Posts Tagged ‘70cm’

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!

FT-790R, 1W of Forlorn Hope? Part II

I’m treasuring my multimode, single band transceiver of yesteryear. They really ‘don’t make them like that any more’. When the FT-790R was manufactured, the UK still happily built cars that broke down and industrial action was a continuing popular pastime. Thank goodness the Japanese manufacturing success story was at its zenith.

In my previous blog, FT-790R, 1W of Forlorn Hope? I outlined my trepidation, my dreams and anticipated joy of achieving hilltop 70cm SSB contacts over phenomenal distances. This was in the face of an antagonist old-timer who thought a little too much RF had leaked into CPU of my head. So what was the reality?

Since taking delivery of the ‘790, I’ve managed to test most of the functions. With a radio of that age I shouldn’t have been surprised that the front panel illumination bulbs have long since ruptured their once-bright filaments, probably in the early 1990’s. A simple removal and refit of the battery tray, which holds eight ‘C’ cells was required to reconnect the internal power.

Powered by 8 'C' cells
Straight away, I connected a spare telescopic antenna for 2m to the front panel BNC and gave a call on FM. I enjoyed two great local contacts with stunning audio quality. Excellent audio reports came back too. Two quick tests with local station John GW4ZPL confirmed that SSB was working fine, albeit a little off frequency with his TS-2000 shack-in-a-box. Then we tried CW (my first CW attempt, ever). Apparently John hadn’t heard ‘chirp’ like mine since working Russian stations in the 1980’s! Luckily the operation manual comes with a full schematic diagram. I’ll have a look and see if a key click filter is misbehaving, for example.

Realistically speaking, I do know that a random 1W call on 70cm SSB is unlikely to attract an answer under normal conditions. My plan was to get to a modest local hilltop with a small beam antenna. If there was any tropospheric enhancement or lift, all the better. Unfortunately, the Welsh weather of late prevented this. Following the advice of Tim, G4VXE, a competition evening would give me a fighting chance.

Casually having a chat on D-STAR the other evening, someone mentioned that a 70cm activity evening was due to start in around half an hour at 20.00. No time to get up a hill and where the heck was my BNC to SO239 adaptor when I needed it to hook up my homebrew 9 element beam? So, if you can picture me leaning out of the upstairs window with a small, non-resonant, telescopic antenna pointing out at a jaunty angle, then this was my big chance. My only chance, so far.

I scanned the SSB portion of the band. Nothing. I twiddled the telescopic antenna and re-scanned. Nothing. But then, a quick call in a clipped English tone and a GD (Isle of Man) callsign. After a couple of failed attempts, antenna adjustment, more leaning out of window, the received signal came up to 5,9. The other station adjusted to my offset frequency and SUCCESS! - Gave me a 5,4. He said he was using 400W and a 23 element beam that wasn’t even pointing my way. The distance was around 73 miles or 117km, admittedly mostly over a great sea-path.

He’d been calling out rather robotically but had a chuckle and warmly wished me 73 when he heard about my operating set up. I forgot his callsign in the excitement and my bemused YL heard a whoop of joy from downstairs.

Anglesey to the Isle of Man
For every person that has told me that an FT-817ND would be the ideal rig for me (and it would), there has been another that has regretted getting rid of their FT-2/6/790R. My FT-790R is a solid, brick-like joy to own. When I get up to a hilltop to try it out again, I’ll be supremely hopeful!

FT-790R, 1W of Forlorn Hope?

“One Watt! One Watt! What on earth bl***y use is that to you?! I don’t know… (sigh)” exclaimed one local straight-talking OM on 2m FM yesterday. His response to my acquisition of a rather dated Yaesu FT-790R ‘portable’ transceiver was less than encouraging. It’s just as well I’m an optimist then. With a maximum output of one Watt on one band (70cm) I think I’ll need to maintain an upward outlook. But here’s the thing – I always find myself drawn to obtaining the more elusive QSO. You need a certain amount of eccentricity and tendency to self-harm to stand on a hilltop for hours calling out on 23cm FM QRP, for example. I was the first in the area to start calling CQ on DV mode on 2m. It took months before my first simplex QSO and then very little since. The higher bands and modes have their rewards in spades, however. The rush of excitement when you finally make that new or distant contact. The perceived camaraderie knowing someone has gone to the same quirky lengths as you to operate on a lesser used band/mode or from an unusual location. You feel you have made a meaningful and personal connection, at least for the duration of the QSO. Oh, it’s such a far cry from exchanging reports of 5/9 with the fiftieth Italian station running 1kW on 20m you’ve spoken to in one morning. Better still, no QSL cards. Any why the FT-790? Well, at just over £100 it’s one of the few portable internal battery-powered transceivers you come across before taking the small fiscal leap to an FT-817, for example. Either way, it will hold its value if my 2m friend’s prophecy becomes true. Anyway, so far so good. It’s powered up and seems to be fully serviceable. I’ve even managed a short-distance QSO with John GW4ZPL over the Menai Strait in the Caernarfon direction, just a few miles away. I’ll be out and about when the weather improves and will post my experiences here. Will it be too futile, or with a bit of luck will I make that elusive QSO I’m after? Will my single sidebanded plaintive cries of CQ reach a caring ear? Maybe I’ll be happily shouting “One Watt! One Watt!” from a heathery Welsh hilltop.

FT-790, forlorn hope?

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