DMR is an open communication standard to help specify a digital communication system with low complexity, low cost and interoperability across brands, so radio communications purchasers are not locked into a proprietary solution. Motorola is often associated with DMR under its MotoTRBO solution branding but is not in any way the only vendor currently offering equipment that uses this underlying technology and has since made it an open standard.
In practice, many brands have not adhered to this open standard approach and have introduced proprietary features that make their product offerings non-inter-operable. Amateur radio examples of this include early digital voice solutions offered by Alinco in the late 1990's and more recently, Icom D-Star and Yaesu Fusion.
Today, many vendors offer DMR equipment such as TYT, Radioddity, Retevis, CSI, Anytone and now Alinco as the only amateur brand to offer DMR.
DMR will lessen the use of FM based communications, but not likely replace it in the amateur radio spectrum. Other digital voice technologies that are proprietary such as Yaesu Fusion and Icom D-Star as well as other open standards such as NXDN and P25 will however impact the long term use of FM or N-FM communications.
DMR is the fastest growing and most widely adopted digital voice technology though and should be explored by existing and new amateur radio operators.
There are a few reasons why DMR has gained in popularity. Price is one major driving factor since no single vendor owns the underlying technology. Other technologies are owned by specific vendors and limit overall adoption due to higher costs.
DMR is unique compared to all other digital voice modes because it is based on TDMA which permits the ability for two separate two way discussions to take place in the same spectrum as a single FM discussion.
DMR also has better signal to noise ratio for "full copy" compared to FM as well as other competing digital voice technologies.
More on the "technicals" can be found on the Wikipedia page here:
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Icom is known for its D-Star branding. The underlying technology is GFSK which stands for Guassian Frequency Shift Keying.
Alinco offered GFSK add on boards for some of its radios before Icom rolled out D-Star in the early 2000's. Both vendors equipment were not compatible with each other. D-star is the oldest common digital voice system, but has stagnated due to little moderately priced equipment compared to DMR which has many vendors and price points in evaluating what radio makes the most sense to purchase.
C4FM is the underlying technology behind Yaesu Fusion. C4FM is based around FDMA or Frequency Division Multiple Access.
This is actually a much better algorithm compared to TDMA based DMR, C4FM offers both a wide and narrow bandwidth option which DMR does not offer. but is limited to just Yaesu equipment which holds it back. Yaesu does have a better pricing model compared to icom, but many who purchase a Fusion capable radio are hard pressed to find as many repeaters as found on DMR and use if on FM.
dPMR may confuse some people as it is FDMA based but not compatible with C4FM. dPMR is similar to NXDN which is focused on commercial users. dPMR is focused on unlicensed users
dPMR is an open standard, but many vendors have issues with compatability.
It is good to consider dPMR like the FM based FRS radios in the US for those looking at digital voice options.
A few countries have set aside channel plans for dPMR to share the spectrum with FRS like devices.
Icom does offer the most mature digital voice technology with associated end user and infrastructure equipment solutions. The topology for non-internet related linking of repeaters is not as robust compared to DMR or C4FM though based on how users are registered.
Icom is an investor in the Japanese Amateur Radio league which owns the D-Star standard. Kenwood has a much smaller investment in D-Star. Both vendors do not offer radios to compete in price compared to DMR or C4FM.
One major feature that D-Star does have is high speed data transmission capability, but again is limited to a small range of compatible equipment and may pose interference to other users of surrounding spectrum.
Yaesu Fusion has been on the market as an amateur technology for about half as long as Icom D-Star. It offers automatic mode sensing between C4FM and analog FM communication which is a nice feature.
It is perhaps the easiest to set up digital voice mode but still relies on older and less flexible control and routing compared to D-Star or DMR when thinking about internet connectivity.
There are theoretical ways to transfer data files over C4FM like D-Star, but has not been implemented in Yaesu equipment. DMR for comparison does offer data transfer, but not as fast but can work in the same spectrum at the same time. C4FM does not permit that.
P25 and NXDN are commercial focused digital voice modes based on FDMA. These are most similar to Fusion and dPMR but are not compatible. Both P25 and NXDN are open standards though.
There are users of this equipment in the amateur radio spectrum using surplus commercial equipment. A major community of first responders and mission critical applications like rail roads use these two modes as a replacement for FM.
In time, we may see more adoption of both P25 and NXDN but is held back by not being supported by the vendor community with moderately priced equipment.