Sunday, April 28, 2019

Which Digital Radio System? A Comparison of What is on Offer

A frequent question often asked at meetings and lectures is - "Which is the standard Digital Mode for Amateur Radio use". The answer is simple, they are all standards in their own right as are CW, AM, FM and SSB. Would anyone ask what is the standard Analog Mode? You could have a comprehensive and very active DMR network fine tuned and perfect for operation in your locality and somebody will buy D-Star instead, so it is really down to personal preference.

In Galway, both DMR and Yaesu fusion have proliferated equally with just three operators showing an interest in D-Star. At the same time, interesting and successful experiments have been carried out using the P25 and NXDN Digital Modes through the 2m Digital Gateway.

What is Digital Voice?
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Digital voice transmission is achieved by passing the analog voice signal through an Analog to Digital Converter where the digital signal is then passed though a Voice Encoder (Vocoder) which analyses and synthesizes the voice signals and compresses them and adds forward error correction to ensure accurate reproduction at the receiving end. The data is sent in packets with a header prepended to provide sync bits, routing instructions and user identity. Other data may be interleaved or substituted for voice to send text or pictures. To decode the signal off air the reverse of this process is employed where the demodulated signal from the receiver is passed through a digital to analog converter and to the speaker. The diagram below is a simple representation of the process.


 What makes digital voice more efficient?

As seasoned VHF operators know, the audio quality of an Analog VHF FM signal will deteriorate as the signal strength reduces. The resultant sound is a hiss accompanying the voice signal as the signal to noise ratio decreases. Digital voice is encoded with error correction and once decoded at the receive side, will remain noise free as noise is not part of the encoding process. Consulting the diagram below, it may be clearly seen that the audio quality of a digital signal can be maintained beyond the point at which the signal to noise ratio has decreased on the Analog Signal. Once the signal strength has become too low to decode due to packet losses, and subsequent increase of Bit Error Rate, the digital voice signal will deteriorate into R2D2 sounds where the signal finally becomes incoherent.


Barrett Communications have incorporated a Digital Voice system into their HF SSB Military/Bush radio system. At the point which the SSB voice signal is barely discernable it is possible to initiate the digital mode and the voice becomes immediately readable with good strong audio and no noise in the background. The link to the YouTube video maybe found Click >>Here.

Comparison of technical specifications

                                                 D-STAR                            DMR                           Fusion

Vocoder                                     AMBE+2                        AMBE +2                    AMBE+2

Forward Error corr                 Voice only                   Voice only                  Voice only

Modulation method                GMSK                              4FSK                         C4FM

Multiplex Method                  FDMA                             TDMA                        FDMA

Transmission Rate                4.8 kbps                          4.8kbps x 2                9.6 kbps

Bandwidth                               6.25 KHz                       6.25 KHz                    12.5 KHz

Developer                                JARL                              ETSI                           Yaesu


Signal Readablility

                                                 D-Star                DMR                     Fusion

Voice quality                            Good                 Good            Narrow mode - Good
                                                                                                  Wide mode     - Very Good

Signal Noise                            None                 None                      None    

Sync Robustness                     Fair                   Good                      Good

Sync Recoverability               Poor                   Best                       Best


Ease of Operation

D-Star

Memory selection  - Dial or GPS search.
Repeater Connection selection  - Dial
Mode selection method - Key Press
Radio Programming - Difficult/ Easy - Most recent Radios may be easier.
Learning curve - Steep
Cost - Expensive

DMR 

Memory selection  - Key Press
Repeater Connection selection  - Dial
Mode selection method - Fixed in memory
Radio Programming - Difficult
Learning curve - Easy
Cost - Reasonable

Yaesu Fusion

Memory selection  - Dial
Repeater Connection selection - Key press  
Mode selection method - Key Press
Radio Programming - Easy
Learning curve - Fairly Easy
Cost - Expensive

The problem with D-Star and Yaesu Fusion, is that they are proprietary and therefore cannot be reproduced by another manufacturer at a cheaper price. They can only be manufactured under license as in the case of D-Star which has been manufactured under license from Icom. Kenwood, by no means, produces D-Star radios any cheaper.

Services available in Galway

Currently we have a 70cms DMR Repeater and a 2m Multi-Mode Digital Gateway located on a high spot on the West Side of the City. Both have good coverage of approximately 30 miles radius. The DMR Network will expand to incorporate a 70cm DMR repeater at Abbeyknockmoy and there will be two more Repeaters coming on stream within the year. In short Co. Galway will be well covered using DMR and the coverage will also spill into neighbouring counties.

