Steve Wright, EI5DD has now updated his comprehensive manual for anyone in Ireland with an interest in the DMR, C4FM and D-Star digital modes. The 62-page document is a substantial increase on the 44-page 2020 edition and should be of interest to anyone starting off and even to established users who want to get a better understanding of the various digital systems.
While the guide is primarily aimed at radio amateurs in Ireland, a lot of the material covered should be of interest to radio amateurs in other countries as well.
Steve Wright, EI5DD has now compiled a comprehensive manual for anyone in Ireland with an interest in the DMR, C4FM and D-Star digital modes. The 44-page document should be of interest to anyone starting off and even to established users who want to get a better understanding of the various digital systems.
While the guide is primarily aimed at radio amateurs in Ireland, a lot of the material covered should be of interest to interested parties in other countries as well.
I find it interesting to follow how digital radio is gradually replacing its analogue counterparts.
The Electronics Communications Committee (ECC) of CEPT recently released an article outlining how digital radio could be introduced to the Marine VHF bands.
At present, the allocated frequencies are in the frequency band 156.025 MHz to 162.025 MHz and are mostly FM i.e. analogue.
The channel spacing is 25 kHz which means that there is a lot of spectrum on the band not being used and there is the potential to squeeze a lot more channels in.
In the article, it states that "with the exception of the automatic identification system (AIS) on channels AIS1 (161.975 MHz) and AIS2 (162.025 MHz) and digital selective calling (DSC), currently all the remaining listed channels are used for analogue voice communication. With the digital data exchange allocations in future, some channels will be used for data transfer and not for voice communication any longer. New digital radios need to be developed, which is different equipment than the current voice communication radios. As the result, the frequencies in the VHF maritime mobile band will be shared by four different systems: analogue voice telephony, DSC, AIS and digital data exchange."
There is a huge legacy issue though to overcome first. For a business, they can just swap out the gear and radios when they upgrade from FM to digital. With the Marine VHF band, there is a huge decentralised user base that will take years to change over.
The article suggests the use of DMR (dPMR) as introduced in recent years on the amateur bands..."The technical candidate solution is dPMR (digital Private Mobile Radio) – a technology currently used in land mobile communications as a replacement for analogue FM voice communication in both VHF and UHF bands. dPMR has been standardised by ETSI, the European standards organisation."
The article suggests that initially, the new digital channels which take up 6.25 KHz and these would fit between the existing analogue FM channels as shown below on the left.
Eventually the FM channels would be phased out and be replaced with all digital (above right). The end result would be a doubling of the number of channels on the Marine band.
It not hard to imagine perhaps a transition period where dual mode (FM & DMR) radios would exist and then there would have to be a date whereby everyone would need to changed over to digital.
In tests done in Estonia, they reported..."Participants in the test were generally positive about the introduction of digital communication with the range being the same (or better) than the range for analogue communication. At the maximum distances, the digital communication remained understandable (d = 19,6 NM) while the analogue communication experienced very high noise and was not understandable."
In the tests done in the Netherlands, they reported..."The users and the observers found the digital voice quality the same or better than the analogue one. The users reported back that listening to digital voice with noise reduction made it easier and less intensive to listen. The users concluded that the digital transmission of voice enabled the same functionality of operation of the ship as for an analogue radio system."
On the fourth Tuesday of every month, we have a local net on 70cms about digital radio. While its primary purpose is to generate some activity on the local DMR repeater and talk groups, the conversation this month turned to the subject of digital voice on the HF bands. It was then that the digital mode FreeDV was mentioned and I have to admit it was the first time I had heard of it.
Perhaps I had come across it before in a news item and I didn't take much notice but it sounds like an interesting mode. It seems that while it started back in 2012, it's only in more recent years that the performance has surpassed that of SSB when the signals are very weak and in the noise.
I thought it might be an idea to put up a blog post so that perhaps a few more people might be interested in trying out this new mode.
My first question I had about this digital mode was how could it possibly work on the HF bands with all of the multi-path interference and fading? Well, it seems as if it's not a case of all or nothing. The signal is broken up into separate sub-carriers and the information is modulated on these by phase shifting.
The modulation mode is OFDM which stands for Orthogonal Frequency Division Multiplexing. The spacing is chosen so that the subcarrier nulls coincide with the peaks of other subcarriers. By doing this, the subcarriers don't interfere with each other.
It works on the principle that even if there is some selective fading on some of the subcarriers, enough information is getting through on the others to allow a conversation to be held.
Waterfall display of a FreeDV signal showing selective fading
FreeDV... What it is and what it's not
First of all, what it's not. It's not for DX-ing, chasing DXCC countries, contesting, chasing squares or awards. It's not there to replace SSB or even your local net on HF.... (for the moment ;o)
COMREG, the Irish licencing authority defines Amateur Radio as follows..."The amateur service is defined by the International Telecommunication Union (ITU)1 as: A radiocommunication service for the purpose of self training, intercommunication and technical investigations carried out by amateurs, that is, by duly authorised persons interested in radio technique solely with a personal aim and without pecuniary interest"
The self training and technical investigation aspects are what FreeDV and digital voice are really about. It's a new technology that is evolving and will appeal to people who want to learn about new developments and how they might be implemented.
The big advantage of FreeDV is that you can use it with existing HF radios with SSB. It's not like DMR, C4FM or D-STAR which require a completely new radio to be bought. If someone is set up for digital modes like FT8 then they have the equipment already. As such, it can be used on any band where there is SSB... HF or VHF.
An interesting feature is that FreeDV demodulator can automatically acquire signals with a frequency offset of up to ±200 Hz. This means that you don't have to be the exact same frequency as the other station. You can see how this might be of value in say a net on SSB where everyone is trying to get on the correct frequency.... and there's always one who is off a bit :o)
Modes...
It's also worth mentioning that there are several FreeDV modes as well. 700D... This is for weak signal work on the HF bands and is claimed to work down as far as a signal to noise ratio of -2dB. The 700C version was introduced in 2017 and was replaced by the better 700D mode in 2018. 800XA... Similar to C4FM except over FM radio. Designed for VHF work. 1600... This is for a higher quality audio signal. 2020... 8 kHz of audio bandwidth is fitted in just 1.6 kHz of RF bandwidth. Delivers high quality audio when the signal are strong. Introduced in 2019.
Video... The video below from the 2018 RSGB Convention gives a good outline of FreeDV and its use. Even if you have only a passing interest in digital voice, it's worth a watch.
