog repeaters on the same tower that I have accessed DMR on. While I cannot receive or transmit to those analog repeaters, I can hit the DMR repeater in the same location. Unscientific at best, but the results are why they are…
As well, the learning curve can be steep, when working with the Professional DMR radios. The CPS is extremely feature-dense, and it is easy to become overwhelmed when navigating through the configuration settings for your radio. This, ultimately, is a benefit, in that these additional features can be utilized, if your local repeater is from the same manufacturer as your radio, and the repeater owner is willing to allow the features to be utilized on the repeater. That being said, it is easy to screw up a setting when writing a new code plug from scratch. The easiest way to get a feel for using the programming software is to obtain an existing codeplug, and reverse-engineer it. This will assist in getting that learning curve to hit its tangent.
Another hurdle to overcome is the single/monoband issue. Many hams have come to expect dual-band operation. Keep in mind, like P25, DMR isn’t designed from the ground-up as an Amateur radio protocol. It was designed for Professional applications, with those users in mind.
Should you decide to get into DMR, you will have to Ham-up, and do what hams are supposed to do, and make the system work for your requirements.
DMR is an extremely robust, and highly capable system that, in many instances, will offer utility at, or in excess of, your communications needs.
Simple things like capacity/traffic doubling, and equipment rated for 100% duty-cycles can breathe new life into your existing legacy infrastructure; or provide system-scalability, and robustness, for new repeaters going on-line. Backward-compatibility with your existing analog equipment ensures that you aren’t making any sacrifices to move into new, often advantageous, territory.
Unlike HF, you can obtain reliable, repeatable, 24/7/365 Worldwide Comms if you're within range of a networked repeater: