Digital Voice (DV) – the new FM?

Once upon a time FM swept away AM, but DV is taking its time despite some clear advantages.

I’ve had yet another stunning 5W mobile QSO on 2m this morning on my way to work. Several miles of clear, unbroken chat, without mobile flutter. DV mode delivers good quality voice against a noiseless background. It is sometimes claimed that coverage is roughly extended by 20% due to advantages of this mode, even. I doubt this is entirely true, but an excellent quality of communication is doggedly maintained before ‘falling off’ very quickly. It is quite robust and packed with extras. Ideal for V/UHF and it’s been around for several years.

Even the 2m band-plan in the UK lists all the simplex channels as dual FM/DV. I must admit that I and my friends keep traffic to the UK DV calling frequency (144.6125 MHz) to ensure anyone equipped with DV will hear us and join. If the current FM population heard our carriers on normal working channels, they would be quick to complain about the noise the noise as QRM.

This is what DV sounds like on your FM radio:

DV mode is famous for being the common mode that binds the larger DSTAR system (Digital Smart Technology for Amateur Radio), but excels as a simplex mode too. No analogue mode will embed your callsign for display, report your GPS position, over a low speed data link – all during a normal voice QSO, rounded off with an inoffensive little beep at the end of the over. (Because there is absolutely no background noise, it’s difficult to detect when someone has released their PTT). This is why DV is such a superb candidate to network via DSTAR. This is where radio marries the internet and we are its children.

The new Icom IC-7100 even has people quirkily chatting away on 4m with DV mode, which I gather works very well.

So what is DV mode made from? Well, your voice is encoded digitally using a vocoder optimised for voice communications in the same way your mobile phone does. The device that does this is called the AMBE chip (Advance MultiBand Excitation). Some people moan that this is unfair being a proprietary device, not being open source technology. However an AMBE chip can be freely bought for just a few dollars and uses proven, reliable technology. Inmarsat have been using it for years.

The digitised voice at 3600bps combines with an additional 1200bps (which you can do anything you want with! Think file transfer, photos, messaging etc.) before being modulated. The 3600bps voice data also includes 1200bps FEC (Forward Error Correction), which sends a little extra data in case any gets lost over the air. When bits are lost, the receiver uses this extra data the plug in the gaps. The modulation scheme is GMSK (Gaussian Mode Shift Keying), which is basically a form of phase modulation. You’ll also appreciate that all new modes often save bandwidth as well as improving quality and a DV carrier will happily fit into 12.5kHz channelised plans.

So, what are we waiting for? The manufacturers! The market is caught up in adoption stalemate, with Icom having settled for DV whilst others shun compatibility. But there are also homebrew DV options out there, with modulator/codec boards that will plug into your FM radio (via the packet port or tap into the discriminator) turning your analogue radio into a dual-mode digital delight.

My home DSTAR hotspot. Comprising 2m PMR radio (underneath), GMSK modem (top) and Raspberry Pi computer (bottom).

There are other digital modes out there too, all with different strengths and weaknesses – and they are interesting. But for the sake of everyday commonality and general take-up, I think DV has it.

So is it time to catch up with modern telecommunications techniques and move away from analogue FM? Maybe there’s something in the more ‘tactile’ feel of FM: The waxing and waning, the background hiss, the heterodyne-ing. You seem to know exactly what’s happening and what’s about to happen. So many modes – enjoy the hobby!

2m - a Band of Two Parts

Like the incoming tide washing away a beautifully constructed and elegant sandcastle, bit by bit, this week’s tropospheric ducting crumbled away with equal inevitability.

At its height, this radio superhighway, formed by different layers of air density and temperature, ‘hotwired’ parts of Europe together on 2m and above like they were connected with RG45 coax itself.

The opening was heralded, accurately, by William Hepburn’s tropospheric ducting forecast (see http://www.dxinfocentre.com/). This site also explains various propagation modes on VHF and above, in a clear and informative manner.

Let me tell you a little about my 2m station here in North Wales. I have a lovely Icom IC-910HX and my antenna is an HB9CV. Yes, you heard right, a peashooter. Two small elements phased together and put up on a pole on the side of the house. You see, I’m starting modestly because our national culture demands that we never do things the easy way. Added to the equation of difficulty, I have a range of 3000ft mountains not too far away from me. Did I mention the HB9CV was fixed? It points plaintively towards the south-east in cold, metallic expectation of flux. It points towards the long, golden beaches of the low countries, the rolling fields of northern France, the great cities of Western Europe and finally, the snow-capped Alps and beyond. But it also has a ‘heart’, a cardiod footprint in fact. The characteristics of the HB9CV trade-off a modest gain with quite a wide beam-width, with a sharp null behind it, giving it its heart shape.

The humble HB9'

You need good hearing. The receive gain is lacking for weak-signal work, but work it does. And this week I worked a station (OE2XRM) in Salzburg, Austria. 5,5 both ways! The pure magic of hearing a man who may be wearing lederhosen over what is normally a line-of-sight form of communication, is thrilling! A couple of stations from Bavaria followed and I was then content to listen as the chatter of stations from France and the Netherlands filled the band.

Later in the afternoon, fuelled by electromagnetic excitement, I ventured onto a local hill (178m/584ft.) with my lovely FT-817 and a 9-element 2m yagi. I also took along a 70cm yagi for fun.

