The use of Digital Voice
Digital voice communication has become the norm in professional radio systems including mobile phone networks. The principal advantages for professional radio systems are (a) clarity and consistency of the voice and (b) encryption (not permitted in the Amateur Service). In addition, such systems normally operate in the VHF and UHF bands.
Amateur radio operates both in (a) the HF bands for long haul communications where SSB and CW remain the preferred modulations, and (b) in the VHF and UHF bands where digital communication techniques have joined SSB and FM as viable technologies.
The diagram below shows the analogue (SSB & FM) system, and below it, the digital system.
The digital communication channel consists of a codec that converts the analogue signal from the microphone into a digital stream followed by a digital modulator that imparts the digitised information onto the carrier for onward transmission. The corresponding receiver reverses the process by demodulating the radio signal to extract the digitised information followed by the codec that converts the digits back into analogue voice.
So the two key parts to digital systems are the codec and the modulator.
Various digital transmission protocols have been devised or adopted by the Amateur Service, and doubtless there will be others that follow. The different protocols are characterised by their choice of codec and modulation technique, although the three common Amateur systems have all adopted the AMBE codec.
The three common digital communication systems used in the Amateur Service are D-STAR, DMR (Digital Mobile Radio) and Fusion, the latter being devised by Yaesu.
All three systems use some form of FSK, Frequency Shift Keying where the 1’s and 0’s are converted into specific frequency offsets from the carrier frequency.
The DMR system uses 4-bit FSK modulation with discrete frequencies -1944Hz, -648Hz, +648 Hz and +1944Hz. Pre-registration is required.
D-STAR promoted by Kenwood, Icom and FlexRadio uses a voice encoder/decoder (a vocoder) rather than a codec. Also, the FSK variant is GMSK 2-bit that manages the transmitted spectrum much better that basic FSK. Pre-registration is required.
Fusion uses the C4FM modulation technique which is another 4-bit FSK modulation technique with discrete frequencies +900, +2700, -900 and – 2700Hz.
The availability of VHF and UHF radios with built in digital voice capability has created a lot of interest with supporters for each technology. Such is the spread of digital that a number of repeaters now support digital work through. The nearest DMR and D-STAR repeater is GB7SO located in Gosport, whilst the nearest Fusion repeater is GB7PO located in Portsmouth
Peter 2E1PHW has five years experience of using C4FM. So when he was asked what he liked about System Fusion and C4FM he was not short of an answer.
His first comment was unlike DMR and DStar C4FM is a new mode, you buy a System Fusion capable transceiver plug it in and you can push the mike button and talk, you don’t have to load code plugs or register to get on the air.
The other big advantage of the mode is that System Fusion Repeaters and transceivers can recognise if a FM signal is being used, called AMS Automatic Mode Select and switch seamlessly between modes so analogue and digital users can talk to each other. When the radio is switched on it remembers which mode was last in use and transmits in that mode.
Peter said that from his experience there was no flutter on a C4FM signal it is “all or nothing” and because of the better signal to noise ratio the digital signal is easier to listen to.
We have two C4FM repeaters in our area GB7PO Portsmouth 439.737.50 Mhz and GB7MT Southampton 439.575 Mhz these repeaters are licenced for digital communications only, not analogue.
If you would like to experience C4FM yourself why not join in on Peters C4FM Confusion Net on GB7PO Portsmouth, Sunday evenings 20.00 hours, or find them on Facebook
They are a friendly group and pleased to help anyone get started in a new mode.
Graham Coleman M5IIT