With four DMR Repeaters, it will be possible to set up roaming facilities. Roaming is similar to the way in which cellular phones work. The mobile station will scan the four Repeater frequencies and as the station moves to the footprint of a stronger Repeater, it will switch over to the frequency of the stronger signal. The shift from one Repeater will be barely noticeable to the operator. With the coverage of Co. Galway and into neighbouring counties, this system will be possible. Not all handheld or mobile radios will have this facility.

The Multi-Mode Gateway operating on 2 metres will provide the opportunity to utilize DMR, Fusion and D-Star. P25 or NXDN are possible through this system but are not active on a standard basis. Experimentation is encouraged so consult the Sysop - Steve, EI5DD, if you wish to try the P25 or NXDN system.

The EI2TBR Analog/fusion Repeater will be placed on a high site sometime in 2019 and will increase the range of Yeasu Fusion. This Repeater is also connected to the Wires-X network. It should be noted that there is a simplex Wires-X Gateway located in Salthill, Galway allowing experimentation with networking on the Yaesu Fusion mode.

The choice of Digital radio communication system is down to the facilities available in the locality.

Personal Hotspots, such as the DVMega, Shark Openspot, and MMDVM Hotspots allow use on all modes in areas not served by a Repeater or Gateway. Access to Reflectors or Talk Groups on a local or worldwide basis are possible via a Hotspot. It is necessary to have an internet or WiFi connection to use a Hotspot. 

In conclusion 

It would be prudent to evaluate the facilities in the area before deciding on which mode or system to operate. DMR would be the popular choice in Galway with 4 Repeaters due on line and also via the Mult-Mode Digital Gateway EI2GCD. Fusion is catered for by one Repeater, EI2TBR, and a Multi-Mode Gateway, EI2GCD and a Fusion Gateway EI2SHD. D-Star is currently only avaiable via the Multi-Mode Digital Gateway, EI2GCD.


DMR is a cheap and efficient entry into Digital Voice Communications. The Tytera and Anytone Handhelds are reasonably priced and boast good quality audio. Tytera produce a dual band MD9600 also of superb quality. One can buy Motorola gear at a higher price and have a "Rolls Royce" of a radio if deemed necessary. The flexibility of DMR is down to the way in which, you the user, programs the radio. It can be simple or complex you decide. The programming of the DMR Radio does tend to deter many but once mastered it is quite relaxing to build a code plug program and satisfying when it somes together. Bear in mind that this is a commercial standard, and NOT Amateur so take it as it is - NO silly amateur quirks. It is still an excellent mode. Audio quality is generally excellent. Only the pedantic would find fault in this mode.

Yaesu Fusion is a C4FM system produced for the Radio Amateur and has all the little unnecessary frills demanded by the hobby. It is relatively expensive but cheaper than D-Star. It is easy to get started as it is only necessary to add your call sign to start the radio. From there it is easy enough to operate without consulting the handbook but it is generally better to do so if you wish to know all the little secrets Fusion has to offer. The Wires-X system is a nice addition and it is possible to link to locations all over the world. In some cases there are many nodes connected to the one room so the chances of a QSO is reasonably likely. The facilities are not as well developed as D-Star but given time there are interesting devlopments on the cards. The audio quality of the Fusion mode is generally excellent.

D-Star is expensive. The AMBE Chip was developed specifically for D-Star and therefore nobody else can utilize it without license. The D-Star GMSK system was developed specifically for the Radio Amateur and consequently has a manual the size of a telephone directory and it is advisable to read this before set-up. The initial set up is cumbersome but once mastered it is plain sailing. The technology is old but has been developed to what it is today and almost all quirks have been programmed out of the firmware. The infrastructure is well populated and facilitates linking between Repeaters and Gateways. Icom continues to produce improved models of their radios and the quality is excellent. Audio Quality is generally excellent and much improved since the system was upgraded to the AMBE+2 Chip.

Check out the pages of the Radio dealers for the best quote on any of the above systems. EBay will often produce some good second hand gear but the cheap prices may introduce their own problems. Tytera MD 380 Handhelds are often available for less than 100 Euro. The UHF handheld will suit the Repeater whilst the VHF version will suit the Gateway. For a few shillings more the Anytone 868 or 878 Dual Band Handhelds are good value and boast good audio.