A quick scan through the beacons bought in HB9HB in Switzerland! The Alps, the chocolate and cuckoo clocks! Unleashing my 5W on Europe didn’t produce much, unfortunately – just a couple of cheery calls from the UK, so I thought I’d swing the beam round to Spain. Now, the DX cluster was not showing any propagation in this direction and I wasn’t hearing any of the beacons on the north Spanish coast. But suddenly, out of nowhere, and at 5,9+, came a signal booming in. Danny, EB1LA, on the northern Spanish coast was stunned to hear me at 5,9 with my humble five Watts. What a result! I can only conclude that the beacons may have been situated too high to ‘tap in’ to the duct. How many people in the UK and Ireland must have been beaming away from Spain that day having assumed that no beacon = no path? I wonder. A quick scan on 70cm revealed no traffic, but I did receive the Netherlands beacon PI7CIS. Quite a trip for UHF.

My hilltop station

The two metre band can be sublime, but comes in two parts. The FM part of it is like an amateur ‘pub’. It seems the place where people gather in the evening to discuss their day and engage in chat (For example, one QSO I heard earlier this year was despairingly on the subject of “my favourite biscuits”). But the lower portion of the band has the elegance and sophistication of a Grand Cru. It’s as if it’s optimised in terms of antenna size, required power and number of users to give you a fighting chance of achieving glory in the aether. And I did.

Postscript 03 Oct. 2013: Just read EI3KD's excellent blog about this tropo opening. It coincidentally contains an audio capture of the end of my QSO with OE2XRM! Recommended reading (and listening).

A Quick RoIP Experiment

RoIP stands for Radio over the Internet Protocol and you can use this technology to connect remote radios over any distance, easily.

There are, of course, very complicated and elegant communications systems that do this already such as Echolink, D-STAR, Allstar, IRLP, Tetra and DMR/Mototrbo, but tonight I homebrewed my very own, modest private link in half an hour – and you can too!

If you have a rig with a packet data port and an external sound interface, such as a Signalink USB, then all you need is some software and a mobile phone, for example.

Signalink USB Interface with FT-817 set to 2m FM calling

Zello app with PTT

I used a fantastic free app called Zello. Zello is a walkie talkie emulator for your smartphone but also has a PC programme counterpart. The clever bit is that it has a VOX function with several adjustments for sensitivity and delay. My project enabled me to talk remotely over 2m using my smartphone!

So, by pressing ‘PTT’ on my smartphone and making a direct call to my computer, the VOX switch was activated on my Signalink USB interface and my FT-817 transmitted. When the squelch was opened on my ‘817 by the station I was talking to (and this is the clever bit) the Zello programme on my computer used its VOX feature to transmit back to my smartphone.

Zello GUI on Windows XP

Zello VOX settings

Now I know that Ham Radio Deluxe can be established to give full remote control over my rig, but that’s for another day. This is one solution to automatically link two radios that anyone can do.

More of a Shout than a Whisper…..

The key to converting your treasured and incredibly versatile FT-817 or similar into a digital powerhouse seems to be an A. computer and B. a sound card interface.

So, armed with a tidy little Signalink USB interface, I’ve been attacking digital modes with vigour, starting with WSPR (Weak Signal Propagation Reporting). The Signalink USB is basically a remote sound card in a box, powered by your computer’s USB. This solution allows controlled audio isolation between your rig and transceiver. Level adjustment is available on the front panel too which means you don’t have to navigate clumsily through several windows on your PC and adjust sliders with your mouse to optimise levels.

FT-817 and Signalink USB Interface

The interface connects directly to the data port of my FT-817 and provides a PTT function, if required. However, I’m also using a CAT interface which provides PTT (Push To Talk) as well as frequency configuration from the WSPR program that I’ve started with.

So, straightforward then? Nearly but not quite. There are a few small pitfalls to be aware of. Firstly, the ‘817 needs to be put into DIG mode as opposed to USB mode. This routes the input and output signal to the data port on the rear panel. The data port is inactive in SSB modes. Secondly, the correct data mode needs to be selected in the second-level menu, namely USER-U. This means that it will be operating in USB mode and the passband will be adequate. If this were to be set to RTTY or PSK, then the filtering for WSPR would be too narrow. WSPR signals are individually narrow, but several occupy the given passband. Finally you need to follow the instructions on Windows setup that comes with the Signalink box to the letter. One unchecked box or misplaced slider will drive you to madness.

WSPR Control Software

Apart from that, it seems to be ‘plug and play’! My first play on 40m with a random wire of some 20m in the back garden pulled in a VK straight away. I was heard up in the Norwegian Arctic Circle with 1W. Elation. Simply tuning to 472kHz pulled in a Dutch station with absolutely no special equipment.

For one whole day I exercised near-military discipline. I stayed on the 30m band all day long without jumping to other bands. 1W into my rear-garden wire antenna reached Israel and the Arctic Circle again. East Coast US stations starting to come in at 20.00 GMT and I was reaching the Mid-West by late evening. Within ten minutes this morning on 17m I was heard in New South Wales and Iceland.

10 mins on 17m!

10 mins on 20m!

I finally unleashed my single Watt on the 20m band for the first time this afternoon, immediately yielding a nice path to the Philippines as well as Europe and the East US.

No wonder this aspect of the hobby is so absorbing. I’m absolutely addicted. Did the developer, Joe Taylor, K1JT, realise what he was unleashing on us? A ‘big shout’ goes out to the man who invented WSPR!