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