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Digital Voice (DV) – the new FM?
7 October 2013 | by Once upon a time FM swept away AM, but DV is taking its time despite some clear advantages.
Rob Law, MW0DNK, is a regular contributor to AmateurRadio.com and writes from Anglesey, Wales. Contact him at [email protected].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!
2 Responses to “Digital Voice (DV) – the new FM?”
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Having done some “under the hood” (or “under the bonnet”, if you prefer) testing with D-Star’s signal integrity under varying signal conditions, I’ve determined several things:
– D-Star doesn’t do as well as analog voice under equal-signal conditions. In other words, a 12dB SINAD analog signal – somewhat noisy, but perfectly copyable *if* the transmitting party is talking at a normal voice level – results in a significant amount of “R2-D2” effect on the voice in DV mode.
– If you want to check the integrity of a radio signal path for a D-Star signal, it is best to do so on analog first. If you hear signs of noise or multipath distortion, you know that the path may be marginal: If your digital signal becomes intermittent or drops off the “digital cliff”, at least you’ll know why!
– The typical D-Star radio, in DV mode, has an audio “compander” in the audio path which means that whether you speak very loudly and close to the microphone or a bit timid, the radio will make sure that you are properly modulated. In analog mode, the vary same radio doesn’t do this and some of the demonstrations to be found online comparing analog to D-Star seem to demonstrate that on analog, the user was just speaking very quietly or too far from the microphone and increasing the effective signal/noise of the link.
Having said all of that, D-Star can be fun (I have a ’91 HT) as are the linking capabilities, but its limitations and quirks must be understood or order to be effectively used.
Good overview.
You say “an AMBE chip can be freely bought for just a few dollars”, but when I go looking for them I see prices of over $40 when buying in quantities of 1,000 or more at a time. Single unit pricing is in the mid-$50’s per chip.
Not outrageous, but when it’s more than the retail selling price for dual band HT radios I wouldn’t exactly call it cheap. By the time the manufacturer and retailers add in their markup, putting an AMBE chip (and I looked at the AMBE-2020 model, there are newer and more expensive models of the chip out now) in a radio has got to raise the selling price by of the radio by somewhere in the range of $100 or more. That’s not including any supporting circuitry or R&D costs or increased power supply load costs or anything else.